Lattice modulation preceding to the R-phase transformation in a Ti₅₀Ni₄₈Fe₂ alloy studied by TEM with energy-filtering

It is known that the cubic (parent phase) to trigonal (R-phase) martensitic transformation is accompanied by some precursor phenomena in the parent state, such as appearance of weak diffuse scattering, although the mechanism is not clear. In the present work, the structural modulation in the parent phase was analyzed both in the real and reciprocal spaces by electron microscopy with a recently developed omega-type energy-filter and imaging plates. Some essential differences in characters were observed between the diffuse scattering in the parent phase and the superlattice reflection in the R-phase, such as in the extinction rules and peak positions etc. We presumed that the diffuse scattering originated from a transverse-type lattice modulation, and the modulated regions in the parent phase had a different crystal structure from the R-phase. Dark-field images with the diffuse scattering showed a characteristic domain-like morphology. By considering these observations, the presence of variants for the lattice modulation was proposed.