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Thermal stability of deformation substructure of cyclically deformed TiAl PST crystals
IntermetaNics 4 (1996) 289-298 0 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0966-9795/961$15.00
0966-9795(95)00046-l ELSEVIER
Thermal stability of deformation substructure of cyclically deformed TiAl PST crystals H. Y. Yasuda, T. Nakano & Y. Umakoshi Department
of
Materials Science and Engineering, Faculty of Engineering, Osaka University, 2-1, Yamada-Oka, Suita, Osaka 565, Japan (Received
1 June 1995; revised version
received
4 September
1995; accepted
18 September
1995)
The effect of deformation mode on the deformation substructure and its thermal stability of TiAl polysynthetically twinned (PST) crystals was investigated using cyclically deformed and monotonically compressed crystals. In the fatigued TiAl PST crystals, two types of deformation substructure of deformation twins and a vein-like structure containing very tangled ordinary dislocations were formed depending on the type of ordered domains in y phase. In contrast, numerous deformation twins and a large number of homogeneously distributed dislocations were observed in the compressed crystals. There was a great difference in the softening process and the thermal stability of deformation substructure between the fatigued and compressed crystals. Recovery occurred in the fatigued crystals even at low temperature (-400°C) due to the climb motion of dislocations assisted by vacancy diffusion but recrystallisation could not be confirmed after annealing at 1000°C for 10’s. In contrast, recrystallisation was observed in the compressed crystals at 1000°C and deformation twins had an important role in the nucleation of new domains. Copyright 0 1996 Elsevier Science Ltd. Key words: A. titanium aluminides, based on TiAl, crack growth, D. microstructure, B. twinning.
1 INTRODUCTION
B. fatigue
resistance
and
domains were classified into two groups of T(twin)-type and V(vein)-type. In V-type domains l/2 < 1TO]ordinary dislocations were activated and tangled with each other, producing numerous dislocation dipoles and loops during their to-and-fro motion, and a vein-like structure containing a high density of tangled dislocations was observed. Formation of this structure was responsible for strong cyclic hardening. Deformation twins were dominantly operative in T-type domains. In general, recrystallisation is effective in improving the ductility of plastically deformed materials. Information about the thermal stability and recovery process of deformation substructure in fatigued materials is required to moderate the fatigue damage. There may be different recovery and recrystallisation behaviours between the fatigued and monotonically deformed crystals because of different morphology and characteristics of lattice defects. In fact, it was reported that recovery in fatigued Al and Cu crystals began from low temperature but that recrystallisation did not occur even at reasonably high temperatures.‘2m’4
In the last several years great progress has been made in improving mechanical properties of TiAl alloys to gain a good balance of strength and ductility through the choice of appropriate alloying elements and the control of grain size and microstructure by thermo-mechanical treatment.1-6 The industrial application of these alloys to high-temperature structural components has necessitated detailed knowledge of their plastic behaviour under repeated loading conditions in addition to monotonic deformation. In previous papers7-” the cyclic deformation and fracture behaviour of TiAl alloys were investigated focusing on the effect of lamellar structure and deformation mode such as twinning and slip systems using polysynthetically twinned (PST) crystals. Strong anisotropy in cyclic hardening, fatigue life and deformation substructure was observed depending on the angle (4) between the loading axis and the lamellar planes, The cyclic behaviour was closely related to the characteristic deformation substructure depending on the type of ordered domain in the y phase. The 289
H. Y. Yusudu
290
In this paper we compare the thermal stability and recovery process of lattice defects and microstructure of cyclically deformed TiAl PST crystals with monotonically deformed ones.
2 EXPERIMENTAL Master ingots of Ti49.lat%Al alloys were prepared by melting high-purity Ti and Al in a plasma arc furnace. Rods sized 15 mm in diameter and 10 mm long were cut from the ingots by spark machining. TiAl PST crystals were grown from these rods by a floating zone method using an NEC35HD single crystal growing apparatus at a rate of 2.5 mm/h under a high-purity argon gas flow. Oriented fatigue specimens with a selected angle (4 = 0’) between the loading axis and the lamellar boundaries along a ~1 iO> zone were cut from the bulk crystals by spark machining in the form of plates with gauge dimensions of 2 X 2 mm2 X 5 mm. AI1 the specimens were mechanically polished and then electrolytically polished in a 6/25/59(vol%) perchloric acid/butanol/methanol solution to remove the surface damage. The stress axes of y domains (T-type and V-type) and a2 phase in TiAl PST crystal are plotted in Fig.1. Stress-controlled tension/compression cyclic deformation test, all beginning in tension, was performed in air at room temperature using a servo hydraulic system (Shimadzu Servo Pulser EHFEDS-1OL type). Specimens were held by Wood’s metal grips to facilitate the alignment. All the tests were conducted at a frequency of 1 Hz. Specimens fatigued at the stress amplitude of *420 MPa to lo5 cycles were used for this work. At this stress level, specimens showed strong cyclic hardening with increasing number of cycles but did not break. The cyclic behaviour of TiAl PST crystals at various stress amplitudes will be described elsewhere.15 To investigate the effect of cyclic and monotonic
(4
03
Fig. 1. Loading axes of T-type and V-type domains (a), and cu, phase (b) in Ti49.lat%Al PST crystal deformed at C#I= 0”.
et al.
deformation modes, compression specimens (2 X 2 mm2 X 5 mm) with the same loading axis as the fatigue test were prepared. They were deformed at a nominal strain rate of 1.2 X lOa s-’ at room temperature up to 5% strain. Both fatigued and compressed specimens showed the same value of micro-Vickers hardness (I& -270). Deformed specimens were enclosed in quartz capsules filled with argon gas and were annealed isochronally for 20 min or isothermally at various Recovery and recrystallisation temperatures. behaviour were monitored by micro-Vickers hardness. All indentations were made on {1iO> planes in y domains which were edge-on to lamellar boundaries. Surface observation was done using an optical microscope equipped with Nomarski interference contrast. To examine the microstructural change during annealing, thin foils for electron microscopic observation were cut by spark machining and then polished in a solution of 10/20/70 perchloric acid/glycerol/methanol using the twin jet technique. Deformation substructure was observed by an H-800 electron microscope operated at 200kV.
