Precipitation Behavior and Mechanism of Sigma Phase in Alloy 925

The aim of the present work was to investigate the precipitation behavior and mechanism of the sigma (σ) phase in Alloy 925 by employing several complementary techniques of microstructural analysis and thermodynamic calculations. The thermal exposures were performed at 650, 750 and 850 ℃ for different times, after which the thermally exposed specimens were studied and compared in terms of their microstructure and the micro-hardness of γ matrix. It was found that the chemical composition of σ phase in Alloy 925 could be represented as (Cr,Mo)₆(Ni,Fe)₅ with the lattice parameters a = 0.878 nm and c = 0.457 nm. Higher local concentrations of Cr and Mo in the γ matrix caused by the formation of γ′ phase and η phase resulted in the formation of σ phase along the grain boundaries and in the vicinity of η phase. Higher contents of Cr, Mo, Al, Ti and Nb in the alloy could accelerate the formation of σ phase. The increase in C content could inhibit the precipitation of σ phase. The formation of σ phase most likely occurred during the thermal exposure at 750 ℃. The amount of σ phase was remarkably increased with the thermal exposure time increasing, which led to the softening of γ matrix.