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Microstructure in Ti–48at.%Al PST crystal subjected to creep deformation
Materials Science and Engineering A329 – 331 (2002) 828 – 834 www.elsevier.com/locate/msea
Microstructure in Ti–48at.%Al PST crystal subjected to creep deformation Tetsuya Asai 1, Seiki Hirata *, Masao Takeyama, Takashi Matsuo Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 -8552, Japan
Abstract Tensile creep tests of the Ti –48at.%Al PST crystals with different lamellar orientation angles with respect to the stress axis f were conducted at 1148 K and 68.6 MPa and the f dependence of creep rate was investigated. The creep rate of the PST crystal strongly depends on f. The minimum creep rate of the PST crystal with f = 63° is three orders of magnitude larger than that of the PST crystal with f=3°, and the rupture life of the PST crystal with f = 63° is one-tenth of that of the PST crystal with f=3°. The marked differences in creep rate and rupture life between the PST crystal with f = 63° and that with f =3° would be ascribed to the role of a2 plate to the operative slip systems. To confirm the above supposition, creep interruption tests in the wide strain range were conducted for the PST crystal with f= 63°. The microstructure of the creep interrupted specimens gives the evidence to understand the role of a2 plate to creep property, that is, the evolution of subgrain occurs just after collapsing of the a2 plates, and the collapse of the a2 plates directly coincides with the onset of accelerating creep. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Creep; Fully transformed lamellar; PST crystal; Orientation of a2 plate to stress axis; TiAl alloy; Accelerarating of creep rate
1. Introduction The Ti –48at.%Al alloy with fully lamellar structure is believed to have a superiority in creep rupture strength to TiAl alloys with different microstructures. By comparing creep of the fully lamellar Ti– 48at.%Al alloy with that of g single phase Ti– 50at.% Al alloy, the superiority in creep rupture strength in Ti– 48at.%Al alloy is elucidated to be derived not by decreasing creep rate, but by delaying the onset of accelerating creep. Furthermore, the delay of the onset of accelerating creep in Ti– 48at.%Al alloy is interpreted by the role of lamellar plate to suppress the appearance of the softening microstructure. Here the softening microstructure is confirmed as the dynamic recrystallization along grain boundaries [1– 6]. To remove the grain boundary from Ti– 48at.%Al polycrystalline alloy, it turns to a single crystal with fully lamellar structure, designated as PST crystal. The creep properties of the PST crystal must be superior to those of the polycrystalline lamellar alloy. To confirm * Corresponding author. 1 Graduate student, currently at Honda Motor Co., Ltd.
the above supposition, the creep of the PST crystals with the different orientation of lamellar plate to the tensile stress axis which is designated as f in the range of 3–63° were investigated and the superiority in the PST crystal to the polycrystalline is confirmed except for the creep rate in the PST crystal with f=63° [1]. Two important subjects still remain unsolved. The first is to clarify the detailed correlation between creep rate and f, and second one is to examine the occurrence of dynamic recrystallization in the PST crystal and its dependence on f. For the first subject, a new PST crystal with f = 77° is prepared, and for the second subject, the microstructural change in the PST crystals with f= 63° is investigated by interrupting creep tests at various strains.
2. Experimental The Ti –47.5at.%Al alloy containing 300 ppm oxygen and 70 ppm nitrogen by weight was prepared for this study. This alloy was melted by induction skull melting, followed by centrifugal casting into bars 12 mm in diameter and 180 mm in length. These bars were HIPed
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at 1473 K/176.4 MPa for 3 h. The bars were singlecrystallized by the optical floating zone method at the growth rate of 30 mm h − 1 in purified argon. Seven PST crystals were grown. The orientation angle f between the stress axis (growth direction) and lamellar plate was measured optically for each crystal cut normal to the lamellar plate along the growth direction, and it was in the range of 3–77°. Tensile creep specimens with a gage portion of 30 mm in length and 6 mm in diameter were machined from the grown crystals. Tensile creep tests were conducted at 1148 K under the constant stress of 68.6 MPa in air using the singlelever type creep machines. Linear variable-differential transducers automatically recorded creep strain through an extensometer attached to annular ridges at both ends of the specimen gage portion. Using the specimen with f= 63°, the creep tests were interrupted by rapid air cooling under load using compressed air, followed by water quenching. Microstructures in the specimen crept specimens were examined using optical, scanning electron and transmission electron microscopes.
