Evolution of Cu-rich particles and Laves phases in a novel Cu-alloyed martensitic heat-resistant steel during interrupted creep and the corresponding effects on microstructural recovery

Abstract

Cu-alloyed martensitic heat-resistant steels are considered promising structural materials in advanced ultra-supercritical (USC) plants. In this work, the evolution of Cu-rich particles (CRPs) and Laves phases (LPs) in the G115 steel and the corresponding effects on microstructure recovery were systematically studied via interrupted creep. Most CRPs precipitated within the lath interior during tempering process and started to precipitate along the lath boundaries during creep. The interior-precipitated CRPs were gradually decomposed due to dislocation cutting effect, resulting in the decrease of their number density. The precipitation of CRPs along the lath boundaries could induce the heterogeneous precipitation of LPs owing to the decrease of nucleation barrier. The number density of LPs rapidly increased during the transient stage but started to decrease during the steady-stage stage due to the coarsening behaviors of Ostwald ripening and swallowing adjacent M₂₃C₆ particles. The larger size and decreased number density of LPs and CRPs considerably weakened their pinning force for dislocations and boundaries, leading to an intense microstructure recovery during the accelerated stage. Eventually, the large area of recrystallized grains and subgrains with low hardness and the accumulation of cavities resulted in the creep fracture.