Microstructure and Mechanical Properties of Co-9Al-xMo-(9-x)W-2Ta-0.02B(x=4, 9) Alloy at Room and High Temperatures

The effects of Mo addition and aging temperatures on phase constitution, microstructure and room/high-temperature mechanical behavior of the Co-9Al-xMo-(9-x)W-2Ta-0.02 B alloys (x = 4, 9, referred as 4Mo and 9Mo alloy hereafter) prepared by arc-melting were investigated. Results show that the as-cast alloys are composed of Co-base solid solution gamma phase and intermetallic Co-3(Al, Me) compound gamma'-phase, where Me stands for W, Mo and Ta. The gamma phase is the matrix and the gamma' phase distributes on the grain boundaries of the gamma matrix. While after 1350 degrees C/8 h solution and 800 degrees C/100 h aging, the cubic gamma' phase with a size of 200 similar to 300 nm homogeneously precipitates on the gamma matrix for the 4Mo alloys. As for the 9Mo alloy, however, the microstructure is significantly changed, and there are a large number of needle-like and cellular kappa-Co3Mo precipitates besides a few cubic gamma' precipitates. After 1350 degrees C/8 h solution and 900 degrees C/100 h aging, a few of the needle-like kappa-Co3Mo precipitates appear in the 4Mo alloy, while the needle-like kappa-Co3Mo precipitates in the 9Mo alloy are coarsened. The 4Mo alloy presents excellent tensile mechanical properties, better than the 9Mo alloy at room and high temperature, and both alloys exhibit yield stress anomalies at temperatures above 600 degrees C.