llllljllllllfllllllllfllllIll"llllllllllll ”mm 786 3768 LIBRARY Mlchlgan State 3 University I This is to certify that the thesis entitled Laser Surface Modification of Titanium Aluminides to Improve Oxidation Resistance at Elevated Temperatures presented by Yong Wook Sin has been accepted towards fulfillment of the requirements for Master's degree in Materials Science 4; ’{ )VXU/I’Wamabm‘aw Major professor Date 2/21/63, 0 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution PLACE IN RETURN BOX to remove this Moat from your rooord. TO AVOID FINES return on or baton duo din. DATE DUE DATE DUE DATE DUE usu Io An Afflmidlvo Action/Equal Opportunlty Ira-amnion Walla-9.1 __.\ LASER SURFACE MODIFICATION OF TITANIUM ALUMINIDES TO IMPROVE OXIDATION RESISTANCE AT ELEVATED TEMPERATURES BY YONG WOOK SIN A THESIS Submitted to MICHIGAN STATE UNIVERSITY in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Metallurgy, Mechanics and Materials Science b35375 495x ABSTRACT LASER SURFACE MODIFICATION OF TITANIUM ALUMINIDES TO IMPROVE OXIDATION RESISTANCE AT ELEVATED TEMPERATURES BY Yong Wook Sin Ifiqu has low density and good mechanical strength at high temperature. But it also has major drawbacks such as low temperature brittleness and poor oxidation resistance at elevated temperatures. The maximum use temperature of this material depends on the improvement of oxidation resistance rather than that of mechanical properties such as creep or strength. Alyri has very good high temperature resistance because of its high contents of aluminum present. High aluminum contents makes protective Algh layer on the surface of the material. In this study, A13Ti coating on to the surface of TiaAl was formed by using laser surface modification technique. The effectiveness of coating was investigated before and after 10 hours cyclic oxidation process. To My Father, Mother and Wife ACKNOWEEDGEMENT I wish to express my greatest appreciation to my advisor, Dr. K.N. Subramanian for his patient guidance and enthusiasm in every aspect of this research, his friendship and frequent encouragement throughout my studies. I would like to thank Dr. Mukherjee for allowing me to use laser facility and Dr. Grumman and Dr. Crimp for their kind guidence. I also would like to thank Mr. Donald E. Larson Jr. of Howmet corporation, whitehall, MI for kindly providing me TiaAl and AlaTi samples used in this study. Special thanks are also due to Dr. Khan and Mr. Shull for his kind help to use the laser and WDS respectively. I also would like to express my deepest gratitute to my parents for their endless love, encouragement and supports. The last but not the least, I would like to thank to my wife, Hae-Jeong. Without her endurence,sacrifice and encouragement to me, the accomplishment of my studies may not be possible. Feburary 19, 1990 iv TABLE OF CONTNETS List of Figures ....... List of Tables ....... 1. INTRODUCTION 1-1. Introduction and Historical Background 1-2. Theoretical Background ::::::: 1-2-1. Intermetallics ....... 1-2-2. Laser Surface Modification ....... 2 . EXPERIMENTAL PROCEDURE 2-1. Materials ....... 2-2. Laser Surface Modification . ...... 2-3. Cyclic Oxidation . ...... 2-4. Scanning Electron Microscopy ..... .. 2-5. Wavelength Dispersive Spectroscopy 2-5-1. Characteristic X-Ray Image ::::::: 2-5-2. Line Spacing ....... 3. RESULTS AND DISCUSSIONS 3-1. Microstructural Observations ....... 3-1-1. Laser Surface Treated Specimen 3-1-2. Cyclic Oxidized Specimen ::::::: 3-2. Characteristic X-Ray Study ....... 3-2-1. Characteristic x-Ray Image ....... 3-2-2. Line Spacing ....... 4 . CONCLUSIONS ....... entrances ....... page vi ix 19 21 30 31 34 34 35 38 39 47 71 71 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 2. 3. 4. 5. 6. 10. 11. 14. LIST OF FIGURES page Ordered substitutional structures in the Cu-Au system: (a) CuAu superlattice. (b) CuaAu superlattice. . . . 12 Binary Ti-Al phase diagram. ... 14 Laser surface modification (cladding). ... 17 Shieding gas chamber. ' ... 24 Beam focusing chart. ... 26 Overall features of laser surface modification. ... 28 Specimen preparation for cyclic oxidation process ... 32 The starting points and locuses for line scanning ... 36 Scanning electron micrographs at the surface of the coated side after laser surface modification: (a) continuous oxide crystals. (b) discontinuous oxide crystals. ... 40 Scanning electron micrograph of the surface oxide layer after laser surface modification. ... 42 Scanning electron micrograph of interface between the coating and matrix. ... 45 Scanning electron micrographs at the coated side: (a) surface. ... 48 (b) interface. ... 50 (c) matrix. ... 52 Scanning electron micrographs at the uncoated side (surface and matrix). ... 54 Scanning electron micrograph of the coating showing the heat affected zone (by laser processing) and the grains. ... 56 fl Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. Scanning electron micrograph of the interface showing strong adherence of coating to the matrix ... 59 Scanning electron micrographs of uncoated side (surface and matrix) after cyclic oxidation showing: (a) surface oxide layer before spelling. (b) surface oxide layer after spelling. .. 62 The schematic diagram of spelling models at the surface of uncoated side during cyclic oxidation process. (a) The first spelling model. (b) The second spelling model. ... 64 Scanning electron micrograph of the damaged spot at the surface. ... 67 Scanning electron micrograph of the damaged spot showing further oxidation into the matrix. ... 69 Characteristic X-rey image at the surface of the coating after laser sufece modification for: (a) titanium. (b) aluminum. --- 72 (c) oxygen. ... 74 Characteristic X-rey image at the surfce of the coating after cyclic oxidation process for: ' (a) titanium. (b) aluminum. ... 77 (c) oxygen. ... 79 Characteristic X-rey image at the interface of the coating after cyclic oxidation process for: (a) titanium. (b) aluminum. ... 81 (c) oxygen. ... 83 Characteristic X-rey image at the surface of the uncoated side after cyclic oxidation process for oxygen: (a) before spelling. (b) after spelling... 86 Characteristic X-ray image at the damaged spot. ... 88 v“ Figure Figure Figure Figure Figure 25. 26. 27. 28. 29. (a) Characteristic X-rey image at the trapped pore for oxygen and (b) corresponding scanning electron micrograph. ... 