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TEM investigation of twin intersection in a Ti–45Al–9Nb–2.5Mn alloy deformed at room temperature
Intermetallics 8 (2000) 539±544
TEM investigation of twin intersection in a Ti±45Al±9Nb±2.5Mn alloy deformed at room temperature G.L. Chen a,*, L.C. Zhang a,b a
State Key Laboratory for Advanced Metals and Materials, Beijing University of Science and Technology, Beijing, 100083, PR China b Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Academia Sinica, Shenyang, 110015, PR China
Abstract Three penetration mechanisms of the type-I twin intersection with the dislocation gliding atomic planes of (111)TB, (001)TB, and (115)TB respectively have been observed in a high-Nb containing TiAl base alloy by high resolution TEM. It was found that the active mode of those intersection mechanisms was related to the thickness of the incident twin (TI). When the TI is very thin such as 2±3 nm, the active intersection mode usually is the intersection mechanism with the dislocation gliding atomic planes of (111)TB; when the TI gets thick, the active intersection mode moves to the intersection mechanism with the gliding atomic planes of (001)TB; as the TI becomes thick enough such as 15 nm which is similar to the thickness of the barrier twin (TB), the active intersection mode changes to the intersection mechanism with the gliding atomic plane of (115)TB. The observed thickness dependence indicates that active dierent twin intersection mechanisms need dierent local stress concentrations near the intersection boundary, which increases with increasing TI thickness. A twin intersection mainly includes two successive processes that are dislocation dissociation on the intersection boundary and the dislocation glides in the barrier twin. The local stress concentration needed to activate those intersection mechanisms is related to the diculties of these two successive processes. The observed sequence in thickness dependence is consistent with the sequence in diculty of those intersection mechanisms based on a detailed analysis of those two successive processes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; B. Twinning
1. Introducton Twinning and twin intersection have been studied extensively due to their important role on the deformation and mechanical properties of TiAl-base alloys [1± 3]. For high temperature deformation, since the CRSS of twinning is lower, the twinning is a principal deformation mode. For room temperature deformation, since the CRSS of twinning becomes higher, the dislocation glide is the main deformation mode. In this case, the twinning becomes a principal stress relaxation mode to avoid the crack initiation. When multiple-twinning systems are activated, the twin intersections between dierent twinning systems become especially important, because it is the main locking mechanism leading to reduction of the mobility of the incoherent twin boundaries. It has been found that two non-equivalent types of twin intersections could occur in the L10±TiAl: the intersection lines along h110] (type-I) and h101] directions (type-II), respectively. The crystallographic features of these two * Corresponding author. Tel.: +86-10-623-32205; fax: +86-10623-32508.
types of twin intersections have been analyzed in recent publications [4±7]. However, the active conditions of those twin intersection mechanisms as well as the detailed atomic mechanism are not clear, and needs further investigation. The material used in the study is a high performance high-Nb containing TiAl-base dual-phase alloy. The alloy not only exhibited both excellent high temperature strength and oxidation resistance, but also has lower stacking fault energy due to the high Nb addition [8]. Therefore, the alloy was favorable to study the twinning and twin intersection as well as the deformation induced structure transitions. Chen, Wang and Zhang et al. [8± 15] reported numerous structural changes of 2 = , = T interlamellar boundaries and various phase transitions in the hot-deformed Ti±45Al±9Nb (at%) alloy. Wang et al. [16,17] have observed and analyzed the deformation twins and twin intersections in the Ti±45Al±9Nb alloy, deformed at high temperature by employing high-resolution transmission electron microscopy (HREM). This paper reports the HREM observations of three types of the type-I twin intersection mechanisms in the Ti±45Al± 9Nb±2.5Mn alloy deformed at room temperature. It is a
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00135-1
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part of the study on the room temperature twinning and twin intersection of the high-Nb containing TiAl alloy. 2. Experimental The alloy ingot, with the composition of Ti±45Al± 9Nb±2.5Mn (at%) was prepared by non-consumable electrode arc melting in a puri®ed argon atmosphere. The as-cast ingot was hot-forged to about 60% reduction at 1050 C at strain rates of about 10ÿ3 sÿ1. Then the forged alloy was heat treated for 0.5 h at 1300 C, followed by furnace cooling (FC), for 4 h at 1250 C, then FC to 950 C and held for 6 h, ®nally air cooling to room temperature. The heat treatment produced a duplex microstructure consisting of lamellar grains (100 mm) and g equiaxed grains (30 mm). The samples with a diameter 5 mm and length 10 mm were spark machined from the heat-treated alloy. Then the specimens were deformed at room temperature by compression to 10±30% plastic strains at a nominal strain rate of 10ÿ2 sÿ1. TEM specimen were prepared by standard twin jet polishing