- Email: [email protected]
High-resolution transmission electron microscope (HRTEM) study of the transformation interface and substructure in NiTiHf40 melt–spun ribbons
Journal of Alloys and Compounds 334 (2002) 147–153
L
www.elsevier.com / locate / jallcom
High-resolution transmission electron microscope (HRTEM) study of the transformation interface and substructure in NiTiHf 40 melt–spun ribbons a, b b a a M. Liu *, X.M. Zhang , Y.Y. Li , J.Z. Chen , M.J. Tu b
a Department of Metal Materials, Sichuan University ( West), Chengdu 610065, People’ s Republic of China The National Key Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’ s Republic of China
Received 26 January 2001; accepted 11 July 2001
Abstract The fine structures of self-accommodation spear-like martensite variant groups and the interface between the martensite region and the amorphous phase region in NiTi–Hf 40 melt–spun ribbons were studied by using high-resolution transmission electron microscopy (HRTEM). In the self-accommodation lath plate martensite group, the variants prefer to form orthogonal morphology of spear-like martensite on both sides of melt–spun ribbons. Some variant pairs were of the (011) B199 Type I twin related with straight and coherent interface while the interface between other variants is curved and incoherent. The main substructure in variants of the group was not the (001) B199 compound twin but the (011) B199 Type I microtwin and (011) stacking faults. The interface between the amorphous phase and the martensite region showed high energy and nonequilibrium configuration. Some area in the interface is straight but not smooth and exhibited an irregular configuration and some area in the interface has an obvious zigzag configuration. Energy-dispersive spectroscopy (EDX) analysis of the composition in NiTi–Hf 40 ribbon showed that the Ni and Ti content were different in the martensite region and in the amorphous phase. The amorphous phase and the martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 compared to those of lower Hf content NiTi–Hf alloys were due to the not retained austenite, the very small variant pair and the coexisting amorphous phase. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Amorphous materials; TEM; Phase transition
1. Introduction With Hf addition, the NiTi-based alloys have been recognized as potential candidate material for high temperature shape memory alloys (HTSMAs) in recent years [1–5]. In the mechanical working process of the NiTi–Hf alloys, however, the major hindrance is their worse workability with increasing Hf addition [4]. Even the hot rolling process is not effective for producing NiTi–Hf ribbons with Hf contents higher than 20 at.% [6]. We have fabricated NiTi-based ribbons using the melt–spun method with Hf content higher than 40 at.%. We noticed that with increasing Hf addition, the reverse transformation temperature increased, some properties improved, but the ribbons became more brittle. Shape memory effect (SME) is generally associated with the thermoelastic martensitic transformation and selfaccommodation of martensite variants. The junction planes *Corresponding author. E-mail address: mm [email protected] (M. Liu). ]
between martensite variants and the variants’ combination play a very important role in the interface movement, as well as the variants coalescence when the specimen is deformed. In this paper, we report our direct observation of the substructure in a self-accommodation spear-like martensitic variant group and the interface structure between the martensite region and amorphous phase by high resolution transmission electron microscope (HRTEM) for a better understanding of NiTi–Hf melt–spun ribbons.
2. Experiment details High purity nickel, titanium and hafnium were melted six times using an argon arc button furnace to obtain Ni 49 Ti 11 Hf 40 (denoted as Hf 40 ) alloys. The melted alloy was rapidly quenched by melt–spinning using a Cu weal in purified atmosphere of inert gas at a cooling rate of about 10 7 K / s. The ribbon was about 40 mm thick and 2 mm wide. Foil samples were cut from the ribbon and thinned directly by ion milling method for HRTEM observation.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01762-5
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
148
The electron microscope (JEOL-2010EX) was operated at an accelerating voltage of 200 kV with a point resolution of 0.197 nm. The composition analysis was done by ultra microanalysis with energy-dispersive spectroscopy (EDX) using the Cliff-Lorimer program in a field emission election mictoscope (HF-2000FEG) equipped with the EDX Oxford, ISIS-142 system.
