EFFECT OF ALLOYING ADDITIONS OF ALUMINIUM AND IRON ON THE CREEP RESISTANCE OF TI - 12CR (WT.%) B y Harish Narasimhan Chakravarty A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Materials Science and Engineering - Master of Science 2019 ABSTRACT EFFECT OF ALLOYING ADDITIONS OF ALUMINIUM AND IRON ON THE CREEP RESISTANCE OF TI - 12CR (WT.%) By Harish Narasimhan Chakravarty Titanium (Ti) and its alloys are among the desired materials for biomaterial, marine, and - Ti) alloys exhibit enhanced strengths and hardness values after thermomechanical processing (T MP) due major driving factor in titanium research. There have been significant efforts to replace high cost alloying elements currently being used in the indust ry , like V, Sn , Mo, Nb, and Zr, with cheaper ferro - chrome substitutes. The goal going forward for metastable - Ti alloys is achieving comparable mechanical properties to the commercially used Ti - - - phase usually tend to exhibit higher strengths and hardness values than the ones that do not - phase. - Ti alloys, Ti - 12Cr - 1Fe - 3Al (wt. %), Ti - 12Cr - 3Al (wt.%), and T - 12Cr (wt. %), were experimentally obtained at a temperature of 683K and applied stress es between 200 - 3 50 MPa. The relationship between microstructure and creep properties were investigated. X - ray diffractometer (XRD), optical microscope (OM), and scanning electron microscope (SEM ) were used to help characterize the microstructure before and after creep deformation. The hardness of alloys increased after heat treatment for 48 hrs at 683 K /or the showed that Ti - 12Cr - 1Fe - 3Al (wt. %) wa s the most creep resistant and Ti - 12Cr (wt. %) was the least creep resistant iii ACKNOWLEDGEMENTS First, I would like to sincerely thank my advisor, Dr. Carl Boehlert, for having given me this opportunity to be a part of his research group. He has constantly guided and supported me in and has always been available to discuss and resolve any technical aspects of my research. As a person, he was always a friend to me and has show n great care and understanding , which has made my time here at MSU a wonderful one . I would like to thank my professors, Dr. Alex Zevalkink, Dr. Nathan Mellott, Dr. Wei Lai, Dr. Eisenlohr , for helping me understand various aspects of materials science and providing me with a strong foundation for my research work. I would like to thank the current and former members I would like t o thank Prof. Masahiko Ikeda for providing the materials used in this research study . I would like to specially thank Jo A nn Ballor for all her help in the lab , especially with polishing for their help in various aspects of my research work , and their support as colleagues and friends. I would like to thank Per Askeland for his help in working with the scanning electron microscope. e , support , and their constant belief in Verma, Swapna Sindhu Mishra, Bansar i Chauhan, Aishwarya Bhatlawande, and Kshitija Deshmukh, for constantly being there for me , and making me feel at home away from home. Cheers to all of you. I would like to finally thank my family and my cousins : Arjun , Anand , Deepika, Navya and Krishna for their support and faith in me. iv TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ ......................... vi LIST OF FIGURES ................................ ................................ ................................ ...................... vii KEY TO SYMBOLS AND ABBREVIATIONS ................................ ................................ ........... x CHAPTER 1 ................................ ................................ ................................ ................................ ... 1 INTRODUCTION, BACKGROUND, AND MOTIVATION ................................ ....................... 1 1.1 Rationale and Research objective ................................ ................................ .................... 1 1.2 Fundamental of Ti and Ti alloys ................................ ................................ ...................... 5 1.2.1 Crystal structure ................................ ................................ ................................ ........ 5 1.2.2 Alloying Elements and Phase Diagrams ................................ ................................ ... 6 1.2.3 Ti - Al Phase Diagram ................................ ................................ ................................ 8 1.2.4 Ti - Cr phase diagram ................................ ................................ ................................ . 9 1.2.5 Ti - Fe phase diagram ................................ ................................ ............................... 11 1.2.6 Th e effect of substitution alloying elements and precipitation on the strength of Ti - alloys 12 1.2.7 The effect of heat treatment on the strength of Ti - alloys ................................ ........ 14 1.3 Creep Testing and creep mechanisms ................................ ................................ ............ 15 1.3.1 Creep testing ................................ ................................ ................................ ........... 15 1.3.2 Creep mechanisms ................................ ................................ ................................ .. 17 1.4 Work performed ................................ ................................ ................................ ............. 21 CHAPTER 2 ................................ ................................ ................................ ................................ . 23 EXPERIMENTAL PROCEDURES ................................ ................................ ............................. 23 2.1 Material ................................ ................................ ................................ .......................... 23 2.2 Microstructural Characterization ................................ ................................ .................... 24 2.2.1 Metallographic sample preparation ................................ ................................ ......... 24 2.2.2 Scanning Electron Microscopy ................................ ................................ ............... 25 2.2.3 Sample preparation and measurement of precipitate volume fraction .................... 26 2.2.4 X - ray diffraction analysis ................................ ................................ ....................... 27 2.3 Creep Testing ................................ ................................ ................................ ................. 30 2.3.1 Sample configuration ................................ ................................ .............................. 30 2.3.2 Creep test experiment ................................ ................................ ............................. 31 2.4 Heat treatment ................................ ................................ ................................ ................ 33 2.5 Hardness measurements ................................ ................................ ................................ . 34 2.6 Creep data analysis ................................ ................................ ................................ ......... 35 CHAPTER 3 ................................ ................................ ................................ ................................ . 37 RESULTS AND DISCUSSION ................................ ................................ ................................ ... 37 3.1 Micros tructural characterization ................................ ................................ .................... 37 3.1.1 XRD results ................................ ................................ ................................ ............. 37 v 3.1.2 SEM result s ................................ ................................ ................................ ............. 45 3.2 Mechanical Properties ................................ ................................ ................................ .... 58 3.2.1 Vickers Hardness ................................ ................................ ................................ .... 58 3.2.2 Cree p tests results ................................ ................................ ................................ ... 60 CHAPTER 4 ................................ ................................ ................................ ................................ . 69 CONCLUSIONS ................................ ................................ ................................ ........................... 69 ..70 vi LIST OF TABLES Table 1. 1 Important characteristics of Ti and Ti based alloys as compared to other structural metallic materials based on Fe, Ni, Al, and Mg (G Lütjering and Williams, 2003) ....................... 1 Table 1. 2 Lattice parameters for Ti allotropic phases ................................ ................................ .... 6 Table 1. 3 List of creep mechanisms and their corresponding creep exponent values ................. 