Mechanism for superior fatigue performance of warm laser shock peened IN718 superalloy after high-temperature ageing

Abstract

The high cycle fatigue properties of IN718 alloy treated by warm laser shock processing (WLSP) were studied after high-temperature ageing at both room temperature and 600 ℃. The strengthening and toughening mechanism of WLSP on the hardened surface layer of IN718 alloy was elucidated to reveal the highly stable strengthening effects at high temperature. It was found that the thermally assisted surface hardening techniques had more obvious advantages in maintaining fatigue resistance and prolonging fatigue life of the alloy with the hardened surface layer during long-term service at elevated temperature, comparing with the conventional laser shock peening (LSP). After WLSP treatment, complex structures of γ″ phase/high-density dislocation with stacking faults (SFs) and nano-sized twins in γ″ phases surrounded by the more stable and higher density of geometrically necessary dislocations (GNDs), formed in the hardened surface layer. Therefore, a remarkable strengthening effect in the hardened surface layer was achieved due to the production of more stable hetero-deformation induced (HDI) stress. Meanwhile, the substructure evolution, compressive residual stress reduction and γ″ phase growth in the hardened layer of WLSP alloy were also inhibited during high-temperature ageing, owing to the existence of the special structure, contributing stability of microstructure and strengthening effect under conditions of high-temperature ageing and subsequent cyclic loading at high temperature. Thus, both the fatigue crack initiation resistance and initiation time of WLSP alloy can be improved in the surface of specimen. This is the main reason why after high-temperature ageing the fatigue resistance of WLSP sample can be kept stably and the median fatigue strength can be improved at both room temperature and 600 ℃.