3. Results and discussion Fig. 1 is a low magnification HRTEM image of the Hf 40 melt–spun ribbons. The corresponding electron diffraction pattern (EDP) showed diffraction spots from martensite superimposed on a typical amorphous ring, which indicated that the martensite embedded in the amorphous phase. The lath plate martensite or the spear-like martensitic variants [7] were dominantly observed in Hf 40 . According to the reports of Han et al. [7] and Zheng et al. [8] of bulk NiTi–Hf 15 (Hf 15 ) HTSMAs, there should be two kinds of self-accommodation variant groups, i.e. mosaic- and spear-like martensite junction plane variant groups. They were the cross-section and vertical section view of the same lath plate groups. Therefore, the mosaiclike junction plane variants should be dominantly observed in the cross section side of the ribbons. Fig. 2(a) shows a low magnification image of martensite region with an orthogonal spear-like junction plane variant group. The group contains two kinds of variant pairs, which were vertical to each other. The [100] B199 EDP from this area is shown in the inset of Fig. 2(a). The size difference of the two variant pairs was very large in the self-accommodation group. The variant was about 2–8 nm wide in one pair and about 40–100 nm in the other. In bulk Hf15, the width of the two variant pairs was almost the same and much wider than Hf 40 [7]. The small variant pair
Fig. 1. Martensite was embedded in amorphous phase.
in the self-accommodation group may act as a coherent phase that strengthened the ribbon [9]. Retained austenite diffraction spots were not found in the EDP inset of Fig. 2(a), but they were always observed in bulk Hf 15 along the same diffraction axis [7,10]. The absence of retained austenite diffraction spots were also noticed in other specimens randomly cut from the same Hf 40 ribbons, which implies that the martensitic transformation is completed in Hf 40 . The absence of austenite, the small variant pair and the coexisting amorphous phase, all contributed to the hardness and brittleness of Hf 40 being higher than in bulk Hf 15 . In order to see the details of the joining regions of these thin plates, the framed areas in Fig. 2(a) were enlarged and shown in Fig. 2(b) and (c). In these micrographs, we can see four variants, indicated by A, B, C and D. The variants A and B are (011) B199 / /(001) B2 Type I twin related [11]. The C and D variants are (011) B199 / /(010) B2 Type I twin related [7,8,11]. The variants in the same pair, i.e. A / B pair or C / D pair, have mirror symmetry with respect to the (011) B199 plane and the twin plane was straight and coherent, as shown in Fig. 2 (b) and (c), but the interface of adjacent variants, i.e. A and C, was curved and incoherent, as shown in Fig. 2(c). The plane angle is about 68 between (001) B199 of variants A and C and about 968 (or 180–965848) between (001) B199 of variants A and B, which were in good agreement with theoretical calculation of monoclinic structure projected in a (001) B2 stereograph [11]. (011) B199 stacking faults were observed and shown in Fig. 2(c). The origin of the stacking faults was thought to be due to (011) B199 (or (011)A,B ) Type I twin plane of the adjacent variant pair. If the shearing stress induced by the twin plane was large enough, it could induce a new variant in the adjacent variant. It might be assumed that (011) B199 stacking faults in variant C, shown by an arrow in Fig. 2(c), was formed by the shearing effect of the nearby twin plane of A / B variant pairs at first. Then the faults may become wider and wider by the shearing mechanism of the martensitic transformation. In the upper part of Fig. 2(c), it can be seen that the right variant B 1 in variant A induced a new spear-like variant B 19 in variant C. The spear shape is energetically helpful in the transformation. The growth of the up variant B 19 stopped after it met variant D. Several ledges were observed in the interface of variant B 19 and C, with one or two atoms high along the [001] B or [010] B direction, with terraces a few atomic distances wide along the (011) B199 plane. Finally the two ends of variant B became the same width as variant B in variant A. According to the W–L–R theory, the (011) B199 Type I twin has the possibility in the phenomenological theory to produce a lattice invariant shear necessary for the martensitic transformation [12]. The above relationship was also observed in the substructure of a variant. Fig. 3 shows the HRTEM images of a single martensite variant with many diffraction spots including those in the inset of Fig. 2 (a),
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
149
Fig. 2. HRTEM image of orthogonal spear-like junction plane variants in a self-accommodation lath plate martensite group. (a) A low magnification image and the corresponding diffraction pattern. (b) and (c) HRTEM images of the framed areas in (a).
