Diffuse electron scattering and “streaming” effects

Abstract

Various anomalies are found in the parent phase of some alloys (especially B2 and DO3 type β-phase alloys) which undergo martensitic transformations, including lattice softening, streaks, weak diffuse reflections of the 2H-type (or ω-like) an X phase, and “streaming”. These are often referred to as premonitory effects prior to a subsequent martensitic transformation, without clear evidence. This chaotic situation results because various phenomena occur concurrently in these materials, which makes it difficult to isolate individual effects. A critical discussion of these phenomena is presented from an experimental point of view. On the nature of the diffuse streaks and diffuse patterns in β-phase alloys, the presence of intense *β rel rods has been confirmed unambiguously. From the direction of the rel rods and conditions for their extinction, it is concluded that they result from the presence of a low frequency transverse acoustic phonon mode, * β. A possible relation between the rel rods and a subsequent transformation is considered. In contrast to earlier reports, the streaming effect has now been detected in a variety of materials, including those which do not undergo transformations. Thus, it is clear that streaming does not have a direct relation with transformations. A possible origin of this effect is discussed and compared with observed characteristics. It is also shown that the 2H type phase is not a premartensitic one, but rather another metastable phase which competes with a subsequent martensitic transformation. Overall, it is concluded that no clear evidence has been obtained as yet in electron microscopy and diffraction to show a truly premonitory effect for a subsequent martensitic transformation.