The R-phase transformation in a Ti50Ni48Fe2 alloy was studied by in-situ electron microscopy with double-tilt cooling stages and an energy-filter, and the controversial incommensurability in the early stage of the transformation was examined by using imaging plates, which have high sensitivity and are easy to access to computer analysis. As a result, the transformation was shown to proceed in the following way. The R-phase transformation starts at Rs point, where the electrical resistance starts to increase, from the B2 parent phase with weak diffuse scattering to the commensurate R-phase with a sharp interface. Thus, the transformation is a first order throughout, and the temperature hysteresis represents the two-phase region of the B2 parent and the commensurate R-phase.
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