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Fig. 3. HRTEM images of a single martensite variant with one and several microtwins, shown respectively in (a) and (b).
which passed through an objective aperture. In Fig. 3(a), the microtwin was about 4 nm wide and formed a (011) B199 Type I twin relationship with the matrix variant. The interface was straight, coherent and composed of atomic scale ledges. These ledges are one or two atoms high, with terraces a few atomic wide along the (011) B199 plane. The blurred regions at the lower right part in Fig. 3(a), which may be caused by the stress concentration, exist mainly between the edge of the microtwin and the
matrix variant. The arrows in the matrix variant indicated some mismatches, which maybe was caused by other variants during the self-accommodation. In Fig. 3(b), the twin relationship was similar to the above variant A and C in Fig. 2(a) and the angle was about 968 (180–965848) between (001) B199 microtwin (i.e. (001) T ) and (001) B199 matrix variant (i.e. (001) M ). Another phenomenon is noteworthy, when the electron beam is parallel to [100] B199 , the substructure of (001) compound twin was
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
not observed in these self-accommodation variant groups as generally observed in other twin relationship variants [13]. Fig. 4(a) shows the electron micrograph of interface between the spear-like martensite (M) and the amorphous phase (A). The interface is straight but exhibits an irregular configuration. Fig. 4(b) is the corresponding EDP taken from the M area. The HRTEM image of framed area in Fig. 4(a) is given in (c). It shows that the interface is not smooth though the lattice fringes are continuous up to the interface and exhibit a ‘gas-like’ amorphous structure at the interface. Fig. 5(a) shows multiple growth stages between the
151
martensitic variant (M) and the amorphous phase (A). Fig. 5(b) is the corresponding EDP taken from the M area. An enlarged HRTEM image framed in Fig. 5(a) is shown in (c). The variant pair is also (011) B199 Type I twin related. The interface between the variant pair is straight and coherent. The interface between the martensite and the amorphous phase has an obvious zigzag configuration. The lattices were twisted as shown by the arrow. These features show the interface was in a high-energy and nonequilibrium configuration. EDX analysis in amorphous phase and martensite was carried out. It was found that the Ni content was about 4 at.% higher in the martensite region than in the amorphous phase. Ti was enriched by about 4 at.% in
Fig. 4. HRTEM interface image between spear-like martensite and the amorphous phase. (a) A low magnification image, (b) the corresponding EDP, (c) an enlarged HRTEM image from the framed area in (a).
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M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
Fig. 5. HRTEM interface image of multiple growth stages between the martensitic variant and the amorphous phase. (a) A low magnification image, (b) the corresponding EDP, (c) a HRTEM image from the framed area in (a).
the amorphous phase and the Hf content was almost the same. The results showed that the element distribution during melt–spinning is not homogeneous. From the above results, it can be reasonably assumed that in some areas, the parent phase with an ordered B2 structure formed first and the interface moved forward very quickly during the nonequilibrium rapid solidification. No time was left for the composition to become homogeneous before the liquid completely solidified. In the Ni poor areas, the phase transformation temperature further decreased even though the Hf content was the same, thus hindering crystallization and formation of the amorphous phase. Therefore, the martensite coexisted with some amorphous in the melt– spinning technique when the cooling rate was high enough. Such results were also observed in other NiTi–Hf ribbons [14].