Fig. 1. Optical micrographs of Ti –48at.%Al PST specimens before creep test: (a) f = 3°, (b) f=63°, (c) f= 77° (growth direction: vertical).
Fig. 2. Creep rate – time curves of Ti–48at.%Al PST specimens with various f at 1148 K/68.6 MPa, together with that of the polycrystalline FL specimen.
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3. Results
3.1. Initial microstructure Optical micrographs of as grown crystals with f=3, 63 and 77° are shown in Fig. 1. The horizontal direction is the growth direction of the PST crystals, which coincide with the stress axis in creep. In the three crystals, a2 plates are detected as a plate with white contrast, while g plates show dark contrast. Among all the PST crystals, the volume fraction of a2 plate before creep testing shows the same value of 17%, and the interlamellar spacing of the a2 plates also shows the same value of 2–3 mm.
3.2. Creep test Fig. 2 shows creep rate–time curves of the four PST crystals with various f of 3, 23, 63 and 77°, accompanied with that of the polycrystalline with fully lamellar structure (FL). The testing time of 5200 h in the PST crystal with f=3° is regarded as the time showing the minimum creep rate, and the time of 3390 h in the PST crystal with f= 23° is in the accelerating creep stage. The PST crystal with f= 63° exhibits the largest creep rate and the shortest rupture life among the PST crystals. In contrast to the smallest decreasing ratio of creep rate during transient creep in the PST crystals with f= 63°, the decreasing ratio of creep rate in the PST crystals with f= 3° becomes larger by increasing the testing time. Finally, a three order of magnitude decrease in the creep rate arises at the time of the minimum creep rate. Here, the decreasing ratio of creep rate indicates the slope in the transient stage of creep rate– time curve drawn in log–log scale. If the slope of the curve in transient stage becomes steeper at that time, the decreasing ratio of creep rate is called as larger. It should be noted that the creep rate starts acceleration only after 100 h for the crystal with f=63°, whereas it is decreasing monotonously even after testing for more than 5000 h for the crystal with f=3°. The creep rate in the PST crystal with f= 77° is almost the same just after loading as the creep rate in the PST crystal f= 23°, but after testing more than 100 h, the creep rate gradually increases in the PST crystal with f=77°. In contrast to this, the creep rate in the PST crystal with f= 23° decreases up to 600 h. The creep rate is plotted as a function of creep strain of the PST crystals in Fig. 3. The PST crystal with f =63° shows the largest creep rupture strain of 0.65. The creep strain at the minimum creep rate is also the largest in the PST crystal with f=63° and it becomes smaller with increasing or decreasing f value. It is obvious that the PST crystal with f=3° will show the smallest strain at the minimum creep rate of approximately 0.007.
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Fig. 3. Creep rate –strain curves of Ti – 48at.%Al PST specimen with various f at 1148 K/68.6 MPa.
The minimum creep rate is plotted as a function of f in Fig. 4, accompanied with that of the polycrystalline lamellar specimen (FL). The minimum creep rate of the PST crystal is dependent strongly on f and it is estimated that the largest creep rate is attained in the PST crystal with f near to 55°. The PST crystal shows larger creep rate than the polycrystal specimen, when f is in the range of 30–80°. From this, f =30 – 80° is defined as the soft orientation. On the other hand, the creep rates of the PST crystals with f =0 – 30° and f = 80– 90° are smaller than that of FL. From this, the orientation of f=0–30° and f = 80 – 90° is defined as the hard orientation. It is noteworthy that the PST crystal with f =77° shows the larger creep rate than the PST crystal with f =23°. The main reason is more early onset of accelerating creep in the PST crystal with f = 77° than that in PST crystal with f = 23°.
3.3. Change in microstructure during creep The collapse of the a2 plate and the occurrence of dynamic recrystallization were detected in creep rupture specimens of the PST crystal with f = 63°. The aim of microstructural observation is to examine evolution of subgrains in the PST crystal. Two new casts were prepared for the above study, and the new casts have almost the same lamellar structure and the values of f which are 67 and 65°. Their microstructural features will be regarded as almost the same as that of crystal with f=63°. This new PST crystal was creep interrupted at the strain of 0.06 at the minimum creep rate, and no obvious change in microstructure can be detected as shown in Fig. 5. However, in the specimen interrupted at the strain of 0.17 in the accelerating creep stage, the boundary appears in the direction parallel to the stress axis and they cut the lamellar
plates in Fig. 6. In the specimen interrupted at the strain of 0.27 in the accelerating creep, the waved lamellar regions were observed and dynamically recrystallized grains were detected in the middle of the specimen as shown in Fig. 7. Furthermore, in the specimen ruptured at the strain of 0.65, the high magnification photograph shows that the lamellar plates were heavily bent and the dynamically recrystallized grain g as well as spherical a2 particles were formed as shown in Fig. 8.