3 RESULTS 3.1 Deformation
microstructure
TiA9.lat%Al PST crystals were composed of a small amount of finely and homogeneously distributed cy2plates and y phase matrix which were unidirectionally aligned maintaining the orientation relationship of < 1 1 l> y//(OOOl)a,and < 110~ y// [ 11iO]cw,.Several factors characterising the lamellar structure have been reported in previous papers.gJ0J6 Ordered domains whose orientations are shown in Fig. l(a) were of two types (T- and V-type) at 4 = 0” from the viewpoint of Schmid factors of possibly activated slip and twinning systems, and the observed deformation substructure (see the details in the previous papers8,“J6). Figure 2 shows surface markings of TiAl PST crystals deformed in a cyclic tension/compression loading and in a monotonic compression. On the surface of the fatigued specimen, fine slip traces could be seen on ( 1 1 1} planes in the y phase crossing the lamellar boundaries and the localised surface steps with extrusions, possibly due to deformation twins which appeared in specific domains, probably in T-type domains. There were few slip traces in the a2 phase since the stress amplitude off 420 MPa was not enough to activate
TiAl PST crystals
(a1 Fig. 2. Slip traces of Ti49.lat%Al
291
IOpm
UN PST crystal.
The arrows
the prismatic slip. In compressed specimens coarse slip traces were clearly observed in y phases accompanied by activation of { lOi0) slips in a2 plates. The different features in the morphology of surface markings between the fatigued and the compressed specimens can be clearly understood by study of the deformation substructure. Figures 3 and 4 show the deformation substructure in y phases of the fatigued and compressed Ti49.1at%Al PST crystals at 4 = O”, respectively. In T-type domains, { 111) < 112]-type deformation
show the LYE plates: (a) fatigued;
(b) compressed.
twins were seen accompanied by 1/2[1 lo]-type ordinary dislocations. The deformation twins were activated at an early stage of cyclic loading, but their density did not increase during cyclic deformation at a constantly applied stress while twins appeared more frequently in the compressed specimen with increasing plastic strain. In V-type domains of the fatigued specimen, l/2 < 1IO] ordinary dislocations tangled with each other during their to-and-fro motion under cyclic loading and formed a heterogeneous deformation substructure
(a1 Fig. 3. Deformation
substructure
in y domains of Ti49.lat%Al PST crystal cyclically deformed: g = 111; (b) T-type domain, foil//[ 1011, g = 111.
0.5pm (a) T-type
domain,
foiW[liO],
H. Y. Yusudu et al.
292
(b)
(a) Fig. 4. Deformation
substructure
0.5pm
in y domains of Ti49.lat%Al PST crystal deformed in compression: foill/[ilO], g = iii; (b) V-type domain, foil/l[iOl], g = ili.
composed of vein-like regions with high density of dislocations and channel-like regions with low dislocation density. A large number of dislocation dipoles, loops and debris was also noted. In con-
trast, dense l/2< 1iO] ordinary dislocations were observed in the compressed specimen but these were homogeneously distributed. 3.2 Change in microhardness during annealing Figure 5 shows the change in micro-Vickers hardness of the fatigued specimen annealed at indicated temperatures for 20 min. The microhardness began to drop slightly even at 300°C followed by a gradual decrease and finally reached a constant
(a) T-type domain,
value above 1000°C. The saturated value was still higher than that of the as-grown crystal. The micro-Vickers hardness was measured at appropriate temperatures from the curve in Fig. 5 during isothermal annealing to examine the difference in annealing processes between the fatigued and the compressed specimens. Figure 6 shows variation in micro-Vickers hardness of the fatigued and compressed Ti-49.1at%Al PST crystals as a function of annealing time. At 400 and 7OO”C, a decrease in microhardness was clearly recognised in the fatigued specimens, but the decrease was small in the compressed specimens. However, a sharp drop in the microhardness appeared in the compressed specimen at an 260 270 P -3 2
260-
: ;
250.
I g
240-
:: ‘7
230.
g f
220-
210 200
0
100
j
200
300
j
400
500
600
’
700
Temperature
800
j
900
1000
’
1100
t 1200
(“C)
Fig. 5. Changes of micro Vickers hardness of cyclically deformed TiL49.lat%Al PST crystals after annealing isochronally at various temperatures for 20 min.
2001 102
1
’ ‘/III’,’
u1111 103 Annealing
104 time (s)
’ Cj’,L”
1
105
Fig. 6. Changes of micro Vickers hardness of fatigued and compressed Ti49.lat%Al PST crystals as a function of annealing time at various temperatures.
293
TiAl PST crystals
early stage of annealing at 1000°C. In contrast, the microhardness of the fatigued specimens decreased during annealing but there was no significant difference in the softening curves at 700 and 1000°C. 3.3 Microstructure and deformation substructure after annealing Figure 7 shows optical micrographs of deformed TidB.lat%Al PST crystals before and after annealing. Since all the specimens were electrically polished after deformation and/or annealing and then etched, the surface marking caused by the motion of dislocations was removed, but the traces of deformation twins and coarse slips were more notable. No noticeable change occurred in the lamellar structure or the traces of deformation twins in the fatigued specimens, while recrystallisation took place and new domains appeared in y phases for the compressed specimen annealed at 1000°C for lo3 s. During further annealing, coalescence and growth of domains occurred and cx2 plates were dissolved in the y phase resulting in separation into short fingers. The lamellar spacing of the compressed specimen increased rapidly during annealing while the lamellar structure of the fatigued specimen was not disturbed as shown in Fig. 8.