90 Characteristic x-ray line scanning before cyclic oxidation process at the coated side showing concentration profiles for: (a) titanium. ... 93 (b) aluminum. ... 95 (c) oxygen. ... 97 Characteristic x-rey line scanning after cyclic oxidation process at the coated side showing concentration profiles for: (a) titanium. ... 100 (b) aluminum. ... 102 (c) oxygen. ... 104 (a) Oxygen concentration profiles at uncoated sides after cyclic oxidation process. ... 106 (b) Oxygen concentration profiles at uncoated sides after cyclic oxidation process. ... 108 (a) Oxygen concentration profiles at uncoated sides after cyclic oxidation process showing trapped pores. ... 111 (b) Oxygen concentration profiles at uncoated sides after cyclic oxidation process showing trapped pores. ... 113 viii Table 1. Table 2. Table 3. LIST OF TABLES The comparions of crystal structures, lattice parameters and thermal expansion coefficients of TiaAl and AlaTi. The chemical analysis of Ti3A1 stock. The conditions of laser surface modification. ix page 1. INTRODUCTION 1-1. INTRODUCTION AND HISTORICAL BACKGROUND Realization of the hypersonic aerospace vehicle for the let century depends on the development of advanced high temperature materials for its high temperature gas turbine engine. The development of titanium alloys for different jet engine parts (fen, high compressor blades and disks) has been carried out for 40 years after the gas turbine rotor was made of titanium alloys for the first time in 1950's [1-4]. Titanium alloys have substituted nickel-based and iron-based superalloys because of their low density and comparable strength [5]. Since then, titanium alloys have been improved considerably and they can reach higher tensile strength at relatively low temperatures (20 - 430°C). They also have good creep strength for temperature up to 600W: [6]. Titanium alloys, however, have some inherent limitations such as low creep strength and poor oxidation resistance properties above about 650°C [7]. The need for high performance aircraft has been increased in the last decade. Hence, the development of new materials for high performance gas turbine engine has been 2 necessitated. Among the available intermetellics, titanium aluminides (TiaAl and TiAl) have received significant attention as potential structural components because they have low density and relatively high strength at elevated temperatures. But both of these intermetellics (TfifiAl and TiAl) possess poor high temperature oxidation resistance, and are brittle at the room temperature [1,5,8,9-23]. The oxidation resistance of T13A1 is worse than that of TiAl [5]. This phenomenon is attributed to the inability of the aluminum present in these intermetellics to provide a protective oxide layer on their surfaces [24]. Because of aforementioned reasons, extensive studies on the development of titanium aluminides as potential structural materials for high temperature applications have been carried out. Titanium alumnides have ordered structures (superlattices), and are composed of a simple ratio of titanium aluminum (AXBY where x and Y are integers) [25]. In the free energy curve, they have a very narrow stability range which causes a rapid rise in G (Gibb's free energy) with small composition fluctuations [26]. Titanium aluminides (intermetellics) have superlattices (Tfiqu has no”, TiAl has 1.10 and AlaTi has 0022) [8,24,27,28]. The comparisons of crytal structures, lattice perpmeters and thermal expansion coefficients of TiaAl and AlaTi ere 3 illustrated in Table 1. These materials have good high temperature properties because of the strong A-B bond and high activation energy. But this strong A-B bond results in low temperature brittleness [25,26]. Enhancement of the low temperature ductility of these intermetellics can be achieved by alloying additions [5,9-11,29]. For example, Blackburn, et a1. improved the low temperature ductility by using niobium alloying additions in Ti-Al system (Ti-16Al-10Nb) [29]. Joseph et al. found that the mechanical properties of niobium alloyed titanium aluminides decreased at elevated temperatures because of embrittlement by oxidation [30]. On the other hand, another intermetellic present in the Ti-Al system, namely AlaTi was found to have very good high temperature oxidation resistance [24,31]. Higher contents of aluminum present in this intermetellic can facilitate the formation of a protective Algh oxide layer [24,32]. In 1985, Stoloff [33] reported that Algh layer, although brittle, is formed on the aluminide surface as in the case of NiAl, protecting the NiAl substrate from the oxidation up to 1000W3. In the case of Kanthal, an iron-aluminum alloy, similar of Algh layer is formed on the surface of substrate to give a good oxidation resistance up to about 1600%:. Kanthal has been used as heating elements for quite same time [23,33]. «and some 36 38 2.34 3:82 35 3:82 36 mmmomé 36... need 3.83 3.3 2.663 3.3 rods 735 :35 735 0 woods 6 meon 4E .4 o .4 a :8 44.2. Mandamusn musuoshun . usoonmuooo cowmcomxo Hosanna scanned Heumhuo mandamus: .234 6:6 H403 no mucmfiowummoo :oflmcmmxm Hashes» use mhmumamumn .4 manna mowuuma .ousuosuuu amummuo no meomfiuomaoo 6:8 5 Lipsitt also suggested that coatings could improve the high temperature oxidation resistance of titanium aluminides (24,25,341. In 1987, Yameguchi [24] reported that Aigri, unlike TiaAlend TiAl, forms only 111203 layer on the surface which protects the substrate from oxidation. He also suggested that Algri can be used for coating materials to protect TiaAl and TiAl as well as Titanium from the high temperature oxidation. Problems relating to its room temperature brittleness have also been encountered in Alyri. Extensive studies have been carried out to overcome the two major drawbacks, of low temperature brittleness and poor oxidation resistance in high temperature applications [30,35-45]. Firstly, the poor oxidation resistance at high temperatures can be improved by alloying. Chromium and vanadium additions to the Ti-Al system form alumina scales on the surface of materials in the air at 1100-1400°C. Ti-30Al- 12Cr-15V alloy forms a single layer of alumina scale in the air at 1400°C [2]. These alumina scales protect the materials from high temperature oxidation. But the addition of these rare-earth elements decreases the melting