3.4. Volume fraction of h2 plate and interlamellar spacing Backscattered electron images of the specimen with f=63° interrupted at the strain of 0.06 (time=133 h) and the specimen with f= 3° interrupted at the strain of 0.007 (time= 5200 h) indicated important evidence as shown in Fig. 9. There was a large difference in creep interrupted time in both PST crystals, that is, 133 h in PST crystal with f=63° interrupted at the strain of
Fig. 4. The correlation between the minimum creep rate and f.
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Fig. 5. Backscattered electron images of Ti –48at.%Al PST specimen with f= 63°. (a) Initial microstructure, (b) creep interrupted at a strain of 0.06 at 1148 K/168.6 MPa. (The stress axis: horizontal).
0.06 and 5200 h in PST crystal with f = 3°. There are no differences in the width of a2 plates and in the lamellar spacing of a2 plate. But the thinning of the a2 plate is confirmed by comparing the width of a2 plates in PST crystals subjected no creep deformation. This means large dependence of the thinning of a2 plates on f during creep. Thinning of a2 plate is caused by the dissolution of a2 phase through instability. So the volume fraction of the a2 plate is measured instead of thinning of a2 plate for the above two PST crystals. Furthermore, interlamellar spacing of a2 plate is also measured, and these two parameters are shown in Fig. 10, accompanied with the values in virgin PST crystal. Interlamellar spacing which also contains the spacing of g plates is approximately 320 nm before the creep test, and becomes larger when subjected to creep. The value in PST crystal with f =63° at the strain of 0.06 is approximately 400 nm and that of the PST crystal with f=3° at the strain of 0.007 is approximately 520 nm. The volume fraction of a2 plate is 0.17 before the creep test, and it becomes small after subjecting creep. It is confirmed that the volume fraction of a2 plate in the PST crystal with f = 63° is the same as that of the PST crystal with f=3°.
The f dependence of the minimum creep rate is shown in Fig. 4. By conducting the creep of new PST crystal with f= 77°, it becomes clear that the smallest minimum creep rate is obtained not in PST crystals with f= 90°, but in PST crystals with f = 0°, and the largest minimum creep rate is obtained in PST crystals with near f= 60°. And in the case of specimens with f =3°, the decreasing ratio of creep rate during transient stage becomes larger with testing time. But in the case of PST crystals with f= 63°, the decreasing ratio of creep rate during transient stage becomes smaller with testing time. The reason for the difference in deceasing ratio in the transient stage is thought to be as follows. Fig. 11
4. Discussion
4.1. Lamellar orientation and slip systems The present study clarified that the creep rate of Ti – 48at.%Al single crystals depend strongly on the lamellar orientation and it is confirmed that the single crystal of Ti–48at.%Al alloy does not always show the smaller creep rate than that on the polycrystalline one.
Fig. 6. Backscattered electron images of Ti – 48at.%Al PST specimen with f= 63° creep interrupted at a strain of 0.17 at 1148 K/68.6 MPa. (The stress axis: horizontal).
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Fig. 7. Backscattered electron images of Ti – 48at.%Al PST specimen with f =63° creep interrupted at a strain of 0.27 at 1148 K/68.6 MPa. (The stress axis: horizontal).
Fig. 9. Backscattered electron images of Ti – 48at.%Al PST specimen with f = 63° creep interrupted at a strain of 0.06 (a), and with f= 3° creep interrupted after testing for 5200 h (b) at 1148 K/68.6 MPa. (The stress axis: vertical).