Annealing
time
(s)
Fig. 8. Variation of lamellar spacing of fatigued and compressed Ti-49.lat%Al PST crystals with annealing time at 1000°C.
Figures 9 and 10 show the deformation substructure in T-type and V-type domains after annealing, respectively. As optical micrographs suggested, numerous deformation twins remained in T-type domains even after annealing at 400 or 700°C for lo5 s. In contrast, remarkable changes in dislocation substructure were observed in V-type domains. In the fatigued specimen the vein-like structure was eliminated, leaving numerous small loops and debris caused by the climb motion and
(a)
(b)
(d)
(e)
Fig. 7. Optical micrographs of Ti49.lat%Al PST crystals deformed and annealed at various temperatures for 16 s: (a) as fatigued; (b) at 400°C fatigued; (c) at 7OO”C, fatigued; (d) as compressed; (e) at 4OO”C, compressed; (f) at 700°C, compressed.
H. Y. Yasudu et al.
294
0C
(4
0.5pm
Fig. 9. Changes of electron micrographs
in T-type domains of Ti-49.1at%Al PST crystals annealed for lo5 s after fatigue or compression: (a) at 4OO”C, fatigued, foil//[liO], g = iil; (b) at 7OO”C,fatigued, foill/[ilO], g = lli; (c) at 4OO”C,compressed, foil//[ilO], g = 11i; (d) at 7OO”C,compressed, foiV/[ilO], g = 11i.
annihilation of l/2 < 1iO] ordinary dislocations at 400°C. The tangled dislocations were all annihilated, while the number of dislocation dipoles and loops that enlarged in size decreased at 700°C. The dislocation rearrangement of polygonisation or formation of subboundaries did not occur at either temperature. Even at 1000°C recrystallisation did not occur since the vein-like structure was eliminated at an early stage during annealing and there was inadequate driving force for recrystallisation. As shown in Fig. 11, dislocamutual
tions, dipoles and loops were almost all annihilated, while deformation twins were stable against annealing. No remarkable change in dislocation substructure of the compressed specimen was observed and a large number of l/2 < 1iO] ordinary dislocations had a screw character although their density gradually decreased. At 700°C curled or helical dislocations and large loops were formed as a result of the climb motion. From these aspects the deformation substructure in V-type domains of
TiAl PST crystals
(c)
(a
0.5pm
Fig. 10. Changes of microstruc_ture in VItype domains of Ti49.lat%Al PST crystals annealed for 10’s after fatigue or_compressi?n. (a) at 4OO”C, fatigued, foil//[ 1111,g = 111; (b) at 700°C. fatigued, foil//[lOl], g = il i; [c) at 4OO”C, compressed, foil//[lOl]; g = 111; (d) at 7OO”C,compressed, foiV/[lOl], g = 111.
the fatigued specimen was more easily recovered at lower temperature than that induced by monotonic deformation. Recrystallisation and growth of y domains were confirmed in the compressed specimen annealed at 1000°C from optical micrographs. Figure 12 shows an electron micrograph of a recrystallisation process appearing in y domains. Dislocations were annealed out in V-type domains, while a large number of deformation twins still existed in T-type domains. A large domain (A) which did
not satisfy Blackburn’s orientation relationship” against neighbouring cz2 plates appeared during recrystallisation. Once the domain is formed, cu, plates start to be dissolved and separate into short plates to release the lattice mismatch at the a,ly interface. The boundaries between the recrystallised domain and the original domains produced during the crystal growing were curved, since some types of domains were preferentially absorbed in a new domain (A) depending on the density of dislocations and twins.
296
H. Y. Yusudu et al.
4 DISCUSSION
Fig. 11. Electron
micrograph of annealed Ti49.lat%Al PST crystal containing T- and V-type domains at 1000°C for lo5 s after cyclic deformation. foil//< 1iO>.
There was a great difference in recovery and recrystallisation behaviour between the fatigued and the compressed specimens. In the former, recovery began from a rather low temperature and the microhardness decreased gradually over a wide temperature range. However, there was no evidence of recrystallisation during annealing up to lOOO”C, while in the compressed specimens the recrystallisation process was confirmed at 1000°C. These differences in the two specimens are thought to be closely related to deformation substructure depending on the type of domain and the loading mode. The deformation substructure developed in each y ordered domain fell into two groups: numerous deformation twins were activated in T-type domains, while l/2 < 1IO] ordinary dislocations were dominantly operative in V-type domains. Although l/2 < 1iO] ordinary dislocations are known to be more easily activated than < 1011, l/2 < 1121 superdislocations in Ti-rich y phase,‘*j19 deformation twins were preferentially activated instead of
(a) Fig. 12. Recrystallised microstructure and diffraction patterns of Ti49.lat%Al PST crystal annealed at 1000°C for lo5 s after compression: (a) microstructure; (b) schematic drawing of (a). Parts (c), (d), (e) and (f) show diffraction patterns from A, B, C and D area in (a), respectively.