point of materials, resulting in degradation of mechanical properties at high temperatures. Hence, the usage of these materials 6 was restricted to the coating [30]. Several coating processes such as slurry spraying, molten dipping and pack cementation have been used for several decades [46]. However, laser surface modification which is used widely because of its convenience and effectiveness in several systems, has not be applied to titanium aluminides. Secondly, low temperature brittleness has been improved by using several following methods: Alloy chemistry modification The addition of vanadium, chromium and manganese to the titanium aluminides improves the ductility, decreasing the volume of the unit cell so as to form strong chemical bonding [5]. On the other hand, the addition of niobium and tungsten decreases the ductility, although it increases the strength. Improved fabrication process Improved processing techniques such as PM/RS (Powder metallurgy / Rapid solidification) method improves the high temperature strength and ductility as a result of diminishing grain size [2,47]. Microstuctural features (1) The increase of ductile alloy composition (7-phase only with 47-49 at.% aluminum content) in the two-phase (a,+7) region in TiAl was proposed to improve ductility by Kim [5]. (2) Precipitation of a ductile second-aphase (bcc structure) in TiaAl + Nb + W alloys improves ductility [12]. (3) The improvement of ductility by addition of vanadium, chromium or manganese to the TiAl is attributed to twinning. Niobium addition can reduce the planarity of slip and increase non-basal slip activity, which improves ductility of man [1,5,47]. The present study will focus on Ti3Al because it is more ductile than TiAl and the mechanical properties of Tfiqu is improved by adding Niobium, Vanadium and Molybdenum. However, the oxidation resistance of Tfiqu is still lower than that of TiAl. AlaTi coating on TiaAl is desirable to improve its high temperature oxidation resistance. Laser surface modification to achieve the desirable surface properties has received significant attention in recent years [48-53]. The ability to achieve surface modifications without significantly altering the bulk properties of the material is the most important advantage of this technique. Melting of the Algri onto the surface of 8 TfiaAl can be carried out by using lasers. The compatibility of these two intermetellics, and the influence of the Algri coating on the high temperature oxidation resistance of TiaAl will be studied in this project. The purpose of this study is to retain the structural capabilities of TiaAl' and at the same time to provide a surface layer of AlaTi to improve the oxidation resistance of the system. 1-2 . THEORETICAL BACKGROUND 1-2-1. Intermetallics The formation of ordered phases can be explained with quasi- chemical model of solutions in thermodynamics conveniently. Assumptions: (a) The solutions have equal molar volumes in the pure state (b) No volume change is required when the solutions are mixing together (AV,I = 0) (c) The only nearest neighbor interactions should be considered as the interatomic forces. Binary solid solution has three types of bonds: ( 1) A-A bonds with an E,m (2) B-B bonds with an E“, (3) A-B bonds with an E”, where EM, BBB, and EM3 are the bonding energies of A-A, B-B and A-B bonds respectively. If PM, P1m and PM, are the numbers of these three typws of bonds respectively, the internal energy of the solution Es can be described as follow: E3 = PMEM + P333133 + PABEAB (i) 10 If n, atoms of A and n, atoms of B are in 1 mole of solid solution, and z is the coordination number of an atom, Then, rnz a P“ + 2PM_ P“ .. n.2/2 - pu/z. (ii) Similary, ‘ Pu = nBZ/z - PM/z. (iii) Substituting equations (ii) and (iii) into (i) gives E5 = ZnAEM/z + znnsm, + pawn - (3M + Eng/2) . On the other hand, energy of the unmixed components are Em = nAZEM/z + nnznn/z The energy of mixed components are ABM ' PAB(EAB "" (EM + Bani/2)- Enthalpy of the mixing process is given by AHH = A2,, - PAVM. But ANQ== 0._ Therefore, AH“ 3 AEH 3 Pumas "' (EM + Bani/2). In the real solutions, to get a minimum free energy, atoms should be arranged with the lowest internal energy consistent with sufficient ramdomness. If the value of EA, - (E:M + Eng/2 is negative (in a system), the internal energy of the system decreases by increasing 11 the number of A-B bonds, which results in atomic ordering [26]. For example, in the Cu-Au alloy system, CuAu has the ordered structure which is composed of alternate layers of the Cu and Au atoms at low temperatures. This kind of lattice is called as a superlattice. On the other hand, (hnAu has another superlattice. These are illustrated in Fig. 1 [26]. Titanium aluminides (Intermetallics) also have ordered structure (superlattice) like Cu-Au alloy system. The binary system of Ti-Al phase diagram is shown in Fig. 2 [47]. The phase diagram of titanium-aluminum system indicates that there are four phases such as high temperature disorderedma-Ti, disordered fl-Ti, ordered hexagonal az-TiaAl and ordered face-centered tetragonel 7- TiAl [5]. 12 Figure 1. Ordered substitutional structures in the Cu-Au system: (a) CuAu superlattice. (b) CuaAu superlattice [26]. 14 Figure 2. Binary Ti-Al phase diagram [24]. 15 ./.... r r , .5. a \\.l 1 x In. . \ m . LkaOI .0007; IRII Iiitbi lltlil DroL r \.\\j K s til .4. ..... a s .I' m r l It..it.... r Hufl 00.0700... w. 41/“ E- f I i r 1L w . xx nnu: Al/L r. 1400 1200 U.._. A] 60 40 20 Ti un96 Ffisnmre 2. 16 1-2-2. Laser Surface Modification In recent years, laser surface modification has emerged as an attractive technique because it is very benificial to modify surface properties without changing bulk properties of materials. Several techniques such as laser-chemical- vapour deposition (LCVD), cladding, alloying, surface hardening and surface melting are included in this category [56]. Laser surface cladding was used in the present study. In cladding, powder is coated on the substrate with a binder and is melted to produce strong bonding with the substrate. (Fig. 3) The beam in this process should be perpendicular (or nearly perpendicular) to the surface of specimen and is mostly defocused. The variables for this process are beam intensity, efficiency, effective heating time and beam diameter. [48] l7 Figuro.3. Laser surface modification (cladding) [26]. 