Pr. ,1? llllfllll l lfllllllllllllllfllll 93 0808 7499 LIBRARY LMkhigau Stat. "M This is to certify that the thesis entitled RECRYSTALLIZATION KINETICS, MICROSTRUCTURE, AND TEXTURE DEVELOPMENT IN BORON DOPED NI3AL presented by Pavan Nagpal has been accepted towards fulfillment of the requirements for 11.5. degree in Materials Science Dr. G. Gottstein Major professor Date May 222 1987 0-7639 MS U is an Affirmatiw Action/Equal Opportunity Institution 'MSU LIBRARIES RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. RECRYSTALLIZATION KINETICS, MICROSTRIICTURB AND TEXTURE DEVEIDPHENT IN BORON DOPED N13A1 3? Pavan Negpel A THESIS Submitted to Michigan State University in partial. fulfillmnt of the require-ants for the degree of MASTER OF SCIENCE Depart-cut of Hetellurgy, Mechanics and Materials Science 1987 ABSTRACT RECRYSTALLIZATION KINETICS, MICROSTRUCTURE, AND TEXTURE DEVELOPMENT IN BORON DOPED NisAl BY Pavan Nagpal Recrystallization texture and microstructural development of Boron doped NisAl rolled to different degrees of defamation were studied. Texture measurements were made on deformed and recrystallized specimens. Also, TEH investigation of deformed and partially recrystallized samples was conducted. The recrystallization temperature was found to be higher (0.55 Tm) than observed in pure metals (0.4 Tm)“ In one hour, an 80 8 0 cold rolled sample recrystallizes at 625 C whereas a 20 % cold rolled 0 sample recrystallizes at 700 C. The brass rolling texture suggests that B-doped Ni,A1 may be a low stacking fault material. Very few deformation twins were observed in the electron microscope, but the tendency towards twinning was suggested by frequent occurence of recrystallization twins. The recrystallization texture was not a typical brass recrystallization texture. In fact, it was not a pronounced texture at all. Boron may be responsible for this effect. mowmcm'rs First of all, I wish to express my sincere thanks and appreciaton to my academic advisor, Professor G. Gottstein, for his constant guidance, continous encouragement and valuable suggestions throughout this study. I would also like to thank my friends in the MMM department for their timely help during experimental work. Finally, the support of U. S. Department of Energy, Office of Basic Science, under grant number DE-FGOZ-BSERASZOS is gratefully acknowledged . LIST OF FIGURES 1. 2. TABLE OF CONTENTS INTRODUCTION LITERATURE SURVEY 2.1 The Material 2.1.1 2.1.2 2.1.3 Mechanical Properties Of NisAl Effect Of Boron On The Strength Of Grain Boundaries. Effect of Alloy Stoichiometry 2.2 Recrystallization 2.2.1 2.2.2 2.2.3 2.2.4 General Definition Nucleation Of Primary Recrystallization Recrystallization Kinetics Recrystallization In Ordered Alloys EXPERIMENTAL PROCEDURE 3.1 Testing Material 3.2 Specimen Preparation 3.2.1 3.2.2 3.2.3 3.2.4 Cold Rolling And Annealing Texture Measurement Optical Microscopy Electron Microscopy 3.3 Hardness Testing EXPERIMENTAL RESULTS 4.1 Recrystallization Behavior 4.2 Microstructural Development Page 11 13 16 16 17 17 18 19 21 Page 4.2.1 Isochronal Test 22 4.2.2 Isothermal Tests 22 5. DISCUSSION 5.1 Recrystallization Behavior 5.1.1 Isochronal Test 48 5.1.2 Isothermal Tests 48 5.2 Texture Measurement 49 6. CONCLUSIONS 51 7. REFERENCES 52 Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. LIST OF FIGURES Figure Captions . Plastic strain to fracture (t) of rapidly solidified Ni,Al - B as a function of Al and B concentration. . Effect of stoichiometry on fracture mode of B - doped Ni,A1 at room temperature. . TEM micrograph of a nucleus at the boundary of a shear band in rolled Cu. . Recovery induced SIBM by local subgrain coalesence at a grain boundary. (a) before,(b) after coalesence. (c) SIBM in 40% compressed Al. . Schematic diagram showing transition from band to matrix with small but additive orientation differences. . Micrograph and [111] pole figure of a Cu single crystal 0 dynamically recrystallized at 560 C showing a twin family. Grain (a) (dashed orientation) is a primary twin to the deformation structure, and grain (b) (dashed-dotted orientation) a twin of (a). . Schematic curves of recrystallization in hardness. . Isochronal annealing (for 2h) curves in the hardness for for specimens of CoaTi cold-rolled to e - 50%. . Isochronal annealing (for 2h) curves in the hardness for the specimens of Co,Ti called rolled to e - 80%. 