Analysis of metastable structures in quenched Cr-Ti alloys

Abstract

Metastable water-quenched bcc Cr40Ti60 is reported to undergo solid state amorphization during sub-eutectoid temperature anneals. Electron diffraction of the metastable bcc phase reveals complex patterns of diffuse scattering indicating the presence of deviation, topological, chemical or both, from the ideally random solid solution. The hypothesis that the microstructure causing the diffuse scattering plays a role in the amorphization process is supported by studies in which an electron- irradiated bcc-based Cu-Ti alloy shows similar diffuse scattering just prior to irradiation-induced amorphization.Diffuse scattering is observed in all major zone axes of the as-quenched alloy except for (111). Analysis of the spatial frequencies of diffuse scattering maxima indicates that in spite of a clear directional relationship between the bcc matrix scattering and the diffuse scattering, at least some diffuse intensity maxima have spatial frequencies which are incommensurate with the bcc matrix. For instance, the streaked diffuse form along <11> in the (110) zone axis pattern (Fig. 1, arrow), which has been determined to be due to kinematical scattering, does not correspond to any possible bcc superlattice spacing along the [11] direction. This suggests that the microstructure of the as-quenched phase includes regions in which static displacements from the bcc equilibrium sites are encountered.