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 me mory alloys. Ti50Ni48Fe2 and Ti50Pd34Fe16, were studied extensively by ener gy-filtered electron microscopy. including in-situ observations, and high-r esolution electron microscopy. Energy-filtered dark-field images, where wea k 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 observ ations. It was found that Ti50Pd34Fe16 exhibited a domain-like structure si milar to that of Ti50Ni48Fe2, which was attributed to a transverse lattice displacement. High-resolution images of Ti50Pd34Fe16 showed that a domain w as coupled with other ones with different orientations of distortion. so as to reduce the total strain due to their formations. Furthermore. effects o f including a fundamental reflection. in addition to the diffuse scattering . on dark-field images were discussed based on the observations and the ima ge processing.