Materials Transactions, Vol. 47, No. 3 (2006) pp. 645 to 649 Special Issue on Shape Memory Alloys and Their Applications #2006 The Japan Institute of Metals
Effect of Heat Treatment Atmosphere on Multistage R-Phase Transformation in an Aged Ti–51.0 at%Ni Alloy Minoru Nishida1; *1 , Kentarou Ishiuchi1; *2 , Kousuke Fujishima1; *3 and Toru Hara2 1 2
Department of Materials Science and Engineering, Kumamoto University, Kumamoto 860-8555, Japan National Institute for Materials Science, Tsukuba 305-0047, Japan
The present study systematically investigates the effect of heat treatment atmosphere on the multistage R-phase transformation (MRT) in an aged Ti–51.0 at%Ni alloy. No MRT occurs when the heat treatments were completed under the regulated atmosphere. On the other hand, the MRT is observed in the specimen heat treated under the unregulated atmosphere. It is apparent from transmission electron microscope observations that the first and the second transformations take place around the grain boundary and at the grain interior, respectively. We conclude that the MRT is extrinsic, and is an artifact during the heat treatment, rather than intrinsic in nature. (Received September 20, 2005; Accepted November 7, 2005; Published March 15, 2006) Keywords: titanium nickel alloy, multistage R-phase transformation, heat treatment atmosphere, aging
1.
Introduction
The aging treatment is effective for the improvement of shape memory and superelastic properties in Ni-rich Ti–Ni alloys due to the precipitation strengthening with coherent Ti3 Ni4 particles. It has been recognized that the aging treatment induces the R-phase transformation. In addition to the R-phase transformation, the multistage martensitic transformation (MMT) appears in the aged Ni-rich Ti–Ni alloys. Four mechanisms of the MMT have been proposed recently.1–4) Bataillard et al. have ascribed it to coherent stress fields around Ti3 Ni4 precipitates.1) Khalil-Allafi et al. have explained it on the basis of evolving Ni concentration profiles between particles and differences in nucleation barriers between R-phase and B190 martensitic phase.2) The modified report of Khalil-Allafi et al. has pointed out that the above two mechanisms cannot rationalize the multistage transformation, and thus the heterogeneity in precipitation morphology of Ti3 Ni4 phase around the grain boundary and at the grain interior is responsible for the multistage transformation from transmission electron microscope (TEM) observations.3) Fan et al. have ascribed it to a result of competition between preferential grain boundary precipitation of Ti3 Ni4 particles and a tendency for homogeneous precipitation when supersaturation of Ni is large.4) The common feature of the microstructure in these four mechanisms is the heterogeneity in precipitation morphology of Ti3 Ni4 phase. We do not intend to argue against those proposed mechanisms, because they have been discussed self-consistently within each experimental condition. However, no reports have been taken into account the heat treatment atmosphere except for the present authors, since we have experienced that the MMT is remarkably influenced by heat-treatment conditions, especially heat-treatment atmosphere in the solution treatment at high temperature.5) No MMT occurs when the evaporation of Ti and Ni and/or the *1Corresponding
author, E-mail:
[email protected] Student, Kumamoto University, presently Graduate Student, Tokyo Institute of Technology *3Graduate Student, Kumamoto University *2Undergraduate
preferential oxidation of Ti in the specimen are prevented and the purification of heat treatment atmosphere is achieved. The heterogeneity in precipitation morphology of Ti3 Ni4 phase, which is responsible for the multistage transformation as mentioned above, can be suppressed with the regulation of heat treatment atmosphere. In addition to the MMT, the appearance of MRT in Ti–50.9 at%Ni alloy aged below 573 K has been reported recently and investigated comprehensively by Kim et al.6) They conclude that the first R-phase transformation takes place in ‘‘affected’’ zones, i.e., around Ti3 Ni4 precipitates and the second one occurs in ‘‘unaffected’’ matrix, i.e., away from precipitates. Consequently, the occurrence of the MRT is attributed to precipitation-induced heterogeneity of the matrix, both in terms of composition and of internal stress fields. However, there is no direct evidence of the ‘‘affected’’ zone and the ‘‘unaffected’’ matrix in their TEM observations, although they comment that the term ‘‘unaffected’’ is only comparative to the ‘‘affected’’ zone. Besides the mechanism of the MRT, one can easily imagine that the appearance of the MRT also depends on the heat treatment atmosphere as well as the MMT. The purpose of the present study is to clarify the effect of heat treatment atmosphere on the MRT in an aged Ni-rich Ti– Ni alloy. TEM observations are also carried out to investigate the correspondence between transformation regions and multi peaks in the DSC curve. 2.