TiAl PST crystals
l/2 < 1iO]-slips in T-type domains under this loading condition (4 = 0’) because of the difference in their stress factors. Since the motion of deformation twins is not equivalent in tension and compression, the twins can be activated only in tension or compression depending on the orientation of loading axis. They are formed at an early stage of deformation but cannot be multiplied during cyclic loading at a constant stress amplitude. In V-type domains, l/2 < 1iO]-type dislocations tangled with each other and a large number of dipoles and loops were formed from jogs of the dislocations. Single vacancies and their clusters would be formed by a nonconservative motion of small jogs and could also be developed by the to-and-fro motion of dislocations under cyclic loading. In monotonic compression, both T-type and V-type domains must be strained under the same condition. As a result, many deformation twins were produced in T-type domains, while the density of dislocations and other lattice defects as their by-products became lower than that at cyclic deformation. A large difference in the recovery process between fatigued and compressed specimens was observed in dislocation substructure, while deformation twins were stable in both specimens below 700°C. Annihilation of tangled dislocations in the fatigued specimens occurred by the climb motion at low temperature (-4OO”C), although dislocations in the compressed specimen were stable at the corresponding temperature and the climb motion of l/2 < 1101 dislocations started at 700°C showing a curled or helical shape. A large number of supersaturated single vacancies and their clusters, which are produced during cyclic deformation, provide a strong driving force for diffusion. The climb motion of dislocations which may be accommodated by vacancy diffusion can occur even at a low temperature. In the recovery process at a low temperature there was no tendency toward polygonisation. In fatigued specimens, l/2 < 1101 ordinary dislocations had several types of segments, such as long dislocations, dipoles and loops forming a heterogeneous structure composed of vein-like and channel-like regions. Their dislocations had the same character as Burgers vector but the complex morphology of dislocation structure was harmful to polygonisation. Therefore, polygonisation for which dislocation array with the same character is required rarely occurs, as suggested by Segall and Partridge. 2oAs evidence of vacancy diffusion, there was loop coarsening from 400 to 700°C. From direct observation of the growth process of disloca-
297
tion loops in quenched Al during annealing, these loops are known to be one of the major sink sites of vacancies.21 In order to reduce the line tension energy, small loops diminished emitting vacancies, while large loops coarsened absorbing vacancies from supersaturated matrix. Thus, the loop size became larger and the density became lower, similar to the process of Ostwald ripening.22 Annihilation of tangled dislocations and coarsening of loops thus resulted in softening below 700°C. Recrystallisation occurred only in the compressed specimen in which the driving force would be given by the numerous deformation twins formed in T-type domains. T-type domains containing numerous deformation twins maintain high stored energy even at lOOO”C, while the stored energy in V-type domains is lowered by annihilation and rearrangement of dislocations. The difference in stored energy leads V-type domains to grow and consume T-type domains. Moreover, if a large domain happened to nucleate, strain energy stored near the interface between the matrix/deformation twin boundaries, $7 and (y21y phase boundaries could be effectively released. This is in good agreement with the recrystallisation process of cold-rolled TiAl PST crystals.23 On the other hand, in the fatigued specimens, the number of deformation twins was smaller than in the compressed one. In addition, highly dense dislocations were annihilated so easily that the driving force for recrystallisation was not sufficient. Deformation twins which are harmful for further fatigue could all be removed by annealing above 12OO”C,but the lamellar structure would be disturbed.
5 CONCLUSIONS Recovery and recrystallisation behaviour of deformed TiAl PST crystals have been investigated focusing on the effect of monotonic and cyclic deformation modes and the type of y domains. The following conclusions were obtained. (1) In fatigued TiAl PST crystals, recovery occurred even at low temperatures (-400°C) without showing polygonisation. Annihilation and climb motion of l/2 < 1iO] ordinary dislocations were assisted by numerous supersaturated vacancies and clusters which were formed by the to-and-fro motion of jogged parts during cyclic loading. In addition, dislocation loops coarsened accompanied by the emittance and absorption of vacancies.
H. Y. Yasuda
298
Recrystallisation did not occur in fatigued crystals up to 1000°C since the vein-like structure diminished easily at an early stage of annealing and a deficient driving force remained. Annealing was not effective in preventing TiAl PST crystals from fatigue damage. (3) Recrystallisation occurred in the deformed TiAl PST crystals in compression and deformation twins had an important role for nucleation and growth of new domains. (2)
et al. 5. Hashimoto,
6. 7.
8.
9. 10. 11. 12.
ACKNOWLEDGEMENT Partial support of this program was provided by a Grant-in-Aid for Scientific Research and Development from the Ministry of Education, Science and Culture of Japan.
13. 14. 15.
16.
17.
REFERENCES Lipsitt, H. A., High Temperature Ordered Intermetallic Alloys, eds C. C. Koch, C. T. Liu & N. S. Stoloff, MRS, Sympo., 1985, Vol. 39, p. 351. Kim, Y-W., JOM, 41 (1989) 24. Kim, Y-M., Acta Metall. Mater., 40 (1992) 1121. Huang, S. C., Structural Intermetallics, eds R. Darolia et a/., TMS, Warrendale, PA., 1993, p. 299.
18. 19. 20. 21. 22. 23.
