Dislocation evolution of a novel Co-Al-W based single crystal alloy with a γ-CoSS/γ’-Co3(Al, W) coherent microstructure around flow stress anomaly temperature from yielding to fracture

Abstract

In this study, a novel single-crystal (SC) Co-9Al-9W-2Ta-0.02B-0.02Ce alloy with a growth direction of [001] was prepared by selecting crystal method (SCM). The tensile strength, ductility and dislocation evolution around temperatures of the flow stress anomaly from deformation to fracture were investigated. Aged at 900 °C for 100 h, the microstructure of the aged SC alloy is composed of Co solid solution γ-CoSS matrix and nano-scale cuboidal γ’-Co3(Al, W) intermetallic precipitates, while the two phases are coherent. The aged SC alloy exhibits tensile flow stress anomaly at 800 °C, with a peck 0.2% yield strength of 685 MPa. The SC alloy always shows strain-hardening state after yielding and excellent elongation larger than 18.9% from RT to 900 °C. At the stress anomaly temperature of 800 °C, the dislocation evolution is dominated by the 〈001〉 dislocation in the {001} plane of the γ’ phase pining back the dislocation in the {111} plane of the γ' phase in a strain range of ∼5%. With the further increase of the strain until fracture, the dislocation configuration is controlled by formation of the superlattice intrinsic stacking faults (SISF) through the frontal a/3 〈121〉 dislocations extending in the γ' phase. At 900 °C, the mobility of dislocation increases by the strong thermal activation and the Orowan mechanism appears due to the dislocations bowling the γ' phase. Both subsequently, reduce the strength and the strain-hardening effect of the SC alloy.