Effect of doping Mg on the microstructure and mechanical behaviours of a novel Co-9Al-4.5W-4.5Mo-2Ta-0.02B alloy at room- and high-temperatures

The microstructure and mechanical behaviour of a novel magnesium (Mg) doped Co-9Al-4.5W-4.5Mo-2Ta-0.02B alloy (doped 0.1, 0.3, 0.7 and 1.0 at.% Mg, referred to as 0.1Mg, 0.3Mg, 0.7Mg and 1Mg alloys hereafter, respectively) at room- and high- temperatures have been investigated. All alloys aged at 800 °C for 50 h consist of a Co solid-solution matrix (γ-Co_SS) and nano-scale cuboidal γ′-Co₃(Al, W) precipitates. Mg is observed to segregate primarily at the grain boundaries and the γ′-Co₃(Al, W) phase. In the 0.7Mg and 1Mg alloys more Mg segregation at the grain boundaries decreases the fraction of Σ1∼3 type grain boundaries and the capability of excluding impurity O from the grain boundaries. With decreased Σ1∼3 boundaries and higher O content, the 0.7Mg and 1Mg alloys fracture in a mixed transgranular-intergranular mode at room temperature with lower strength and ductility under tension. However, the 0.1 and 0.3Mg alloys fracture in a transgranular mode with higher strength and ductility. The four alloys exhibit compressive flow stress anomalies at 700 °C and the strength obviously decreases when the nominal Mg contents increase to 0.7–1 at.% at 600–800 °C. As for the 0.1Mg and 0.3Mg alloys, the strengths from room temperature to 800 °C are higher and not obviously affected by the nominal Mg contents, primarily owing to the two alloys having almost the same and higher Σ1∼3 grain boundary fraction and the lower O concentration in the grain boundary.