K. & Kimura, M., Structural Zntermetallics, eds R. Darolia et al., TMS, Warrendale, PA., 1993, p. 309. Kim, Y-W., JOM, 46 (1994) 30. Umakoshi, Y., Yasuda, H. Y. & Nakano, T., Proc. 3rd Japan Inter. SAMPE Sympo., eds M. Yamaguchi & H. Fukutomi, Makuhari, Jap., 1993, p. 1328. Umakoshi, Y., Yasuda, H. Y. & Nakano, T., Strength of Materials, ZCSMA-IO, eds W. Oikawa et al., Sendai, Jap. Inst. Metals, 1994, p. 345. Yasuda, H. Y., Nakano, T. & Umakoshi, Y., Phil. Mag. A, 71 (1995) 127. Umakoshi, Y., Yasuda, H. Y. & Nakano, T., Mater. Sci. Eng., 194A (1995) 43. Yasuda, H. Y., Nakano, T. & Umakoshi, Y., Phil. Mag. A., (1996) in press. Broom, T., Moloneux, J. H. & Whittaker, V. N., J. Inst. Metals, 84 (1955556) 357. Kemsley, D. S., J. Inst. Metals, 85 (1956-57) 417. Broom, T. & Ham, R. K., Proc. Roy. Sot. A., 251 (1959) 186. Nakano, T., Higashitanaka, N., Yasuda, H. Y. & Umakoshi, Y., Proc. 127th Spring Meeting of Iron and Steel Institute of Japan, 1994, p. 643. Umakoshi, Y., Yasuda, H. Y., Nakano, T. & Nakazawa, J., Gamma Titanium Aliminides, eds Y.-M. Kim et al., TMS, Las Vegas, 1995, in press. Blackburn, M. J., Technology and Application of Titanium, eds R. T. Yaffee et al., Pergamon Press, Oxford, 1970, p. 633. Shechtman, D., Blackburn, M. J. & Lipsitt, H. A., Met. Trans., 5 (1974) 1973. Inui, H., Oh, M. H., Nakamura, A. & Yamaguchi, M., Acta Metall. Mater., 40 (1992) 3095. Segall, R. L. & Partridge, P. G., Phil. Msg., 5 (1959) 912. Silcox, J. & Whelan, M. J., Phil. Mag., 5 (1959)l. Ostwald, W., 2. Phys Chem., 34 (1900) 495. Oh, M. H., Nakamura, A. & Yamaguchi, M., Acta Metall. Mater., 40 (1992) 167.
0966-9795(95)00046-l ELSEVIER
Thermal stability of deformation substructure of cyclically deformed TiAl PST crystals H. Y. Yasuda, T. Nakano & Y. Umakoshi Department
of
Materials Science and Engineering, Faculty of Engineering, Osaka University, 2-1, Yamada-Oka, Suita, Osaka 565, Japan (Received
1 June 1995; revised version
received
4 September
1995; accepted
18 September
1995)
The effect of deformation mode on the deformation substructure and its thermal stability of TiAl polysynthetically twinned (PST) crystals was investigated using cyclically deformed and monotonically compressed crystals. In the fatigued TiAl PST crystals, two types of deformation substructure of deformation twins and a vein-like structure containing very tangled ordinary dislocations were formed depending on the type of ordered domains in y phase. In contrast, numerous deformation twins and a large number of homogeneously distributed dislocations were observed in the compressed crystals. There was a great difference in the softening process and the thermal stability of deformation substructure between the fatigued and compressed crystals. Recovery occurred in the fatigued crystals even at low temperature (-400°C) due to the climb motion of dislocations assisted by vacancy diffusion but recrystallisation could not be confirmed after annealing at 1000°C for 10’s. In contrast, recrystallisation was observed in the compressed crystals at 1000°C and deformation twins had an important role in the nucleation of new domains. Copyright 0 1996 Elsevier Science Ltd. Key words: A. titanium aluminides, based on TiAl, crack growth, D. microstructure, B. twinning.
1 INTRODUCTION
B. fatigue
resistance
and
domains were classified into two groups of T(twin)-type and V(vein)-type. In V-type domains l/2 < 1TO]ordinary dislocations were activated and tangled with each other, producing numerous dislocation dipoles and loops during their to-and-fro motion, and a vein-like structure containing a high density of tangled dislocations was observed. Formation of this structure was responsible for strong cyclic hardening. Deformation twins were dominantly operative in T-type domains. In general, recrystallisation is effective in improving the ductility of plastically deformed materials. Information about the thermal stability and recovery process of deformation substructure in fatigued materials is required to moderate the fatigue damage. There may be different recovery and recrystallisation behaviours between the fatigued and monotonically deformed crystals because of different morphology and characteristics of lattice defects. In fact, it was reported that recovery in fatigued Al and Cu crystals began from low temperature but that recrystallisation did not occur even at reasonably high temperatures.‘2m’4
In the last several years great progress has been made in improving mechanical properties of TiAl alloys to gain a good balance of strength and ductility through the choice of appropriate alloying elements and the control of grain size and microstructure by thermo-mechanical treatment.1-6 The industrial application of these alloys to high-temperature structural components has necessitated detailed knowledge of their plastic behaviour under repeated loading conditions in addition to monotonic deformation. In previous papers7-” the cyclic deformation and fracture behaviour of TiAl alloys were investigated focusing on the effect of lamellar structure and deformation mode such as twinning and slip systems using polysynthetically twinned (PST) crystals. Strong anisotropy in cyclic hardening, fatigue life and deformation substructure was observed depending on the angle (4) between the loading axis and the lamellar planes, The cyclic behaviour was closely related to the characteristic deformation substructure depending on the type of ordered domain in the y phase. The 289
H. Y. Yusudu
290
In this paper we compare the thermal stability and recovery process of lattice defects and microstructure of cyclically deformed TiAl PST crystals with monotonically deformed ones.
