Thermal stability of γ′ phase in long-term aged Co-Al-W alloys

Abstract Three cast alloys with compositions of Co-9Al-8W-0.1B, Co-9Al-9W-0.1B and Co-9Al-11W-0.1B (all atomic percent) were aged at 900 °C for 200 h, 500 h, 1000 h, 2000 h, 5000 h and 10000 h to investigate the thermal stability of L12 type γ′-Co3(Al,W) precipitates. The microstructure during the extended ageing process was observed by Scanning and Transmission Electron Microscopy in order to investigate the evolution of the γ/γ′ two-phase microstructure and the formation of any additional phases. It was found after 5000 h ageing that four phases, γ, γ′, D019 and B2, were present within the microstructure as identified by X-ray Diffraction combined with Transmission Electron Microscopy and Electron Backscattered Diffraction. The γ′ precipitates coarsened according to modified Lifshitz, Slyozov and Wagner theory generally. The fraction of γ′ phase decreased while that of D019 phase increased contrarily. The phase transformation process from γ′-L12 to D019 was illustrated by Transmission Electron Microscopy. Stacking faults were identified as the initial step of the decomposition of the γ′ phase.