Comparative study of R-phase martensitic transformations in TiNi-based shape memory alloys induced by point defects and precipitates

We studied the influence of point defects (Fe) and precipitates (Ti3Ni4) on the characteristics of R-phase martensitic transformation by comparing the transport and thermal properties of as-quenched Ti50Ni46Fe4 and annealed Ti48.7Ni51.3 shape memory alloys. Both alloys undergo a weak first-order R-phase transformation with a small thermal hysteresis (less than 7 K) and non-zero transformation strain, suggesting the introduction of point defects and precipitates lead to a stable R-phase in these alloys due to the defects induced local lattice deformations. Furthermore, our study revealed that the transition temperature, transformation width, and transformation strain of the investigated R-phase TiNi-based alloys are strongly affected by the induced defects. As a result, the annealed Ti48.7Ni51.3 has a higher transition temperature than that of Ti50Ni46Fe4, as expected.