Microstructure evolution and its impact on mechanical properties of a cobalt-based superalloy subjected to long-term heat-treatments

Under heat treatment or long-term service environment, cobalt-based superalloys usually experience complex phase transformations including the formation and dissolution of a variety of carbides, which is important to study to better understand the mechanical performance of such materials. Here we investigate the carbide evolution behavior and its impact on the room-temperature mechanical properties of a Co-based superalloy subjected to long-term heat treatments. In the as-cast state, two types of eutectics are observed in the alloy: one is composed of primarily M23C6 and austenite matrix (Type I) and the other consists of mainly M7C3 and austenite matrix (Type II). While main features of Type I eutectic are largely remained during annealing at 1000 ˚C, the M7C3 carbides in Type II eutectic transform to lamellar M23C6 after heat treatment. The underlying carbide transition mechanism is discussed. Besides, during heat treatment at 1000 ˚C, the number of MC carbides does not vary significantly, but the edge of MC carbides has remarkable changes, which is related to the complex carbide formation behavior at the MC-matrix interfaces. The formation of lamellar M23C6 carbides and the associated formation of geometrically necessary dislocations and stacking faults in the heat-treated alloy enhances its tensile strength. However, with longer holding time, the eutectic structure gradually disappears, and the carbides become spheroidized, which leads to the gradually decrease of the strength level. Our study provides useful information on the processing control of similar Co-based superalloys. •In the as-cast Co-based superalloy, two types of eutectics were observed: one is composed of primarily M23C6 and austenite (Type I) and the other consists of M7C3 and austenite (Type II).•Main features of Type I eutectic were largely remained during annealing at 1000 ˚C, the M7C3 carbides in Type II eutectic transformed to lamellar M23C6 after heat treatment.•The edge of MC carbides had remarkable changes due to the complex formation and transition of different types of carbides.•After heat treatment, the strength of the alloy was improved due to the formation of lamellar M23C6 carbides.•With longer heat treatment time, the eutectics gradually disappeared and carbides were spheroidized, resulting in a weakened precipitation hardening effect.