Precipitation of Co in Supersaturated Au-Based Au-Co Alloys; Microstructural Evolution and Transformation Kinetics

Abstract

Natural formation of the unusual bcc modification for Co was demonstrated to occur upon precipitation of Co in supersaturated Au90Co10 alloy, applying high resolution transmission electron microscopy. The bcc Co phase precipitates as thin (0.5 – 1 nm) plates along {100} habit planes, exhibiting the orientation relationship: {100}Au,fcc//{100}Co,bcc and <001>Au,fcc//<011>Co,bcc. Prolonged annealing induces a Bain type transformation from bcc Co into fcc Co. The precipitation kinetics of Co in supersaturated solid solutions of Au90Co10 was investigated by DSC upon isothermal annealing at temperatures in the range 567 K to 612 K and upon isochronal annealing at heating rates in the range 5 to 40 K·min‑1. Results of the fitting of a modular model of transformation kinetics to, simultaneously, all isothermal DSC runs and to, simultaneously, all isochronal DSC runs, could be discussed in terms of the different extents of the effects of quenched‑in defects on isothermally and isochronally conducted transformations.