On the role of chemical and microstructural heterogeneities in multistage martensitic transformations

The present paper considers chemical and microstructural reasons for unusual multiple step transformations as observed using differential scanning calorimetry (DSC) for Ni-rich NiTi-alloys on cooling from the B2 regime. In a simplified two-particles/matrix system a multiple step transformation can be explained based on two elements: (1) The composition inhomogeneity which evolves during aging as Ni₄Ti₃ precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19′ (large). These two elements explain the features of the evolution of DSC charts during aging, which change from two transformation peaks on cooling after short aging times, to three after intermediate aging times and finally again back to two peaks after long aging times (2-3-2 transformation behavior). Another reason for multiple step transformations in Ni-rich NiTi alloys may be that Ni₄Ti₃ particles precipitate heterogeneously near grain boundaries. Then the first two transformation peaks can be attributed to the formation of R-phase and B19′ in the grain boundary regions while the third peak corresponds to a transformation of B2 to B19′ in the precipitate free interior of the grains.