Fig. 8. Backscattered electron images of Ti – 48at.%Al PST specimen with f= 63° creep ruptured at 1148 K/68.6 MPa. (The stress axis: horizontal).
indicates the relationship between lamellar plates and slip systems. The arrow indicates the primary slip direction. In the case of specimens with f = 3°, primary slip systems work across the lamellar plate. In this case the lamellar interface acts as an obstacle to dislocation, and the moving distance of the primary slip system is short. Other slip systems cannot operate. Under such situations the creep rate decreases steeply by the small strain. However, in the case of specimens with f = 63°, the primary slip system operates parallel to the lamellar plate. In this case the lamellar interface does not act as an obstacle to the primary slip system, and acts as obstacles to secondly slip system. Consequently, large creep rates continues up to large strain.
Fig. 10. Volume fraction and interlamellar spacing of a2 plate in the specimen with f =63° interrupted at the strain of 0.06 (time = 133 h) and the specimen with f= 3° interrupted at the strain of 0.007 (time =5200 h).
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Fig. 11. Schematic illustration of role of lamellar plates on slip system; (a) the specimen with f =3°, (b) the specimen with f= 63°.
Fig. 12. Schematic illustration of the microstructural change in PST crystals with f = 63° as a function of creep strain.
4.2. Cause to accelerate the creep rate for the specimen with = 63 ° 4.2.1. Collapse of the h2 plate of the specimen with = 63 ° The dynamically recrystallized grains are detected in the specimen with f = 63° at the beginning of the accelerating stage, but cannot be detected in the specimen with f =3°. This f dependence of the occurrence of dynamic recrystallization is associated with the difference in deformation mode between two specimens. The microstructural evolution of the specimens with f= 63° is summarized in Fig. 12. The formation process of the dynamically recrystallized grains is in the following sequence: (1) collapse of the a2 plates; (2) formation of subboundaries running through the gauge portion; (3) further collapse of the a2 plate along the subboundaries; (4) occurrence of dynamic recrystallization at the space where the a2 plate collapsed.
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From these microstructural observations of the specimens with f= 3° at various creep stages, such new experimental evidence was elucidated that the onset of accelerating creep coincided not with the dynamic recrystallization, but the breakup of the a2 plates. Why does the breakup of the a2 plate make the creep rate larger? In the specimen with f=63°, a2 plates suppress the secondly slip system, therefore the large deformation will be attained only by primary slip. As a result, many dislocations may be piled up at the a2 plate and arise larger creep rates at the time when the collapse of a2 plates occurred. 4.2.2. Thinning of the h2 plate in the specimen with = 63 ° As shown in Fig. 10, the volume fraction of the a2 plate of the specimen with f= 63° is as same as that of the specimen with f= 3°, but there is a marked difference in testing time between two specimens. Here the reason why there is no difference in volume fraction between the above two specimens must be discussed. PST crystals prepared by OFZ method lead a non-equilibrium microstructure with a high volume fraction of a2 phase [7]. Therefore, the volume fraction of the a2 plate must be decreased during the creep test. Here is important experimental evidence that the specimen with f= 63° shows one order larger creep strain than the specimen with f=3°. Nieh et al. [8,9] reported the importance of a ledge mechanism by dislocation sliding on the a2 plate by heating. Then it is supposed that plastic deformation promotes the transformation from a2 to g.
5. Summary Using the new crystal with f= 77°, the previous study on the f dependence of minimum creep rate is again confirmed. Smaller creep rate than that of FL specimen in the specimens with a2 plates parallel or normal to the stress axis, and its effect is larger in the crystal parallel to the stress axis. PST crystals with f=30–80° show a larger creep rate than FL specimen. Collapse of a2 plates coincides well with the onset of accelerating creep.
Acknowledgements This research is supported by the research grant on ‘Research for the Future Program’ from Japan Society for the Promotion of Science (96R12301). We acknowledge Central Research Laboratories, Mitsubishi Materials Institute for providing the alloy ingots.
