Mechanisms of σ phase precipitation and dissolution behavior in a Cr-rich Ni-based superalloy during long-term aging

Abstract

In this study, σ phase precipitation and dissolution behavior during long-term aging at 900 °C were systematically investigated in a Cr-rich polycrystalline Ni-based superalloy. Multiscale characterization reveals that Cr and W segregation drives the preferential precipitation of lath-like σ phase in the γ matrix. With prolonged aging, σ phase increasingly nucleates at M₂₃C₆ carbide interface and grows into the γ matrix. Atomic-scale analysis demonstrates a highly coherent M₂₃C₆/σ nucleation interface established through long-range atomic matching. While the σ/γ growth interface adopts a stepped configuration to minimize lattice strain, further facilitated by internal planar defects. During later-stage aging, σ phase dissolution is dominated by competitive consumption of Cr due to substantial M₂₃C₆ precipitation. Furthermore, mechanical tests indicates that the limited fraction of σ phase leads to a negligible effect on stress-rupture life at 900 °C/200 MPa. This work provides insights into the interfacial mechanisms of σ phase precipitation and the competitive kinetics governing its dissolution, thereby enhancing the understanding of microstructural stability in such alloys.