10. Optical micrographs illustrating the microstructural changes accompanying the isochronal anneals of NiAl. 11. Schematic diagram showing rolling configuration. Page 10 11 12 14 14 15 18 Fig. Fig. Fig. Fig. Fig. Fig. 17. 12. 13. 14. 15. 16 Figure Captions Isochronal annealing (for 15 minutes) curve in the hardness for the 72% cold rolled specimens. 0 Isothermal annealing curves at 700 C with different degrees of deformation. Isothermal annealing curves at 625°C with different degrees of deformation. Isothermal annealing of 80% called rolled specimens at 625, 650 and 675 deg. C. Optical micrographs illustrating microstructural changes accompanying isochronal annealing of 72% cold rolled specimens: . o a) for 15 min. at 700 C. o b) for 15 min. at 800 C. o c) for 15 min. at 900 C. o d) for 15 min. at 950 C. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 80% cold rolled specimens: 0 a) after 20 min. at 700 C. o b) after 40 min. at 700 C. o c) after 50 min. at 700 C. o d) after 15 min. at 675 C. o e) after 25 min. at 675 C. o f) after 40 min. at 650 C. vi Page 24 25 26 27 28 28 29 29 30 3O 31 31 32 32 Fig. 18. Fig. 19. Fig. 20. Figure Captions o g) after 10 min. at 625 C. h) after 55 min. at 625°C. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 60% cold rolled specimens. a) as rolled o b) after 45 min. at 700 C. c) after 45 min. at 625 0C. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 40% cold rolled specimens: a) as rolled. b) after 45 min. at 625°C. c) after 20 min. at 700°C. d) after 30 min. at 700°C. e) after 40 min. at 700°C. f) after 50 min. at 700°C. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 20% cold rolled specimens: a) as rolled o b) after 20 min. at 700 C. o c) after 30 min. at 700 C. o d) after 40-min. at 700 C. v” Page 33 33 34 34 35 36 36 37 37 38 38 39 39 4O 40 Figure Captions Page e) after 50 min. at 700°C. 41 Fig. 21. Pole figure of 90 % cold rolled specimen. a) {111}. 43 b) {200}. 44 Fig. 22. Pole figure of fully recrystallized specimen. a) {111}. 45 b) {200}. 46 Fig. 23. TEM micrograph showing a partially recrystallized structure. 47 vnl 1 . INTRODUCTION The Nickel Aluminide, NisAl, is an intermetallic compound having the L12 ordered crystal structure [1]. This material has unique properties that makes it attractive for structural applications at high temperatures. Most important, unlike most conventional alloys, the yield strength of NisAl increases with increasing test temperature [2-4] . Despite its attractive properties, fabrication for structural use is precluded by its propensity towards brittle frature in polycrystalline forms [5-7] . The brittleness is associated with intergranular fracture without appreciable plastic deformation within the grains. Small additions of boron considerably improves the ductility of NisAl [8] . Ductilized NisAl is conceived to be a potential material for future high temperature structural applications. For this purpose it will have to be formed into required shapes and will be exposed to high temperature service environments. At some stage of its production process it will be recrystallized to optimize its microstructure or achieve the desired mechanical properties. The current study is to elucidate the kinetics, mechanisms and microstructural development during recrystallization of B-doped Ni A1 . 3 It is widely known that recrystallization is a process that causes softening of a deformed metal and facilitates further deformation. The process of recrystallization, seemingly quite simple, is actually complicated by the fact that it is a structure sensitive process. The texture obtained in the material during deformation is also very important in predicting the mechanisms controlling recrystallisation. The work done here is a part of a larger study of dynamic recrystallization of boron doped NisAl. It confines its scope to static recrystallization kinetics and texture development. 