Comparative study of intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for BM and HAZ in a novel Fe-Ni-based alloy joint

Intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for the base metal (BM) and heat affected zone (HAZ) in a novel cost-effective Fe-Ni-based alloy joint were investigated comparatively. It was found that the BM tended to initiate acicular nano-precipitates perpendicular to grain boundaries, while their formation was suppressed at the HAZ. After creep at 650 degrees C under 300 MPa, nano-precipitates composed of gamma<acute accent>-Ni3(Al, Ti), M23C6 and sigma phase at BM would expand along the grain boundaries (GBs) with significant GB migration. Creep cavities were more prone to initiate at the precipitate-free zone and the acicular nano-precipitate roots due to the precipitation strengthening reduction and local strength mismatch. Furthermore, grain boundary embrittlement induced by acicular precipitates and the presence of recrystallized grains at the crack tip also promoted the crack propagation. In contrast, the welding thermal cycle contributed to the M23C6 redistribution at the GBs, which decreased element diffusion rate and dragging grain boundary movement, thereby exhibiting better creep crack resistance at the HAZ. The finding provided profound implications for the comprehensive understanding of acicular nano-precipitate evolution in the newly designed Fe-Ni-based alloy joint under service condition.