Effects of warm laser peening on the elevated temperature tensile properties and fracture behavior of IN718 nickel-based superalloy

Abstract IN718 nickel-based superalloy specimens were treated by laser peening (LP) and warm laser peening (WLP) respectively. Tensile tests were performed on the matrix and treated specimens at both room (25 °C) and elevated temperature (700 °C). Tensile properties were examined by engineering stress-strain curves. The sectional micro-structural observations were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in sequence to investigate the micro-structural evolution and precipitation strengthening mechanism. The fracture morphologies of untreated, LP and WLP treated specimens after tensile deformation at elevated temperature were also analyzed. The results show that LP process, in particular WLP process were found to have a remarkable effect on improving tensile properties of IN718 superalloy at both room and elevated temperatures. WLP treated specimens exhibited more stable high-temperature tensile properties. A serrated flow phenomenon was found in the stress-strain curves at elevated temperature. More δ precipitates dispersed in the grain boundaries after WLP treatment. Detailed TEM microstructural analysis showed that various typical microstructures including dislocation lines, dislocation tangles, dislocation arrays, mechanical twins and precipitated particles tangled together, forming the strengthening phase. A new formed substructure of submicron rhombic blocks with lengths in the range of 400–600 nm was observed in the WLP treated specimens. In addition, the WLP treated specimens exhibited a ductile fracture mode, which was totally different with the ductile-brittle mixed fracture mode in untreated and LP treated specimens. WLP process is verified as an effective method to enhance the high-temperature properties of IN718 nickel-based superalloy.