and/or ion-milling, and then observed in a JEOL-2000EX (II) HREM. 3. Results Figs. 1(a), (b) and (c) are HREM micrographs of the type-I twin intersections (the beam direction is [110]M). In Fig. 1(a), the thickness of the barrier twin TB and the incident twin TI are about 12 and 13 nm, respectively. The dashed line in Fig. 1(a) indicates that twin TI does not undergo a displacement normal to its habit plane across the intersection with the barrier twin TB. It means that the twin TI might trespass into the twin TB without de¯ection. The trace of the boundary between the twin TI and the intersection region is nearly parallel to the trace of (111)TI. A displacement of the twin TB normal to its habit plane across the intersection was measured to be two thirds of the width of the twin TI. In Fig. 1(b), the thicknesses of the barrier twin TB and the incident twin TI are about 19 and 13 nm, respectively. The trace of the gliding plane of the twin TI across the intersection is nearly parallel to the trace of the (001)TB plane [indicated by the dashed line in Fig. 1(b)]. It indicates that the twin TI penetrates into the twin TB by the gliding of perfect dislocations on the (001)TB basal plane. The trace of the boundary between the twin TI and the intersection region is also nearly the trace of the (111)TI. The displacement of the twin TB normal to its habit plane was measured to be equal to two thirds of the width of the twin TI. The displacement of the twin TI normal to its habit plane was measured to be less than one third of the width of twin TB. In Fig. 1(c), the thicknesses of the barrier twin TB and the
incident twin TI are about 7 and 1.5 nm, respectively. The trace of the gliding plane of twin TI across the intersection is nearly parallel to the trace of the (111)TB plane [indicated by the dashed line in Fig. 1(c)]. The twin TB keeps ¯at and unde¯ected. The boundary between twin TI and the intersection region is just the habit plane of the twin TB. The displacement of the twin TI normal to its habit plane was measured to be about two thirds of the width of the twin TB. Fig. 1(d)±(f) are the drafts schematically showing the intersection geometry of these three dierent type-I twin intersection mechanisms shown in Fig. 1(a)±(c), respectively. Fig. 2 is a HREM micrograph of type-I twin intersection viewed on [110]M direction. The barrier twin TB is intersected with three incident twins TI1, TI2 and TI3. The thickness of the twin TB is about 10.5 nm, the thicknesses of the twins TI1, TI2 and TI3 are about 12.5, 4.5 and 2.5 nm, respectively. According to the twin intersection features shown by the dashed lines 1, 2 and 3 in Fig. 2, the twin intersections should be consistent with the abovementioned three types of intersection con®gurations in Fig. 1, respectively. The intersection of the twin TI1 (12.5 nm) with the twin TB shown by the dash line 1 follows the intersection con®guration shown in Fig. 1(a) and (d). The intersection of the twin TI2 (4.5 nm) with the twin TB shown by the dash line 2 follows the intersection con®guration shown in Fig. 1(b) and (e). The intersection of the twin TI3 (2.5 nm) with the twin TB shown by the dash line 3 follows the intersection con®guration shown in Fig. 1(c) and (f). Fig. 2 clearly illustrates a thickness dependence of the type-I twin intersection. The thickness dependence indicates that, which type of the intersection mechanisms could be active, is related to the thickness of the incident twin relative to the twin TB. Fig. 3 is a HREM micrograph showing that an incident twin TI is separated from three thin twins T1, T2 and T3 by twin intersection. The thickness of the twin TB and the twin TI are about 8 and 3 nm, respectively. The twin penetration phenomena TI-T1, TI-T2 and TI-T3 shown by dotted lines 1, 2, and 3 in Fig. 3 are clearly consistent with the three types of intersection con®gurations respectively. The thickness of the thin twin T3 is more than 10 atomic planes, whereas the thickness of the thin twins T1, T2 are two or three (111)M atomic planes only. It indicates that the TI-T3 twin penetration or the (111) plane penetration is the main twin intersection mechanism. Basically, It is consistent with the thickness dependence of the twin intersection due to the thickness of the TI is 3 nm. 4. Discussion Figs. 1 and 2 clearly illustrate the three dierent penetration mechanisms of type-I twin intersections. The observations of both the straight penetration and the (001)TB intersection mechanisms are consistent with
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Fig. 1. HREM micrographs and corresponding schematic presentation showing three types of the type-I intersection mechanisms, along the beam direction [110]: (a) and (d) accommodation on (115)TB, (b) and (e) accommodation on (001)TB, (c) and (f) accommodation on (111)TB.
the observations of the twin intersections in recent publications [4±6]. Wardle et al. [4] observed the (001)TB cube plane penetration phenomena of type-I twin intersection in a polycrystalline TiAl-based alloy deformed at
room temperature. Sun et al. [5] observed the (115)TB straight penetration phenomena of a type-I twin intersection in a TiAl-based alloy during room-temperature deformation, and observed the (001)TB cube plane
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Fig. 2. HREM micrograph showing three types of the type-I intersection mechanisms, along the beam direction [110], where three incident twins TI1, TI2 and TI3 with dierent thickness intersecting with the barrier twin TB.