18 v V V V V V. 71"g'e77.'l ' . TA.LA.1L..AL1LA .A.11 'J'A'A .1.LL Base metal Clad layer Figure 3 . l9 2. EXPERIMENTAL PROCEDURE 2-1 . MATERIALS Ti3Al and A13Ti intermetellics used in the present study were obtained, in plate and ingot forms respectively, from Howmet Inc. The detailed composition of quhl is listed in Table 1. 20 Table 2. The chemical analysis of TiaAl stock Elements Ti Al Mo Fe V Cu Si Nb Wt % 56.3 14 2.07 0.08 3.87 0.15 0.1 23.43 At % 60.8 25 1 3 10 (Ti-25A1-lONb-3V-1Mo) 21 2-2. LASER SURFACE MODIFICATION Among various types of lasers that have been developed for various purposes, usually two types of lasers are used for laser surface modification of metals and ceramics. One such type is solid state laser such as pulsed ruby, and Nd(Neodymium):glass laser: the other type is gas laser such as C0: laser. In particular, the application of gas lasers is gaining importance: they have continuous waves, high efficiency (for example, COz laser has around 30 % efficiency), high power and reasonable costs for installation. A 2.0 KW. C0; laser was used in the present study. Ti3Al and AlaTi were prepared for matrix and coating respectively. Small square specimens (10 mm x 10 mm x 3 mm) were cut out from the T13Al plate. The AlaTi ingot was crushed and ground to a powder in a ball mill using alumina balls for 24 hours. The AlaTi powder was coated on the Tfiqu plates in the form of slurry prepared with absolute alcohol. The coated specimens were allowed to dry completely for about 20 minutes before laser surface modification. 22 Several operational parameters need to be considered for laser surface modification process. These include the power of laser, interaction time (beam traveling speed), the range of spot size (beam diameter), and the number of times of remelting. In the present study the beam diameter on the surface of specimen coated by Algri powder was fixed. Interaction time between the laser beam and specimen, and the number of times of remelting were changed. Helium (He) gas was used as a shielding gas. The shielding gas chamber was specially designed for increasing the efficiency of shielding gas. The gas chamber used in this study is shown in Fig. 4. Surface remelting was carried out along the same track of the same specimen for several times. All laser treatments were accomplished using a defocusd beam. The details of the processing conditions used are listed in Table 2. The graph used for converting the beam diameter to the distance from the nozzle of the laser beam to surface of specimen is given in Fig. 5. The overall features of laser surface modification are illustrated in Fig. 6. After each laser surface modification process, visual inspection was made and the conditions of beam spot size and traveling speed of the laser beam were changed to find the optimum condition for producing a good coating. 23 Table 3. The conditions of laser surface modification The number of times of remelting 1 1 1 2 beam speed (in/min) 30 35 40 50 60 * used laser power: 680 W * used power density: 10‘ J/cm2 * maximum power density; 10° J/cm2 24 Figure 4. Shieding gas chamber. 25 J -Slit for Laser Beam "‘— Enclosure (Quartz) Specimen Q T Sample stage (Graphite) V Figure 4 . Figure 5. 26 Beam focusing chart. 27 1.0 3 mm (0.12 in) ‘63 Ill 5 g 0.1 C III 1.. III a S o 2 4 ill 0 .‘61 BEAM DIAMETER VERSUS DISTANCE FROM Focus FOR LENSES OF VARIOUS FOCAL LENGTH .001 .01 .10 1.0 10 2.0 in DISTANCE FROM FOCUS (INCHESI Figure 5 . 28 Figure 6. Overall features of laser surface modification. 29 LASER BEAM 1“ ET 45-DEG REE MIFIRCR ”anti !:_ I} : ._—. -* """ -~-‘ A :2“ _‘ - :4: LENS GAS —>-' "A ': INLET this —.- W rET‘ !’ . GASJET I FIT—i. AXIAL ' 1"? NOZZLE " m ADJUSTMENT .,‘ # NOZZLE Figure 5 - 30 2-3. CYCLIC OXIDATION Cyclic oxidation was carried out for 10 hours. One cycle of the process consisted of 1 hour of oxidation at 1000°C and 15 minutes of cooling down to ambient temperature. The specimens treated by the laser were cut in the form of square plates (2.5 mm x 2.5 mm x 3.0 mm) by high speed diamond saw. Cyclic oxidation studies were carried out only on specimens with even coating. The thickness of the coating was almost 1 mm for the specimens selected for oxidation studies. All the selected specimens were polished with sand paper to have even surfaces before cyclic oxidation process. The oxidized specimens were assessed by several ways. After visual inspection, the oxide formation of coated side and that of uncoated side were investigated by Scanning Electron Microscopy (SEM) and Wavelength Dispersive Spectrometer (WDS) . 31 2-4. SCANNING ELECTRON MICROSCOPY To determine how Al3Ti coating protected the TiaAl matrix from oxidation, the cyclic oxidized specimens were cut by a diamond saw which is shown in Fig. 7. The cut surface of specimens were ground with # 240 sand paper, followed by # 320, # 400 and # 600 grit papers: they were polished on the rotating wheel with 600 grit, followed by 5 pm, 0.3 pm, 0.05 pm abrasive particles. Interfaces and grain boundaries in specimens were barely seen by naked eye after the polishing procedure. The cross section of the oxidized specimens was investigated by using SEM. 32 Figure 7. Specimen preparation before and after cyclic oxidation process. (dotted line is indicated cut surface. 33 / Cutting line \ AI,Ti . (before cutting) TI,AI AI,Ti TisAl (after cutting) Figure 7 . 34 2-5. IAVELENOTE DISPERSIVE SPECTROSCOPY 2-5-1. Characteristic x-ray Image Characteristic x-rey image studies were carried out for determining the concentration profiles of the three major elements such as titanium, aluminum, and oxygen. Sample currents for titanium and aluminum detections were fixed at 0.016 uA (0.03 ”A full scale). The amount of oxygen was not enough to be detected on the screen in WDS. So the sample current for oxygen detection was increased up to 0.2 MA (0.3 uA full scale) which was about 12.5 times more than that for titanium and aluminum. Nevertheless oxygen detection based on the image on the screen was still not strong enough. To obtain a sharp image of the oxygen mapping on the screen, the exposure time for oxygen was up to two and a half times longer than that for titanium and aluminum were selected. The exposure time for oxygen was 100 seconds and that for titanium and aluminum were 40 seconds. P 10 (90 % Argon-10 % Methane) gas was used for the X-ray detector for 02. 