On the relation between Ni4Ti3-precipitation in Ni-rich NiTi shape memory alloys (during aging and creep) and martensitic transformations

The influence of aging and creep on the martensitic transformation of a Ti-50.7 at.%Ni shape memory alloy was investigated. The particle precipitation which occurs during aging of the solution annealed and water quenched material at 500°C is strongly affected by the presence of small external stresses (pre-loads). Stress free aging results in local precipitation near grain boundaries while aging under a small pre-load (2 MPa) results in a homogeneous precipitation of particles. The corresponding microstructures are associated with specific types of DSC charts. As compared to the solution annealed material, the material after 11 hours of creep (530°C, 120 MPa) and after stress free aging for 11 hours at 530°C exhibit an 18°C increase in phase transition temperatures. The accumulated creep strain influences the shape, spatial distribution and volume fraction of Ni$_4$Ti$_3$ precipitates; it does not strongly affect the transformation behavior. This experimental result is discussed in terms of the counteracting effects of Ni-depletion due to the precipitation of Ni$_4$Ti$_3$ particles and the formation of a dislocation substructure. Dynamic precipitation moreover has an effect on the shape of creep curves, which exhibit sharp creep rate minima at small creep strains

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 Ni$_4$Ti$_3$, 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 Ni$_4$Ti$_3$, 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