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

Abstract
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 ₄Ti 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 ₄Ti 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.