penetration phenomena of type-I twin intersection in a TiAl-based alloy during high-temperature deformation. Zhang et al. [6] observed both of the penetration phenomena in a Ti±50Al±2Mn±1Nb alloy during roomtemperature deformation. However, the (111)TB plane penetration phenomena was not detected in these TEM studies [4±6]. Sun et al. [7] pointed out the diculty of the (111)TB plane penetration mechanism based on a detailed crystallographic and stress analysis of this type penetration mechanism. Thus, the Figs. 1(c) and 2 illustrate ®rstly, using the HREM technique, that the (111)TB plane penetration phenomena could occur in the special case of a very thin incident twin. Furthermore, our observations shown in the ®gures, further illustrate the thickness dependence of these intersection mechanisms, which was not predicted in previous publications. Generally, the twin intersection may include two successive processes. When a TI impacts a TB, a dissociation reaction of the twinning dislocations may happen near the intersection boundary between the twins TI and TB. The produced dissociation dislocations, then, glide along the slip plane in the twin TB, ®nally passing the twin TB. Any type of the twin intersections could be active only if both the two successive processes can proceed under the drive force. The drive force must be the local stress concentration near the intersection boundary, which is believed to relate to the thickness of the twin TI relative to the twin TB. The straight penetration mechanism [Fig. 1(a) and (d)] is not characterized by complicated dislocation
dissociation reaction being involved [5,6]. The 1/6[112]M twinning dislocations gliding on the (111)M planes in the incident twin TI directly translate into 1/18[552]TB partial dislocations at the intersection boundaries. Then, the 1/18[552]TB partial dislocations glide on (115)TB planes in the barrier twin TB, and pass through the twin TB. It is clear that the CRSS of the 1/18[552]TB partial dislocations is very large due to the big burgers vector. For the (001)TB intersection mechanism, Zhang [6] suggested the following dislocation dissociation reaction: 6 1=6112M ! 4 1=2110TB 2 1=3111TB 2 1=2110M
1
Corresponding slip planes are the (111)M , (001)TB (111)TB, and (001)M for the dislocations 1/6[112]M , 1/ 2[110]TB, 1/3[111]TB and 1/2[110]M, respectively. This dislocation dissociation reaction at the intersection boundary is complicated. It is worth noting that four 1/2[110]TB dislocations on the (001)TB planes are formed through this reaction. The lattice intrinsic friction of the dislocation slip on the (001)TB planes is rather large since the (001)TB plane is not the primary slip plane. For the (111)TB intersection mechanism, the suggested dislocation dissociation reaction is [5,6]: 3 1=6112M ! 3 1=6112TB 2 1=6 112 TB
2
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D:S: 115TB ) D:S: 001TB ) D:S: 111TB :
Fig. 3. HREM micrograph showing that a incident twin TI is separated into three thin twins T1, T2 and T3 by intersecting with the barrier twin TB.
Corresponding slip planes are the (111)M (111)TB, and (111)TB for the dislocations 1/6[112]M, 1/6[112]TB, and 1/6[112]TB, respectively. In the barrier twin TB, the two 1/6[112]TB partial dislocations gliding on the (111)TB planes are detwinning shear, which can translate two atomic layers of the stacking sequence of twin TB into that of the matrix M. However, The detwinning shear is not favored due to the unsuitable direction of the applied stress. Therefore, its shear distance is very limited [5]. The pile-up of the detwinning dislocations will stop the further dislocation dissociation reaction. So the dislocation dissociation reaction (2) for the (111)TB intersection mechanism is very dicult to occur. This might be the reason why Wardle et al. [4], Sun et al. [5], Zhang and Chaturvedi [6] did not observe the (111)TB intersection mechanism. However, because the (111)TB plane is the primary slip plane, the CRSS of the 1/6[112]TB dislocation gliding on the (111)TB planes is very small. It is easy to operate. According to the above analysis, the sequence of dislocation dissociation (D.D.) in the intersection mechanisms from dicult to easy is as follows: D:D: 111TB ) D:D: 001TB ) D:D: 115TB ; Nevertheless, the sequence from dicult to easy of the dissociation dislocation gliding in the twin TB is in an opposite direction:
When the thickness of the twin TI is small, the local stress concentration near the intersection boundary is so small that only the 1/6[112]TB dislocation gliding on the (111)TB can be operated. In addition, the 1/6[112]TB detwinning dislocations are less and are not enough to form a dislocation pile-up to stop the further dislocation dissociation reaction (2). Thus, when the thickness of the twin TI is small such as less than 3 nm, the main intersection mechanism could be the (111)TB penetration mechanism. When the twin TI becomes thick, the local stress concentration near the intersection boundary is big enough to operate both the 1/6[112]TB and 1/2[110]TB dissociation dislocations gliding on the (111)TB and (001)TB planes respectively. Nevertheless, the pile-up of many 1/6[112]TB(111)TB detwinning dislocations could not be favorite to the occurrence of the (111)TB penetration mechanism. Thus, when the thickness of the twin TI becomes slightly large, the intersection mechanism could be (001)TB plane penetration mechanism instead of the (111)TB penetration mechanism. It is because the dislocation dissociation reaction (1) for (001)TB plane penetration