Precipitation kinetics of Ti₃Ni₄ in polycrystalline Ni-rich TiNi alloys and its relation to abnormal multi-stage transformation behavior

Normally, martensitic transformation occurs in two stages (B2-R-B19′) in aged Ni-rich TiNi alloys. But a three-stage martensitic transformation was also reported. The origin of this abnormal transformation behavior has remained controversial, and in particular it is difficult to understand why in some cases the three-stage transformation occurs while in other cases normal two-stage transformation appears. Very recently by considering the kinetics features of Ti₃Ni₄ precipitation in polycrystalline Ni-rich TiNi, we propose a general mechanism, which is able to explain not only abnormal three-stage transformation but also normal two-stage transformation. In the present study, we made further experimental effort to critically verify the proposed mechanism by studying the transformation behavior of low Ni-rich polycrystals and intermediate Ni-rich polycrystals. We have found that low Ni-rich polycrystals exhibit three-stage transformation and the number of transformation peaks is independent of aging time; intermediate Ni-rich samples exhibit three-stage transformation after short time aging and two-stage transformation after longer time aging; but all the high Ni-rich samples show two-stage transformation, being independent of aging time. These results are consistent with our mechanism 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.