35 2-5-2. Line Scanning In the characteristic x-ray image study, electron beam swept certain areas on the specimen. In the line scanning, the beam was fixed initially at one point and then beam was scanned along a single straight line on the specimen to record the change in concentration of elements (for example, in the X- and/or Y-direction). The electron beam was kept perpendicular to the specimen surface. The conditions of WDS for line scanning were the same as that for characteristic X-ray image study. The speed of moving beam on the specimen was 0.193 mm / min. The chart speed of recorder was 1 in / min. The line scanning was executed from the top of the coated surface to the bOttom end of the matrix ( which was negative Y-direction ) and from the left side of the uncoated surface to the right side of the uncoated surface ( which was positive X-direction ). The starting point of the line scanning on the coated surface was at the middle of the full length of the specimen. The starting point of the line scanning on the uncoated side was at the half of the full height of the uncoated portion in which the width of the coating was not included. (Fig. 8) ZAF technique for absorption correction, atomic number correction and fluorescence correction was not carried out in this X-ray analysis. 36 Figure 8. The starting points and locuses for line scanning. 37 (Cut specimen) ‘ ‘\ \ I \f L AI,TI Ti,AI I -Y ' Figure 8 . +x 38 3. RESULTS AND DISCUSSION 3"]. . NICROSTRUCTURAL OBSERVATIONS The advantages of laser surface coating is that it can provide better specimen surface modification for protection against severe environments without changing the bulk properties. The optimum conditions to produce an even coating of AlaTi on the surface of TiaAl were obtained by trial and error method. Among the several trials made, five of them yielded specimens suitable for the present study. Although all five specimens had uniform Algri coatings, two of them exhibited breaks in the coating. The details of laser processing conditions used are already presented in Table 2. The specimens processed by laser with conditions, such as surface remelting speeds of 50 and 60 in/min, and surface melting speed of 35 in/min were selected for investigation by SEM. SEM of these specimens was conducted before and after the cyclic oxidation process. Specimen preparation for SEN studies has been described in the previous chapter. 39 3-1-1. Laser Surface Treated Specimen Several kinds of oxide crystals were found on the surface of the coated side, but all these exhibited charateristics of the early stages of oxide formation. The sequence of oxides formed on the coated side depended upon the laser beam speed and the number of times of remelting. Algh was formed on the surface of Algri coating. Continuous oxide crystal layer was formed on the coating when the surface was remelted with a defocused beam at a speed of 50 and 60 in/min. On the other hand, specimens processed by the defocused beam at a speed of 35 in/min formed discontinuous oxide crystal layer (Fig. 9). From the investigations of the microstructure at the interface between the coating and the oxide layer, it was observed that transverse cracks propagated along the interface and new oxides were growing on the Alyri coating at the crack (Fig. 10). It appears that such crack propagation is caused by abrupt temperature change by laser processing. This crack disappeared after cyclic oxidation process, because diffusion between the two phases took place during that process. Such crack can cause decrease the adherence of oxide layer to the coating. The adherence of the coating to the matrix appeared to be very strong, since two components of the system ( viz. the matrix and the coating ) were mixed well, and there were no 40 Figure 9. Scanning electron micrographs at the surface of the coated side after laser surface modification:(a) continuous oxide crystals. (b) discontinuous oxide crystals. ; “:’ V’ «‘9 +"} DEG.“ '% Discontinuous oxide crystals Figure 9 . 42 Figure 10. Scanning electron micrograph of the surface oxide layer after laser surface modification. 43 Figure 10. 44 cracks or pores at the interface. It was also observed that the clear interface line between the coating and the matrix was not fully developed so as to discriminate the two components individually. On the other hand, although grains and grain boundries were formed in the matrix, they could not be observed by SEM. These results are illustrated in Fig. 11. After laser processing, it was observed that only small areas on the surface of the uncoated side were oxidized. Most of the oxidized areas (in the early stages) were spelled. On the other hand, the unoxidized areas were discolored by the laser beam. These surfaces were not appropriate for cyclic oxidation process because of the following reasons : Firstly, the specimens did not have homogeneous surface (viz. partly oxidized and spelled surface). Secondly, melted Alyri coating overflowed and covered the top of these surfaces. For these resons, these surfaces were not used for oxidation studies. The specimens which had oxide layer only on the surface of the coated side were prepared to study the oxygen penetration profiles at the coated side and at the uncoated side during the cyclic oxidation process. 45 Figure 11. Scanning electron micrograph of interface between the coating and matrix. 46 Interface TI,AI Figure 11. 47 3-1-2. Cyclic Oxidized Specimen The specimen used for cyclic oxidation were small rectangular plates cut from the large square TiaAl plate. These were coated with Algri. After 10 hours of cyclic oxidation process, the specimens were sectioned and polished for SEM examination. The microstructural features of the coated side (surface, interface and matrix) and the uncoated side (surface and matrix) are shown in Figs. 12 and 13. It was observed that the oxide layer on the surface of the coated side was more uniform than it was before oxidation process. Cracks between the oxide layer and the coating disappeared. (Fig. 14) These results indicated that the adherence of the oxide layer to the coating increases because of the above mentioned phenomenon. No oxide layers were formed in the coating below the oxide layer, but evidence indicated the presence of small quantities of oxide formation in the subsurface. (Fig. 14) Grains were found in the Algni coating, even though they were not fully developed at the heat affected zone (HA2) after cyclic oxidation process (Fig. 14). During the cyclic oxidation process, the surface oxide layer protected the matrix as well as the coating itelf from oxygen penetration. 