21 Table 2. 1 Measured compositions of the alloys studied** ................................ .......................... 23 2 phases from literature ................................ ................................ ................................ ................................ ........ 30 Table 3. 1 Summary of the grain sizes of the as - processed (BCC) and creep tested (transformed BCC) micr ostructures ................................ ................................ ................................ ................... 52 Table 3. 2 Summary of precipitate volume fractions of the alloys ................................ ............... 58 Table 3. 3 Summary of creep tested specimens ................................ ................................ ............ 63 vii LIST OF FIGURES Figure 1. 1 Cast Ti - 6Al - 4V wing attach fitting for the F - 22 military aircraft (Lütjering, G., and Williams, 2003). ................................ ................................ ................................ .............................. 2 Figure 1. 2 Schematic diagram of artificial hip implant (Chunxiang et al., 2003) ......................... 3 - 2006 (Chunxiang et al., 20 11). ................................ ................................ ................................ ........ 4 - phase, - - phase for pure Ti. ............................ 5 nts, c) - - eutectoid elements with titanium (A=Ti and B=alloying element) (Takashi Maeda, 1988) ................................ ................................ ................................ ................... 7 Ms refers to the start of martensitic phase transformation (Leyens and Peters, 2006). .................. 8 Figure 1. 7 Ti - Al phase diagram (Massalski, 1990). ................................ ................................ ...... 9 Figure 1. 8 Ti - Cr phase diag ram (Baker, 1992). ................................ ................................ ........... 11 Figure 1. 9 BSE SEM photomicrographs of aged Ti - 12Mo - 6Zr - 2F sample which exhibited fine - phase part icles. ................................ ................................ ................................ ......... 13 Figure 1. 10 Schematic temperature vs time plot for Ti - - phase forms at lower temperatures and for longer aging t imes (Hickman, 1969). ................................ ......................... 14 Figure 1. 11 Schematic diagram of a creep test (https://www.twi - global.com/technical - knowledge/job - knowledge/creep - and - creep - testing - 081) ................................ ............................ 16 Figure 1. 12 Typical creep strain vs t ime plot for pure metals (Kassner, 2015) ........................... 16 Figure 1. 13 Schematic representation of a) Nabarro - Herring creep and b) Coble creep (Meyers and Chawla, 2008) ................................ ................................ ................................ ........................ 19 Figure 2. 1 An image showing the three as - processed alloys with EDM cuts from where the samples for the creep tests were extracted. The top, cross - section and thickness orientations are shown. ................................ ................................ ................................ ................................ ........... 24 Figure 2. 2 A photograph showing Tescan Mira3 SEM chamber ................................ ................ 26 Figure 2. 3 A schematic showing the geometry of the test specimens used in the creep experiments. viii The dimensions are in millimeters ................................ ................................ ................................ 28 Figure 2. 4 A photo of a sample representative of those used in the cr eep experiments. The sample dimensions are provided. ................................ ................................ ................................ .............. 31 Figure 2. 5 Photograph showing the creep frame and the sample setup represen tative of all the creep experiments. ................................ ................................ ................................ ........................ 32 Figure 2. 6 Image of the furnace used for the heat treatments. The real - time sample temperature was measured during the heat treatment. ................................ ................................ ...................... 34 Figure 2. 7 Image of the Vickers hardness testing machine. ................................ ........................ 35 ks ................................ ................................ ................................ ........................ 37 for 12 hrs ................................ ................................ ................................ ................................ ....... 39 MPa and 683 K. ................................ ................................ ................................ ............................ 41 Figure 3. 4 XRD data in 483 K, 583 K, 623 K, and 683 K and for 12 hrs ................................ ................................ ........... 42 Figure 3. 5 XRD plots comparing as - processed, HT, and creep tested conditions ....................... 44 Figure 3. 6 BSE SEM photomicrographs of the cross section of the as - processed microstructures of a.) TC b.) ................ 46 Figure 3. 7 BSE SEM photomicrographs of the microstructures of a.) TC b.) TCA and c.) TCFA alloys taken from the cross section of the grips of creep tested samples and etched. .................. 48 Figure 3. 8 BSE SEM photomicrographs of the microstructures of a.) TC b.) TCA and c.) TCFA alloys taken fro m the cross section of the gage of samples creep tested at 350 MPa and 583 K . 50 Figure 3. 9 BSE SEM photomicrographs of the as - processed a.) TC, b.) TCA, and c.) TCFA alloys. ................................ ................................ .................. 53 Figure 3. 10 SE SEM photomicrographs of a.) TC, b.) TCA, and c.) TCFA alloys after creep testing at 350 MPa - like structures indicating presence of secondary ................................ ................................ ... 54 Figure 3. 11 BSE SEM photomicrographs of a.) TC, b.) TCA, c.) TCFA alloys with corresponding ImageJ contrast and thresholdin g images. ................................ ................................ .................... 56 Figure 3. 12 Hardness values for the alloys in the different conditions showing an increase from ix as - processed to the he at - treated grip section and a further increase in the crept gage section ..... 59 Figure 3. 13 Total Strain vs time heat treatment plots for a.) TC, b.) TCA, and c.) TCFA showing a slight drop in deflection at lower times and stable deflection at higher times. .......................... 61 Figure 3. 14 Representative strain rate vs time plot for TCFA alloy at 350 MPa and 683 K displaying a plateauing of strain rate curve with increasing time indicating secondary creep regime. The plot is representative of all th e creep experiments. ................................ ................................ 64 Figure 3. 15 Creep strain vs time plots for TCFA alloy at 683 K and varying applied stress levels ................................ ................................ ................................ ................................ ....................... 65 Figure 3. 16 Creep strain vs time plots for the three alloys tested at 683 K and a.) 200 MPa, b.) 250MPa, c.) 300 MPa, d.) 350 MPa showi ng a consistent creep resistance trend. ...................... 66 Figure 3. 17 Comparison creep strain vs time plot for TCFA alloy tested at 683 K and 350 MPa before and after heat treatment, with Ti - 6Al - 4V tested at similar conditions. ............................. 68 x KEY TO SYMBOLS AND ABBREVIATIONS 3D Three - dimensional Al Aluminum ASTM American society for testing and materials bcc Body - centered cubic BSE Backscattered electron CP Commercially pure Cr Chromium Cu Copper EBSD Electron backscattered diffraction EDM Electrical discharge machining elongation - to - failure ssmin / / aa Steady state strain Fe Iron fcc Face centered cubic hcp Hexagonal closed - packed Mg Magnesium Mo Molybdenum n creep/stress exponent N Nitrogen Nb Niobium Ni Nickel RT Room temperature xi O Oxygen stress SE Secondary electron SEM Scanning electron microscopy SiC Silicon carbide TEM Transmission electron microscopy Ti Titanium V Vanadium wt.% Weight percent XRD X - ray diffraction Zr Zirconium TC Ti - 12Cr - 1Fe - 3Al TCA Ti - 12Cr - 3Al TCFA Ti - 12Cr - 1Fe - 3A 1 CHAPTER 1 INTRODUCTION , BACKGROUND, AND MOTIVATION 1.1 Rationale and Research objective Ti and its a lloys have been commercially used for over 50 years and have made significant contributions to many fields such as aerospace, chemical plant, oil and gas, marine, thermal, and nuclear power stations. The principle strengths of Ti compared to other structural metallic metals based on iron (Fe), nickel (Ni), aluminum (Al), and magnesium (Mg), are its high specific strength (high strength and low density), excellent corrosion resistance, and the capacity to withstand extreme temperatures (Leyens and Peters, 2003a) . Table 1.1 summarizes some of the important characteristic of Ti and its alloys with other commonly used structural alloys. Table 1. 