mechanism could take place instead of the reaction (2). When the twin TI becomes thick enough to be about the size of the twin TB, the corresponding local stress concentration near the intersection boundary is big enough to operate the gliding of all the dissociation dislocations for the three types of the penetration mechanisms. As mentioned previously, the (111)TB plane penetration mechanism could not take place due to the above-mentioned reason. Meanwhile, the (001)TB plane penetration mechanism is replaced by the (115)TB straight penetration mechanism. The possible reason is that the (115)TB straight penetration mechanism does not need any complicated dislocation dissociation reaction. In sum, the local stress concentration, which is related to the thickness of the twin TI, is an important factor to determine what dissociation dislocation could be operated in the twin TB. However, the diculty of the dissociation dislocation reaction is another factor determining whether the penetration mechanism could continue. Wardle [4] and Zhang et al. [6] have discussed the occurrence of the intersection mechanisms based on the energetic measurement in terms of the balance in the self-energies of the involved dislocations. Sun et al. [5,7] considered the eect of the stress distribution ahead of the pile-up of incident twinning dislocations near the intersection boundary. The observed thickness dependence of the twin penetration mechanisms clearly proves the important eects of both sides. The local stress concentration near the intersection boundary must be larger than the CRSS of the moving dislocation in twin
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TB. It is an essential stress condition to operate a twin intersection mechanism. The energetic condition of the dissociation dislocation reaction of the intersection mechanism is a necessary condition for the intersection mechanism to take place continuously. However, it is necessary to further develop a quantitative analysis to determine the thickness dependence of the twin intersection mechanisms. Fig. 3 implies that the critical thickness of TI for the (111)TB penetration mechanism may be 3 nm. In this case, the pile-up of the detwinning dislocations might deduce high stress concentration, which not only stops the (111)TB penetration mechanism from continuing, but also stimulates the (115)TB penetration to operate. However, the local stress concentration should be immediately reduced to only operating the (001)TB gliding by the (115)TB penetration of 2 atomic planes. 5. Conclusion The type-I twin intersection of a high-Nb containing TiAl base alloy, that deformed at room temperature, has been investigated. The observations of HREM illustrated the three types of penetration mechanisms with the gliding on the atomic planes (111)TB, (001)TB, and (115)TB respectively. The observed thickness dependence indicated that the active penetration mechanism is closely related to the thickness of the incident twin TI relative to the twin TB: 1. When the incident twin TI is very thin such as less than 3 nm, the (111)TB penetration mechanism could operate. 2. When the twin TI gets thick, the (001)TB penetration mechanism could be active instead of the (111)TB penetration mechanism. 3. When the twin TI is as thick the barrier twin TB, the intersection mechanism is changed to the straight penetration mechanism. The 1/6[112]M twinning dislocations gliding on the (111)M planes in the incident twin TI, directly translated into 1/ 18[552]TB partial dislocations gliding on the (115)TB atomic plane in the twin TB.
The thickness dependence of the penetration mechanism indicates the importance of both sides of the energetic consideration [4,6] and the stress concentration near the intersection boundary [5,7]. The essential condition for a dissociation dislocation gliding in the twin TB is the local stress concentration near the intersection boundary large enough to operate the dislocation gliding. The necessary condition is the favorable energetic condition for the dissociation dislocation reaction to continue. The observed thickness dependence is consistent with this explanation. Acknowledgements The study was supported by the National Nature Science Foundation of China. The authors wish to thank Prof. H. Q. Ye for the helpful discussion. References [1] Christian JW, Mahajan S. Progress in Materials Science 1995;39:1. [2] Farenc S, Coujou A, Couret A. Phil Mag 1993;A67:127. [3] Jin Z, Bieler TR. Phil Mag 1995;A71:925. [4] Wardle S, Phan I, Hug G. Phil Mag 1993;A67:497. [5] Sun YQ, Hazzledine PM, Christian JW. Phil Mag 1993;A68:471. [6] Zhang YG, Chaturvedi MC. Phil Mag 1993;A68:915. [7] Sun YQ, Hazzledine PM, Christian JW. Phil Mag 1993;A68:495. [8] Chen GL, Wang JG, Zhang LC, Ye HQ. Acta Metall Sinica 1995;8:273. [9] Zhang LC, Chen GL, Wang JG, Ye HQ. Intermetallics 1997;5:289. [10] Wang JG, Zhang LC, Chen GL, Ye HQ. Journal of Mater Sci Tech 1998;14:138. [11] Zhang LC, Chen GL, Wang JG, Ye HQ. Mater Lett 1998;36:132. [12] Zhang LC, Chen GL, Wang JG, Ye HQ. Mater Sci Eng 1998;A247:1. [13] Wang JG, Chen GL, Zhang LC, Ye HQ. Mater Lett 1997;31:179. [14] Wang JG, Zhang LC, Chen GL, Ye HQ, Nieh TG. Mater Sci Eng 1997;287:A239. [15] Wang JG, Zhang LC, Chen GL, Ye HQ. Scripta Metall Mater 1997;37:135. [16] Wang JG, Zhang LC, Chen GL, Ye H.Q. Structral Intermetallics. In: Nathal MV, Darolia R, Liu CT, Martin PL, Miracle DB, Wagner R, Yamaguchi M. The Minerals, Metals & Materials Society, 1997. 119 [17] Wang JG, Zhang LC, Chen GL, Ye HQ. Mater Sci Eng 1998;A252:222.
