Microstructures and mechanical properties of alloys Co-9Al-(9-x)W-xMo-2Ta-0.02B at room and high temperatures

Based on the composition of Co-9Al-9W-2Ta-0.02B, 2at%, 4at%, 6at% and 9at% Mo are added to replace W (hereafter referred to as alloys 2Mo, 4Mo, 6Mo and 9Mo respectively; Mo-free alloy is referred to as alloy 0Mo). The effect of Mo additions on phase constitution, microstructure, HV hardness and room/high-temperature strength is investigated. It is found that the as-cast alloys show a microstructure composed of Co-base solid solution γ phase and intermetallic Co₃ (Al, Me) compound γ′ phase (where Me stands for W, Mo and Ta). The γ phase is the matrix and the γ′ phase is distributed on the grain boundaries of the γ phase. After 1350°C/8 h solution and 800°C/100 h aging, a cubic γ′ phase with a size of 200-300 nm homogeneously precipitates in the γ matrix for alloys 0Mo, 2Mo and 4Mo, and the as-cast γ′ phase disappears. As for alloys 6Mo and 9Mo, the microstructure is significantly changed; however, no cubic γ′ precipitates are found in the γ matrix; instead there is a large number of needle-like TCP precipitates, while the as-cast γ′ phase Co₃ (Al, Me) compound changes to Co₃ (Al, Mo) compound. Abnormal yield characteristic is found for all alloys. As the Mo content increases, the room/high-temperature strength of the heat-treated alloys decreases, and the abnormal yield temperature shifts towards lower temperatures.