Precursor effects of martensitic transformations in Ti-based alloys studied by electron microscopy with energy filtering.

Precursor effects of martensitic transformations in two well-known shape memory alloys, Ti50Ni48Fe2 and Ti50Pd34Fe16, were studied extensively by energy-filtered electron microscopy, including in-situ observations, and high-resolution electron microscopy. Energy-filtered dark-field images, where weak diffuse scattering was utilized, clearly showed the microstructure in the premartensitic state of Ti50Ni48Fe2. Tiny domains observable in this state were close to spherical rather than thin and slender, and the temperature dependence of the domain-like structure was clarified by the in-situ observations. It was found that Ti50Pd34Fe16 exhibited a domain-like structure similar to that of Ti50Ni48Fe2, which was attributed to a transverse lattice displacement. High-resolution images of Ti50Pd34Fe16 showed that a domain was coupled with other ones with different orientations of distortion, so as to reduce the total strain due to their formations. Furthermore, effects of including a fundamental reflection, in addition to the diffuse scattering, on dark-field images were discussed based on the observations and the image processing.