Experimental Procedure
Ti–51.0 at%Ni alloy was prepared from 99.7 mass% sponge Ti and 99.9 mass% electrolytic Ni by a highfrequency vacuum induction furnace using a graphite crucible, followed by casting into an iron mold. The ingot was hot-forged and drawn to rod of 3 mm in diameter. A part of the forged ingot was rolled to sheet of 0.5 mm in thickness. The rod was cut into disks of about 1 mm in thickness for DSC measurements and about 0.2 mm in thickness for TEM observations, respectively. The sheet was used for the cover plate of DSC and TEM specimens as described below. Four types of heat treatment conditions were examined in the present study are schematically illustrated in Fig. 1. In the
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condition A, the specimens were solely sealed in the evacuated quartz tube of 2:5 103 Pa. The top and bottom surfaces of specimens were covered with a pair of rolled Ti– Ni sheets in the evacuated quartz tube in the condition B. In the condition C, the specimens were wrapped with pure Ti foil of 200 mm in thickness as getter material in the evacuated quartz tube. The rest were sandwiched between the rolled sheets and then wrapped with pure Ti foil in the evacuated quartz tube, which is referred to as the condition D. They were solution-treated at 1073 K for 3.6 ks, and then quenched into ice water. Subsequently, the specimens were aged at 473 K for various periods from 36 to 1800 ks, since the Rphase appears in the Ti–50.9 at%Ni alloy aged at 473 K for 36 ks or more in the comprehensive work of Kim et al.6) For instance, the specimen A–A described in the later section indicates that both the solution treatment and the aging are completed in the condition A. Differential scanning calorimetry (DSC) measurements were performed by using a Shimadzu DSC-50 calorimeter with a cooling and heating rate of 10 K/min. The temperature range measured was about 170 to 350 K. TEM specimens were electropolished using the twin jet method in an electrolyte of 20% HNO3 and 80%CH3 OH about 250 K. TEM observations were carried out in the JEOL-2000FX microscope operated at 200 kV. 3.
Results and Discussion
Figures 2(a) to (d) show DSC cooling curves of the solution treated specimens with conditions A to D, respectively. There is no difference between the four curves essentially. The peak temperature is about 225 K irrespective of the heat treatment atmosphere. The single exothermic peak is due to the martensitic transformation, since the thermal hysteresis between heating and cooling processes was
relatively large, about 30 K in comparison to that of the Rphase transformation described below. Figure 3 shows DSC cooling curves of each specimen aged at 473 K for various periods. The MRT denoted as R1 and R2 takes place in the specimens A–A and B–B aged for 72 ks or more as shown in (a) and (b), respectively. It is most likely that both R1 and R2 peaks are attributable to the R-phase transformation, since their thermal hysteresis is small about 5 K in the incomplete and the complete thermal cycle experiments. The transformation behavior is essentially the same in these two specimens. In the specimen C–C aged for 72 ks or more the MRT is also observed. However, the peak area of R2 is relatively smaller than that in the specimens A–A and B–B as discussed later in Fig. 4. On the other hand, there is no MRT in the specimen D–D for any aging periods within the present experiment as seen in (d). It is apparent that the MRT is also suppressible with the regulation of heat treatment atmosphere as well as the MMT. The Ms temperature of all the specimens aged for 36 to 1800 ks is not detected irrespective of the heat treatment atmosphere within the temperature range measured in the present experiment. The decrease in Ms is probably caused by the fine and dense Ti3 Ni4 particles which obstruct the shape change for martensitic transformation.7–9) Figure 4 shows the latent heat of transformation estimated from the DSC cooling curve in the specimens A–A to D–D aged at 473 K for 720 ks. The latent heat of the summation of R1 and R2 transformations in the specimens A–A to C–C and that of single transformation in the specimen D–D are nearly the same about 160 J/mol (3.1 J/g) which is comparable to that in the previous reports.10) This fact suggests that the R-phase transformation in the aged Ni-rich Ti–Ni alloys is a single step in nature the same as that in ternary alloys such as Ti– Ni–Fe11) and Ti–Ni–Al.12) When comparing the condition B with the condition C, the purification of heat treatment
