Precipitation behavior and mechanical property evolution in a solid solution strengthened Ni-Cr-W superalloy during short-term aging

The precipitation behavior and mechanical property evolution of a superalloy, Haynes 230, were investigated during short-term aging at 800 ℃. The results demonstrated that M6C and four different forms of M23C6 precipitated throughout aging, and the evolution of M23C6 followed a sequence of intergranular precipitates, intragranular nano-precipitates, incoherent twin boundary (TB) precipitates, and peritectic precipitates enveloping the outer layer of M6C, all of which exhibited a semi-coherent cubic-to-cubic relationship with the austenitic matrix. Among them, the intergranular M23C6 was mainly lamellar. The lamellar M23C6 formed through a discontinuous reaction, resulting from the decomposition of the supersaturated matrix and alternating with the γ matrix. Intragranular nano-M23C6 precipitated preferentially at dislocations, while incoherent TB M23C6 grew along the {111} direction of the twin plane. In addition, peritectic M23C6 was a product of the degradation reaction of M6C at 800 ℃, with higher W and Cr than the other forms of M23C6. Overall, the significant changes in microstructure and properties of alloy 230 during aging at 800 ℃ were concentrated in the first 200 h. At this stage, the precipitation amount of carbides increased dramatically, which enhanced the strength and hardness to peaks by hindering the dislocation motion. Meanwhile, ductility and toughness decreased significantly on account of the reduction of grain boundary bonding strength caused by lamellar M23C6. With the extension of aging time, the tensile fracture mode at room temperature shifted from transgranular to intergranular fracture, and intergranular M23C6 gradually became the preferential crack source.