Precipitation and transformation of σ phase in a selective laser melted high-chromium superalloy subjected to heat treatment.

A commercial high-chromium contained GH648 superalloy was fabricated by selective laser melting (SLM), followed by solution treatment at 1140 °C for 1 h and subsequent aging treatment at 900 °C for 16 h. The precipitation and transformation of σ phase has been characterized. It has been shown that σ precipitates formed after solution treatment, attributed to the segregation of Mo element in the subgrain boundaries and also the difference between the diffusion ability of subgrain boundaries and lattice. Further aging treatment leads to the transformation from σ phase to α-Cr phase. The transformation of σ phase is owing to Mo diffusing during the aging processing. The nucleation points of α-Cr phase are mainly located at triple junction, incoherent γ/σ interfaces, subgrain boundaries and other high energy interfacial regions. The unique precipitation behavior leads to the small size and uniform distribution of the α-Cr phase in the GH648 superalloy.