Effect of Heat Treatment Conditions on Multistage Martensitic Transformation in Aged Ni-rich Ti-Ni Alloys

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 Ti 3 Ni 4 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.

FIB/SEM Applied to Quantitative 3D Analysis of Precipitates in Ni-Ti

“…Fig. 1 shows a scheme developed by Nishida et al [16] and several secondary electron (SE) images of precipitates in these different regions. In the present study two different slice boxes are selected inside a single grain, one in the grain interior region (GI) with relatively large but few precipitates and the other one near the grain boundary (GB) with relatively small but many precipitates.…”

“…Also, the volume fraction of precipitates will influence the overall composition of the matrix, having a strong effect on the martensite transformation temperatures [10]. These days, the Ni 4 Ti 3 precipitates with heterogeneous microstructures concentrating around grain boundaries have been widely studied due to their special contribution on the MMT, however, the mechanism of this phenomenon is still controversial [3,9,[11][12][13][14][15][16]. In the present study, a Slice-and-View procedure in a focused ion beam (FIB)/SEM Dual-Beam system is applied to investigate a polycrystalline Ni 50.8 Ti 49.2 alloy with such heterogeneous Ni 4 Ti 3 precipitates, in order to reveal the different characteristics of the 3D configuration of precipitates in different regions of a single grain in this alloy.…”