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References [1] T. Matsuo, T. Nozaki, T. Asai, S.Y. Chang, M. Takeyama, Intermetallics 6 (1998) 695. [2] K. Araki, M. Takeyama, T. Matsuo, M. Kikuchi, The Abstracts of the 117th Fall Meetings of Japan Inst. Metals and Inter. Symp. on Adv. Materials and Tech. for the 21st Century, 1995, p. 292. [3] T. Nozaki, K. Araki, M. Takeyama, T. Matsuo, Proc. Int. Conf. on Thermomechanical Processing of Steels and Other Materials. THERMEC’97, TMS, Warrendale, PA, 1997, p. 1415. [4] T.A. Parthasarathy, M.G. Mendiratta, D.M. Dimiduk, Acta
Mater. 46 (1998) 4005. [5] J.G. Lin, Y.G. Zhang, C.Q. Chen, J. Mater. Sci. Lett. 18 (1999) 263. [6] G. Wegmann, T. Suda, K. Maruyama, in: T. Sakuma, K. Yagi (Eds.), Creep and Fracture of Engineering Materials and Structure, Trans Tech Publications, Zuerich, 1999, p. 639. [7] Y. Yamamoto, M. Takeyama, T. Matsuo, in: T. Chandra (Ed.), Proc. THERMEC’2000 (in press). [8] T.G. Nieh, L.M. Hsiung, in: T. Sakuma, K. Yagi (Eds.), Creep and Fracture of Engineering Materials and Structure, Trans Tech Publications, Zuerich, 1999, p. 685. [9] J.N. Wang, T.G. Nieh, Acta Mater. 47 (1998) 1887.
Microstructure in Ti–48at.%Al PST crystal subjected to creep deformation Tetsuya Asai 1, Seiki Hirata *, Masao Takeyama, Takashi Matsuo Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 -8552, Japan
Abstract Tensile creep tests of the Ti –48at.%Al PST crystals with different lamellar orientation angles with respect to the stress axis f were conducted at 1148 K and 68.6 MPa and the f dependence of creep rate was investigated. The creep rate of the PST crystal strongly depends on f. The minimum creep rate of the PST crystal with f = 63° is three orders of magnitude larger than that of the PST crystal with f=3°, and the rupture life of the PST crystal with f = 63° is one-tenth of that of the PST crystal with f=3°. The marked differences in creep rate and rupture life between the PST crystal with f = 63° and that with f =3° would be ascribed to the role of a2 plate to the operative slip systems. To confirm the above supposition, creep interruption tests in the wide strain range were conducted for the PST crystal with f= 63°. The microstructure of the creep interrupted specimens gives the evidence to understand the role of a2 plate to creep property, that is, the evolution of subgrain occurs just after collapsing of the a2 plates, and the collapse of the a2 plates directly coincides with the onset of accelerating creep. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Creep; Fully transformed lamellar; PST crystal; Orientation of a2 plate to stress axis; TiAl alloy; Accelerarating of creep rate
1. Introduction The Ti –48at.%Al alloy with fully lamellar structure is believed to have a superiority in creep rupture strength to TiAl alloys with different microstructures. By comparing creep of the fully lamellar Ti– 48at.%Al alloy with that of g single phase Ti– 50at.% Al alloy, the superiority in creep rupture strength in Ti– 48at.%Al alloy is elucidated to be derived not by decreasing creep rate, but by delaying the onset of accelerating creep. Furthermore, the delay of the onset of accelerating creep in Ti– 48at.%Al alloy is interpreted by the role of lamellar plate to suppress the appearance of the softening microstructure. Here the softening microstructure is confirmed as the dynamic recrystallization along grain boundaries [1– 6]. To remove the grain boundary from Ti– 48at.%Al polycrystalline alloy, it turns to a single crystal with fully lamellar structure, designated as PST crystal. The creep properties of the PST crystal must be superior to those of the polycrystalline lamellar alloy. To confirm * Corresponding author. 1 Graduate student, currently at Honda Motor Co., Ltd.
the above supposition, the creep of the PST crystals with the different orientation of lamellar plate to the tensile stress axis which is designated as f in the range of 3–63° were investigated and the superiority in the PST crystal to the polycrystalline is confirmed except for the creep rate in the PST crystal with f=63° [1]. Two important subjects still remain unsolved. The first is to clarify the detailed correlation between creep rate and f, and second one is to examine the occurrence of dynamic recrystallization in the PST crystal and its dependence on f. For the first subject, a new PST crystal with f = 77° is prepared, and for the second subject, the microstructural change in the PST crystals with f= 63° is investigated by interrupting creep tests at various strains.