48 Figure 12. Scanning electron micrographs at the coated side: (a) surface. 49 '1’“ A h. ‘" N *— Oxide layer a <— AlsTi 20 pm coating I——-'I Figure 12. 50 Figure 12. (continued) Scanning electron micrographs at the coated side: 1(b) interface. 51 Oxide layer ‘— AlsTi coating Figure 12. (continued) 52 Figure 12. (continued) Scanning electron micrographs at the coated side: (c) matrix. 53 - AI,Ti Coating Interface M. W Grain boundary (continued) Figure 12. 54 Figure 13. Scanning electron micrographs at the uncoated side (surface and matrix). 55 < (... Oxide layer ‘.-— TigAl Figure 13. 56 Figure 14. Scanning electron micrograph of the coating showing the heat affected zone (by laser processing) and the grains. 57 7 ‘— 0xide layer Heat affected zone *— AI,1'i Figure 14. 9? 4| 58 Tiny pores observed in the coating could be eliminated by using slower laser beam speeds. These results were reported by Oakley et a1 [57]. After cyclic oxidation, a distinct interface develops between the coating and the matrix. This interface line appears to be very stable as a result of its roughness and interdiffusion. These phenomena led to strong adherence of coating to the matrix (Fig. 15). This diffusion caused the interface line to move a little further into the matrix side, increasing the coating thickness. After 10 hours cyclic oxidation process, new phase was formed near the interface (in the ngAl side) which appears to be probably a TiAl phase [58]. It was also observed that the grains and grain boundaries were well developed in the matrix due to the oxidation treatment. During the cyclic oxidation process (1 hour of oxidation and 15 minutes of cooling), the surface of the uncoated side was severely oxidized. The morphology and composition of the oxide layer on the uncoated side was totally different from that on the coated side: the latter was AIME but the former was TiO2 (Rutile). The oxide layer formed and spelled. during the first cycle of cyclic oxidation process, revealing new surface to air. However, the entire oxide layer did not spell, leaving remnants of the oxide layer on the surface of the uncoated side. This caused the oxide 59 Figure 15. Scanning electron micrograph of the interface showing strong adherence of coating to the matrix. 60 Figure 15 . Interface 61 layers to accumulate, with transverse cracks in-between them. Such results are illustrated in Fig. 16. Two reasons can be used to explain the presence of spelling and transverse cracks after the oxidation process. They are as follows: (i), the stress and strain of the matrix due to oxidation process are transmitted to the oxide layers [59-61], and (ii), the stress and strain due to thermal shock during the oxidation process cause the oxide layers to spell. These two processes occured at the same time. Two models to explain the spelling behavior can be envisaged. The sequence of steps occuring in the first model are listed below: (a) the first oxide layer is formed during the first cycle of oxidation process. (b) the second oxide layer is formed beneath the first layer during the second cycle. Simultaneously, transverse cracks propagated between two layers. (c) the first oxide layer spells after the third cycle of oxidation process. In the second model, after the first cycle of oxidation process, the oxide layer detached from the matrix and spelled. These models are illustrated in Fig. 17. Oxidation at the uncoated side took place because of oxygen Figure 16. 62 Scanning electron micrographs of uncoated side (surface and matrix) after cyclic oxidation showing: (a) surface oxide layer before spelling. (b) surface oxide layer after spelling. 63 Oxide layer (TiO,) Figure 16. Figure 17. 64 The schematic diagram of spelling models at the surface of uncoated side during cyclic oxidation process. (a) The first spelling model. (b) The second spelling model. 65 _ “Oxide layer » ...a—Oxide :2! :3 M S 4 layer TisAl l Figure 17. 66 penetration. The penetration depth of oxygen depends on the oxidation time and temperature [37,42]. The damaged spots at the surface caused by the cutting operation, in spite of being very small, exhibited a larger degree of oxidation as compared to the other regions of the uncoated surfaces (Fig. 18). The damaged spots on the surface could be exposed pores as a result of cutting, or due to cracks between grains. Oxides in the damaged spots did not spell, since they adhered to the small spaces between the grains. Such damaged spots played an important role as the origin of further oxidation of the matrix. (Fig. 19) On the other hand, the oxide layer on the coated side did not spell because the tiny pores in the coating absorbed the stress and strain transmitted from the matrix. The strong adherence (due to diffusion between the oxide layer and the coating) prevented the oxide layer from spelling. 67 Figure 18. Scanning electron micrograph of the damaged spot at the surface. 68 Damaged spot Figure 18. 69 Figure 19. Scanning electron micrograph of the damaged spot showing further oxidation into the matrix. 70 Damaged spot Figure 19. 71 3-2. CBRACTERISTIC X-RAY STUDY 3-2-1. Characteristic X-ray Image Characteristic X-rey image studies were carried out on the surface, interface and matrix of the coated and uncoated sides before and after cyclic oxidation process. Concentration profiles of titanium, aluminum, and oxygen were taken from the same regions selected for the SEM studies. Before the cyclic oxidation process, uneven distributions of titanium and aluminum were observed in the surface of the Algni coating. A small amount of oxygen distribution was also found here. Such features were caused partly by the laser surface modification and partly by the beginning of the oxide formation. Unfortunatly, it was hard to guess the kind of oxide which was formed at the surface because the 'concentration profiles of three elements (titanium, aluminum and oxygen) were all mixed up (Fig. 20) But after the cyclic oxidation process, the oxygen' concentration was found to be very high only at the surface, which indicated the presence of an oxide layer at the surface of the A1531 coating. The titanium concentration 72 Figure 20. Characteristic X-ray image at the surface of the coating after laser sufece modification for: (a) titanium. (b) aluminum. 