Effect of Co element on microstructure and softening behavior of NiW medium heavy alloy

In this study, a novel Ni-42W-XCo-1Mo medium heavy alloy (X = 0, 5, 10, 15, 20, wt%) was designed, and the impact of Co element on the microstructure and mechanical properties was investigated. The results indicated that the addition of Co effectively inhibited the formation of topologically close-packed (TCP) phases, which decreased with the addition of Co and reached a minimum in the Ni-42W-10Co-1Mo alloy. The yield strength of the Ni-42W-XCo-1Mo medium heavy alloy decreased with increasing Co content at both room temperature and high temperatures (750 °C). Specifically, the room-temperature yield strength of Ni-42W-1Mo alloy decreased from 516 MPa to 368 MPa after addition of 20 wt% Co, as well as the microhardness decreased from 271 HV to 233 HV. It was attributed to Co weakened precipitated strengthening effect and induced softening effect. Moreover, the Portevin-Le Chatelier (PLC) effect was observed in the Ni-42W-XCo-1Mo alloys during high temperature compression, which was attributed to the continuous dissolution and precipitation of the TCP phase repeatedly, leading to dislocation pinning and unpinning. Moreover, the Ni-42W-XCo-1Mo alloys showcased exceptional ductility, withstood up to 60 % strain without fracturing due to the face-centered cubic (FCC) matrix.