On the effect of aging on martensitic transformations in Ni-rich NiTi shape memory alloys
Smart Materials & Structures 14(5): S186-S191
We investigate the effect of 450 degrees 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-ray diffraction (XRD), neutron diffraction (ND) and differential scanning calorimetry (DSC). On cooling from the high temperature phase, DSC charts show two distinct peaks after short aging times, three peaks after intermediate aging times and two peaks again after long aging times (2-3-2 transformation behaviour). After 1.5 h 450 degrees C aging, three DSC peaks were obtained on cooling from the high temperature phase. The first peak on cooling represents the formation of R-phase and the second peak is associated with the formation of B 19'. Due to a macroscopic microstructural heterogeneity a second B 19' peak (overall third peak) occurs at lower temperatures. We discuss our results in the light of recent remarks on burst-like events during the growth of thermoelastic martensite and on the effect of oxidation on NiTi microstructures.