The influence of aging on critical transformation stress levels and martensite start temperatures in NiTi: Part II - Discussion of experimental results

An experimental study was performed to determine the effect of aging on martensitic transformations in NiTi. Polycrystalline and single crystal NiTi ([100], [110], and [111] orientations) were both considered. Stress-induced transformations in polycrystalline NiTi were found to closely resemble transformations in single crystals of the [110] and [111] orientations. Solutionized and over-aged single crystals exhibited a strong orientation dependence of the critical stress required to trigger the transformation, sigma(cr). The Schmid law Has able to accurately predict the orientation dependence of sigma(cr) in the solutionized and over-aged single crystals. Peak-aged single crystals demonstrated a much weaker orientation dependence of sigma(cr) and in general, the Schmid law was not obeyed. By considering the local stress fields outside of the semi-coherent precipitates, the decrease in the orientation dependence of sigma(cr) was accounted for. The martensite start temperatures, M-s, in aged single crystal and polycrystalline NiTi were much higher than in solutionized samples. In peak-aged NiTi the increase was primarily attributed to the local stress fields outside the coherent precipitates which create preferential nucleation sites for the martensite. In the over-used NiTi the increase in M-s was primarily attributed to the decrease in the average Ni concentration of the matrix surrounding the coarsened precipitates.