TEM investigation of twin intersection in a Ti±45Al±9Nb±2.5Mn alloy deformed at room temperature G.L. Chen a,*, L.C. Zhang a,b a
State Key Laboratory for Advanced Metals and Materials, Beijing University of Science and Technology, Beijing, 100083, PR China b Laboratory of Atomic Imaging of Solids, Institute of Metal Research, Academia Sinica, Shenyang, 110015, PR China
Abstract Three penetration mechanisms of the type-I twin intersection with the dislocation gliding atomic planes of (111)TB, (001)TB, and (115)TB respectively have been observed in a high-Nb containing TiAl base alloy by high resolution TEM. It was found that the active mode of those intersection mechanisms was related to the thickness of the incident twin (TI). When the TI is very thin such as 2±3 nm, the active intersection mode usually is the intersection mechanism with the dislocation gliding atomic planes of (111)TB; when the TI gets thick, the active intersection mode moves to the intersection mechanism with the gliding atomic planes of (001)TB; as the TI becomes thick enough such as 15 nm which is similar to the thickness of the barrier twin (TB), the active intersection mode changes to the intersection mechanism with the gliding atomic plane of (115)TB. The observed thickness dependence indicates that active dierent twin intersection mechanisms need dierent local stress concentrations near the intersection boundary, which increases with increasing TI thickness. A twin intersection mainly includes two successive processes that are dislocation dissociation on the intersection boundary and the dislocation glides in the barrier twin. The local stress concentration needed to activate those intersection mechanisms is related to the diculties of these two successive processes. The observed sequence in thickness dependence is consistent with the sequence in diculty of those intersection mechanisms based on a detailed analysis of those two successive processes. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; B. Twinning
1. Introducton Twinning and twin intersection have been studied extensively due to their important role on the deformation and mechanical properties of TiAl-base alloys [1± 3]. For high temperature deformation, since the CRSS of twinning is lower, the twinning is a principal deformation mode. For room temperature deformation, since the CRSS of twinning becomes higher, the dislocation glide is the main deformation mode. In this case, the twinning becomes a principal stress relaxation mode to avoid the crack initiation. When multiple-twinning systems are activated, the twin intersections between dierent twinning systems become especially important, because it is the main locking mechanism leading to reduction of the mobility of the incoherent twin boundaries. It has been found that two non-equivalent types of twin intersections could occur in the L10±TiAl: the intersection lines along h110] (type-I) and h101] directions (type-II), respectively. The crystallographic features of these two * Corresponding author. Tel.: +86-10-623-32205; fax: +86-10623-32508.
types of twin intersections have been analyzed in recent publications [4±7]. However, the active conditions of those twin intersection mechanisms as well as the detailed atomic mechanism are not clear, and needs further investigation. The material used in the study is a high performance high-Nb containing TiAl-base dual-phase alloy. The alloy not only exhibited both excellent high temperature strength and oxidation resistance, but also has lower stacking fault energy due to the high Nb addition [8]. Therefore, the alloy was favorable to study the twinning and twin intersection as well as the deformation induced structure transitions. Chen, Wang and Zhang et al. [8± 15] reported numerous structural changes of 2 = , = T interlamellar boundaries and various phase transitions in the hot-deformed Ti±45Al±9Nb (at%) alloy. Wang et al. [16,17] have observed and analyzed the deformation twins and twin intersections in the Ti±45Al±9Nb alloy, deformed at high temperature by employing high-resolution transmission electron microscopy (HREM). This paper reports the HREM observations of three types of the type-I twin intersection mechanisms in the Ti±45Al± 9Nb±2.5Mn alloy deformed at room temperature. It is a
0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(99)00135-1
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G.L. Chen, L.C. Zhang / Intermetallics 8 (2000) 539±544
part of the study on the room temperature twinning and twin intersection of the high-Nb containing TiAl alloy. 2. Experimental The alloy ingot, with the composition of Ti±45Al± 9Nb±2.5Mn (at%) was prepared by non-consumable electrode arc melting in a puri®ed argon atmosphere. The as-cast ingot was hot-forged to about 60% reduction at 1050 C at strain rates of about 10ÿ3 sÿ1. Then the forged alloy was heat treated for 0.5 h at 1300 C, followed by furnace cooling (FC), for 4 h at 1250 C, then FC to 950 C and held for 6 h, ®nally air cooling to room temperature. The heat treatment produced a duplex microstructure consisting of lamellar grains (100 mm) and g equiaxed grains (30 mm). The samples with a diameter 5 mm and length 10 mm were spark machined from the heat-treated alloy. Then the specimens were deformed at room temperature by compression to 10±30% plastic strains at a nominal strain rate of 10ÿ2 sÿ1. TEM specimen were prepared by standard twin jet polishing and/or ion-milling, and then observed in a JEOL-2000EX (II) HREM. 3. Results Figs. 1(a), (b) and (c) are HREM micrographs of the type-I twin intersections (the beam direction is [110]M). In Fig. 1(a), the thickness of the barrier twin TB and the incident twin TI are about 12 and 13 nm, respectively. The dashed line in Fig. 1(a) indicates that twin TI does not undergo a displacement normal to its habit plane across the intersection with the barrier twin TB. It means that the twin TI might trespass into the twin TB without de¯ection. The trace of the boundary between the twin TI and the intersection region is nearly parallel to the trace of (111)TI. A displacement of the twin TB normal to its habit plane across the intersection was measured to be two thirds of the width of the twin TI. In Fig. 1(b), the thicknesses of the barrier twin TB and the incident twin TI are about 19 and 13 nm, respectively. The trace of the gliding plane of the twin TI across the intersection is nearly parallel to the trace of the (001)TB plane [indicated by the dashed line in Fig. 1(b)]. It indicates that the twin TI penetrates into the twin TB by the gliding of perfect dislocations on the (001)TB basal plane. The trace of the boundary between the twin TI and the intersection region is also nearly the trace of the (111)TI. The displacement of the twin TB normal to its habit plane was measured to be equal to two thirds of the width of the twin TI. The displacement of the twin TI normal to its habit plane was measured to be less than one third of the width of twin TB. In Fig. 1(c), the thicknesses of the barrier twin TB and the
incident twin TI are about 7 and 1.5 nm, respectively. The trace of the gliding plane of twin TI across the intersection is nearly parallel to the trace of the (111)TB plane [indicated by the dashed line in Fig. 1(c)]. The twin TB keeps ¯at and unde¯ected. The boundary between twin TI and the intersection region is just the habit plane of the twin TB. The displacement of the twin TI normal to its habit plane was measured to be about two thirds of the width of the twin TB. Fig. 1(d)±(f) are the drafts schematically showing the intersection geometry of these three dierent type-I twin intersection mechanisms shown in Fig. 1(a)±(c), respectively. Fig. 2 is a HREM micrograph of type-I twin intersection viewed on [110]M direction. The barrier twin TB is intersected with three incident twins TI1, TI2 and TI3. The thickness of the twin TB is about 10.5 nm, the thicknesses of the twins TI1, TI2 and TI3 are about 12.5, 4.5 and 2.5 nm, respectively. According to the twin intersection features shown by the dashed lines 1, 2 and 3 in Fig. 2, the twin intersections should be consistent with the abovementioned three types of intersection con®gurations in Fig. 1, respectively. The intersection of the twin TI1 (12.5 nm) with the twin TB shown by the dash line 1 follows the intersection con®guration shown in Fig. 1(a) and (d). The intersection of the twin TI2 (4.5 nm) with the twin TB shown by the dash line 2 follows the intersection con®guration shown in Fig. 1(b) and (e). The intersection of the twin TI3 (2.5 nm) with the twin TB shown by the dash line 3 follows the intersection con®guration shown in Fig. 1(c) and (f). Fig. 2 clearly illustrates a thickness dependence of the type-I twin intersection. The thickness dependence indicates that, which type of the intersection mechanisms could be active, is related to the thickness of the incident twin relative to the twin TB. Fig. 3 is a HREM micrograph showing that an incident twin TI is separated from three thin twins T1, T2 and T3 by twin intersection. The thickness of the twin TB and the twin TI are about 8 and 3 nm, respectively. The twin penetration phenomena TI-T1, TI-T2 and TI-T3 shown by dotted lines 1, 2, and 3 in Fig. 3 are clearly consistent with the three types of intersection con®gurations respectively. The thickness of the thin twin T3 is more than 10 atomic planes, whereas the thickness of the thin twins T1, T2 are two or three (111)M atomic planes only. It indicates that the TI-T3 twin penetration or the (111) plane penetration is the main twin intersection mechanism. Basically, It is consistent with the thickness dependence of the twin intersection due to the thickness of the TI is 3 nm. 4. Discussion Figs. 1 and 2 clearly illustrate the three dierent penetration mechanisms of type-I twin intersections. The observations of both the straight penetration and the (001)TB intersection mechanisms are consistent with
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Fig. 1. HREM micrographs and corresponding schematic presentation showing three types of the type-I intersection mechanisms, along the beam direction [110]: (a) and (d) accommodation on (115)TB, (b) and (e) accommodation on (001)TB, (c) and (f) accommodation on (111)TB.
the observations of the twin intersections in recent publications [4±6]. Wardle et al. [4] observed the (001)TB cube plane penetration phenomena of type-I twin intersection in a polycrystalline TiAl-based alloy deformed at
room temperature. Sun et al. [5] observed the (115)TB straight penetration phenomena of a type-I twin intersection in a TiAl-based alloy during room-temperature deformation, and observed the (001)TB cube plane
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Fig. 2. HREM micrograph showing three types of the type-I intersection mechanisms, along the beam direction [110], where three incident twins TI1, TI2 and TI3 with dierent thickness intersecting with the barrier twin TB.