2. LITERATURE SURVEY 2.1 The Material 2.1.1 Mechanical Properties of NiBAl Superlattice alloys (including intermetallic compounds and long range ordered alloys ) have been a topic of considerable scientific and engineering interests in recent years. This is due to the fact that many superlattice alloys show unusual mechanical properties, high microstructural stability and high oxidation resistance at high elevated temperatures. A large number of ng-type compounds (i.e. CuaAu crystal structure) show substantial increase in flow strength with increasing temperature [9-10]. This behavior makes these compounds attractive as new heat resisting materials. However, these materials are often subject to intergranular embrittlement. NisAl falls under this class of materials. NisAl, also, forms an L12 ordered crystal structure below the peritectic 0 temperature (1395 C). It is the most important strengthening constituent(1') phase of commercial nickel-base superalloys, and is responsible for their high temperature strength and creep resistance. It has been known for years that single crystals of NisAl are ductile, while its polycrystalline forms are extremely brittle [6,11]. The brittleness of Ni Al polycrystals is due to intergranular fracture. 3 4 Microalloying, which involves addition of minor concentrations (usually) in the ppm range ) of elements to control grain boundary structure or comnposition. Aoki and Izumi [12] first discovered that small boron additions substantially improved the ductility of NisAl at room temperature. By control of boron concentration, alloy stochiometry, and thermomechanical treatment, Liu and Koch [13] reported a tensile elongation exceeding 50% - the highest tensile ductility ever achieved by polycrystalline aluminides. The beneficial effect of boron was also confirmed by Taub, Huang and Chang [14] who tested NisAl foil materials prepared by melt spin technique. 2.1.2 Effect Of Boron On The Strength Of Grain Boundaries The beneficial effect of boron on the mechanical properties of NisAl seems to be related to its effects on grain boundary properties. Without any addition of boron, NisAl fails by intergranular fracture. With small additions of boron it shows great ductility and fails through a mixture of transgranular and intergranular fracture [15] . According to Schulson and Baker [16] boron, by segregating to the grain boundaries, eases the movement of dislocations within the plane of the boundary . It is based on the view that to propagate slip through a grain boundary it is necessary to push dislocations into the boundary from one side and bring dislocation out into the grain on the other side. In the general case, the orientation of the grains is such that the dislocation that emerges into the grain will have a Burgers vector different from that of dislocation that enters the boundary. A grain boundary dislocation must be left behind whose Burgers vector is vector difference betweeen those 5 of incoming and outgoing dislocations. If further slip dislocations are to pass through the boundary at the same location, then the strain concentrations represented the boundary dislocations must be removed or spread out, presumably by dislocation motion within the boundary plane. The easier is this motion, the easier will be the propagation of slip. Also, the tendency for cracking will be lower, owing to the reduction in stress concentration, and the ductility will be greater. 2.1.3 The Effect Of Alloy Stochiometry Alloy Stoichiometry has a strong effect on the ductility and fracture behavior of boron-doped NisAl [15] . Boron is most effective in improving the ductility and supressing intergranular fracture in Ni,A1 containing 24 at.% Al [Figs. 1. and 2.]. 6 Ni=Al RATIO 77:23 7624 7525 7426 ' 1.5 _ J [0 _ 23 0.0 _ 2 IO — I2 — a: 27 _ .9 2° 22 0.2 '— 0 I3 I l 22 - 23 24 ‘ 26 Al (at pcl) Fig. l. giaific strain to fracture (%) of rapidly solidified 3 _ B as a functionb of A1 and B concentration. . .1 ' . i n .‘ 3 ”if. [g g. X . ' ._ ‘ “.‘ ’ ~ “14,994 in x \V. .‘.".. -. ' :h .12 0‘. ' . .. .1 S‘..f | . O ‘ ‘ § .1. ' {F’yf ;‘"I'i‘l‘ : _ . ’w .4 1' . '0 i; . e..." . ’3 ' ‘w’ 95" if}? .e: Ticfir '\l -l ,i‘»:‘..‘, .. 