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 M 6 C and four different forms of M 23 C 6 precipitated throughout aging, and the evolution of M 23 C 6 followed a sequence of intergranular precipitates, intragranular nano-precipitates, incoherent twin boundary (TB) precipitates, and peritectic precipitates enveloping the outer layer of M 6 C, all of which exhibited a semi-coherent cubic-to-cubic relationship with the austenitic matrix. Among them, the intergranular M 23 C 6 was mainly lamellar. The lamellar M 23 C 6 formed through a discontinuous reaction, resulting from the decomposition of the supersaturated matrix and alternating with the γ matrix. Intragranular nano-M 23 C 6 precipitated preferentially at dislocations, while incoherent TB M 23 C 6 grew along the {111} direction of the twin plane. In addition, peritectic M 23 C 6 was a product of the degradation reaction of M 6 C at 800 ℃, with higher W and Cr than the other forms of M 23 C 6. 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 M 23 C 6. With the extension of aging time, the tensile fracture mode at room temperature shifted from transgranular to intergranular fracture, and intergranular M 23 C 6 gradually became the preferential crack source. • The nucleation and growth mechanism of precipitates in alloy 230 were revealed. • Four different types of M 23 C 6 carbides were found for the first time in alloy 230. • Peritectic M 23 C 6 has higher W and Cr than other forms of M 23 C 6. • The intergranular lamellar M 23 C 6 is the product of discontinuous reaction. • It is verified that the change of alloy 230 at 800 ℃ is concentrated in the early aging period. [ABSTRACT FROM AUTHOR]