On the effect of aging on martensitic transformations in Ni-rich NiTi shape memoryalloys

Abstract

We investigate the effect of 450 °C aging on the microstructure and on the martensitic transformations in a Ni-rich (50.8 at.%Ni) NiTi shape memory alloy using transmission electron microscopy (TEM), x-raydiffraction (XRD), neutron diffraction (ND) and differential scanning calorimetry(DSC). On cooling from the high temperature phase, DSC charts show two distinctpeaks after short aging times, three peaks after intermediate aging times and twopeaks again after long aging times (2–3–2 transformation behaviour). After 1.5 h450 °C aging, three DSC peaks were obtained on cooling from the high temperature phase. The first peak oncooling represents the formation of R-phase and the second peak is associated with the formation ofB19′. Due to a macroscopic microstructural heterogeneity a secondB19′ peak (overall third peak) occurs at lower temperatures. We discuss our results in the lightof recent remarks on burst-like events during the growth of thermoelastic martensite andon the effect of oxidation on NiTi microstructures.