3 r-' ”Ilka. i 50 . [.LM (0) 24% AI, 6 = 49.4% (b) 24.5 “lo AI, e = 37.0% (C) 25.0% Al, 6 = 6.0% TRANSGRANULAR NMXED FRACTURE INTERGRANULAR FRACTURE MODE FRACTURE Fig. 2. Effect of Stoichiometry on fracture mode of B - doped NisAl at room temperature. 2.2 Recrystallization 2.2.1 General Definition Primary recrystallization is defined as the nucleation and growth of strain free grains from a deformed microstructure (It is actually rebuilding of the crystalline structure by generation and motion of high angle grain boundaries from the deformed state with concomittant elimination of the dislocation structure). Primary recrystallization diminishes the number of structure defects caused by prior defamation, and restores the properties the metal had before deformation. The main driving force for primary recrystallization is the elastic energy stored in the strain field of the dislocation which comprise the deformed structure. 2.2.2 Nucleation Of Primary Recrystallization. The characteristics of recrystallization nucleation are : a) Nucleation of the new state is irreversible and the initial state cannot be restored. b) The size of the viable nuclei is microscopically small and its configuration preformed in the deformed structure. c) Nucleations occur in regions which deviate from the average structure. The following nucleating mechanisms have been proposed: 1) Nucleation by strain induced grain boundary motion ( SIBM ). A preexisting grain boundary in a polycrystal may bulge when an imbalance of driving force is introduced. Since bulging increases the grain boundary area the bulge must exceed a critical radius in order to gain energy on further growth. SIBM has been observed to occur only at law degrees of deformation as shown below Fig. 4. 2) Nucleation at deformation inhomogeneties [Figs. 3. and 5. ]. During deformation band shaped inhomogeneties are famed ( kink bands, defamation bands, shear bands etc. ). The transition from band to matrix is generally accamadated by elongated subgrains with small but additive orientation differences. The growth of one subgrain in the transition region leads to a high angle grain boundary. Fig. 3. TEM micrograph of a nucleus at the boundary of a shear band in rolled Cu. Fig. 4. Recovery induced SIBM by local subgrain coalesence at a grain boundary. (a) before,(b) after coalesence. (c) SIBM in 40% compressed Al. -—- .., .fi‘m' . . a , . . . >30 _ .. 0:. O... . .. ' p. O . ..L_, . w — - k-z. — - . - - "T‘ - -, h f 1 If; -.V' i . _ . . _ . . I x l .17."."9-ri l r for? 7"“ Fig. 5. Schematic diagram showing transition from band to matrix with small but additive orientation differences. II a) Production of grains with orientation not preexisting in the deformed S truc ture . One example is the so called Rowland transformation by which two twin related crystals collapse to form a common orientation. This mechanism was suggested for the formation of the cube texture.The feasibility of this mechanism has been doubted, however, and there is little evidence to support this model. High angle grain boundary can also be created by twin formation. TEM observations have established that annealing twins are formed at a very early state of recrystallization. They are emitted from subboundaries and can produce a high angle boundary. Within the twins second, third or fourth order twins can be formed [19] providing a highly mobile grain boundary [Fig. 6. ]. Fig. 6. Micrograph and [111] pole figure of a Cu single crystal 0 dynamically recrystallized at 560 C showing a twin family. Grain (a) (dashed orientation) is a primary twin to the deformation structure, and grain (b) (dashed-dotted orientation) a twin of (a). 2.2.3 Recrystallizaton Kinetics 12 The recrystallization temperature is generally understood as the temperature where recrystallization has been completed in one hour. The recrystallization volume fraction can be measuured by structure sensitive physical properties like electrical resistivity, specific heat or flow stress ( hardness ). The relative change of a property is generally believed to be proportional to the recrystallized volume fraction. After annealing for time t, the recrystallized volume fraction X(t) is given by h(t) " ho X(t) - where h(t) - The measured quantity after time t ho - h (t-O) hE - h (t-m) Schematic curves of recrystallization are shown below [Fig. 7. ]. h ”- Eff .f‘. ' :‘7_f.’paaggi ,‘w —--5- . ~ 2 0.5(hrha) _L_. +1, Fig. 7. Schematic curves of recrystallization in hardness. 