Origin of abnormal multi-stage martensitic transformation behavior in aged Ni-rich Ti-Ni shape memory alloys

After aging Ni-rich Ti-Ni alloys exhibit finely dispersed Ti ₃Ni₄ particles embedded in B2 matrix. Under such condition the B2 matrix normally undergoes two-stage martensitic transformation B2-R-B19′. However, there is also evidence of three-stage transformation. The origin of such abnormal three-stage transformation remains controversial. In the present study, we made a comparative study between single crystals and corresponding polycrystals and found that all single crystals exhibit normal two-stage transformation, being independent of Ni content. By comparison, polycrystals with low Ni content (50.6Ni) show three-stage transformation, but those with high Ni content (51.5Ni) again exhibit normal two-stage transformation. These new findings are consistent with a simple scenario that different transformation behaviors are a result of competition between preferential grain-boundary precipitation of Ti₃Ni₄ particles and a tendency for homogeneous precipitation when supersaturation of Ni is large. This is a natural consequence of the nucleation kinetics of supersaturated solid solutions in polycrystalline state. The above scenario not only explains the abnormal three-stage transformation but also the normal two-stage transformation.