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atmosphere with the getter material is more effective to suppress the MRT than the prevention of evaporation of Ti and/or Ni with the Ti–Ni sheet of the same composition. In
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other words, it is likely that interstitial impurity elements such as oxygen and nitrogen induce the MRT. Consequently, a kind of compositional and/or microstructure modifications is induced during the solution treatment with conditions A to C and is emphasized during aging, although the detailed mechanism has not been clarified yet. Figure 5 shows DSC heating curves of the specimens A–A and D–D aged at 473 K for 720 ks. As apparently from the above results, two endothermic peaks are detected in the curve of the specimen A–A denoted as AR1 and AR2 as expected from the above results. On the other hand, there is a single peak in the curve of the specimen D–D. The electropolishing for TEM specimens and the observations are carried out around 250 K and room temperature as bounded by solid and broken lines, respectively. One can expect that the R-phase corresponding to the R1 peak is observed in the specimen A–A, while the R-phase developed in the whole area is probably seen in the specimen B–B. Figure 6 shows
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bright image and corresponding electron diffraction patterns in the specimen A–A. The R-phase with dark contrast is only observed around the grain boundary as seen in (a), which can be confirmed by the electron diffraction patterns in (b) and (c) taken from the areas B and C in (a), respectively. The characteristic 1/3 h110iB2 superlattice reflections of the Rphase can be seen in (b).11) Such superlattice reflections cannot be recognized and the spots derived from B2 structure is only seen in (c). However, the intensity of 1/7 h321iB2 13–15) reflections derived from Ti3 Ni4 precipitates is too weak to reproduce in the patterns. It is apparent that the R-phase transformation due to the R1 peak takes place around the
grain boundary, and that corresponding to the R2 peak probably appears at the grain interior. Figure 7(a) shows low magnification dark field image of a crystal grain in the specimen D–D. Bright field images in Figs. 7(b) and (c) are enlarged micrographs at the areas B and C in (a). As expected in Figs. 3(d) and 5, the R-phase with needle-like morphology is recognized in the whole grain. These observations suggest that the MRT is induced by the macroscopic heterogeneity between around the grain boundary and inside the grain in the specimens aged under unregulated atmosphere, rather than the microscopic or nanoscopic heterogeneity around the precipitates defined as the ‘‘affected’’ and the ‘‘unaffected’’ zones by Kim et al.6) From these results it is considered that a kind of compositional fluctuation is induced during the solution treatment, when the specimen was sealed in an evacuated quartz tube under unregulated atmosphere. Then the fluctuation is emphasized during the subsequent aging treatment. The heterogeneity of internal stress fields may also occur during the precipitation. Consequently, the MRT induces in the specimen heat treated without the regulation of atmosphere. Although the mechanism and process of compositional fluctuation during heat treatment have not been clarified yet, it may be related to the evaporation of Ti and Ni and the contamination of interstitial impurity elements. The latter is more effective than the former as discussed in Figs. 3 and 4. The precise analysis around the grain boundary and inside the grain is required to confirm this hypothesis. It is now under study and will be reported in due course.
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4.
Conclusions
In the present study we have demonstrated that the appearance and disappearance of the MRT in aged Ni-rich Ti–Ni alloys depend on the heat treatment atmosphere. No MRT occurs when the evaporation of Ti and/or Ni is prevented and the purification of heat treatment atmosphere is achieved. The R1 and R2 transformations in the MRT take place along the grain boundary and at the grain interior, respectively, in the specimens aged under unregulated atmosphere. We concluded that the MRT are extrinsic in nature, and is an artifact during heat treatment in aged Ni-rich Ti–Ni alloys, rather than intrinsic in nature. Acknowledgements This work was supported by Grant-in-Aid for Scientific Research (B) from Japan Science Promotion Society, COEdirecting Research Program B at Kumamoto University on Nano-space Electrochemistry and Special Program at Kumamoto University for Promoting Research on Nanospace Electrochemistry.
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