13 2.2.4 Recrystallization In Ordered Alloys Recrystallization kinetics have been reported for a very few ordered alloys [20], a warm worked intermetallic compound in polycrystalline fom [21] and a cold worked one in a single crystal fam [22] . Takasugi and Izumi [23] observed that recrystallization and grain growth occurs in polycrystalline CasTi ( Ll, structure ). The critical temperatures for recrystallization and grain growth were quite high, as shown in the figs. 8 5n 9. Salute atoms drastically reduce the mobility through a sort of solute atmosphere near the grain boundary [24] . Excess Co atoms in the off-stoichiometric CosTi can be regarded as salute atoms. Therefore lower rate of recrystalliztion is assumed to be due to interaction of grain boundary with Ca atoms. Schulson and Haff observed that warm worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys [21] . The decrease in hardness caresspands to the increase in recrystallized area as seen in Fig. 10. I I4 ”MUM Fig. 8. Isochronal for specimens 0 -8 Marquees (H, ) O CO 20 0|. 7. T. b O CO 2‘ 00. 7. TI '- 0 Co 23 01.55 h 1 1 l 1 l __J O 200 400 600 000 $000 1200 Temperature ('0 annealing (for 2h) curves in the hardness for f CosTi cold-rolled to e - 50%. 000 r- e CoZOerJtY. m *- 0 C02! out 1. m .- 0 602309.361: 500 - 4” r- 300 - 200 *- 0 an» 400 am: an «no tan Temperature ('C) Fig. 9. Isochronal annealing (for 2h) curves in the hardness for the specimens of CaaTi called rolled to e - 80%. "ANDREI! I‘ U 0 I A; 1 l l I i 1 444 13’}: 400 500 .00 700 .00 ’00 IOOO "0° YEIPIRATUII ('C) Fig. 10. Optical micrographs illustrating the microstructural changes accompanying the isochronal anneals of NiAl. 3. EXPERIMENTAL.PROCEDURE 3.1 Testing Material A casting prepared by arc-melting several times in an argon atmosphere to promote hamogenity and then drop-cast in a copper mold, of the following composition was used. The composition of the final casting was as follows: Ni 76 at. % Al 24 at. % B .24 at. % 3.2 Specimen Preparation 3.2.1 Cold Rolling And Annealing Experiment 1 : Isochronal Annealing. A Specimen was cut from a B-doped NiaAl casting by abrasive wheel cutter. This was then rolled at room temperature in steps of .3 - .4 mm initially. Cracks were observed after 23% reduction in thickness. Further reduction was given in minimum steps of'.].- .2 mm to avoid cracking. Maximum reduction of 72% was possible since extensive cracking was observed. Samples of the rolled sheet were isochronally annealed in air for 15 o 0 minutes between 700 C and 1000 C. 16 l7 Experiment 2 : Isothermal Annealing. Another specimen was similarly rolled at room temperature to 80 % reduction, upon which the specimen had cracked extensively. A sample was out after 18 %, 4O %, 60 % reductions. 0 These samples were then isothemally annealed in air at 700 C upto 60 minutes. 0 Experiment 3 : This sample was homogenized at 1050 C for an hour. Then it was rolled to 20, 40, 60 and 80 percentage reductions. Further, samples were isothermally annealed at 600, 625, 650 and 675 degree celsius. 3.2.2 Texture Measurement Samples rolled to 80% and 90% reduction were used to determine the deformation texture. The surface of the samples were etched to eliminate surface effects. {111} and {200) pole figures were measured using an x- ray goniometer. 3.2.3 Optical Microscopy Experiment 1 : The samples were etched in the following solution : Nitric Acid 20 ml Phosphoric Acid 20 m1 Distilled Water, 20 m1 Acetic Acid 10 m1 Hydraflauric Acid 10 ml Hydrochloric Acid 10 ml 18 The sample was immersed in a solution heated to 45°C for 303. The microstructures were seen along rolling plane normal. Experiment 2 & 3 : These samples were etched in Marble 's reagent (composition shown below ) at room temperature for 55. The microstructures were seen along the transverse direction. Marble's reagent : CuSo‘ : 5g HCl : 20 ml H20 : 20 m1 Fig. 11. Schematic diagram showing rolling configuration. 