THE EFFECT OF STRESS ON SOFT MODES FOR THE PHASE-TRANSFORMATION IN A TI-NI ALLOY .2. EFFECTS OF AGING AND THERMAL CYCLING ON THE PHASE-TRANSFORMATION

The softening of phonon modes is sensitive to stress. Both the internal tensile stress formed by Ti11Ni14 precipitates in Ni-rich Ti-Ni alloy and the internal stress introduced by thermal cycling in equiatomic Ti-Ni alloy have strong effects on the softening of phonon modes, especially on phonons with <111> polarization vectors which are closely related to the R-phase transformation. The increase in the amount of R phase is the main effect of ageing and thermal cycling Ti-Ni alloy; this results in remarkable improvement in the shape memory effect and the pseudo-elasticity of the material. The negative effect of a compressive stress on the R-phase transformation has been experimentally confirmed.