penetration phenomena of type-I twin intersection in a TiAl-based alloy during high-temperature deformation. Zhang et al. [6] observed both of the penetration phenomena in a Ti±50Al±2Mn±1Nb alloy during roomtemperature deformation. However, the (111)TB plane penetration phenomena was not detected in these TEM studies [4±6]. Sun et al. [7] pointed out the diculty of the (111)TB plane penetration mechanism based on a detailed crystallographic and stress analysis of this type penetration mechanism. Thus, the Figs. 1(c) and 2 illustrate ®rstly, using the HREM technique, that the (111)TB plane penetration phenomena could occur in the special case of a very thin incident twin. Furthermore, our observations shown in the ®gures, further illustrate the thickness dependence of these intersection mechanisms, which was not predicted in previous publications. Generally, the twin intersection may include two successive processes. When a TI impacts a TB, a dissociation reaction of the twinning dislocations may happen near the intersection boundary between the twins TI and TB. The produced dissociation dislocations, then, glide along the slip plane in the twin TB, ®nally passing the twin TB. Any type of the twin intersections could be active only if both the two successive processes can proceed under the drive force. The drive force must be the local stress concentration near the intersection boundary, which is believed to relate to the thickness of the twin TI relative to the twin TB. The straight penetration mechanism [Fig. 1(a) and (d)] is not characterized by complicated dislocation
dissociation reaction being involved [5,6]. The 1/6[112]M twinning dislocations gliding on the (111)M planes in the incident twin TI directly translate into 1/18[552]TB partial dislocations at the intersection boundaries. Then, the 1/18[552]TB partial dislocations glide on (115)TB planes in the barrier twin TB, and pass through the twin TB. It is clear that the CRSS of the 1/18[552]TB partial dislocations is very large due to the big burgers vector. For the (001)TB intersection mechanism, Zhang [6] suggested the following dislocation dissociation reaction: 6 1=6112M ! 4 1=2110TB 2 1=3111TB 2 1=2110M
1
Corresponding slip planes are the (111)M , (001)TB (111)TB, and (001)M for the dislocations 1/6[112]M , 1/ 2[110]TB, 1/3[111]TB and 1/2[110]M, respectively. This dislocation dissociation reaction at the intersection boundary is complicated. It is worth noting that four 1/2[110]TB dislocations on the (001)TB planes are formed through this reaction. The lattice intrinsic friction of the dislocation slip on the (001)TB planes is rather large since the (001)TB plane is not the primary slip plane. For the (111)TB intersection mechanism, the suggested dislocation dissociation reaction is [5,6]: 3 1=6112M ! 3 1=6112TB 2 1=6 112 TB
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D:S: 115TB ) D:S: 001TB ) D:S: 111TB :
Fig. 3. HREM micrograph showing that a incident twin TI is separated into three thin twins T1, T2 and T3 by intersecting with the barrier twin TB.
Corresponding slip planes are the (111)M (111)TB, and (111)TB for the dislocations 1/6[112]M, 1/6[112]TB, and 1/6[112]TB, respectively. In the barrier twin TB, the two 1/6[112]TB partial dislocations gliding on the (111)TB planes are detwinning shear, which can translate two atomic layers of the stacking sequence of twin TB into that of the matrix M. However, The detwinning shear is not favored due to the unsuitable direction of the applied stress. Therefore, its shear distance is very limited [5]. The pile-up of the detwinning dislocations will stop the further dislocation dissociation reaction. So the dislocation dissociation reaction (2) for the (111)TB intersection mechanism is very dicult to occur. This might be the reason why Wardle et al. [4], Sun et al. [5], Zhang and Chaturvedi [6] did not observe the (111)TB intersection mechanism. However, because the (111)TB plane is the primary slip plane, the CRSS of the 1/6[112]TB dislocation gliding on the (111)TB planes is very small. It is easy to operate. According to the above analysis, the sequence of dislocation dissociation (D.D.) in the intersection mechanisms from dicult to easy is as follows: D:D: 111TB ) D:D: 001TB ) D:D: 115TB ; Nevertheless, the sequence from dicult to easy of the dissociation dislocation gliding in the twin TB is in an opposite direction:
When the thickness of the twin TI is small, the local stress concentration near the intersection boundary is so small that only the 1/6[112]TB dislocation gliding on the (111)TB can be operated. In addition, the 1/6[112]TB detwinning dislocations are less and are not enough to form a dislocation pile-up to stop the further dislocation dissociation reaction (2). Thus, when the thickness of the twin TI is small such as less than 3 nm, the main intersection mechanism could be the (111)TB penetration mechanism. When the twin TI becomes thick, the local stress concentration near the intersection boundary is big enough to operate both the 1/6[112]TB and 1/2[110]TB dissociation dislocations gliding on the (111)TB and (001)TB planes respectively. Nevertheless, the pile-up of many 1/6[112]TB(111)TB detwinning dislocations could not be favorite to the occurrence of the (111)TB penetration mechanism. Thus, when the thickness of the twin TI becomes slightly large, the intersection mechanism could be (001)TB plane penetration mechanism instead of the (111)TB penetration mechanism. It is because the dislocation dissociation reaction (1) for (001)TB plane penetration