3.2.4 Electron Microscopy Rolled and partially recrystallized samples were reduced in thickness to 500nm by grinding and then mechanical polished to 150pm. Disks of 3mm in 19 diameter were cut by electron discharge machine. These samples were then electropolished in the following solution: ] Perchlaric Acid 150 ml Distilled Water 875 m1 Butoxy Ethanol 150 m1 ww.—~..—-.u~‘_—- _— . -... 5 Ethanol 946 m1 l 0 Voltage: 27 V Temp.: 10-12 C Current (important): 0.23-0.25 amps. 3.3 Hardness Testing (1) For the Isochronal test, Rockwell hardness was measured. The Rockwell values were converted into Brinell hardness number (BI-IN) by using the following relations: 835-3100: BHN - .1199. 13O-RB C20-C40: BHN - 192999 (lOO-RC)2 >C40: BHN - 29999 lOO-RC (2) Microhardness tests were employed for the remaining experiments. A diamond pyramid hardness tester was used to measure the change an 20 hardness an annealing. The diamond pyramid hardness number (DPH) was determined from the following equation [24]: DPH - 22sinli£21 where P - applied load,Kg L - average length of diagonal, mm a 0 - angle between opposite faces of diamond - 136 Applied load was 0.5 Kg Amzleast five identatians were made. Care was taken to discard indentations near the cracks. 4. EXPERIMENTALRESULTS 4.1 Recrystallization Behavior Fig. 12 shows the hardness of a 72 % cold rolled specimen isochronally o annealed for 15 minutes at temperatures upto 1000 C. No change in a o a hardness occurs upto 650 C. The rise in hardness between 550 C and 650 C is misleading. The curve is drawn by a computer using a spline which a 0 results in the peculiar shape. Between 650 C and 700 C, there is a drastic drop in hardness and then it continues to drop steadily till a 1000 C. Fig. 13 shows the change in hardness an isothemal annealing at 700°C for different degrees of deformation. The curve for 80% cold rolling shows the steepest drop in 10 minutes. The hardness decreases steadily tilI 50 minutes. For 60, 40, and 20 % cold rolling curves, there is a considerable change within 20 minutes of annealing. Then the hardness remains almost constant for an hour. A slight increase in hardness for 20 % cold rolling curve may be due to the area where the indentation was made. Isothermal annealing curves at 625 deg C for 40 and 60 % cold rolled specimen are shown in fig. 14. The 60 % cold rolling curve shows no change in hardness until 15 minutes and then continues to drop till 45 mins. The 40 % cold rolling curve shows a slight drop in 15 mins. and it remains steady till 45 mins. 21 22 Fig. 15 shows isothemal annealing curves for 80 % cold rolled specimen a at different temperatures. At 625 C, there is no change in hardness even after 25 minutes of annealing . A considerable change in hardness o is observed after 55 mins. of annealing. At 650 C, no drop in hardness is seen till 10 minutes. Between 10 and 35 mins. , the hardness decreases a considerably. The 675 C curve shows a steady drop in hardness till 35 mins. of annealing. 4.2 Microstructural Development 4.2.1 Isochronal Test: For the isochronal test on a 72 % cold rolled sample, very fine recrystallized grains of less than lum grain size were observed after a annealing for 15 minutes at 700 C [fig. 12]. The grain size increases a to about 15pm after annealing for 15 min. at 950 C [fig. 16 d]. 4.2.2 Isothermal Tests: A. 80 % cold rolling Extremely fine grains are seen after isothemal annealing for 20 min. at o 700 C. The microstructure looks almost similar on further annealing for o 40 minutes [fig. 17 b]. On lowering the temperature, 675 C, higher amount of unrecrystallized area is seen after 15 mins. of annealing 0 [fig. ]. At 650 C, few unrecrystallized grains are seen after annealing for 40 mins. [fig. 17 f]. Very few recrystallized grains are seen after 23 o of annealing at 625 C. The specimen is almost completely recrystallized after after 55 mins. B. 60 % cold Rolling The deformed structure of 60 % cold rolled sample is seen in fig. 18 a. Grains are oriented towards the rolling direction. Completly o recrystallized grains are observed after annealing for 45 mins. at 700 C a [fig. 18 b] whereas at 625 C are still seen after