2 EXPERIMENTAL Master ingots of Ti49.lat%Al alloys were prepared by melting high-purity Ti and Al in a plasma arc furnace. Rods sized 15 mm in diameter and 10 mm long were cut from the ingots by spark machining. TiAl PST crystals were grown from these rods by a floating zone method using an NEC35HD single crystal growing apparatus at a rate of 2.5 mm/h under a high-purity argon gas flow. Oriented fatigue specimens with a selected angle (4 = 0’) between the loading axis and the lamellar boundaries along a ~1 iO> zone were cut from the bulk crystals by spark machining in the form of plates with gauge dimensions of 2 X 2 mm2 X 5 mm. AI1 the specimens were mechanically polished and then electrolytically polished in a 6/25/59(vol%) perchloric acid/butanol/methanol solution to remove the surface damage. The stress axes of y domains (T-type and V-type) and a2 phase in TiAl PST crystal are plotted in Fig.1. Stress-controlled tension/compression cyclic deformation test, all beginning in tension, was performed in air at room temperature using a servo hydraulic system (Shimadzu Servo Pulser EHFEDS-1OL type). Specimens were held by Wood’s metal grips to facilitate the alignment. All the tests were conducted at a frequency of 1 Hz. Specimens fatigued at the stress amplitude of *420 MPa to lo5 cycles were used for this work. At this stress level, specimens showed strong cyclic hardening with increasing number of cycles but did not break. The cyclic behaviour of TiAl PST crystals at various stress amplitudes will be described elsewhere.15 To investigate the effect of cyclic and monotonic
(4
03
Fig. 1. Loading axes of T-type and V-type domains (a), and cu, phase (b) in Ti49.lat%Al PST crystal deformed at C#I= 0”.
et al.
deformation modes, compression specimens (2 X 2 mm2 X 5 mm) with the same loading axis as the fatigue test were prepared. They were deformed at a nominal strain rate of 1.2 X lOa s-’ at room temperature up to 5% strain. Both fatigued and compressed specimens showed the same value of micro-Vickers hardness (I& -270). Deformed specimens were enclosed in quartz capsules filled with argon gas and were annealed isochronally for 20 min or isothermally at various Recovery and recrystallisation temperatures. behaviour were monitored by micro-Vickers hardness. All indentations were made on {1iO> planes in y domains which were edge-on to lamellar boundaries. Surface observation was done using an optical microscope equipped with Nomarski interference contrast. To examine the microstructural change during annealing, thin foils for electron microscopic observation were cut by spark machining and then polished in a solution of 10/20/70 perchloric acid/glycerol/methanol using the twin jet technique. Deformation substructure was observed by an H-800 electron microscope operated at 200kV.
3 RESULTS 3.1 Deformation
microstructure
TiA9.lat%Al PST crystals were composed of a small amount of finely and homogeneously distributed cy2plates and y phase matrix which were unidirectionally aligned maintaining the orientation relationship of < 1 1 l> y//(OOOl)a,and < 110~ y// [ 11iO]cw,.Several factors characterising the lamellar structure have been reported in previous papers.gJ0J6 Ordered domains whose orientations are shown in Fig. l(a) were of two types (T- and V-type) at 4 = 0” from the viewpoint of Schmid factors of possibly activated slip and twinning systems, and the observed deformation substructure (see the details in the previous papers8,“J6). Figure 2 shows surface markings of TiAl PST crystals deformed in a cyclic tension/compression loading and in a monotonic compression. On the surface of the fatigued specimen, fine slip traces could be seen on ( 1 1 1} planes in the y phase crossing the lamellar boundaries and the localised surface steps with extrusions, possibly due to deformation twins which appeared in specific domains, probably in T-type domains. There were few slip traces in the a2 phase since the stress amplitude off 420 MPa was not enough to activate
TiAl PST crystals
(a1 Fig. 2. Slip traces of Ti49.lat%Al
291
IOpm
UN PST crystal.
The arrows
the prismatic slip. In compressed specimens coarse slip traces were clearly observed in y phases accompanied by activation of { lOi0) slips in a2 plates. The different features in the morphology of surface markings between the fatigued and the compressed specimens can be clearly understood by study of the deformation substructure. Figures 3 and 4 show the deformation substructure in y phases of the fatigued and compressed Ti49.1at%Al PST crystals at 4 = O”, respectively. In T-type domains, { 111) < 112]-type deformation
show the LYE plates: (a) fatigued;
(b) compressed.
twins were seen accompanied by 1/2[1 lo]-type ordinary dislocations. The deformation twins were activated at an early stage of cyclic loading, but their density did not increase during cyclic deformation at a constantly applied stress while twins appeared more frequently in the compressed specimen with increasing plastic strain. In V-type domains of the fatigued specimen, l/2 < 1IO] ordinary dislocations tangled with each other during their to-and-fro motion under cyclic loading and formed a heterogeneous deformation substructure
(a1 Fig. 3. Deformation
substructure
in y domains of Ti49.lat%Al PST crystal cyclically deformed: g = 111; (b) T-type domain, foil//[ 1011, g = 111.
0.5pm (a) T-type
domain,
foiW[liO],
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(b)
(a) Fig. 4. Deformation
substructure
0.5pm
in y domains of Ti49.lat%Al PST crystal deformed in compression: foill/[ilO], g = iii; (b) V-type domain, foil/l[iOl], g = ili.
composed of vein-like regions with high density of dislocations and channel-like regions with low dislocation density. A large number of dislocation dipoles, loops and debris was also noted. In con-
trast, dense l/2< 1iO] ordinary dislocations were observed in the compressed specimen but these were homogeneously distributed. 3.2 Change in microhardness during annealing Figure 5 shows the change in micro-Vickers hardness of the fatigued specimen annealed at indicated temperatures for 20 min. The microhardness began to drop slightly even at 300°C followed by a gradual decrease and finally reached a constant
(a) T-type domain,
value above 1000°C. The saturated value was still higher than that of the as-grown crystal. The micro-Vickers hardness was measured at appropriate temperatures from the curve in Fig. 5 during isothermal annealing to examine the difference in annealing processes between the fatigued and the compressed specimens. Figure 6 shows variation in micro-Vickers hardness of the fatigued and compressed Ti-49.1at%Al PST crystals as a function of annealing time. At 400 and 7OO”C, a decrease in microhardness was clearly recognised in the fatigued specimens, but the decrease was small in the compressed specimens. However, a sharp drop in the microhardness appeared in the compressed specimen at an 260 270 P -3 2
260-
: ;
250.
