A comparative study on microstructures and tensile behaviours of a novel polycrystalline and single-crystal Co–9Al–9W–2Ta-0.02B-0.02Ce alloy at room and high temperatures

A comparative investigation in microstructure and mechanical properties of a novel as-cast polycrystalline and single-crystal (SC) Co–9Al–9W–2Ta-0.02B-0.02Ce (at.%) alloy with a growth direction of [0 0 1] has been performed. Aged at 900 °C for 100 h, the polycrystalline and SC alloy consisted of a Co solid solution (γ-Co_SS) matrix and nano-scale cuboidal γ′-Co₃(Al, W) intermetallic precipitates, while the two phases cohered with each other. For the polycrystalline alloy, Ce was observed to segregate primarily at the grain boundaries and the γ′-Co₃(Al, W) phase, and it could exclude impurity O from the grain boundaries. From room temperature (RT) to 900 °C, the SC sample always showed higher yield strength and elongation under tension than the polycrystalline sample except strength at 700 °C, while the flow stress anomalies took place at 700 °C for the polycrystalline alloy, and 800 °C for the SC alloy. Without the grain boundary, the SC alloy had excellent elongation, larger than 15.6% from RT to 900 °C. However, the polycrystalline alloy lost the ductility at temperatures above 600 °C, which was caused by transformation of the failure from a mixed transgranular dimple plus intergranular brittle mode to fully intergranular brittle mode.