1 Important characteristics of Ti and Ti based alloys as compared to other structural metallic materials based on Fe, Ni, Al, and Mg (G Lütjering and Williams, 2003) Fe Ni Al 1538 1455 660 RT E (GPa) 215 200 72 Yield Stress Level (MPa) 1000 1000 1000 Density (g/cm3) 4.5 7.9 8.9 2.7 Comparative Corrosion Resistance Low High Comparative Reactivity with Oxygen Low High Comparative Price of Metal Low Medium 2 About 80% of Ti production is for aviation and aerospace industry. Approximately 21% of B - 1 Ti , mainly for the manufacture of the fuselage, wings, skins and load - bearing component (Chunxiang et al. , 2011) . In the F - 15 fighter structural materials, Ti - alloys occupy a capacity of 7000 kg, which is about 34% structural weight of the airframe . For Boeing 757 aircraft the structural weight percentage is 5%. And as for DC10 aircraft produced by McDonnell Dou gl as, Ti - alloys accounts for more than 10% of its structural weight (Liu et al. , 2008) . Figure 1.1 shows an example of F - 22 military aircraft wing fitting made from Ti - alloys. Figure 1. 1 Cast Ti - 6Al - 4V wing attach fitting for the F - 22 military aircraft (Lütjering, G., and Will iams, 2003). 3 T i and its alloys are superior to many materials such as stainless and pyrolytic carbon in terms of mechanical properties and biocompatibility. They are widely used in biomedical devices and components especially as hard tissue replacements be cause of their desirable properties, such as relatively low modulus, good fatigue strength, corrosion resistance, and biocompatibility. One of the most common applications of Ti and its alloys is in artificial hip joints that consist of an articulating bea ring (femoral head and cup) and stem as depicted in Figure 1.2 (Chunxiang et al. , 2003, 2011) . Figure 1. 2 Schematic diagram of artificial hip implant (Chunxiang et al., 2003) Cost is a major driving factor in Ti research. Processing Ti from its naturally occurring oxide state is expensive due to inert atmosphere requirements and pos t - processing costs. Figure 1.3 shows the US producer price index for finished rolled titanium (Producer Price Index, PPI) changes during 1971 2006 years (Ivasyshyn and Aleksandrov, 2008) . Replacing commonly used alloying elements like vanadium (V), niobium (Nb), and molybdenum (Mo) with lowe r 4 cost alternatives like Chromium (Cr) and Iron (Fe) is a potential means to reduce the cost s (Cotton et al. , 2015) . Figure 1. 3 ween 1971 - 2006 (Chunxiang et al., 2011). As Ti and its alloys are often used in critical applications and it becomes essential to thoroughly understand their mechanical properties and deformation behavior. Also, Ti - alloys, being relatively newer materials compared to steel or Al, it is imperative to o btain a detailed characterization of their microstructures and mechanical properties . J. Bal l or et a l. studied microstructu re - property relationships of four Ti - 12 Cr (wt. %) alloys and the effect of individual alloying elements was investigate d (J. Ballor et al. , 2019) . This thesis study was intended to further the understanding of the mechanical properties of the s ame alloys. 5 1.2 Fundamental of Ti and Ti alloys 1.2.1 Crystal structure Ti is a transition metal belonging to the IV - A group in the periodic table with an atomic number of 22, atomic weight of 47.88, and has the low temperature equilibrium phase with hexagonal c rystal structure designated as (a = 0.295nm, c = 0.468nm, c/a = 1.587 ( at 298 K)) . Ti is allotropic and exhibit s more than one crystal structure . At 1155 K , the hcp - phase transforms to the high temperature - phase ( a = 0.3307nm (1173K) ). Apart from the equilibrium phases, Ti alloys also exhibit n on - equilibrium phases such as orthorhombic martensite ( ), - phase, retained - phase and - phase (Leyens and Peters, 2003b; Lütjering, G., and Williams, 2003) . Figure 1.4 represents schematic unit cells of corresponding lattice parameters are provided in Table 1.1 (Hickman, 1969; Welsch, Boyer and Collings, 1993; Gerd Lütjering and Williams, 2003) . a) c) b) Figure 1. 4 - - - phase for pure Ti. 6 Table 1. 2 Lattice parameters for Ti allotropic phases 1.2.2 Alloying Elements and Phase Diagrams Alloying elements of titanium are usually classified by the ir ability to stabilize either the or phase. A few neutral elements stabilize neither of the two phases . Alloying elements of titanium are roughly classified into four main groups according to the type of phase diagram formed which are schematically illustrated in Figure 1.5 (Takashi Maeda, 1988; Welsch, Boyer and Collings, 1993; Donachie, 2000) . They are described as follows. a) o r phase . Zr and Hf also exhibit this form of phase diagram b) stabilizing elements - phase regions with an increasing - transus temperature . Examples are carbon (C) , nitrogen ( N ) , oxygen (O) , aluminum ( A l), and gallium (G a ) . c) - isomorphous - molybdenum (Mo), and t echnetium (Tc). d) - eutectoid elements : - stabilizing ele ments, - phase , and decompose the - - phase plus a compound at low temperature s . Examples Phase a (pm) c (pm) c/a APF 332 - - 0.68 460 282 0.613 0.68 295 466 1.588 0.74 7 are hydrogen ( H ) , beryllium ( Be ) , silicon ( Si ) , chromium ( Cr ) , manganese ( Mn ) , and iron ( Fe ). Figure 1. 5 - - eutectoid elements with titanium (A=Ti and B=alloying element) (Takashi Maeda, 1988) A schematic three - dimensio nal (3D) phase diagram in Figure 1.6 shows the influence of - - phase stabilizers on the phase s constitutions (Leyens and Peters, 200 3b) - phase stabilizers ( about 1 to 2 wt.% ) to th e - phase alloys, results in the formation of near (Joshi, 2006) . The two - phase mixture, - alloys , usually contain 4 - - phase stabili zers (Joshi, 2006) . Further increas ing - phase stabilizers to about 8 10 - 15 wt.% results in metastable/near - , and in single - - phase stabilizers over 20 wt.% (Joshi, 2006) . Figure 1. 6 Ti. Ms refers to the start of martensitic phase transformation (Leyens and Peters, 2006). 1.2.3 Ti - Al Phase Diagram - phase stabilizer and is wi dely used alloying element in Ti alloys. The addition of Al - - phase by substitutional solid - solution strengthening. To prevent the precipitation of the Ti 3 Al intermetallic phase at RT, the Al conten t in Ti alloys is usually restricted to 7 wt.% (Joshi, 2006) . Figure 1.7 shows the Ti - Al binary phase diagram (Massalski, 1990) . 9 Figure 1. 7 Ti - Al phase diagram (Massalski, 1990). The a ddi tion of Al results in an increase of the c/a ratio and leads to a more closed packed basal plane. Due to the smaller atomic radius of Al, 0.125 nm , compared to the atomic radius of Ti, 0.176 nm , a compression of the HCP unit cell is observed when Ti is substituted with Al atoms (Clementi, Raimondi and Reinhardt, 1963) . 1.2.4 Ti - Cr phase diagram among the eutectoid forming elements. T he eutectoid temperature being 940 K and the eutectoid point lying at ab out 15% Cr (Lütjering, G., and Williams, 2003) . 10 The eutectoid decomposition is very sluggish and the precipitation of the intermetallic compound TiCr 2 is avoided in Ti alloys with Cr contents below 5 % (wt. % ). B120VCA alloy with 11% (wt.%) Cr was used in the SR - 71 airplane but is unstable because it forms TiCr 2 precipitates after prolonged exposure to elevated temperatures (Lütjering, G., and Williams, 2003) . The intermetallic ordered structure in the form of the three Laves phases are comprised of three different, but closely - related crystal structures, namely C15 (cubic), C14 (hexagonal), and C36 (dihexagonal) in the Strukturbericht designation (Niinomi, 2008) (Barrett, 1966) . The presence of all three Laves phase variants in a sin gle binary system is unusual (Chen, 1996) . These phases are stable, and once formed do not easily transform. But these Laves phases do 2 phase field. They require prolonged aging and have been observed to have re latively sluggish kinetics (Chen, 1996) . P. Samimi et al, proposed the sequence for the formation of TiCr 2 : - - 2 (Samimi et al. , 2015) . Most have - phase. For example, in the Ti - Cr system ( see Fig ure 1.8 ) the maximum solubility is only about 0.5%. 11 Figure 1. 8 Ti - Cr phase diagram (Baker, 1992). 1.2.5 Ti - Fe phase diagram A nother . Fe is a stronger er than chromium, and the eut ectoid temperature in the Ti - Fe system is about 873 K (Lütjering, G., and Williams, 2003) . The formation of the equilibrium intermetallic compound is avoided in Ti - alloys by restricting the Fe content to a maxi mum of 5.5% (wt.%) . This was verif ied in the - 1.5Al - 5.5Fe - 6.8Mo ( wt.