Anti-precursor effect of Fe on martensitic transformation in TiNi alloys

Abstract

The lack of information about point defects presents a big hurdle to a complete understanding of the premartensitic phenomena in shape memory alloys. Using first-principles density functional theory calculations, we have studied the behavior of substitutional Fe in both B2 and R phases of Ti50Ni50−xFex, at different concentrations. We show that with no need of aggregation, an Fe atom can incur a drastic atomic scale local lattice distortion in the R phase and makes its nearest neighbor environment like an intermediate structure between B2 and R. In addition, its solution energy is lower in B2 than in R phase, indicating that Fe tends to stabilize the austenite against the martensite. This means that Fe exerts an anti-precursor effect in the B2 to R transformation, in contrast to the assumption that Fe fosters the growth of R-like nanodomains. The Fe–Fe interaction is very weakly attractive in both phases, suggesting that only short-range variation in concentration is possible and the formation of precipitates is unlikely. The difference in cohesive energy of B2 and R phases, and the martensitic transformation temperature as well, decreases with the increasing Fe concentration, until the Fe content exceeds a critical level at which point the R phase turns into B2 phase automatically and the martensitic transformation is suppressed altogether. Such a complete suppression is an excellent analogue to the concept of frozen strain glass. Since Fe atoms at different Ni sites in R phase have divergent solution energy, the anti-precursor effect of Fe may vary at different spot in the material, and so will be the local transformation temperature.