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 mar tensitic 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 tran sformations in single crystals of the [110] and [111] orientations. Solutio nized and over-aged single crystals exhibited a strong orientation dependen ce 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 s ingle crystals demonstrated a much weaker orientation dependence of sigma(c r) and in general, the Schmid law was not obeyed. By considering the local stress fields outside of the semi-coherent precipitates, the decrease in th e orientation dependence of sigma(cr) was accounted for. The martensite sta rt temperatures, M-s, in aged single crystal and polycrystalline NiTi were much higher than in solutionized samples. In peak-aged NiTi the increase wa s primarily attributed to the local stress fields outside the coherent prec ipitates which create preferential nucleation sites for the martensite. In the over-used NiTi the increase in M-s was primarily attributed to the decr ease in the average Ni concentration of the matrix surrounding the coarsene d precipitates.