Micro and macromechanical observation of polycrystalline NiTi using in situ optical microscopy

An experimental investigation of the micro and macromechanical transformation behavior of binary NiTi was undertaken using in situ optical microscopy. Special attention was paid to macroscopic banding, variant microstructure, effects of cyclic loading, strain rate and temperature effects. The experiments were accomplished with a custom-built loading stage designed to allow simultaneous loading and viewing of transformation behavior on the specimen surface with an optical microscope. The results show clearly that martensitic transformation occurs throughout the material at all strain levels. Macroscopic bands are regions of more intense transformation, but areas outside the bands are not martensite-free. The bands themselves are shown to contain regions (stripes) of higher and lower transformation, especially in the earlier transformation stages. Even at full transformation of the specimen, our results show that a polycrystalline NiTi material is far from being 100% martensitic. Low level cyclic loading of the NiTi specimens was also pursued which revealed significant microstructural changes in the material after as few as 10 cycles. Furthermore was observed that the variants activated remained the same at the various strain rates tested, however at the highest strain rate possible in our set-up, small redistributions of martensitic plates within grains were seen after loading.