Microstructure evolution and strengthening mechanism of carbides-reinforced Co–Cr–Nb–W alloy under high-temperature stress-rupture

Abstract

A novel Co–Cr–Nb–W alloy with a carbide-dominant microstructure was designed in view of the excellent characteristics of NbC and γ-Co matrix to use as wear-resistant components on integral shrouds. The actual service conditions of the alloy were simulated using heat treatment experiments to investigate the influence of high temperature on their microstructure and mechanical properties. The stress-rupture property of the alloy was improved by exposing them to high temperatures. Carbides in the matrix bore the main load under external stress. The fracture mechanism of the Co–Cr–Nb–W alloy is ductile combined with localized brittle failure around large carbides. M6C was formed from the degeneration of MC and decomposition of M23C6 during the heat treatment process along with the two carbides transformation as follows: MC + matrix → M6C and M23C6 + matrix → M6C. The elimination of lamellar M23C6 by carbide transformation and supplemental precipitation of fine M6C around skeleton MC were conducive to longer rupture life. Stacking faults (SFs) played a crucial role in the strengthening mechanism of the Co–Cr–Nb–W alloy. The external stress activated different slip planes in the γ matrix to form “X-shaped” crossing stacking faults (CSFs). Lomer-Cottrell (L–C) locks in CSFs were confirmed by transmission electron microscopy (TEM) under two-beam conditions. SFs can interact with precipitates, dislocations, and grain boundaries to enhance the strengthening mechanism. The SF width was increased by heat treatment, which further increased the dislocation accumulation in the matrix and promoted the formation of CSFs and L–C locks. Therefore, the heat treatments were beneficial to the enhancement of the stress-rupture property of the Co–Cr–Nb–W alloy.