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The sigma phase (σ) formation in long-term aged polycrystalline K44 superalloy has been studied as a function of aging temperature and time. The σ phase in K44 alloy forms mainly in dendrite cores with a few at interdendritic regions. The kinetics of σ formation can be described by the Johnson–Avrami –Mehl equation that yields activation energies of more than 130 kJ/mol in an early stage and less than 100 kJ/mol in steady state. Tensile experiments at room temperature and 1173 K and endurance experiments at 1173 K and 274 MPa were performed to test the effect of σ phases on these properties and no remarkable harmful effect is found. Moreover, the effect of long-term aging on the mechanical properties is realized mostly through the variation of  γ ′, not σ phase formation.

Sigma phase formation and its effect on mechanical properties in the corrosion-resistant superalloy K44