Electron diffraction study of n/7<123> diffuse scattering from Ti_48·5Ni_51·5alloy

Abstract

Diffuse scattering from TiNi alloys of near equi-atomic composition is investigated by electron diffraction, and several high-resolution images produced by diffuse scattering together with some Bragg reflections are presented. Three characteristic features of diffuse scattering are as follows. (1) The maxima of the diffuse scattering are located near n/7, 2n/7, 3n/7 (n an integer) and equivalent positions. The positions can be understood if eight variants of the X-phase Ti₁₁ Ni₁₄ exist in a parent phase (cubic B2) at the same time. (2) A ring-type diffuse scattering observed in the h0l reciprocal plane reversibly transforms at a low temperature (∼120K) into a particular pattern which has diffuse maxima near n/3, 0, n/3 (n an integer), where the rhombohedral phase shows the superstructure reflections. (3) All the diffuse scattering disappears in the electron diffraction pattern when the electron beam is incident parallel to the <111> direction. From high-resolution images it is concluded that the diffuse scattering originates not from limited areas such as micro-domains but from rather wide areas which are homogeneously distributed in the specimen. In order to interpret these experimental results, an idea of transverse displacement waves occurring in crystals is used.