Effect Of δ Phase On Fatigue Properties Of GH4169 Alloy

GH4169 is a kind of precipitation strengthening type superalloy. Because it has satisfactory performance such as obdurability, corrosion resistance, weldability and fatigue crack resistance, GH4169 is widely applied in aerospace and petroleum industry. The current research published about regularities of precipitation and dissolution of δ phase, hot deformation behavior, effect of external environment and loading conditions on fatigue crack growth rate. However, effect of δ phase on fracture toughness and fatigue crack growth rate has few report. Effect of volume fraction of δ phase on fracture toughness and fatigue crack growth rate was researched in this paper. Different volume fraction of δ phase is obtained by different solution temperature and time. The microstructure of GH4169 is observed by using OM(optical microscope), SEM(scanning electron microscope) and TEM(transmission electron microscope). The influence mechanism of volume fraction of δ phase and grain size of fracture toughness and fatigue crack growth rate is revealed.Content of δ phase is increased with decreasing of solution temperature or increasing of solution time in six kinds of heat treatments. With content of δ phase increasing, the strength of GH4169 decreased, but plasticity and impact toughness increased. The strength of GH4169 decreased with increasing of grain size, according with hall-petch formula.Fracture toughness of GH4169 is decreasing with the increase of δ phase. Fracture toughness of material characterized the ability of resistant crack unstable propagation. δ phase mostly distributed at grain boundary, but γ′ and γ " phase precipitated in the matrix. δ phase is intermetallic compound, and γ" phase denuded zone surrounded δ phase, Which lead to intergranular propagation. The increase of grain size can decrease the probability of crack intergranular propagation. Grain size could increase expansion resistance of crack and improve fracture toughness.The Paris region of material under different heat treatment were obtained by CT samples.The increasing of δ phase lead to increase of fatigue crack growth rate in stable crack propagation and fast propagation region of FCG curve. Plastic area of crack tip in stable crack propagation is greater than or equal grain size. δ phase which distributed at grain boundary can accelerate fatigue crack growth rate. In fast propagation region, plastic area of crack tip in stable crack propagation is much higher than grain size. The effect of δ phase on fatigue crack growth rate is more significant. When stress intensity factor is close to nearthreshold region, plastic area of crack tip less than grain size. The effect of δ phase on fatigue crack growth rate is decreased. The increasing of grain size was beneficial to decrease fatigue crack growth rate.