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 inpolycrystalline 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, σ_cr. The Schmid law was able to accurately predict the orientation dependence of σ_cr in the solutionized and over-aged single crystals. Peak-aged single crystals demonstrated a much weaker orientation dependence of σ_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 σ_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-aged NiTi the increase in M_s was primarily attributed to the decrease in the average Ni concentration of the matrix surrounding the coarsened precipitates.