4. Conclusion In a self-accommodating lath plate martensite group, the variants prefer to form an orthogonal morphology of spearlike martensite on both sides of melt–spun ribbons. Variants in the pairs A\B or C\D were of the (011) B199 Type I twin related with straight and coherent interface while the interface between the variants C and A or B is curved and incoherent with ledges in it. The planes of (011)A,B and (011) C,D were almost vertical to each other. In the orthogonal self-accommodation spear-like variants group, the main substructure in the variants of the group was the (011) B199 Type I microtwin and (011) B199 stacking faults. The interface between the amorphous phase and the martensite region showed an energetically unfavorable
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
nonequilibrium configuration. Some areas in the interface are straight but not smooth and exhibit an irregular configuration and some areas in the interface have an obvious zigzag configuration. EDX analysis of Hf 40 ribbons showed that the Hf content in the martensite region was the same as in the amorphous region but the Ni content was a little higher and the Ti content was a little lower in the martensite region than that in the amorphous region. The amorphous phase and martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 were due to the absence of austenite, the very small variant pairs and the coexisting amorphous phase.
Acknowledgements This work was sponsored by a special support fund of the president of the Chinese Academy of Sciences in 1999.
References [1] Krupp GmbH Essen, Patentschrift DE 4006076. CI. Fried (1990). [2] J.H. Muder, J.H. Mass, J. Beyer, ICOMAT-92, CA, USA, 1992.
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[3] Johnson Service Company, US Patent No. 5,114,514, Milwaukee, WI, 1992. [4] K.H. Wu, Z. Pu, H.K. Tseng, F.S. Biancaniello, in: A.R. Pelton, D. Hodgson, D. Duerig (Eds.), Proceedings of SMST-94, MIAS, Montery, 1995, p. 61. [5] P.L. Potapcv, A.V. Shelyakov, A.A. Gultyaev, E.L. Svistunova, N.M. Matveevaa, D.D. Hodgson, Mater. Lett. 32 (1997) 247. [6] Z.J. Pu, K.H. Wu, Y.Q. Liu, in: C. Youyi, T. Hailing (Eds.), Shape Memory Materials ’94, International Academic Publisher, Beijing, 1994, p. 292. [7] X.D. Han, W.H. Zou, R. Wang, Z. Zhang, D.Z. Yang, Acta Mater. 44 (1996) 3711. [8] Y.F. Zheng, L.C. Zhao, H.Q. Ye, Script Mater. 38 (1998) 1249. [9] K. Ofawa, T. Kikuchi, S. Kajiwara, T. Matsunnata, S. Miyazaki, J. Phys. IV. France 7, Collque C5, Supplement, au Journal de Physique III (1997) C5–221. [10] X.D. Han, W.H. Zou, R. Wang, Z. Zhang, D.Z. Yang, K.H. Wu. J. Phys. IV. Collque C8, Supplement, au Journal de Physique III, 5 (1995) C8–753. [11] X.D. Han, C.Y. Chung, R. Wang, Z. Zhang, J.K.L. Lai, D.Z. Yang, Mater. Trans. JIM 38 (1997) 842. [12] K. Yamauchi, M. Nishida, I. Ital, K. Kitamura, A. Chiba, Mater. Trans. JIM 37 (1996) 210. [13] Y.F. Zheng, W. Cai, J.X. Zhang, Y.Q. Wang, L.C. Zhao, H.Q. Ye, Mater. Lett. 36 (1998) 142. [14] M. Liu, X.M. Zhang, L. Lu, Y.Y. Li, A.V. Shellyakov, J. Mater. Sci. Lett. 19 (2000) 1383.
