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 degrees C for 100 h, the polycrystalline and SC alloy consisted of a Co solid solution (gamma-CoSS) matrix and nano-scale cuboidal gamma '-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 gamma '-Co3(Al, W) phase, and it could exclude impurity O from the grain boundaries. From room temperature (RT) to 900 degrees C, the SC sample always showed higher yield strength and elongation under tension than the polycrystalline sample except strength at 700 degrees C, while the flow stress anomalies took place at 700 degrees C for the polycrystalline alloy, and 800 degrees C for the SC alloy. Without the grain boundary, the SC alloy had excellent elongation, larger than 15.6% from RT to 900 degrees C. However, the polycrystalline alloy lost the ductility at temperatures above 600 degrees C, which was caused by transformation of the failure from a mixed transgranular dimple plus intergranular brittle mode to fully intergranular brittle mode.