Microstructures of Co–Re–Cr, Mo–Si and Mo–Si–B high-temperature alloys

Abstract

The present exploratory study considers microstructures of two potential high-temperature alloy systems, based on the high-melting-point base alloys Co–Re–Cr and Mo–Si/Mo–Si–B. Flat Co–Re–Cr samples were produced by arc-melting (ingot metallurgy) whereas the Mo–Si and Mo–Si–B samples were produced following a powder metallurgical processing route involving consolidation and aging under hot isostatic pressure. Transmission electron microscopy was carried out using a FEI Tecnai F20 to characterize the microstructures of a number of specific alloys: Co–17Re–23Cr–2.6C, Co–17Re–23Cr–2.6C–1.2Ta, Co–31Re–23Cr, Mo–1.5Si, Mo–1.5Si–1Zr and Mo–9Si–8B–1Zr (numbers indicate at.%). For the Co–Re–Cr samples with the lower Re-content, electron diffraction analysis reveals a hexagonal ɛ(Co,Re,Cr) matrix with plate-like precipitates of type M 23C 6. A closer inspection of the diffraction patterns of precipitates and matrix reveals orientation-relationships of types (1 1 1) M 23C 6 || (0 0 0 1)hex and [ 0 1 ¯ 1 ] M 23C 6 || [1 0 0]hex. Ta additions lead to the formation of tiny TaC precipitates. In the microstructure of the Re-rich alloy Co–31Re–23Cr two phases are detected: the ɛ(Co,Re,Cr) phase and a phase which is rich in Cr and Re with lattice parameters similar to those in Cr 2Re 3. The Mo–0.5Si alloy consists of a bcc matrix phase with Si in solid solution. Adding Zr leads to the formation of small Zr-rich precipitates as was shown using energy dispersive analysis of X-rays. These precipitates are also found in the Mo-phase of the Mo–9Si–8B–1Zr alloy. The results obtained in the present study will serve as a reference for further work which will focus on the evolution of microstructure during creep.