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www.elsevier.com / locate / jallcom
High-resolution transmission electron microscope (HRTEM) study of the transformation interface and substructure in NiTiHf 40 melt–spun ribbons a, b b a a M. Liu *, X.M. Zhang , Y.Y. Li , J.Z. Chen , M.J. Tu b
a Department of Metal Materials, Sichuan University ( West), Chengdu 610065, People’ s Republic of China The National Key Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’ s Republic of China
Received 26 January 2001; accepted 11 July 2001
Abstract The fine structures of self-accommodation spear-like martensite variant groups and the interface between the martensite region and the amorphous phase region in NiTi–Hf 40 melt–spun ribbons were studied by using high-resolution transmission electron microscopy (HRTEM). In the self-accommodation lath plate martensite group, the variants prefer to form orthogonal morphology of spear-like martensite on both sides of melt–spun ribbons. Some variant pairs were of the (011) B199 Type I twin related with straight and coherent interface while the interface between other variants is curved and incoherent. The main substructure in variants of the group was not the (001) B199 compound twin but the (011) B199 Type I microtwin and (011) stacking faults. The interface between the amorphous phase and the martensite region showed high energy and nonequilibrium configuration. Some area in the interface is straight but not smooth and exhibited an irregular configuration and some area in the interface has an obvious zigzag configuration. Energy-dispersive spectroscopy (EDX) analysis of the composition in NiTi–Hf 40 ribbon showed that the Ni and Ti content were different in the martensite region and in the amorphous phase. The amorphous phase and the martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 compared to those of lower Hf content NiTi–Hf alloys were due to the not retained austenite, the very small variant pair and the coexisting amorphous phase. 2002 Elsevier Science B.V. All rights reserved. Keywords: Transition metal alloys; Amorphous materials; TEM; Phase transition
1. Introduction With Hf addition, the NiTi-based alloys have been recognized as potential candidate material for high temperature shape memory alloys (HTSMAs) in recent years [1–5]. In the mechanical working process of the NiTi–Hf alloys, however, the major hindrance is their worse workability with increasing Hf addition [4]. Even the hot rolling process is not effective for producing NiTi–Hf ribbons with Hf contents higher than 20 at.% [6]. We have fabricated NiTi-based ribbons using the melt–spun method with Hf content higher than 40 at.%. We noticed that with increasing Hf addition, the reverse transformation temperature increased, some properties improved, but the ribbons became more brittle. Shape memory effect (SME) is generally associated with the thermoelastic martensitic transformation and selfaccommodation of martensite variants. The junction planes *Corresponding author. E-mail address: mm [email protected] (M. Liu). ]
between martensite variants and the variants’ combination play a very important role in the interface movement, as well as the variants coalescence when the specimen is deformed. In this paper, we report our direct observation of the substructure in a self-accommodation spear-like martensitic variant group and the interface structure between the martensite region and amorphous phase by high resolution transmission electron microscope (HRTEM) for a better understanding of NiTi–Hf melt–spun ribbons.
2. Experiment details High purity nickel, titanium and hafnium were melted six times using an argon arc button furnace to obtain Ni 49 Ti 11 Hf 40 (denoted as Hf 40 ) alloys. The melted alloy was rapidly quenched by melt–spinning using a Cu weal in purified atmosphere of inert gas at a cooling rate of about 10 7 K / s. The ribbon was about 40 mm thick and 2 mm wide. Foil samples were cut from the ribbon and thinned directly by ion milling method for HRTEM observation.
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01762-5
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
148
The electron microscope (JEOL-2010EX) was operated at an accelerating voltage of 200 kV with a point resolution of 0.197 nm. The composition analysis was done by ultra microanalysis with energy-dispersive spectroscopy (EDX) using the Cliff-Lorimer program in a field emission election mictoscope (HF-2000FEG) equipped with the EDX Oxford, ISIS-142 system.
3. Results and discussion Fig. 1 is a low magnification HRTEM image of the Hf 40 melt–spun ribbons. The corresponding electron diffraction pattern (EDP) showed diffraction spots from martensite superimposed on a typical amorphous ring, which indicated that the martensite embedded in the amorphous phase. The lath plate martensite or the spear-like martensitic variants [7] were dominantly observed in Hf 40 . According to the reports of Han et al. [7] and Zheng et al. [8] of bulk NiTi–Hf 15 (Hf 15 ) HTSMAs, there should be two kinds of self-accommodation variant groups, i.e. mosaic- and spear-like martensite junction plane variant groups. They were the cross-section and vertical section view of the same lath plate groups. Therefore, the mosaiclike junction plane variants should be dominantly observed in the cross section side of the ribbons. Fig. 2(a) shows a low magnification image of martensite region with an orthogonal spear-like junction plane variant group. The group contains two kinds of variant pairs, which were vertical to each other. The [100] B199 EDP from this area is shown in the inset of Fig. 2(a). The size difference of the two variant pairs was very large in the self-accommodation group. The variant was about 2–8 nm wide in one pair and about 40–100 nm in the other. In bulk Hf15, the width of the two variant pairs was almost the same and much wider than Hf 40 [7]. The small variant pair
Fig. 1. Martensite was embedded in amorphous phase.
