Stress-assisted nucleation and growth of γ″ and γ′ precipitates in a Cu-1.2wt%Be-0.1wt%Co alloy aged at 320°C

摘要

An investigation by high-resolution transmission electron microscopy of the precipitation process during ageing a Cu-1.2wt%Be-0.1wt%Co alloy at 320°C has revealed that the transition phases follow a γ″ → γ″+ γ′ → γ sequence. The γ′ phase heterogeneously precipitates on the γ″ phase. The effects of an external stress on the nucleation and growth of disc-shaped γ″ and plate-shaped precipitates have been examined for the alloy aged at 320°C. A compressive stress applied in the [001] direction during ageing preferentially accelerates the nucleation and growth of the γ″ variant normal to the [001] axis among three crystallographically equivalent variants and the specific four γ′ variants formed on the γ″ variant normal to the [001] axis. A tensile stress does not significantly affect those of and precipitates. The critical diameter of the disc-shaped γ″ nucleus is estimated as about 1nm from evaluation of the interaction energy between the applied stress and the misfit strains γ″ of precipitates. It is proposed that applied external stress does not affect the diffusion rate but the interphase boundary velocity.