2. Experimental The Ti –47.5at.%Al alloy containing 300 ppm oxygen and 70 ppm nitrogen by weight was prepared for this study. This alloy was melted by induction skull melting, followed by centrifugal casting into bars 12 mm in diameter and 180 mm in length. These bars were HIPed
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at 1473 K/176.4 MPa for 3 h. The bars were singlecrystallized by the optical floating zone method at the growth rate of 30 mm h − 1 in purified argon. Seven PST crystals were grown. The orientation angle f between the stress axis (growth direction) and lamellar plate was measured optically for each crystal cut normal to the lamellar plate along the growth direction, and it was in the range of 3–77°. Tensile creep specimens with a gage portion of 30 mm in length and 6 mm in diameter were machined from the grown crystals. Tensile creep tests were conducted at 1148 K under the constant stress of 68.6 MPa in air using the singlelever type creep machines. Linear variable-differential transducers automatically recorded creep strain through an extensometer attached to annular ridges at both ends of the specimen gage portion. Using the specimen with f= 63°, the creep tests were interrupted by rapid air cooling under load using compressed air, followed by water quenching. Microstructures in the specimen crept specimens were examined using optical, scanning electron and transmission electron microscopes.
Fig. 1. Optical micrographs of Ti –48at.%Al PST specimens before creep test: (a) f = 3°, (b) f=63°, (c) f= 77° (growth direction: vertical).
Fig. 2. Creep rate – time curves of Ti–48at.%Al PST specimens with various f at 1148 K/68.6 MPa, together with that of the polycrystalline FL specimen.
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3. Results
3.1. Initial microstructure Optical micrographs of as grown crystals with f=3, 63 and 77° are shown in Fig. 1. The horizontal direction is the growth direction of the PST crystals, which coincide with the stress axis in creep. In the three crystals, a2 plates are detected as a plate with white contrast, while g plates show dark contrast. Among all the PST crystals, the volume fraction of a2 plate before creep testing shows the same value of 17%, and the interlamellar spacing of the a2 plates also shows the same value of 2–3 mm.
3.2. Creep test Fig. 2 shows creep rate–time curves of the four PST crystals with various f of 3, 23, 63 and 77°, accompanied with that of the polycrystalline with fully lamellar structure (FL). The testing time of 5200 h in the PST crystal with f=3° is regarded as the time showing the minimum creep rate, and the time of 3390 h in the PST crystal with f= 23° is in the accelerating creep stage. The PST crystal with f= 63° exhibits the largest creep rate and the shortest rupture life among the PST crystals. In contrast to the smallest decreasing ratio of creep rate during transient creep in the PST crystals with f= 63°, the decreasing ratio of creep rate in the PST crystals with f= 3° becomes larger by increasing the testing time. Finally, a three order of magnitude decrease in the creep rate arises at the time of the minimum creep rate. Here, the decreasing ratio of creep rate indicates the slope in the transient stage of creep rate– time curve drawn in log–log scale. If the slope of the curve in transient stage becomes steeper at that time, the decreasing ratio of creep rate is called as larger. It should be noted that the creep rate starts acceleration only after 100 h for the crystal with f=63°, whereas it is decreasing monotonously even after testing for more than 5000 h for the crystal with f=3°. The creep rate in the PST crystal with f= 77° is almost the same just after loading as the creep rate in the PST crystal f= 23°, but after testing more than 100 h, the creep rate gradually increases in the PST crystal with f=77°. In contrast to this, the creep rate in the PST crystal with f= 23° decreases up to 600 h. The creep rate is plotted as a function of creep strain of the PST crystals in Fig. 3. The PST crystal with f =63° shows the largest creep rupture strain of 0.65. The creep strain at the minimum creep rate is also the largest in the PST crystal with f=63° and it becomes smaller with increasing or decreasing f value. It is obvious that the PST crystal with f=3° will show the smallest strain at the minimum creep rate of approximately 0.007.
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Fig. 3. Creep rate –strain curves of Ti – 48at.%Al PST specimen with various f at 1148 K/68.6 MPa.
The minimum creep rate is plotted as a function of f in Fig. 4, accompanied with that of the polycrystalline lamellar specimen (FL). The minimum creep rate of the PST crystal is dependent strongly on f and it is estimated that the largest creep rate is attained in the PST crystal with f near to 55°. The PST crystal shows larger creep rate than the polycrystal specimen, when f is in the range of 30–80°. From this, f =30 – 80° is defined as the soft orientation. On the other hand, the creep rates of the PST crystals with f =0 – 30° and f = 80– 90° are smaller than that of FL. From this, the orientation of f=0–30° and f = 80 – 90° is defined as the hard orientation. It is noteworthy that the PST crystal with f =77° shows the larger creep rate than the PST crystal with f =23°. The main reason is more early onset of accelerating creep in the PST crystal with f = 77° than that in PST crystal with f = 23°.
