Micro-structural Characteristics and High-temperature Performance of IN718 Nickel-based Alloy Treated by Laser Peening

Abstract:

The aim of this study was to investigate the effects of laser peening (LP) on the micro-structural characteristics and high-temperature performance of IN718 nickel-based alloy. Firstly, LP process was performed on the standard tensile specimens at ambient temperature, then the treated specimens were exposed at 700 ℃ for 300 min. As comparison, we detected the surface residual stress and nano hardness on the untreated, LPed and heat-exposure specimens respectively. The corresponding micro-structures were also observed. The results indicate that high amplitude compressive residual stress distributed in the LP treated surface, the maximum value was -706 MPa, appearing in the central region of the LP area. LP process significantly increased nano hardness of treated surface, the average nano hardness was enhanced by 67.4%. Both the residual stress and nano hardness decreased after heat exposure, however, the strengthening effect still existed while compared with untreated specimens. An obvious “layer-seperating” phenomenon on the grain morphology in the depth direction was observed after laser peening. The grain size in the plastic deforming layer refined greatly, the affecting depth of which reached a value of 80 μm. The precipitated δ phase was found in the grain boundaries for the heat-exposure samples. A further observation verified that the interaction of mechanical twins and dislocation resulted in the grain refinement. After heat exposure, the single- directional-distributed dislocation array in the grain well verified the dislocation activities, which indicates that LP can inhibit the grain growing of IN718 alloy at high temperature.

Key words:IN718 nickel-based alloy; laser peening; high-temperature performance; micro structure