Sensitive Temperature and Reason of Rapid Fatigue Crack Propagation in Nickel-Based Superalloy

Superalloys are widely used in aerospace industry owing to the excellent mechanical properties and microstructure stability at high temperatures. However, the recent developments in the aerospace industry have piled higher demands on superalloys, especially for damage tolerance at high temperatures. The fatigue crack growth rate (FCGR) is an important parameter that describes damage tolerance. Although several domestic studies on FCGR in superalloys have been reported, systematic understanding is still lacking and urgently required. Hence, this study investigated the phenomenon of sensitive temperature for rapid fatigue crack propagation in several nickel-based superalloys and reasons for its emergence by adopting a systematic program of experiments and simulations. The fatigue crack propagation behavior of FGH4096, FGH4097, and FGH4098 powder-metallurgy nickel-based superalloys, and GH4720Li and GH4738 wrought nickel-based superalloys were systematically investigated in a wide temperature range of 550-800oC using a fatigue crack propagation test. The fatigue crack propagation paths and crack microstructures after the fatigue crack propagation tests were observed. The results clearly demonstrated that the relationship between fatigue life and temperature is nonlinear. A sensitive temperature for rapid fatigue crack propagation for all investigated nickel-based superalloys was also observed, where the fatigue crack propagation rate markedly increased and fatigue life dramatically shortened. Microstructure evolution and mechanical property degradation at high temperatures were not found to be the major reasons behind the occurrence of sensitive temperature of rapid fatigue crack propagation. However, a comparison of fracture morphologies and fatigue crack propagation paths at different temperatures combined with the analysis of oxidation damage components revealed the high-temperature oxidation damage of grain boundary as the major reason for the occurrence of sensitive temperature. The contributions of fatigue damage and oxidation damage at different temperatures were compared using the classical linear superposition damage component model. The results showed that the contribution of oxidation damage increased markedly with increasing temperature. As a result, the fatigue life decreased dramatically at high temperatures and the fatigue propagation rates increased rapidly. Furthermore, the effect of O on the grain boundary strength in the Ni and NiCr system at different temperatures was investigated by molecular dynamics simulations. The grain boundary separation work decreased with increasing temperature and after which the value decreased dramaticalloy. It was concluded that the accumulation of O on the grain boundary resulted in a decrease in the grain boundary separation work and weakened the grain boundary.