I g
240-
:: ‘7
230.
g f
220-
210 200
0
100
j
200
300
j
400
500
600
’
700
Temperature
800
j
900
1000
’
1100
t 1200
(“C)
Fig. 5. Changes of micro Vickers hardness of cyclically deformed TiL49.lat%Al PST crystals after annealing isochronally at various temperatures for 20 min.
2001 102
1
’ ‘/III’,’
u1111 103 Annealing
104 time (s)
’ Cj’,L”
1
105
Fig. 6. Changes of micro Vickers hardness of fatigued and compressed Ti49.lat%Al PST crystals as a function of annealing time at various temperatures.
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TiAl PST crystals
early stage of annealing at 1000°C. In contrast, the microhardness of the fatigued specimens decreased during annealing but there was no significant difference in the softening curves at 700 and 1000°C. 3.3 Microstructure and deformation substructure after annealing Figure 7 shows optical micrographs of deformed TidB.lat%Al PST crystals before and after annealing. Since all the specimens were electrically polished after deformation and/or annealing and then etched, the surface marking caused by the motion of dislocations was removed, but the traces of deformation twins and coarse slips were more notable. No noticeable change occurred in the lamellar structure or the traces of deformation twins in the fatigued specimens, while recrystallisation took place and new domains appeared in y phases for the compressed specimen annealed at 1000°C for lo3 s. During further annealing, coalescence and growth of domains occurred and cx2 plates were dissolved in the y phase resulting in separation into short fingers. The lamellar spacing of the compressed specimen increased rapidly during annealing while the lamellar structure of the fatigued specimen was not disturbed as shown in Fig. 8.
Annealing
time
(s)
Fig. 8. Variation of lamellar spacing of fatigued and compressed Ti-49.lat%Al PST crystals with annealing time at 1000°C.
Figures 9 and 10 show the deformation substructure in T-type and V-type domains after annealing, respectively. As optical micrographs suggested, numerous deformation twins remained in T-type domains even after annealing at 400 or 700°C for lo5 s. In contrast, remarkable changes in dislocation substructure were observed in V-type domains. In the fatigued specimen the vein-like structure was eliminated, leaving numerous small loops and debris caused by the climb motion and
(a)
(b)
(d)
(e)
Fig. 7. Optical micrographs of Ti49.lat%Al PST crystals deformed and annealed at various temperatures for 16 s: (a) as fatigued; (b) at 400°C fatigued; (c) at 7OO”C, fatigued; (d) as compressed; (e) at 4OO”C, compressed; (f) at 700°C, compressed.
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0C
(4
0.5pm
Fig. 9. Changes of electron micrographs
in T-type domains of Ti-49.1at%Al PST crystals annealed for lo5 s after fatigue or compression: (a) at 4OO”C, fatigued, foil//[liO], g = iil; (b) at 7OO”C,fatigued, foill/[ilO], g = lli; (c) at 4OO”C,compressed, foil//[ilO], g = 11i; (d) at 7OO”C,compressed, foiV/[ilO], g = 11i.
annihilation of l/2 < 1iO] ordinary dislocations at 400°C. The tangled dislocations were all annihilated, while the number of dislocation dipoles and loops that enlarged in size decreased at 700°C. The dislocation rearrangement of polygonisation or formation of subboundaries did not occur at either temperature. Even at 1000°C recrystallisation did not occur since the vein-like structure was eliminated at an early stage during annealing and there was inadequate driving force for recrystallisation. As shown in Fig. 11, dislocamutual
tions, dipoles and loops were almost all annihilated, while deformation twins were stable against annealing. No remarkable change in dislocation substructure of the compressed specimen was observed and a large number of l/2 < 1iO] ordinary dislocations had a screw character although their density gradually decreased. At 700°C curled or helical dislocations and large loops were formed as a result of the climb motion. From these aspects the deformation substructure in V-type domains of
TiAl PST crystals
(c)
(a
0.5pm
Fig. 10. Changes of microstruc_ture in VItype domains of Ti49.lat%Al PST crystals annealed for 10’s after fatigue or_compressi?n. (a) at 4OO”C, fatigued, foil//[ 1111,g = 111; (b) at 700°C. fatigued, foil//[lOl], g = il i; [c) at 4OO”C, compressed, foil//[lOl]; g = 111; (d) at 7OO”C,compressed, foiV/[lOl], g = 111.
the fatigued specimen was more easily recovered at lower temperature than that induced by monotonic deformation. Recrystallisation and growth of y domains were confirmed in the compressed specimen annealed at 1000°C from optical micrographs. Figure 12 shows an electron micrograph of a recrystallisation process appearing in y domains. Dislocations were annealed out in V-type domains, while a large number of deformation twins still existed in T-type domains. A large domain (A) which did
not satisfy Blackburn’s orientation relationship” against neighbouring cz2 plates appeared during recrystallisation. Once the domain is formed, cu, plates start to be dissolved and separate into short plates to release the lattice mismatch at the a,ly interface. The boundaries between the recrystallised domain and the original domains produced during the crystal growing were curved, since some types of domains were preferentially absorbed in a new domain (A) depending on the density of dislocations and twins.
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4 DISCUSSION
Fig. 11. Electron
micrograph of annealed Ti49.lat%Al PST crystal containing T- and V-type domains at 1000°C for lo5 s after cyclic deformation. foil//< 1iO>.
