Effects of different elevated temperature and long-term exposure on microstructural evolution and mechanical characteristics of IN617 Ni-based superalloy

Abstract

The microstructural changes and mechanical properties of service-exposed IN617 Ni-based superalloy were studied at temperatures of 750, 800, 850 and 900 °C and different time ranging from 4250 to 105000 h. Some microscopic techniques and mechanical experiments, including Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), impact, hardness and room/elevated temperatures tensile tests were employed to investigate the microstructure and mechanical performance of the superalloy. In the samples operated at 750 °C/105000 h and 800 °C/98000 h conditions, the γ′ strengthening phase was observed, leading to noticeable improvement in mechanical features of the material. At 850 °C/24000 h and 900 °C/4250 h, the γ′ weight percentage was very low to be detected, so that no significant variation in the mechanical properties of the superalloy was observed. Moreover, metallographic evaluations showed that carbide particles nucleated and grew along the grain boundaries and also within the grains with increasing exposure time during service condition, while their growth rate depended upon the operating temperature. Changes in microstructural characteristics and mechanical properties were assessed using Larson-Miller parameter. As the value of P parameter went up, the area fraction of carbides and the weight percentage of M23C6 carbide increased in return, whereas weight percentage of M6C carbide revealed a decreasing trend. This is due to the fact that a part of M6C carbide was transformed to equilibrium state of M23C6 carbide by increasing P parameter. Impact energy, ultimate tensile strength at room temperature and ultimate tensile strength at 750 °C drastically reduced with increasing P parameter. Although elongation percentage and yield strength showed a decreasing trend in terms of the P parameter, they did not follow regular changes contrary to other mechanical attributes. Measurements indicated that the average hardness value remained constant at around 215 HV with increasing P parameter up to 27500 and then decreased with a steep slope down to about 178 HV.