% ) (G Lütjering and Williams, 2003) . - phase also has a higher solubility for hydrogen (H), and thus its presence improves the hydrogen tolerance of this class of titanium alloys. This becomes important since many applications - Ti alloys are in corrosive environments where there is a relatively high 12 possibility f or pickup of H during se rvice (Leyens and Peters, 2006) . 1.2.6 The effect of substitution alloying elements and precipitation on the strength of Ti - alloys The strengthening mechan isms for Ti - alloys can be classified into: solid solution strengthening, dislocation density hardening, boundary strengthening, precipitate hardening, grain refinement (Lütjering, G., and Williams, 2003) . The substitutional alloying elements in fluence the strength of the Ti - alloys as the alloying elements affect the precipitation of secondary phases (Lee, Ju and Lin, 2002; Li et al., 2002) . Lee et al. investigated the effect of Nb content on the mechanical properties of Ti - xNb alloys, where x was between 5 to 35 wt.% (Lee, Ju and Lin, 2002) . The amount of Nb affected the c rystal structure and morphology of Ti - xNb alloys. For Nb contents less than 15 wt.%, the alloy microstructure was dominated by the hexagonal phase. For Nb contents between 17.5 to 25 wt.%, t he alloy microstructure was dominated by the orthorhombic phas e. A nd, for Nb content was higher than 25 wt.% , the alloy microstructure was dominated It was observed that highest strengths were achieved in Ti - 10Nb and Ti - 27.5Nb, According to Lütjering and Williams - Ti alloys is by precipitation hardening (Lütjering, G., and Williams, 2003) . Thermal and/or mechanical processing and alloying content have a strong influence on the morphology, vol ume fraction, size, and distribution of the - Ti alloys (Ankem and Greene, 2002; Nag et al. , 2009a) . Precipitates obstruct the movement of dislocations and add strength and toughness. - Ti alloys correlating the mechanical properties with the microstructural changes. For example, in Ti - 13Mo - 7Zr - 3Fe (wt.% ) alloy, an increase in the hardness value was observed after aging due to 13 - phase as shown in Figure 1.9 (Banerjee et al., 2004) . Figure 1. 9 BSE SEM photomicrographs of aged Ti - 12Mo - 6Zr - 2F sample which exhibited fine - phase particles. - phase precipitates in Ti - alloys are classified into three types based on their transformation behavior (Kuan, Ahrens and Sass, 1975; Lütjering, G., and Williams, 2003; Dubinskiy et al. , 2016) : martensitic transformation caused during quenching diffusion - controlled process during aging Induced by deformation process iso has an ellipsoidal or a cuboidal shape (Lütjering, G., and Willia ms, 2003) . Hickman found that t - phase can form during quenching, during aging at temperatures between 373 K and 773 K , or during mechanical deformation (Hickman, 1969). In Ti - 20V , t - phase formed at lower temperatures and longer aging times as sh own in Figure 1.10 . Hickman also found that t - phase results in an increase in both the strength and 14 hardness, while decreasing the elongation - to - failure ( ) (Hickman, 1969). Figure 1. 10 Schematic temperature vs time plot for Ti - - phase forms at lower temperatures and for longer aging times (Hickman, 1969). 1.2.7 The effect of heat treatment on the strength of Ti - alloys Thermal treatment of Ti - alloys significant ly contributes to its strength due to precipitati on formation by diffusion (Lütjering, G., and Williams, 2003) . The creation of precipitates , their size, geometry, and volume fraction can be controlled by a ging treatments which in turn affect the strength of the Ti - alloys (Donachie, 2000; Dehghan - Manshadi and Dippenaar, 2011; Chen et al., 2014) . - Ti alloys is s an aging step. 15 The thermal effects on the strength of Ti - 35Nb - 7Zr - 5Ta - xO (wt. %) was studied by Qazi et al. (Qazi et al., 2005) . - phase formation was observed at 700 K and this provided strengthening. The maximum strength was observed in a 0.46 wt.% O alloy after a 755 K heat treatment due - phase precipitation (Qazi et al., 2005) . Similar strengthening has been observed - Ti allo ys during aging between 623 K and 873 K for about 12 days (Qazi et al. , 2005) . 1.3 Creep Testing and creep mechanisms 1.3.1 Cree p testing Creep is a time dependent deformation process in materials under a constant load below the yield stress particularly at elevated temperatures. At ambient temperature , with most materials , this deformation is so slow that it is not significant. A t higher temperature, the rate of deformation increases at an applied load and it becomes vitally important to know the speed of deformation at a given load and temperature if components are to be safely designed for high temperature service. Failure to be able to do this may result in, for example, the premature failure of a pressure vessel or the fouling of gas turbine blades on the turbine casing. Creep tests are usually conducted using a tensile specimen to which a constant stress is applied, often by t he simple method of suspending weights from it. Surrounding the specimen is a thermostatically controlled furnace and the temperature is measured by a thermocouple attached to the gauge length of the specimen as shown in Figure 1.11. The extension of the s pecimen is measured by a very sensitive extensometer as the actual amount of deformation may be between two to three per cent. 16 Figure 1. 11 Schematic diagram of a creep test (https://www.twi - global.com/technical - knowledge/job - knowledge/creep - and - creep - testing - 081) The results of the test are then plotted on a graph of strain versus time (see Figure 1.12). Testing is generally carried out in air at atmospheric pressure. However , i f the material is to operate in an aggressive environment then the testing may need to be carried out in a controlled environment simulating service conditions. Figure 1. 12 Typical creep strain vs time plot for pure metals (Kassner, 2015) 17 The three stages of creep shown in Figure 1.12 are: Primary Creep: The creep rate decreases with time. Secondary Creep: It is also called steady state where the creep rate is constant with respect to time . Tertiary Creep: The creep r ate increases with time . The creep behavior of Ti - 3Al - 2.5V (wt.%) was evaluated by Gollapudi et al., under a wide range of stresses and temperatures, and the deformation modes were characterized as a function of the testing parameters (i.e. stress and temperature) (Gollapudi, Charit and Murty K, 2008) . T he effect of alloying element content on the creep deformation behavior of Ti - 6Al - 2Sn - 4Zr - 2Mo (wt.%) was studied by Hayes et al. (Hayes R, Vishwanathan G and Mills M, 2002) . Jenkins et al. studied the creep properties of duplex - annealed Ti - 8Al - 1Mo - 1V (wt.%) alloy at temperatures of 600 F, 800 F, 1000 F, and 1200 F (D. and A., 1968) . 1.3.2 Creep mechanisms The deformation mechanisms controlling the secondary steady - stat e creep behavior of pure metals and single - phase alloys are the most thoroughly understood because the secondary steady state strain rate ( aa ) can be measured experimentally and used to suggest the dominant deformation mechanism . There are two broad mecha nisms by which steady state creep takes place: diffusion creep and dislocation creep . Diffusion creep: This type of creep occurs by transport of material via diffusion of atoms within a grain. It is driven by a gradient of free energy (chemical potential) , which in 18 this case , is created by the applied stress . There are two types of diffusion creep mechanisms depending on the diffusion paths . Nabarro - Herring Creep : The deformation of a metal due to the stress directed flow of vacancies through the crystal lattice (Nabarro, 1948; Herring, 1950) . A standard form of the Nabarro - Herring (N - H) equation for strain rate is given by: (1.1) W here is the strain rate, is the applied stress, D L is the lattice diffusion coefficient, consta nt, and T is the absolute temperature (Nabarro, 1948) Coble Creep : This model of creep deformation occurs due to vacancy diffusion through the grain boundaries, rather than through the crystal lattice (Coble, 1963) . The equation Coble derived for the strain rate occurring from vacancy diffusion through the grain boundaries is given by : (1.2) where is the strain rate , is the applied stress, D B is the grain boundary diffusion coefficient, W is the width of the grain boundary, is the vacancy volume, d is the absolute temperature . 19 Figure 1. 