Observations of Thin Crystals of TiNi in Premartensitic States

Abstract

Thin films of TiNi have been examined in the temperature range 0°–200°C, using transmission electron microscopy and selected-area diffraction. The CsCl diffraction patterns are accompanied by strong localized diffuse scattering, which becomes more pronounced as the temperature is reduced, approaching that of a phase transformation. A series of experiments designed to determine the directions of maximum diffusely scattered intensity suggest that these are 〈210〉 and 〈321〉, and that scattering is largely confined to reciprocal lattice directions subtending small angles with the incident electron beam. Since the phase transformation is thought to be of the second kind, and diffusionless, the results are interpreted in terms of large-amplitude, short-wavelength atomic displacements, reflecting incipient mechanical instability. Several diffraction patterns and dark-field photomicrographs are presented to indicate the scale of the domain structure of the transformation product.