Effect of Heat Treatment Conditions on Multistage Martensitic Transformation in Aged Ni-rich Ti-Ni Alloys
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Effect of Heat Treatment Conditions on Multistage Martensitic Transformation in Aged Ni-rich Ti-Ni Alloys Minoru Nishida, Toru Hara1, Yasuhiro Morizono, Mitsuhiro Matsuda and Kousuke Fujishima Department of Materials Science and Engineering, Kumamoto University, 2-39-1 Kurokami ,Kumamoto 860-8555, Japan 1 National Institute for Materials Science, 1-2-1Sengen, Tukuba 305-0047, Japan ABSTRACT It has been demonstrated with systematic experiments that the appearance and disappearance of multistage martensitic transformation (MMT) in aged Ni-rich Ti-Ni alloys depend on the heat treatment conditions. No multistage transformation occurs when the evaporation of Ti and Ni and/or the preferential oxidation of Ti in the specimen are prevented and the purification of heat treatment atmosphere in an evacuated quartz tube is achieved. The heterogeneity in precipitation morphology of the Ti3Ni4 phase, which is responsible for the multistage transformation can be suppressed with the regulation of heat treatment atmosphere as mentioned above. We have concluded that the multistage martensitic transformation in aged Ni-rich Ti-Ni alloys is extrinsic in nature, and is an artifact during the heat treatment.
INTRODUCTION Ti-Ni alloys of the near-equiatomic composition are technologically important materials with their superior shape memory and superelastic properties associated with B2 to R-phase and B19’ martensitic transformations. The application of the alloys has been spread to not only engineering but also medical and dental fields. It is widely recognized that improvements of shape memory and mechanical properties in the alloys are achieved by thermomechanical and aging treatment. Especially, the aging treatment is an effective process in Ni-rich Ti-Ni alloys due to precipitation strengthening of the parent phase with coherent Ti3Ni4 phase. It has been known that the aging treatment induces the R-phase transformation. In addition to the generation of R-phase transformation, it has been also reported that the MMT appears in the aged Ni-rich Ti-Ni alloys. Four mechanisms of multistage transformation have been proposed recently [1-4]. Bataillard et al. have ascribed it to coherent stress fields around Ti3Ni4 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 B19’ martensitic phase [2]. The latest 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 Ti3Ni4 phase is responsible for the multistage transformation from TEM observations [3]. Fan et al. have ascribed it to a result of competition between preferential grain boundary precipitation of Ti3Ni4 particles and a tendency for homogeneous precipitation when supersaturation of Ni is large [4]. We do not intend to argue against those proposed mechanisms, because they have been discussed self-consistently within each
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