Effects of cryogenic treatment on mechanical properties and micro-structures of IN718 super-alloy

The main purpose of this study is to investigate the mechanical properties and micro-structures of IN718 super-alloy subjected to Cryogenic Treatment (CT). Tensile tests were performed and their engineering stress-strain curves were examined. Fracture morphologies of samples were observed with the help of Scanning Electron Microscopy (SEM). The micro-structural evolution of IN718 super-alloy before and after CT was investigated by both Optic Microscope (OM) and SEM. The composition of the precipitated phase was detected by Energy Dispersive Spectrometer (EDS). In order to better analyze the evolution of micro-structures, Transmission Electron Microscopy (TEM) observation was also carried out. Additionally, XRD technique was used to analyze the surface residual stress and microstructural phase. The results show that after two cycles of CT (CT-2), the tensile properties of IN718 super-alloy were found to be substantially improved at room temperature. Also, in comparison to the ductile fracture mode with very small dimples and some quasi-cleavage planes of untreated specimen, CT-2 specimen exhibited a more ductile fracture mode. The grains of CT-2 sample were significantly refined and the size became much more uniform. Compared to untreated specimen, the quantity of precipitated phases at grain boundaries was increased while the size of the precipitates was smaller and the distribution was more uniform after CT-2. High density dislocations were found to pile up at the grain boundaries. The occurrence of this phenomenon is expected to be explained as a result of micro plastic deformation and the increase of internal stress induced by volume shrinkage during the CT process.