73 (Ti) +— Surface I .1 ,p.a.& an ,. (All Figure 20. Figure 20. 74 (continued) Characteristic X-rey image at the surface of the coating after laser sufece modification for: (c) oxygen. 75 (Oxygen) Figure 20 . (continued) 76 profile was uniform throughout the specimen except in the oxide layer on the AlaTi coating. The concentration of titanium at the oxide layer was negligible. On the other hand, the aluminum concentration in the oxide layer on the Algri coating was much higher than that in other regions of the specimen. These results indicated that the oxide layer was composed of alumina. Alumina was formed on the surface of the coating because of higher aluminum contents in the Algri coating (Fig. 21). It has been reported that the higher aluminum protects this type of materials from oxidation [24]. Subrahmanyam et al. [45] also reported that AlaTi was more effective than TiAl in preventing oxygen penetration. Oxygen was not detected below the surface oxide layer (viz. the coating, interface and matrix) because the oxide layer protected the coated side from the oxidation. A change in the concentration profile of titanium and aluminum was observed at the interface between the coating and the matrix. Titanium concentration increased and aluminum concentration decreased below the interface because AlaTi was coated on the TiaAl (Fig. 22) On the uncoated side, three kinds of oxygen concentration profiles were detected. The first profile was matched to the scanning electron micrographs obtained from the surface of uncoated side which had several oxide layers. 77 Figure 21. Characteristic X-ray image at the surfce of the coating after cyclic oxidation process for: (a) titanium. (b) aluminum. 78 (Ti) (All <— Surface Figure 21. Figure 21. 79 (continued) Characteristic X-ray image at the surfce of the coating after cyclic oxidation process for: (c) oxygen. 80 (Oxygen) surface <— Oxide layer c LAMCM) Figure 21. (continued) 81 Figure 22. Characteristic X-ray image at the interface of the coating after cyclic oxidation process for: (a) titanium. (b) aluminum. 82 (Ti) «— AI,TI ~— lnterface . a -— TiaAl 1 (AI) -—— AlsTi b ‘— Interface TiaAl Figure 22. Figure 22. 83 (continued) Characteristic X-ray image at the interface of the coating after cyclic oxidation process for: (c) oxygen. 84 (Oxygen) .— AI3Ti Interface «— TisAI Figure 22. (continued) 85 The second profile was for the uncoated surface in which fully developed oxide layers had already spelled off and the the oxide layer developed during initial stage was left on the top of the surface. Insignificant oxygen concentration was detected in this region (Fig. 23). The oxygen level of the damaged spot was much higher than that of the others (Fig. 24). Trapped pores (which were also damaged spots) in the matrix had a slightly higher oxygen level than that of matrix because they had trapped oxygen within them (Fig.25). The oxidation process also had occurred inside the wall of pores. All the above results matched the results of the SEM studies. 86 Figure 23. Characteristic X-ray image at the surface of the uncoated side after cyclic oxidation process for oxygen: (a) before spelling. (b) after spelling. 87 (Oxygen) (Oxygen) Surface Figure 23. Figure 24. Characteristic x-ray image at the damaged spot. 89 (Oxygen) <— Surface“ --—— TiaAl Figure 24. Figure 25. (a) (b) 90 Characteristic X-ray image at the trapped pore for oxygen and corresponding scanning electron micrograph. 91 (OxygeNl -— Surface I,TI ~t—-.A hu or . .fiop. Jf" Q. aw ‘Trapped pore Figure 25. 92 3-2-2. Line Scanning Line scanning was carried out on the same areas as those selected for SEM and characteristic x-ray image studies. A scan line consisting of a set of successive points was chosen along the specimen surface. Line scanning could not describe the overall concentration profile of the elements as the area (image) scanning did, but was useful in describing the change of concentration along a line. The line scans for titanium, aluminum and oxygen were carried out before and after the cyclic oxidation process. Before the cyclic oxidation process, the concentration level for titanium and aluminum was not uniform in the coating but was uniform in the matrix. It was also observed that the Scans for the titanium and aluminum concentrations changed sharply at the interface. Only one high peak for oxygen was obtained in the full scanned line, which proved that the oxide layer was already formed on the surface of the Algri coating after laser surface modification. Other than that peak, no other oxygen peaks were detected in the coating, interface and matrix (Fig. 26). 93 Figure 26. Characteristic X-rey line scanning before cyclic oxidation process at the coated side showing concentration profiles for: (a) titanium. 94 --.-o- -- - ---- —-—..—-—-o--- . ; T1,AI i U i f I K i I , 0 e +0.”.-- -. O 0....- .. .—... i . I I ! -.. - ..§ 3 I ‘ -—-.- a.‘.- e o- o - - ye..- I l I I -" ""' ‘ " “'§"—— Interface L i l; AI,T1 I u. o otti.nJ Tali-aft - - . _..— a- . -_--o- - ...-- ..-_ - -..-a . . . .. . . . . m I . a _V _ . . . . . _ _ ..9. I- , ...... .a _ .. 0.193mm Figure 26. 95 Figure 26. (continued) Characteristic X-ray line scanning before cyclic oxidation process at the coated side showing concentration profiles for: (b) aluminum. 96 ‘ _ "(Ab _ i : ‘ ”‘f‘ "" ___,x — ——v;:~—-- —- ---~- LI." _; ' ' ___-__E_- _ _ - . ! -—- —;;c— ———--—— --— -— _ __ - _l_ _ _ _ ,_ ; f """":f _ “‘— _ - I g i I 11.x .-_ ”I I ' 'l IL _!_ a_ ..... ’ I . - Iii] I I" It. _____é_ m -_‘__ _- i TI,AI nails...— ' ' 0.193 mm Figure 26. (continued) 97 Figure 26. (continued) Characteristic X-ray line scanning before cyclic oxidation process at the coated side showing concentration profiles for: (c) oxygen. 98 'II- I- I n.l'e".L.l I. . III. II it I. . I I -.— ”on- “-P‘“- ...-w..- -o--» I Acessaudoo. EE may... 0 .u« change Baa. 02.5 33.6.... 1... . I- --I-.. . Ilsbli II -i_1|- ...... - w u e . IIIIIIIL III' II. . I _ . l.-III: I - .. I. . L - . m - A - II. . i. I I o t. IIII I . . IIIIiII.