13 Schematic representation of a) Nabarro - Herring creep and b) Coble creep (Meyers and Chawla, 2008) Dislocation creep: This type of creep mechanism involving motion of dislocations. This mechanism of creep tends to dominate at high stresses and relatively low temperatures. Dislocations can move by gliding in a slip plane which requir es very little thermal activation . However, the rate - determining step for their motion is often a climb process, which requires diffusion and is thus time - dependent and favored by higher temperatures. Obstacles in the slip plane, such as other dislocations, precipitates or grain boundaries, can lead to such situations (Metallurgy, 1986) . and the equation that describes the strain rate dependence is: (1.3) where is the strain rate, B is a constant, is the applied stress, n is the creep stress exponen t, Q app is the activation energy for creep (in kJ/mol), R is the gas constant, and T 20 is the absolute temperature Dislocation viscous glide mechanism, with a n value of about 3, is a process in which the dislocations movement is impeded due to their interac tion with the solute atoms in several possible ways (Sherby, Anderson and Dorn, 1951) . Grain Boundary Sliding: At elevated temperatures, the grains i n a polycrystalline metal can move relative to one another which is referred to as grain boundary sliding (GBS) (Metallurgy, 1986) . GBS process, t ypically , is considered to accommodates deformation due to a diffusional or dislocation controlled deformation mechanism (Metallurgy, 1986) . The values of creep stress exponents give insight into the deformation mechanisms that might occur during the secondary stage of creep. These parameters are typically calculated using Equation 1.3 after the mi nimum creep rate is determined directly from strain versus time data collected from actual creep experiments. Table 1.1 provides the creep exponent values for the different creep mechanisms. 21 Table 1. 3 List of creep mechanis ms and their corresponding creep exponent values Creep mechanism Creep exponent Nabarro - Herring 1 Coble 1 GBS 2 Dislocation climb ~ 5 Dislocation viscous glide ~3 1.4 Work performed In this thesis work , the creep properties of three - Ti alloys , Ti - 1 2 Cr - 1Fe - 3Al (wt. %) (TCFA) , Ti - 1 2 Cr - 3Al (wt. %) (TCA) , T - 1 2 C r (wt. %) (TC), were measured and compared. The microstructur es for these alloys were studied before and after creep using various charac terization techniques . The material was supplied by P rofessor Masahiko Ikeda of Kansai University, Kansai, Japan. In Chapter 1, a brief overview of Ti and Ti - alloys is provided, including their crystal structure and commonly used alloying elements. The inf luence of alloying, precipitation, and heat - treatment on the strength of bcc Ti alloys are discussed. A background on creep and its mechanisms is p rovided. In Chapter 2, the experimental procedures used in this thesis work are described. Bulk alloy compo sitions are provided in t his chapter. In addition , this chapter covers details of the experimental set - up for the creep experiments and volume percentage of the precipitate phase(s) . 22 In Chapter 3, the results from the microstructure characterization and the creep experiments are presented. The grain siz - phase in the alloys were measured using the line - intercept method from back scattered electron images ( B SE) . Vol ume percentage of the precipitate phase(s) were measured using ImageJ software from BSE photomicrographs. X - ray diffraction (XRD) was performed on the as - processed alloys, heat - treated samples, and creep tested samples to investigate the phases present. Cr eep tests were conducted on the material at stresses ranging from 200 MPa - 350 MPa at 683 K . The surface microstructure after each test was captured by SEM images. Results for the creep strain vs time analysis for the alloys was for every test were presente d. The microstructure of the three alloys and how they affect the creep trends obtained is discussed. In Chapter 4, the conclusions of this thesis work are presented, and potential future work is suggested. 23 CHAPTER 2 EXPERIMENTAL PROCEDURES 2.1 Materi al Three - Ti alloys were studied in this work. All the studied alloys were levitation melted in a 2 - kg, 90Dx80L LEV levitation induction furnace. They were then hot forged into blocks of dimension 25 x 60 x 250 - mm 3 at approximately 1320 K. The alloys were then subjected to homogenization annealing at 1173 K for 1 hour in vacuum. The quench ing process was carried out at 34.7 K/s (Cooling rate) in ice wate r (J Ballor et al. , 2019) . Figure 2.1 shows the as - processed alloys , which were processed by Daido Steel Corporation (Nagoya, Japan) . Maintaining a high cooling rate helps obtain a the - phase (Ikeda et al. , 2004) . Table 2. 1 provides the measured Cr, Al, Fe contents of each alloy , measured using i nductively coupled plasma atomic emission spectroscopy (ICP - AES). Using nondispersive infrared spectroscopy (NDIR), the O, C, and N contents were determined by t he Daido Steel Corporation (Nagoya, Japan). Table 2. 1 Measured compositions of the alloys studied** Chemical composition Alloy Ti Cr Fe Al C O N Ti - 12Cr (TC) wt.% Bal. 12.1 0.02 0.01 0.004 0.059 0.004 at.% Bal. 11.2 0.02 0.02 0.02 0.18 0.01 Ti - 12Cr - 3Al (TCA) wt.% Bal. 12.3 0.02 3.06 0.003 0.062 0.004 at.% Bal. 11.2 0.02 5.35 0.01 0.18 0.01 Ti - 12Cr - 1Fe - 3Al (TCFA) wt.% Bal. 12.4 0.93 3.02 0.003 0.11 0.004 at.% Bal. 11.3 0.79 5.28 0.01 0.32 0.01 **Table taken from Ballor et al. 2019 24 Figure 2. 1 An image showing the three as - processed alloys with EDM cuts from where the samples for the creep tests were extracted. The top, cross - section and thickness orientations are shown. 2.2 Microst ructural Characterization 2.2.1 Metallographic sample preparation The creep samples were machined using electric dischar ge machining (EDM) . All the samples were mechanically polished prior to creep testing. First, the samples were ground and polished using silicon carbide (SiC) papers progressively from 120 to 2400 g rits. About 5 to 10 minutes 25 were spent at each p olishing s tep until the scratches from the previous SiC paper was removed. Water was used to rinse the specimens before moving to the next grinding step. Next, 3 µm, 1 µm, 0.5 µm, and 0.25 µm diamond pastes were used to obtain a mirror finish . Each polishing step was around 15 minutes with water rinsing in between steps. The final polishing step used a solution consisting of a 5:1 ratio of Stru e rs OP - S Colloidal silica ( 0.02 0.04 micron) and 30% hydrogen per oxide . This step required around 30 minutes of polishing time. The polishing cloth used in this step was purchased from Buehler (catalog No. M500 - 12PS). After polishing, water was used to rinse away the colloidal silica. The s amples were ultrasonically cle aned in ethanol to remove the residual colloidal silica. The samples were also etched for about two minutes using reagent (commonly used chemical etchant for titanium ) before observing under the microscope . Th e etchant consist ed of 100 ml water, 1 - 3 ml hydrofluoric acid and 2 - 6 ml nitric acid . 2.2.2 Scanning Electron Microscopy The microstructure of the alloys investigated was characterized using scanning electron microscopy (SEM). The SEM used in this work was a field emission gun Tescan Mira 3 SEM (Fig ure 2.2 ) . Both secondary electron (SE) and back - scattered electron (BSE) images were acquired before and after the creep experiments. Grain sizes were determined using BSE photomicrographs . The different phases could be distinguished using BSE imaging. Pha ses containing larger average atomic numbers appear brighter in BSE images than phases containing lower average atomic numbers (Flegler S.L., Heckman J.W.Jr., 1993) . The possible - phase - phase of the Ti alloys evaluated in this study appeared dark or grey due to its low average atomic number - 26 phase stabilizer and it has an atomic number of 13 - phase is considered to be Ti rich which has an atomic nu mber of 22 - phase appeared brighter because it is enriched (Boyer, Welsch and Collings, 1994) . The average - phase grain size was measured from the SEM images according to ASTM standard E112 - 12 (ASTM Internati onal) Grain . Figure 2. 2 A photograph showing Tescan Mira3 SEM chamber 2.2.3 Sample preparation and measurement of precipitate v olume fraction The BSE photomicrographs used to calculate the volume fraction of precipitates were captured using the Regulus 8200 ultra - high - resolution SEM at Hitachi High Tech Americas , Maryland. 27 The samples were first mounted on a stub using hot glue . Then the samples were mechanically polished using SiC paper of 180 grit and 1200 grit on the cross - sectional face. Next, c ross - sectional milling was performed using an ion milling system, IM4000 Plus , at an accelerating voltage of 6 kV and a milling peri od of 2 hrs. The samples were analyzed for topographical information using the Regulus SEM , operating at a voltage of 5 kV. The volume fraction of the precipitate phase(s) was calculated using ImageJ software. adjusted to color the phase of interest. The function allowed the determination of the area fraction of a particular phase in the BSE image. This area fraction translated to the volume fraction of the phase (Hariharan, 2018) . A similar technique was applied to multiple BSE images and the volume fractions w ere averaged. 