in the self-accommodation group may act as a coherent phase that strengthened the ribbon [9]. Retained austenite diffraction spots were not found in the EDP inset of Fig. 2(a), but they were always observed in bulk Hf 15 along the same diffraction axis [7,10]. The absence of retained austenite diffraction spots were also noticed in other specimens randomly cut from the same Hf 40 ribbons, which implies that the martensitic transformation is completed in Hf 40 . The absence of austenite, the small variant pair and the coexisting amorphous phase, all contributed to the hardness and brittleness of Hf 40 being higher than in bulk Hf 15 . In order to see the details of the joining regions of these thin plates, the framed areas in Fig. 2(a) were enlarged and shown in Fig. 2(b) and (c). In these micrographs, we can see four variants, indicated by A, B, C and D. The variants A and B are (011) B199 / /(001) B2 Type I twin related [11]. The C and D variants are (011) B199 / /(010) B2 Type I twin related [7,8,11]. The variants in the same pair, i.e. A / B pair or C / D pair, have mirror symmetry with respect to the (011) B199 plane and the twin plane was straight and coherent, as shown in Fig. 2 (b) and (c), but the interface of adjacent variants, i.e. A and C, was curved and incoherent, as shown in Fig. 2(c). The plane angle is about 68 between (001) B199 of variants A and C and about 968 (or 180–965848) between (001) B199 of variants A and B, which were in good agreement with theoretical calculation of monoclinic structure projected in a (001) B2 stereograph [11]. (011) B199 stacking faults were observed and shown in Fig. 2(c). The origin of the stacking faults was thought to be due to (011) B199 (or (011)A,B ) Type I twin plane of the adjacent variant pair. If the shearing stress induced by the twin plane was large enough, it could induce a new variant in the adjacent variant. It might be assumed that (011) B199 stacking faults in variant C, shown by an arrow in Fig. 2(c), was formed by the shearing effect of the nearby twin plane of A / B variant pairs at first. Then the faults may become wider and wider by the shearing mechanism of the martensitic transformation. In the upper part of Fig. 2(c), it can be seen that the right variant B 1 in variant A induced a new spear-like variant B 19 in variant C. The spear shape is energetically helpful in the transformation. The growth of the up variant B 19 stopped after it met variant D. Several ledges were observed in the interface of variant B 19 and C, with one or two atoms high along the [001] B or [010] B direction, with terraces a few atomic distances wide along the (011) B199 plane. Finally the two ends of variant B became the same width as variant B in variant A. According to the W–L–R theory, the (011) B199 Type I twin has the possibility in the phenomenological theory to produce a lattice invariant shear necessary for the martensitic transformation [12]. The above relationship was also observed in the substructure of a variant. Fig. 3 shows the HRTEM images of a single martensite variant with many diffraction spots including those in the inset of Fig. 2 (a),
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
149
Fig. 2. HRTEM image of orthogonal spear-like junction plane variants in a self-accommodation lath plate martensite group. (a) A low magnification image and the corresponding diffraction pattern. (b) and (c) HRTEM images of the framed areas in (a).
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Fig. 3. HRTEM images of a single martensite variant with one and several microtwins, shown respectively in (a) and (b).