—. III1 III I II I ----i, _ 1 . . . c I C . - II. '1. . ...II I I. I I . O c u a .. ulli...'0 :81 I’ll. 1"an I I a " Inf. ell-Ill T‘Ilnl. Ill- IlTII-I'IIIPI. . . . . _ m . . Ensues. 99 After 10 hours of cyclic oxidation process, the concentration levels of titanium and aluminum in the coating became more uniform like those in the matrix because of diffusion. .During the oxidation process, some amount of titanium in the matrix diffused to the coating and some amount of aluminum in the coating diffused in the direction opposite to that of the titanium flow. Hence, the concentration profiles of titanium and aluminum, more or less, smoothened at the interface, and the coating grew thicker than before. The concentration profile of oxygen was almost the same as that before cyclic oxidation except for differences in the peak width of the oxide layer. The peak in the oxygen scan increased in width slightly because of the growth of oxide layer during the cyclic oxidation process. Besides this peak, no other peaks were observed throughout the specimen (Fig. 27). This also proved that the coated side (coating, interface and matrix) was protected well from the oxidation even after the cyclic oxidation process. On the other hand, several oxygen peaks were seen at or near the surface of the uncoated side. The size of the oxygen peaks at the surface were largest, and were followed by smaller ones in the decreasing order of oxygen concentrations (Fig. 28). Peeks which did not follow this 100 Figure 27. Characteristic X-ray line scanning after cyclic oxidation process at the coated side showing concentration profiles for: (a) titanium. .;- Illterf'c.’ --— — a . --- -.- m . ,——‘.- a- -—- -.. -...- .u-_ -' . “In. . ... “ ...- ! o .----.. - V. 1--...-” _ d- — ----. 0.193 mm Figure 27. Figure 27. 102 (continued) Characteristic X-ray line scanning after cyclic oxidation process at the coated side showing concentration profiles for: (b) aluminum. 103 Inuufime -. .J- 0.193 mm (continued) Figure 27. 104 Figure 27. (continued) Characteristic X-ray line scanning after cyclic oxidation process at the coated side showing concentration profiles for: (c) oxygen. 105 EE «and TII. 32:53:00. .hu ousmwh 2.: {ontoge— cuae. 0240 _ ... Aceuaxow 106 Figure 28. (a) Oxygen concentration profiles at uncoated sides after cyclic oxidation process. 107 (Oxygen) lee- 016.574 1. IL 1 1 ” --- w. -- _ _ _ . . m r _ m . _ _ -s- -Ifiilll _ _ _ . lo-.I.I|l -..-IIItoLItI It _ m _ _ . M . . _ w _ I . M . -m..--.:MII.I% -39 .-.-.---.. ‘_*~Io¢ ..-...” --.. . 0 11’” 0.193 mm Oxide layer Figure 28. 108 Figure 28. (continued) (b) Oxygen concentration profiles at uncoated sides after cyclic oxidation process. 109 Loan. 23.5 .uoaaaueoo. .eu uneven Loan. 02x0 Acuuaxov 110 trend were detected in the middle of the full scan line because of the presence of trapped pores (Fig.29). Pores trapped some amount of oxygen when they were formed inside the matrix and the oxygen penetrated to the matrix through the pore walls. This trapped oxygen formed the oxide on the pore well during the oxidation process. The shapes of the peaks on both the uncoated surfaces (on the left and right sides) were not same because of differences in the degree of oxidation at the both sides. 111 Figure 29. (a) Oxygen concentration profiles at uncoated sides after cyclic oxidation process showing trapped pores. 112 .e« casuah O 22- 12.39.... cone. 03x0 8:. man... 3.: Loan. 02.5 _ III. N l— 4 . 1 . . . . .. m. _,. .. .... fl. .1 .is PI—Cl. -: - --. .- ’ e .I’ . -II.IIA l1|-ll'-l 'I'llfilllllit - . . _ 8 A _ LIIII II. I I“ . _ 7 i L )- ..-.qr.: ..-. l" ...—(.4... :.. .-0‘--.e-)- :- -4 e.- —- A—nlp. . A _ . I ..-I. III—...... I -.- . _ . TIMIII.I.I-I| _.II!IIJI.--II I I I I I I l I I I . . LILLONIIL. ITII. .IIIII - ,- if I .11.. .II __-..-- ..;--._-.r - .. 113 Figure 29. (continued) (b) Oxygen concentration profiles at uncoated sides after cyclic oxidation process showing trapped pores. 114 .vosaflunou. .oa ouaowm an»: 02.5 a ..oaa. 02.5 22. eoaaflh 5—: «and . o c . H . . I . A . 0 g - . — . a , II 0 ll. '0 I I- III..'| ’9IO.'I 4'-.‘.-.'I.|u !.O.|-o. I III. ll-llT II.-- .I.00'-|q' ll..ln|. I. .I.l.n II o ., . . u M . w . . . m . g b I.— .. by |IWI ll...ll.'u 'Iv’..ll.-li .IIIIIIIIIIII'QIIU'.-."II II. In. 'lh-‘l..tn-ll'.1lal '.'Ir0Ioll —. .. . _ m 4 u . . . . . n . .- - L; I..- ---IIII sail..- -... _ ...m .. m . . _ I. O L M . ..i..ul ..I ill.L-i. ""I...In.ll.-.l.u.ll|lo III?! .w o . o . a . n . 'II. I, IrIOQ-r III I I ' I'lll'l ll '0 III lllc~§udl" luq-l ‘l - I 5%. 2!- ..i T -II I. .. I-. u. - .ll ...-IO. . ‘I 'J . 5....."I .9... .' .I..,O!OI'III,II'- Il'- Lfil‘-.. Ill. .l..ll 'IIII I III.II|.- I'll' Ill'nlfl._wl l..-l.|| Iti '.. .1 III: --..-;.-I .--I ._ II-...--..--._ -.-...III {III III. I.. if I + m . 4 i m . 115 4. CONCLUSIONS AlaTi coating was formed on the Ti3A1 substrate by using the method of laser surface modification. This process used in the present study is simple, convenient, effective and satisfactory. After laser surface modification process, the oxide layer formed on the surface of the Algri coating protected the TiaAl substrate as well as the coating itself from the oxidation during the 10 hours cyclic oxidation process (1 hour of oxidation at 1000°C and 15 minutes cooling at ambient temperature during each cycle). The oxide layer formed on the surface of the uncoated Tfiqu substrate spalled off after every cycle of cyclic oxidation process. This was caused by the stress and strain transmitted from the substrate and by the stress and strain which occured in the oxide layer during the cyclic oxidation process. The oxide layer formed on the coated side was A15% and that formed on the uncoated side was Tioz. 116 The strain and stress caused by thermal expansion mismatch between the coating and the substrate did not cause the surface oxide layer on the Alyri coating spall off during the 10 hours cyclic oxidation process. The interface between the AlaTi coating and TiaAl substrate moved down slightly to the substrate side during the oxidation process due to interlayer diffusion. 117 REFERENCES 1. F.H. Froes, "The Advanced Aerospace Structural Materials Dilemma", J. Metals, pp 30-35 (1989) 2. J. Wadsworth and F.H. Froes, "Developments in Metallic Materials for Aerospace Applications", J. 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