2.2.4 X - ray diffraction analysis X - Ray diffraction (XRD) was performed on the alloys with a Bruker - D2 PHASER diffractometer (see Figure 2.3) in the Bragg Brentano orientation using Cu - characteristic wavelength of 1.54 Å and a Ni filter. Each sample scan used a step size of - 90°. All scans were conducted with a tube current of 35 mA and a tube voltage of 35 kV . First, the sample holder was scanned, and the spectrum wa Background intensities were found from EVA software ( EVA uses DIFFRAC method to calculate the background ). Then background intensities were then subtracted from the raw 2 stripping was performed (EVA software). The final sample spectrum was obtained by subtracting the sample holder spectrum from the sample spectrum . 28 Figure 2. 3 A schematic showing the geometry of the test specimens used in the creep experiments. The dimensions are in millimeters tapping) for the scans . The d - B 2. - be solved for directly, shown in Equation 2. 2. 1 for Cu - calculations (Cullity and Stock, 2001) . ) (2.1) 29 (2.2) TiCr 2 phases were found in the literature (Sugano and Ikeda, 2005; Ogawa et al. , 2007, 200 8; Kolli, Joost and Ankem, 2015) were found by taking the figures from the literature and placing them into an online digitizer with a magnified pointer. The error involved in this method was ±1.63º due to the size of the pixels. The marke to the error. Because of this, the phase analysis is highly qualitative and must be re - evaluated using EBSD technique. Intensity ratios could not be obtained from these figures . The list of all the theoretical peaks can be found in Table 2 .2 . To improve the qualitative analysis three filtering steps were used. 1) Peaks with no evidence in the literature were removed. 2) Peaks with unconvincing evidence were removed i.e. any dat a point that was less than or equal to the 4% over the background intensity was set to zero. 3) Any available to deconvolute them). After the above filtration, minor peaks in the data were mor e visible. Matching the dense clusters of peaks. Phases were identified by overlaying the final processed intensity data with lines showing all possible peak loca tions from literature . 30 Table 2. 2 2 phases from literature TiCr 2 35 51 37 38 65 43 40 79 45 41 80 53 63 70 71 74 76 77 78 82 87 2.3 Creep Testing 2.3.1 Sample configuration For the creep mechanical testing, flat dog - bone specimens with dimensions of about 7 ± 0.2 mm gage width, 1.1 mm ± 0.2 mm thickness , and 122 mm length were EDM cut . Figure 2.4 shows a photograph of an EDM cut specimen. The samples were polished using the m ethod described in section 2.2.1 31 Figure 2. 4 A photo of a sample representative of those used in the creep experiments. The sample dimensions are provided. 2.3.2 Creep test experiment Conventional uniaxial constant load constant temperature creep tests were performed using Applied Test Systems (ATS) (Butler, PA) 2400 series creep frames (see Figure 2.5) . The creep frame was equipped with a high temperature split tube furnace with a maximum temperature of 2073 K, and a con tact type extensometer attached to a linear variable differential transformer (LVDT) to measure the strain. The creep tests were conducted in an open - air environment. A gage length of 1 inch (25.4 mm) was use d as standard for deflection measurement . The cr eep testing conditions of the three alloys are listed in Table 3 .1 . 32 Figure 2. 5 Photograph showing the creep frame and the sample setup representative of all the creep experiments. - Ti - 12Cr - 1Fe - 3Al (wt. %) (TCFA) alloy was found to be 653 K to 723 K (Khademi, 2018) . For this study, the creep tests were conducted at, 200 MPa, 250 MPa, 300 MPa, and 350 MPa, and at 683 K. P rior to creep testing, all the samples were heat treated for 48 hrs inside the creep frame furnace 33 (i.e. in an open - air environment) at the creep temperature under no load. The approximate duration of the tests was about 336 hours (14 days), and al l the tests were stopped in the secondary creep regime . None of the samples were taken to failure. 2.4 Heat treatment To understand the thermal effects on the phases in the alloys, a separate set of samples were subjected to only heat treat ment followed by analyz ing with the Bruker X - Ray Diffractometer . Small rectangular samples of dimension of about 15 m m x 15 m m were cut from the grip s of the dog - bone samples using a low speed cutting blade. All the rectangular samples were polished up to 1200 grit (emery paper) to reduce any background noise from surface roughness. These samples were placed in a cylindrical quartz tube which was first evacuated to ensure a low - pressure environment . For real - time temperature measurement, a test set up was design ed and fabricated as shown in Figure 2. 6 . The samples were heat treated at four different temperatures, 683K, 623 K , 573 K , and 483 K , for about 12 hrs. For each scan, the top orientation was placed in the face up position inside the Bruker X - Ray Diffracti on chamber. 34 Figure 2. 6 Image of the furnace used for the heat treatments. The real - time sample temperature was measured during the heat treatment. 2.5 Hardness measurements Vickers microhardness testing was performed on the samples according to ASTM Standards E92 - 17 (ASTM E92 - 17, 2017) . This testing was performed using a 500 g - force load and a dwell time of 10 s (Figure 2. 7 ) . Six readings wer e made at each of the locations and the hardness values (HV) were reported. The hardness testing was performed on the as - processed samples, the grips of the creep tested samples (subjected to heat treatment and minimal creep deformation), and the gage sect ions of the creep - tested samples (subjected to both heat treatment and creep deformation). 35 Figure 2. 7 Image of the Vickers hardness testing machine. 2.6 Creep data analysis Deformation data as a function of time was collected from the creep frame for all the samples. K aleidagraph , a data analysis and graphing software, was used to display c reep strain vs time plots for all the samples. A p ower law (see Equation 2 . 3 ) was used to calculate the creep exponent value s . By substituting the different stresses and their corresponding steady state creep rate s for a creep temperature in Equation 2. 4 , a plot of logarithm steady state creep rate and logarithm stress was obtained and the s lope (i.e. creep stress exponent) was calculated . ss n / k T) ( 2. 3 ) ss Q / k T ( 2. 4 ) A Matlab code was used to calculate the strain rate from the strain values for the alloys (see 36 appendix). Strain rate vs time plots were generated to verify if crept samples were stopped in the secondary creep regime and had a steady state creep rate (discussed in Section 3.3.2) . Def lection data of the samples as a function of time was also collected from the creep frame during the 48 hr heat treatment under no load prior to creep testing. Deflection vs time plots were generated for the samples to determine possible phase transformations occurring during this period. 37 CHAPTER 3 RESULTS AND DISCUSSION 3.1 Microstructural characterization 3.1.1 XRD results The XRD results for the as - processed materials revealed peaks corresponding to the (see Figure 3.1) Figure 3. 1 38 From the integra - phase peak which was almost three times greater than that of the other two alloys. The addition of Al may be responsible for the observed texturing. The further addition of Fe to Ti - 12Cr - 3Al (wt. %) seems to re duce the - phase peak. The changes in the atomic radii of the individual elements - phase peak positions for the three alloys. The - Ti was calculated for (refer to Equation 2 .1 ). ) ( 2 .1 ) A decrease of about 0.02 Å was found going from the pure Ti (Theoretical value ASM Handbook, vol. 9, pp. 899 917 ) to TC. Similarly, a decrease of 0.04 Å was found on addition of Al to TC, and a decrease of 0.02 Å on addition of both , Al and Fe to TC . Although the change ob served in the lattice parameter was small it may have influence d the hardness and the strength of the alloys . - and TiCr 2 phases found in the literature (Ikeda et al. , 2004; Gunawarman et al. , 2005; Ogawa et al. , 2008) - Ti alloys was used to identify the peaks as shown in Figure 3.2 . To improve the qualitative analysis, peaks below 5% of the background intensity were set to zero. 39 Figure 3. 2 K for 12 hrs TC alloy showed larger number of - phase peaks compared to TCA and TCFA alloys. The - phase stabilizer appears to have reduced the n umber of - phase peaks in the TCA alloy. It appears that the further addition of Fe to TCA has reduced the number - - phase stabilizer could possibly reduce the - phase precipitation at this t emperature. The aging duration could also play an important role in the number of secondary phase peaks and their intensities. The XRD pattern - phases similar to what was reported by 40 Li et al. after i sothermal aging of Ti - 29Nb - 13Ta - 4.6Zr (wt. %) for 48 hrs between 573 K to 673 K (Li et al. , 2015) . The XRD patterns of the post - creep tested samples is indicated in Figure 3. 