There was a great difference in recovery and recrystallisation behaviour between the fatigued and the compressed specimens. In the former, recovery began from a rather low temperature and the microhardness decreased gradually over a wide temperature range. However, there was no evidence of recrystallisation during annealing up to lOOO”C, while in the compressed specimens the recrystallisation process was confirmed at 1000°C. These differences in the two specimens are thought to be closely related to deformation substructure depending on the type of domain and the loading mode. The deformation substructure developed in each y ordered domain fell into two groups: numerous deformation twins were activated in T-type domains, while l/2 < 1IO] ordinary dislocations were dominantly operative in V-type domains. Although l/2 < 1iO] ordinary dislocations are known to be more easily activated than < 1011, l/2 < 1121 superdislocations in Ti-rich y phase,‘*j19 deformation twins were preferentially activated instead of
(a) Fig. 12. Recrystallised microstructure and diffraction patterns of Ti49.lat%Al PST crystal annealed at 1000°C for lo5 s after compression: (a) microstructure; (b) schematic drawing of (a). Parts (c), (d), (e) and (f) show diffraction patterns from A, B, C and D area in (a), respectively.
TiAl PST crystals
l/2 < 1iO]-slips in T-type domains under this loading condition (4 = 0’) because of the difference in their stress factors. Since the motion of deformation twins is not equivalent in tension and compression, the twins can be activated only in tension or compression depending on the orientation of loading axis. They are formed at an early stage of deformation but cannot be multiplied during cyclic loading at a constant stress amplitude. In V-type domains, l/2 < 1iO]-type dislocations tangled with each other and a large number of dipoles and loops were formed from jogs of the dislocations. Single vacancies and their clusters would be formed by a nonconservative motion of small jogs and could also be developed by the to-and-fro motion of dislocations under cyclic loading. In monotonic compression, both T-type and V-type domains must be strained under the same condition. As a result, many deformation twins were produced in T-type domains, while the density of dislocations and other lattice defects as their by-products became lower than that at cyclic deformation. A large difference in the recovery process between fatigued and compressed specimens was observed in dislocation substructure, while deformation twins were stable in both specimens below 700°C. Annihilation of tangled dislocations in the fatigued specimens occurred by the climb motion at low temperature (-4OO”C), although dislocations in the compressed specimen were stable at the corresponding temperature and the climb motion of l/2 < 1101 dislocations started at 700°C showing a curled or helical shape. A large number of supersaturated single vacancies and their clusters, which are produced during cyclic deformation, provide a strong driving force for diffusion. The climb motion of dislocations which may be accommodated by vacancy diffusion can occur even at a low temperature. In the recovery process at a low temperature there was no tendency toward polygonisation. In fatigued specimens, l/2 < 1101 ordinary dislocations had several types of segments, such as long dislocations, dipoles and loops forming a heterogeneous structure composed of vein-like and channel-like regions. Their dislocations had the same character as Burgers vector but the complex morphology of dislocation structure was harmful to polygonisation. Therefore, polygonisation for which dislocation array with the same character is required rarely occurs, as suggested by Segall and Partridge. 2oAs evidence of vacancy diffusion, there was loop coarsening from 400 to 700°C. From direct observation of the growth process of disloca-
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tion loops in quenched Al during annealing, these loops are known to be one of the major sink sites of vacancies.21 In order to reduce the line tension energy, small loops diminished emitting vacancies, while large loops coarsened absorbing vacancies from supersaturated matrix. Thus, the loop size became larger and the density became lower, similar to the process of Ostwald ripening.22 Annihilation of tangled dislocations and coarsening of loops thus resulted in softening below 700°C. Recrystallisation occurred only in the compressed specimen in which the driving force would be given by the numerous deformation twins formed in T-type domains. T-type domains containing numerous deformation twins maintain high stored energy even at lOOO”C, while the stored energy in V-type domains is lowered by annihilation and rearrangement of dislocations. The difference in stored energy leads V-type domains to grow and consume T-type domains. Moreover, if a large domain happened to nucleate, strain energy stored near the interface between the matrix/deformation twin boundaries, $7 and (y21y phase boundaries could be effectively released. This is in good agreement with the recrystallisation process of cold-rolled TiAl PST crystals.23 On the other hand, in the fatigued specimens, the number of deformation twins was smaller than in the compressed one. In addition, highly dense dislocations were annihilated so easily that the driving force for recrystallisation was not sufficient. Deformation twins which are harmful for further fatigue could all be removed by annealing above 12OO”C,but the lamellar structure would be disturbed.
5 CONCLUSIONS Recovery and recrystallisation behaviour of deformed TiAl PST crystals have been investigated focusing on the effect of monotonic and cyclic deformation modes and the type of y domains. The following conclusions were obtained. (1) In fatigued TiAl PST crystals, recovery occurred even at low temperatures (-400°C) without showing polygonisation. Annihilation and climb motion of l/2 < 1iO] ordinary dislocations were assisted by numerous supersaturated vacancies and clusters which were formed by the to-and-fro motion of jogged parts during cyclic loading. In addition, dislocation loops coarsened accompanied by the emittance and absorption of vacancies.
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Recrystallisation did not occur in fatigued crystals up to 1000°C since the vein-like structure diminished easily at an early stage of annealing and a deficient driving force remained. Annealing was not effective in preventing TiAl PST crystals from fatigue damage. (3) Recrystallisation occurred in the deformed TiAl PST crystals in compression and deformation twins had an important role for nucleation and growth of new domains. (2)
et al. 5. Hashimoto,
6. 7.
8.
9. 10. 11. 12.
ACKNOWLEDGEMENT Partial support of this program was provided by a Grant-in-Aid for Scientific Research and Development from the Ministry of Education, Science and Culture of Japan.
13. 14. 15.
16.
17.
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