Review: on the structure and microstructure of quenched beta-brass type alloys

Quenched beta-phase alloys in systems based on the noble metals are often thermodynamically metastable. A review of the information derived from transmission electron microscopy indicates the presence of various anomalous contrast and diffraction effects in such alloys. Contrast effects may be referred to as mottling, striation, cross-hatching, etc., and they may change in intensity with respect to different extinction contours, foil thickness and foil tilting. Diffraction effects refer to diffuse streaking in certain directions, the appearance of additional maxima or their splitting, and slight deviations from cubic symmetry. The possible causes for the presence of such features are discussed in the present paper. Some effects are considered to be at least in part the result of surface rippling, caused by electropolishing, but other effects are associated with the presence of precipitates, or with atomic displacements from ideal bcc packing. A detailed analysis of the diffraction effects suggests that structural features must be present analogous to the hexagonal ω-phase (known in transition alloys), and the nearly hexagonal α′ phase (known in bainitic structures). The stability of the beta-phase alloys is considered in terms of the above features, concluding that both vibrational and electronic entropy terms may be contributing, and that instability at low temperatures may be manifested in a number of ways not all of which fit into conventional phase-transformation categories.