3. The results showed 2 phases - - phase peak intensity. - pha se has been reported to be the possible cause for the nucleation of the - phase (Williams and Blackburn, 1969; Rhodes and Williams, 1975; Pennock, Flower and West, 1980; Azimzadeh and Rack, 1998; Ohmori et al. , 2001; Prima et al. , 2006; Nag et al. , 2009b; Li et al. , 2015) . TiCr 2 intermetallic phase has been observed in other Ti - Cr alloys with higher concentrations of Cr. B. Martin found TiCr 2 lave precipitates in Ti - xCr (wt%.) alloys for Cr content between 12 - 15 wt . % aged between 773 K and 898 K (Martin, 2017) . SEM imaging of Ti - 15Cr (%wt.) initially showed sluggish precipitation of the TiCr 2 Laves phase at lower aging temperatures of about 843 K (Martin, 2017) . Higher temperatures observed th e Lave phases growing in clusters (Martin, 2017) . 41 Figure 3. 3 350 MPa and 683 K. T he XRD patterns of the TCFA alloy heat treated at 683K, 623 K, 573 K, and 483 K, for about 12 hrs is shown in Figure 3.4. 42 - phase peaks. For higher heat treatment - phase Figure 3. 4 t treated at 483 K, 583 K, 623 K, and 683 K and for 12 hrs 43 - phase. The - phase forms during aging process for aging temperatures between 373 K - Ti alloys (Hickman, 1969) . The temperature versus aging time diagram for Ti - - phase formed at lo wer temperatures and for longer aging times for Ti - 20V. The aging time of 12 hrs might not have been enough - - phase for lower aging temperatures like 483 K and 583 K. - phases - Ti alloys for aging temperatures ranging between 573 K and 773 K and for aging temperatures ranging between 48 hrs to 100 hrs (Liu and Welsch, 1988; Li et al., 2002) . - - phase peaks in Figure 3.4 for aging temperatures 623 K and 683 K are in line with the studies. Similar trends were observed for the TCA and the TCFA alloys. In Figure 3. 5 - phase peaks for the as - processed, HT, and creep tested conditions were lined up in order to better visualize the secondary phase peaks in the alloys . The XRD patterns of the samples after HT /or - phase peaks for the alloys . The XRD patterns of the post - creep tested alloys indicate s TiCr 2 phases . A - phase peaks were observed in the creep test ed samples as compared to the HT samples . - phase nucleates from the - phase (Rhodes and Williams, 1975; Pennock, Flower and West, 1980; Azimzadeh and Rack, 1998; Ohmori et al., 2001; Prima et al., 2006; Nag et al., 200 9b; Chen et al., 2014). It is possible - - phase which could explain the peaks observed in f igures observed after HT and in the creep tested conditions, whose cause is not understood yet. For creep tested condition which is unusual and not completely understood. 44 Figure 3. 5 XRD plots comparing as - processed, HT, and creep tested conditions . 45 Figur e 3. 5 3.1.2 SEM results Figures 3. 6 show s the microstructures of the alloys in the as - processed condition. The BSE photomicrographs of the cross section of the as - processed samples reveals a - phase microstructure. Figure 3. 7 shows the microstructures of the cross section of grips of the alloys a fter creep testing at 350 MPa and 683 K . The grip section of the creep tested samples was only subjected to about half the stress that the gage section experience d for about 13 days (~ 2:1 is ratio of areas of the grip and gage sections) . 46 a. b . Figure 3. 6 BSE SEM photomicrographs of the cross section of the as - processed microstructures 47 Figure 3. 6 In the as - processed alloys, the addition of Al increased the grain size almost by a factor of 1.4 compared to the TC alloy. The addition of both Fe and Al only resulted in a very comparable grain size value compared to the TC alloy. The addition of Al alone had a more pronounced effect on the grain size . c . 48 a. b. Figure 3. 7 BSE SEM photomicrographs of the microstructures of a.) TC b.) TCA and c.) TCFA alloys taken from the cross section of the grips of creep tested samples and etched. 49 Figure 3. 7 - processed condition was compared with the grain sizes from grips of the creep tested samples. For the grips of the creep tested samples, the average - phase grain size was comparable. c. 50 a. b . Figure 3. 8 BSE SEM photomicrographs of the microstructures of a.) TC b.) TCA and c.) TCFA alloys taken from the cross section of the gage of samples creep tested at 350 MPa and 583 K 51 Figure 3. 8 Figure 3. 8 shows the microstructures of the cross section of gage of the alloys after creep testing at 350 MPa and 683 K for about 13 days . The TC grain size was comparable in the grip , gage section , and in the as - processed conditions . Table 3.1 summaries the obser ved grain size results. The observed decrease , after creep testing compared to the as - processed condition, may be due to the possible phase transformations occurring during creep in the gage section . All the observed grain size changes were within a factor of 2 for the studied alloys going from the as - processed to the creep tested conditions. TC exhibited the largest grain size while TCFA exhibited the smallest grain size after creep . c . 52 Representative BSE SEM images of the as - processed alloys are provided in Figure 3. 9 . The SE Representative BSE SEM images of the as - processed alloys are provided in Figure 3.9 . The SE SEM images for the creep tested gage section are provided in Figure 3. 10 . The SE images show pit - like nanoscale structures with visible topographical contrast sugge sting possible secondary - - phase, it is expected that these fine features in Figure 3. 10 microscopy (TEM) analysis is intended to verify the phase(s) observed. Table 3. 1 Summary of the grain sizes of th e as - processed (BCC) and creep tested (transformed BCC) microstructures Elongated direction Transverse direction 60.2 135.5 64 84.1 58.8 53 b . a. Figure 3. 9 BSE SEM photomicrographs of the as - processed a.) TC, b.) TCA, and c.) TCFA reagent 54 Figure 3. 9 Figure 3. 9 c . a. Figure 3. 10 SE SEM photomicrographs of a.) TC, b.) TCA, and c.) TCFA alloys after creep matrix with pit - like structures indicating presence of 55 F igure 3. 10 b . c . 56 T he volume fraction of the precipitates in the alloys were evaluated from BSE photomicrographs using the thresholding function in the image editing software ImageJ as shown in Figure 3.1 1 . TC and TCA showed similar values of 59.7 % and 59.1 % respectively for the precipitate volume fraction (see T able 3.2 ). Figure 3. 11 BSE SEM photomicrographs of a.) TC, b.) TCA, c.) TCFA alloys with corresponding ImageJ contrast and thresholding images. 57 Figure 3. 1 1 As such, there seems to be a reduced effect of Al on the precipitate volume fraction. TCFA had a higher precipitate volume fraction of 63.1 %. The addition of Fe and Al to TC showed an increase of about 6 % in the precipitate volume fraction compared to TC al loy. The higher precipitate fraction in TCFA could possibly be the cause for greater creep resistance compared to the other two alloys. 58 Table 3. 2 Summary of precipitate volume fractions of the alloys 3.2 Mechanical Properties 3.2.1 Vickers Hardness The Vickers hardness testing and the results are represented in Figure 3.1 2 . The alloys show an increase in the HV after creep testing. TCFA showed a 75% increase, followed by TCA with a 68%, and then by TC with a 50% increase. This could be due to the precipi tation hardening - Ti alloys which strongly hinders the movement of dislocations. Due to the aging process, precipitates nucleate and change in volume fraction, size, and geometry leading - Ti alloy (Kobayashi et al. , 1998; Donachie, 2000; G Lütjering and Williams, 2003; Ivasishin et al. , 2003; Dehghan - Manshadi and Dippenaar, 2011; Chen et al. , 2014) . Alloys Trials Mean vol ume percent of precipitates (%) Mean vol ume percent of bulk (%) Minimum Median Maximum TC 6 59.7 40.3 57.2 59 .2 64. 5 TCA 5 59.1 40.9 54. 1 58. 2 64.2 TCFA 5 63.1 36.9 61. 1 63. 1 65.2 59 Figure 3. 12 Hardness values for the alloys in the different conditions showing an increase from as - processed to the heat - treated grip section and a further increase in the crept gage section - - phase transformation occurs i n the temperature - Ti alloys (Hickman, 1969) . The hardness values increase is likely - phase precipitate formation. The gage section hardness values wer e 5 - 10% larger than those in the grip section. This is expected to be a result of the increased dislocation content in the gage as compared to the grip. The increase in the hardness values post creep could also be due to the presence of intermetallic TiCr 2 s . B. Martin observed an increase in the hardness value from about 175 HV to 360 HV in Ti - xCr - 15 wt.