which passed through an objective aperture. In Fig. 3(a), the microtwin was about 4 nm wide and formed a (011) B199 Type I twin relationship with the matrix variant. The interface was straight, coherent and composed of atomic scale ledges. These ledges are one or two atoms high, with terraces a few atomic wide along the (011) B199 plane. The blurred regions at the lower right part in Fig. 3(a), which may be caused by the stress concentration, exist mainly between the edge of the microtwin and the
matrix variant. The arrows in the matrix variant indicated some mismatches, which maybe was caused by other variants during the self-accommodation. In Fig. 3(b), the twin relationship was similar to the above variant A and C in Fig. 2(a) and the angle was about 968 (180–965848) between (001) B199 microtwin (i.e. (001) T ) and (001) B199 matrix variant (i.e. (001) M ). Another phenomenon is noteworthy, when the electron beam is parallel to [100] B199 , the substructure of (001) compound twin was
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
not observed in these self-accommodation variant groups as generally observed in other twin relationship variants [13]. Fig. 4(a) shows the electron micrograph of interface between the spear-like martensite (M) and the amorphous phase (A). The interface is straight but exhibits an irregular configuration. Fig. 4(b) is the corresponding EDP taken from the M area. The HRTEM image of framed area in Fig. 4(a) is given in (c). It shows that the interface is not smooth though the lattice fringes are continuous up to the interface and exhibit a ‘gas-like’ amorphous structure at the interface. Fig. 5(a) shows multiple growth stages between the
151
martensitic variant (M) and the amorphous phase (A). Fig. 5(b) is the corresponding EDP taken from the M area. An enlarged HRTEM image framed in Fig. 5(a) is shown in (c). The variant pair is also (011) B199 Type I twin related. The interface between the variant pair is straight and coherent. The interface between the martensite and the amorphous phase has an obvious zigzag configuration. The lattices were twisted as shown by the arrow. These features show the interface was in a high-energy and nonequilibrium configuration. EDX analysis in amorphous phase and martensite was carried out. It was found that the Ni content was about 4 at.% higher in the martensite region than in the amorphous phase. Ti was enriched by about 4 at.% in
Fig. 4. HRTEM interface image between spear-like martensite and the amorphous phase. (a) A low magnification image, (b) the corresponding EDP, (c) an enlarged HRTEM image from the framed area in (a).
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M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
Fig. 5. HRTEM interface image of multiple growth stages between the martensitic variant and the amorphous phase. (a) A low magnification image, (b) the corresponding EDP, (c) a HRTEM image from the framed area in (a).
the amorphous phase and the Hf content was almost the same. The results showed that the element distribution during melt–spinning is not homogeneous. From the above results, it can be reasonably assumed that in some areas, the parent phase with an ordered B2 structure formed first and the interface moved forward very quickly during the nonequilibrium rapid solidification. No time was left for the composition to become homogeneous before the liquid completely solidified. In the Ni poor areas, the phase transformation temperature further decreased even though the Hf content was the same, thus hindering crystallization and formation of the amorphous phase. Therefore, the martensite coexisted with some amorphous in the melt– spinning technique when the cooling rate was high enough. Such results were also observed in other NiTi–Hf ribbons [14].
4. Conclusion In a self-accommodating lath plate martensite group, the variants prefer to form an orthogonal morphology of spearlike martensite on both sides of melt–spun ribbons. Variants in the pairs A\B or C\D were of the (011) B199 Type I twin related with straight and coherent interface while the interface between the variants C and A or B is curved and incoherent with ledges in it. The planes of (011)A,B and (011) C,D were almost vertical to each other. In the orthogonal self-accommodation spear-like variants group, the main substructure in the variants of the group was the (011) B199 Type I microtwin and (011) B199 stacking faults. The interface between the amorphous phase and the martensite region showed an energetically unfavorable
M. Liu et al. / Journal of Alloys and Compounds 334 (2002) 147 – 153
nonequilibrium configuration. Some areas in the interface are straight but not smooth and exhibit an irregular configuration and some areas in the interface have an obvious zigzag configuration. EDX analysis of Hf 40 ribbons showed that the Hf content in the martensite region was the same as in the amorphous region but the Ni content was a little higher and the Ti content was a little lower in the martensite region than that in the amorphous region. The amorphous phase and martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 were due to the absence of austenite, the very small variant pairs and the coexisting amorphous phase.
Acknowledgements This work was sponsored by a special support fund of the president of the Chinese Academy of Sciences in 1999.
References [1] Krupp GmbH Essen, Patentschrift DE 4006076. CI. Fried (1990). [2] J.H. Muder, J.H. Mass, J. Beyer, ICOMAT-92, CA, USA, 1992.
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