mechanism could take place instead of the reaction (2). When the twin TI becomes thick enough to be about the size of the twin TB, the corresponding local stress concentration near the intersection boundary is big enough to operate the gliding of all the dissociation dislocations for the three types of the penetration mechanisms. As mentioned previously, the (111)TB plane penetration mechanism could not take place due to the above-mentioned reason. Meanwhile, the (001)TB plane penetration mechanism is replaced by the (115)TB straight penetration mechanism. The possible reason is that the (115)TB straight penetration mechanism does not need any complicated dislocation dissociation reaction. In sum, the local stress concentration, which is related to the thickness of the twin TI, is an important factor to determine what dissociation dislocation could be operated in the twin TB. However, the diculty of the dissociation dislocation reaction is another factor determining whether the penetration mechanism could continue. Wardle [4] and Zhang et al. [6] have discussed the occurrence of the intersection mechanisms based on the energetic measurement in terms of the balance in the self-energies of the involved dislocations. Sun et al. [5,7] considered the eect of the stress distribution ahead of the pile-up of incident twinning dislocations near the intersection boundary. The observed thickness dependence of the twin penetration mechanisms clearly proves the important eects of both sides. The local stress concentration near the intersection boundary must be larger than the CRSS of the moving dislocation in twin
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TB. It is an essential stress condition to operate a twin intersection mechanism. The energetic condition of the dissociation dislocation reaction of the intersection mechanism is a necessary condition for the intersection mechanism to take place continuously. However, it is necessary to further develop a quantitative analysis to determine the thickness dependence of the twin intersection mechanisms. Fig. 3 implies that the critical thickness of TI for the (111)TB penetration mechanism may be 3 nm. In this case, the pile-up of the detwinning dislocations might deduce high stress concentration, which not only stops the (111)TB penetration mechanism from continuing, but also stimulates the (115)TB penetration to operate. However, the local stress concentration should be immediately reduced to only operating the (001)TB gliding by the (115)TB penetration of 2 atomic planes. 5. Conclusion The type-I twin intersection of a high-Nb containing TiAl base alloy, that deformed at room temperature, has been investigated. The observations of HREM illustrated the three types of penetration mechanisms with the gliding on the atomic planes (111)TB, (001)TB, and (115)TB respectively. The observed thickness dependence indicated that the active penetration mechanism is closely related to the thickness of the incident twin TI relative to the twin TB: 1. When the incident twin TI is very thin such as less than 3 nm, the (111)TB penetration mechanism could operate. 2. When the twin TI gets thick, the (001)TB penetration mechanism could be active instead of the (111)TB penetration mechanism. 3. When the twin TI is as thick the barrier twin TB, the intersection mechanism is changed to the straight penetration mechanism. The 1/6[112]M twinning dislocations gliding on the (111)M planes in the incident twin TI, directly translated into 1/ 18[552]TB partial dislocations gliding on the (115)TB atomic plane in the twin TB.
The thickness dependence of the penetration mechanism indicates the importance of both sides of the energetic consideration [4,6] and the stress concentration near the intersection boundary [5,7]. The essential condition for a dissociation dislocation gliding in the twin TB is the local stress concentration near the intersection boundary large enough to operate the dislocation gliding. The necessary condition is the favorable energetic condition for the dissociation dislocation reaction to continue. The observed thickness dependence is consistent with this explanation. Acknowledgements The study was supported by the National Nature Science Foundation of China. The authors wish to thank Prof. H. Q. Ye for the helpful discussion. References [1] Christian JW, Mahajan S. Progress in Materials Science 1995;39:1. [2] Farenc S, Coujou A, Couret A. Phil Mag 1993;A67:127. [3] Jin Z, Bieler TR. Phil Mag 1995;A71:925. [4] Wardle S, Phan I, Hug G. Phil Mag 1993;A67:497. [5] Sun YQ, Hazzledine PM, Christian JW. Phil Mag 1993;A68:471. [6] Zhang YG, Chaturvedi MC. Phil Mag 1993;A68:915. [7] Sun YQ, Hazzledine PM, Christian JW. Phil Mag 1993;A68:495. [8] Chen GL, Wang JG, Zhang LC, Ye HQ. Acta Metall Sinica 1995;8:273. [9] Zhang LC, Chen GL, Wang JG, Ye HQ. Intermetallics 1997;5:289. [10] Wang JG, Zhang LC, Chen GL, Ye HQ. Journal of Mater Sci Tech 1998;14:138. [11] Zhang LC, Chen GL, Wang JG, Ye HQ. Mater Lett 1998;36:132. [12] Zhang LC, Chen GL, Wang JG, Ye HQ. Mater Sci Eng 1998;A247:1. [13] Wang JG, Chen GL, Zhang LC, Ye HQ. Mater Lett 1997;31:179. [14] Wang JG, Zhang LC, Chen GL, Ye HQ, Nieh TG. Mater Sci Eng 1997;287:A239. [15] Wang JG, Zhang LC, Chen GL, Ye HQ. Scripta Metall Mater 1997;37:135. [16] Wang JG, Zhang LC, Chen GL, Ye H.Q. Structral Intermetallics. In: Nathal MV, Darolia R, Liu CT, Martin PL, Miracle DB, Wagner R, Yamaguchi M. The Minerals, Metals & Materials Society, 1997. 119 [17] Wang JG, Zhang LC, Chen GL, Ye HQ. Mater Sci Eng 1998;A252:222.