New insights into the optimisation of the solution heat treatment process and properties of CMSX-4 superalloys

Abstract

For nickel-based single-crystal superalloys, simultaneously reducing the segregation and optimising the microstructure is a primary challenge in the field of heat treatment. To further improve rupture life, the element partition, microstructure evolution, and creep behaviour of the second-generation nickel-based single-crystal superalloy CMSX-4 under five different solution processes were studied using electron microprobe, scanning electron microscopy, and transmission electron microscopy. Under the same conditions, both continuous heating and remelting heat treatment exhibited higher elements homogenisation efficiency. Moreover, it was found that the improvement of element segregation can make γ′ phase smaller and more uniform. These heat treatments reduced the incipient melting tendency or eliminated the incipient melting eutectic, thereby increasing the peak heat treatment temperature of the CMSX-4 alloy. However, remelting heat treatment results in a significant increase in the porosity of the alloy, leading to a deterioration in performance. Finally, based on the above study, a novel process combining stepwise heating and continuous heating was proposed, which not only increased the peak solid-solution temperature but also prevented the incipient melting and optimized the microstructure. After using the novel process, the durability of the alloy increased by 48 % from 240 to 356 h under the test conditions of 980 °C/250 MPa. The higher proportion of the γ′ phase, narrower matrix channel, and denser dislocation network were the main reasons for the improvement in the rupture life.