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.