Verification of the ability of Cu to dissolve in BCC delta in a delta-gamma Solid Solution above 1200 degrees C and boosting delta nano-hardness in Cu-containing PHSS

Because Cu is an austenite gamma stabilizer, it is thought that Cu only dissolves in the FCC gamma phase but not in the BCC delta when Cu-containing PHSS are treated in a dual delta-gamma solid solution at high temperature. However, the nano precipitation of Cu in the allotriomorphic delta found upon quenching from 1295 degrees C. To study this unexpected phenomenon, thermodynamic calculations were performed. The predicted solubilities of Cu in the gamma and delta at 1295 degrees C were 3.52 and 1.99 wt%. Consequently, quenching the solid solution from 1295 to 25 degrees C results in the ultra-supersaturation of Cu in the delta. The vacancy concentration in the delta solution was estimated to change from 10(24) vac./m(3) at 1295 degrees C to 10(5) vac./m(3) at 25 degrees C; quenching yields excess vacancies. Thus, Cu-rich nanoparticles precipitate in the delta matrix through the interdiffusion of the ultra-supersaturated Cu and excess vacancies. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.