annealing for 45 mins. [fig. 18 c]. C. 40 % cold rolling Fig. 19 a shows the 40 % cold rolled structure. New grains are seen at a the grain boundaries after 20 mins. of annealing at 700 C [fig. 19 c] . These grains grow on further annealing till 50 min. [fig. 19 f]. After a annealing for 45 mins. at 625 C, only few grains are seen at the grain boundaries [fig. 19 b]. D. 20 % cold Rolling Fig. 20 a shows the slightly elongated grains of a 20 % cold rolled structure. There is no change in microstructure after 20 min. of o annealing at 700 C. [fig. 20 b]. Further annealing of 10 min. shows some grain boundary migration [fig. 20 c] . New grains are seen near the surface of the sample[fig. 20 d]. This may be due to the surface being more deformed that the bulk owing to friction on the surface and small a thickness reductions. 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VAII'II .232 3:8 200 .Row to mc__omcc< _oE._o£om_ (ww*tuw/b>1)sseupioH Fig. 16 Optical micrographs illustrating microstructural changes accompanying isochronal annealing of 72% cold rolled specimens: 0 a) for 15 min. at 700 C. o b) for 15 min. at 800 C. 0 c) for 15 min. at 900 C. - o d) for 15 min. at 950 C. 30 Fig. 17. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 80% cold rolled specimens: 0 a) after 20 min. at 700 C. b) after 40 min. at 700°C. 31 o d) after 15 min. at 675 C. 32 o e) after 25 min. at 675 C. ‘o f) after 40 min. at 650 C. 33 o h) after 55 min. at 625 C. 34 Fig. 18. Optical micrographs illustrating microstructural changes accompanying isothemal annealing of 60% cold rolled specimens. a) as rolled o , b) after 45 min. at 700 C. 35 5 ¢ - . .V * . «.4 a! Mtt‘.“ .4 -' ‘ Rafi-«001m '. .-.* -.' ,1 H c) after 45 min. at 625 ”C. 36 Fig. 19. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 40% cold rolled specimens: a) as rolled. b) after 45 min. at 625°C. 37 o c) after 20 min. at 700 C. o d) after 30 min. at 700 C. 38 . o f) after 50 min. at 700 C. 39 Fig. 20. Optical micrographs illustrating microstructural changes accompanying isothermal annealing of 20% cold rolled specimens: a) as rolled o b) after 20 min. at 700 C. 40 o c) after 30 min. at 700 C. o d) after 40 min. at 700 C. 4l o e) after 50 min. at 700 C. 42 4.3 Texture Measurement Fig. 21 shows [111} and {200} pole figures of 90 % cold rolled specimen. Both the pole figures show a stronger gass component and a weaker brass component. Fig. 22 shows the {111} and {200) pole figures of fully recrystallized specimen. There is no pronounced texture at all. 4.4 Transmission Electron Microscopy Figs. 23 show a partially recrystallized structure of a specimen rolled a to 75 % and annealed at 625 C for 10 mins. Large grains are seen surrounded by the deformed matrix. 43 as :2: stimuli!!! >>>>>>>>>>>>>> L1 ou-~a-ieas aluminum :53. 223 ” zap—3L: x-..c " x: as xx. .3 a t. / = f. u E _ .. . / 255 n b ..............n.u.... . ,_ w—fioz ” N.” . .................H.J........u.u.... .. . ._ .. zea. mama " ~.u m.. .;gz.n _ m_,.w..,.. .8 ”WWW”; + ”.333” e z Corsica ....N. _ “my I; :3 a s p m w . was A u a E Fig. 21. Pole figure of 90 % cold rolled specimen., a)m[111}. 44 worm m_o=wm >>>>>>>>>>>>>> om-~o-~¢@q Hausauum q_rm.z,=m " z_wprw.~ss x-sw¢ " x: “a :3 pm =xr "$9 \ ”harm ” 2 ”~26: ” N.“ ”OH. mama " ~.u e: um: copapnm a o.wu¢ p . ~._au N . a.w~¢ w . a.awa a . c.5ua u . Ho.a~w a . _~.eaa a . pa.¢e~ 13) (20a). 45 worm m_n=wm >>>>>>>>>>>>>> su-~a-peaq opuhau~o m_rm 23:” u z-=rwm.AHh x-aam " x: mu 3: mm = x a u app ”anew " H saga: “ ~.u ace. mama " N.“ as am: Cowman” a o.maa a . ~.hga N . O.wud w . a.uou a . ”.4.“ u . he.aep a . ”w.omo a . Hu.aa_ Fig. 22. Pale figure of fully recrystallized specimen. a) {111}. AA IHUN IIII I86l-l3-90 :18 ' OOZ'HHIUEIN 6T 83 b) (200}. ' 310d VVVVVVVVVVVVVV IHHDII 47 Fig. 23. TEM micrograph showing a partially recrystallized structure. 5 . DISCUSSION 5.1 Recrystalliztion Behaviour 5.1.1 Isochronal Test Complete recrystallization is occurs within 15 mins. of annealing at o 700 C. The continual drop in hardness may be due to a grain size strengthening affect. Firstly, the Hall-Patch coefficient, K, is very high [25] . Secondly, there is a substantial grain growth from