% Cr (Martin, 2017) . This was attributed to the 2 precipitation in the alloy with the increase in the Cr concentration (Martin, 0 100 200 300 400 500 600 Hardness (HV) TCFA TCA TC Heat Treated - Grip section Heat Treated & Crept - Gage section As Processed 60 2017) . Chen et al. studied hardness in the Ti - Cr alloy space and observed the effects of ternary additions and two - phase b inary alloys containing Ti - Cr (Chen, 1996) . They found that formation of the Lave TiCr 2 phase in conjunction with a Ti - Cr bcc improved the toughness and the hardness of the alloy (Chen, 1996) . T he addition of ternary elements , like Mo, V, and Fe, increased the toughness of these alloys (Chen, 1996) . Th is was explained by studies showing an improved ductility through dispersion and relaxation of the closed - packed structure on addition of ternary elements as substitutional solutes (Chen, 1996) . 3.2.2 Creep tests results T he representative heating plot s are shown for the alloys in Figure 3.1 3 . A slight decrease was observed in the deflection and in turn the strain, initially at shorter heat treatment times, and the strain seemed to stabilize at longer heat treatment times. Th e initial drop in the strain value could be due to the phase transformations taking place in the alloys with heat treatment . Although each of the creep samples were heat treated at the creep temperature (683 K) for 48 hrs prior to loading (creep testing) , it is possible that the phase transformations were not complete. Therefore, the microstructure may still have been evolving d uring the subsequent creep exposure. 61 Figure 3. 13 Total Strain vs time heat treatment plots for a.) TC, b.) TCA, and c.) TCFA showing a slight drop in deflection at lower times an d stable deflection at higher times. 62 Figure 3. 1 3 A summa ry of all the creep tests performed on the alloys , their conditions, and the results obtained can be found in Table 3.3 . 63 Table 3. 3 Summary of creep tested specimens Test Number Material Stress (M P a) Creep temperature ( K ) Creep test duration (hrs) Total strain % Creep Strain % ssmin (10 - 4 ) (1/hr) 1 TCFA 350 683 306 1.59 1.38 3.4 2 TCA 350 683 342 2.49 2.14 2.7 3 TC 350 683 282 2.97 2.59 3.3 4 TCFA 300 683 645 2.42 2.22 1.5 5 TCA 300 683 645 2.42 1.65 2.1 6 TC 300 683 588 3.1 2.75 2.7 7 TCFA 250 683 890 1.91 1.82 1.4 8 TCA 250 683 442 1.65 1.48 1.9 9 TC 250 683 475 2.26 2.01 2.2 10 TCFA 200 683 617 0.85 0.78 0.6 1 1 TCA 200 683 162 0.88 0.69 0.4 1 2 TC 200 683 432 2.14 1.94 1.9 The strain rate vs time for the TCFA alloy creep tested at 350 MPa and 683 K displayed a strain rate curve plateauing with increasing time indicating secondary creep regime and a steady state creep rate (see Figure 3.1 4 ) . The plot is representative of all the samples creep tested, and the creep tests were stopped in the secondary creep regime. 64 Figure 3. 14 Representative strain rate vs time plot for TCFA alloy at 350 M Pa and 683 K displaying a plateauing of strain rate curve with increasing time indicating secondary creep regime. The plot is representative of all the creep experiments. Figure 3.1 5 compares the creep strain vs time curves for TCFA as a function of applied stress at 683 K. The curves indicate an increase in the creep strain as the stress was increased. Similar trends were observed for the other two alloys . 65 Fig ure 3. 15 Creep strain vs time plots for TCFA alloy at 683 K and varying applied stress levels Figure 3. 1 6 compares the creep stain versus time plots for the three alloys at each of the applied stress levels. At 200 MPa, TCFA e xhibited greater creep resistance followed by TCA and then TC as observed in Figure 3. 1 6 . This trend in the creep resistances was consistently seen over all the four applied stress values. This indicates that the addition of both 3 wt. % Al and 1 wt. % Fe to Ti - 12Cr (wt. %) provided the greatest creep properties among the alloys 66 Figure 3. 16 Creep strain vs time plots for the three alloys tested at 683 K and a.) 200 MPa, b.) 250MPa, c.) 300 MPa, d.) 350 MPa showing a consistent creep resistance trend. 67 Figure 3. 1 6 68 T he creep strain vs time plots for TCFA alloy tested at 350 MPa and 683 K , before and after heat treatment, were compared with Ti - 6Al - 4V tested at 350 MPa and 673 K (see Figure 3.1 7 ) . Figure 3. 17 Comparison creep strain vs time plot for TCFA alloy tested at 683 K and 350 MPa before and after heat treatment, with Ti - 6Al - 4V tested at similar conditions. TCFA exhibited greater creep resistance than Ti - 6Al - 4V after heat treatment at 683 K for 48 hrs prior to creep testing. TCFA alloy with no prior heat treatment exhibited lesser creep resistance than T i - 6Al - 4V . This could indicate that a fter the to - phase transformation, the TCFA alloy exhibit ed greater creep resistance than Ti - 6Al - 4V . The greater creep resistance of TCFA alloy could also be due to the presence of TiCr 2 tes - phase matrix . D. Northwood et al, found that the multi - phase Ti - 13V - 11Cr - 2 phases ha d a slower creep rate in the secondary regime t - phase Ti - 13V - 11Cr - 3Al (wt.%) alloy (Northwood and Smith, 1987) . 69 CHAPTER 4 CONCLUSIONS 1. The as - - phase peaks. After heat - treatment at /or phase(s). Post creep tested XRD revealed peaks correspond , , and TiCr 2 secondary phase(s) for the alloys . The SE SEM images of the creep tested samples revealed nanoscale pit - like structures suggesting the presence of secondary phase(s). 2. TC exhibited the largest grain size while TCFA exhibited the small est grain size after creep . 3. The addition of Al and Fe did not contribute to a significant change in the Vickers hardness of the as - processed alloys. Post heat - treatment and post creep tested Vickers hardness values for the alloys showed an increase when co mpared to the as - received material indicating possible phase transformations during the aging process and precipitation hardening effects - Ti alloys which strongly hinders the movement of dislocations . TCFA exhibited the highest hardness follo wed by TCA and then TC. A slight increase of about 5% - 10% in the hardness values was observed going from the grip to the gage sections of the creep tested alloys which could be the result of the higher dislocation content in the gage section. 4. The creep st rain increased as the applied stress was increased from 200 MPa to 350 MPa for all the alloys during creep testing . The creep strength results followed the same trend as the hardness results. TCFA exhibited the greatest creep resistance followed by TCA and then TC at 683 K and for all stress levels . TCFA exhibited greater creep resistance than Ti - 6Al - 4V after heat treatment at 683 K for 48 hrs prior to creep testing and exhibited lower creep 70 resistance than Ti - 6Al - 4V in the no prior heat treatment case. 5. TC and TCA showed similar values of 59.7 % and 59.1 % respectively for the precipitate volume fraction. As such, there seemed to be a reduced effect of Al on the precipitate volume fraction. TCFA had a higher precipitate volume percent of 63.1 %. The addit ion of Fe and Al to TC exhibited an increase of about 6 % in the precipitate volume percent compared to TC alloy. Recommendations for future work 1. Determining other mechanical properties like tensile strength, elongation - to - failure, fatigue strength and c orrelating them with the microstructural changes is crucial in developing an understanding of the influence of alloying elements on properties. Such studies will be important in developing cheaper alternatives to the current Ti alloys commercially being us ed. 2. To have a better understanding of the creep mechanisms, it is necessary to estimate the activation energies and the creep exponent values of the different mechanisms observed. Future work could focus on performing more creep tests at different creep te mperatures to estimate the activation energies which would in turn help understand the deformation mechanisms observed . Also, p erforming creep tests are more stress levels will help obtain a more accurate creep exponent plot and value. 3. In - situ creep tests for the alloys could be conducted and used to characterize the distribution of the different deformation modes as a function of testing conditions (i.e. stress, temperature and loading rate). 4. To verify the phase(s) observed, future t ransmission electron microscopy (TEM) analysis , 71 and EBSD analysis could be conducted. 72 APPENDIX 73 APPENDIX MATLAB code for calculating strain rate The following Matlab code was used to calculate the strain rate from the deflection and time data collected from the creep test frames and obtain a plot of strain rate vs time. a=importdata('strain_rate.xlsx'); t=a(:,1); d=a(:,2); for i=1:2000; t1=t(i); t2=t(i+50); d1=d (i); d2=d(i+50); v(i)=(d2 - d1)/(t2 - t1); end plot(t(1:2000),v); value, second time value, first deflection value, second deflection value, strain rate respectively. 74 Strain rate vs time plots for all alloys at 683 K 75 76 77 78 79 80 BIBLIOGRAPHY 81 BIBLIOGRAPHY Materials Science and Engineering: A , 263(2), pp. 127 131. doi: 10.1016/s0921 - 5093(98)01170 - 8. 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