3.3. Change in microstructure during creep The collapse of the a2 plate and the occurrence of dynamic recrystallization were detected in creep rupture specimens of the PST crystal with f = 63°. The aim of microstructural observation is to examine evolution of subgrains in the PST crystal. Two new casts were prepared for the above study, and the new casts have almost the same lamellar structure and the values of f which are 67 and 65°. Their microstructural features will be regarded as almost the same as that of crystal with f=63°. This new PST crystal was creep interrupted at the strain of 0.06 at the minimum creep rate, and no obvious change in microstructure can be detected as shown in Fig. 5. However, in the specimen interrupted at the strain of 0.17 in the accelerating creep stage, the boundary appears in the direction parallel to the stress axis and they cut the lamellar
plates in Fig. 6. In the specimen interrupted at the strain of 0.27 in the accelerating creep, the waved lamellar regions were observed and dynamically recrystallized grains were detected in the middle of the specimen as shown in Fig. 7. Furthermore, in the specimen ruptured at the strain of 0.65, the high magnification photograph shows that the lamellar plates were heavily bent and the dynamically recrystallized grain g as well as spherical a2 particles were formed as shown in Fig. 8.
3.4. Volume fraction of h2 plate and interlamellar spacing Backscattered electron images of the specimen with f=63° interrupted at the strain of 0.06 (time=133 h) and the specimen with f= 3° interrupted at the strain of 0.007 (time= 5200 h) indicated important evidence as shown in Fig. 9. There was a large difference in creep interrupted time in both PST crystals, that is, 133 h in PST crystal with f=63° interrupted at the strain of
Fig. 4. The correlation between the minimum creep rate and f.
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Fig. 5. Backscattered electron images of Ti –48at.%Al PST specimen with f= 63°. (a) Initial microstructure, (b) creep interrupted at a strain of 0.06 at 1148 K/168.6 MPa. (The stress axis: horizontal).
0.06 and 5200 h in PST crystal with f = 3°. There are no differences in the width of a2 plates and in the lamellar spacing of a2 plate. But the thinning of the a2 plate is confirmed by comparing the width of a2 plates in PST crystals subjected no creep deformation. This means large dependence of the thinning of a2 plates on f during creep. Thinning of a2 plate is caused by the dissolution of a2 phase through instability. So the volume fraction of the a2 plate is measured instead of thinning of a2 plate for the above two PST crystals. Furthermore, interlamellar spacing of a2 plate is also measured, and these two parameters are shown in Fig. 10, accompanied with the values in virgin PST crystal. Interlamellar spacing which also contains the spacing of g plates is approximately 320 nm before the creep test, and becomes larger when subjected to creep. The value in PST crystal with f =63° at the strain of 0.06 is approximately 400 nm and that of the PST crystal with f=3° at the strain of 0.007 is approximately 520 nm. The volume fraction of a2 plate is 0.17 before the creep test, and it becomes small after subjecting creep. It is confirmed that the volume fraction of a2 plate in the PST crystal with f = 63° is the same as that of the PST crystal with f=3°.
The f dependence of the minimum creep rate is shown in Fig. 4. By conducting the creep of new PST crystal with f= 77°, it becomes clear that the smallest minimum creep rate is obtained not in PST crystals with f= 90°, but in PST crystals with f = 0°, and the largest minimum creep rate is obtained in PST crystals with near f= 60°. And in the case of specimens with f =3°, the decreasing ratio of creep rate during transient stage becomes larger with testing time. But in the case of PST crystals with f= 63°, the decreasing ratio of creep rate during transient stage becomes smaller with testing time. The reason for the difference in deceasing ratio in the transient stage is thought to be as follows. Fig. 11
4. Discussion
4.1. Lamellar orientation and slip systems The present study clarified that the creep rate of Ti – 48at.%Al single crystals depend strongly on the lamellar orientation and it is confirmed that the single crystal of Ti–48at.%Al alloy does not always show the smaller creep rate than that on the polycrystalline one.
Fig. 6. Backscattered electron images of Ti – 48at.%Al PST specimen with f= 63° creep interrupted at a strain of 0.17 at 1148 K/68.6 MPa. (The stress axis: horizontal).
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Fig. 7. Backscattered electron images of Ti – 48at.%Al PST specimen with f =63° creep interrupted at a strain of 0.27 at 1148 K/68.6 MPa. (The stress axis: horizontal).
Fig. 9. Backscattered electron images of Ti – 48at.%Al PST specimen with f = 63° creep interrupted at a strain of 0.06 (a), and with f= 3° creep interrupted after testing for 5200 h (b) at 1148 K/68.6 MPa. (The stress axis: vertical).
Fig. 8. Backscattered electron images of Ti – 48at.%Al PST specimen with f= 63° creep ruptured at 1148 K/68.6 MPa. (The stress axis: horizontal).
indicates the relationship between lamellar plates and slip systems. The arrow indicates the primary slip direction. In the case of specimens with f = 3°, primary slip systems work across the lamellar plate. In this case the lamellar interface acts as an obstacle to dislocation, and the moving distance of the primary slip system is short. Other slip systems cannot operate. Under such situations the creep rate decreases steeply by the small strain. However, in the case of specimens with f = 63°, the primary slip system operates parallel to the lamellar plate. In this case the lamellar interface does not act as an obstacle to the primary slip system, and acts as obstacles to secondly slip system. Consequently, large creep rates continues up to large strain.
Fig. 10. Volume fraction and interlamellar spacing of a2 plate in the specimen with f =63° interrupted at the strain of 0.06 (time = 133 h) and the specimen with f= 3° interrupted at the strain of 0.007 (time =5200 h).
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Fig. 11. Schematic illustration of role of lamellar plates on slip system; (a) the specimen with f =3°, (b) the specimen with f= 63°.
Fig. 12. Schematic illustration of the microstructural change in PST crystals with f = 63° as a function of creep strain.
4.2. Cause to accelerate the creep rate for the specimen with = 63 ° 4.2.1. Collapse of the h2 plate of the specimen with = 63 ° The dynamically recrystallized grains are detected in the specimen with f = 63° at the beginning of the accelerating stage, but cannot be detected in the specimen with f =3°. This f dependence of the occurrence of dynamic recrystallization is associated with the difference in deformation mode between two specimens. The microstructural evolution of the specimens with f= 63° is summarized in Fig. 12. The formation process of the dynamically recrystallized grains is in the following sequence: (1) collapse of the a2 plates; (2) formation of subboundaries running through the gauge portion; (3) further collapse of the a2 plate along the subboundaries; (4) occurrence of dynamic recrystallization at the space where the a2 plate collapsed.
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From these microstructural observations of the specimens with f= 3° at various creep stages, such new experimental evidence was elucidated that the onset of accelerating creep coincided not with the dynamic recrystallization, but the breakup of the a2 plates. Why does the breakup of the a2 plate make the creep rate larger? In the specimen with f=63°, a2 plates suppress the secondly slip system, therefore the large deformation will be attained only by primary slip. As a result, many dislocations may be piled up at the a2 plate and arise larger creep rates at the time when the collapse of a2 plates occurred. 4.2.2. Thinning of the h2 plate in the specimen with = 63 ° As shown in Fig. 10, the volume fraction of the a2 plate of the specimen with f= 63° is as same as that of the specimen with f= 3°, but there is a marked difference in testing time between two specimens. Here the reason why there is no difference in volume fraction between the above two specimens must be discussed. PST crystals prepared by OFZ method lead a non-equilibrium microstructure with a high volume fraction of a2 phase [7]. Therefore, the volume fraction of the a2 plate must be decreased during the creep test. Here is important experimental evidence that the specimen with f= 63° shows one order larger creep strain than the specimen with f=3°. Nieh et al. [8,9] reported the importance of a ledge mechanism by dislocation sliding on the a2 plate by heating. Then it is supposed that plastic deformation promotes the transformation from a2 to g.
5. Summary Using the new crystal with f= 77°, the previous study on the f dependence of minimum creep rate is again confirmed. Smaller creep rate than that of FL specimen in the specimens with a2 plates parallel or normal to the stress axis, and its effect is larger in the crystal parallel to the stress axis. PST crystals with f=30–80° show a larger creep rate than FL specimen. Collapse of a2 plates coincides well with the onset of accelerating creep.
Acknowledgements This research is supported by the research grant on ‘Research for the Future Program’ from Japan Society for the Promotion of Science (96R12301). We acknowledge Central Research Laboratories, Mitsubishi Materials Institute for providing the alloy ingots.
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