Fractographic analysis on fatigue crack propagation behavior of Inconel 706 at 25, 450 and 650 °C

Fatigue crack propagation (FCP) behavior of Inconel 706 (IN706) was examined at two different R ratios of 0.1 and 0.7 and three different testing temperatures of 25, 450 and 650 °C. Several points were notable, including the increase in near-threshold ΔK, ΔKth, value with increasing temperature, extremely high starting da/dN values for stage II regime at elevated temperature and the change in Paris' slope, m, at certain test condition. The crack path observation suggested that the increase in ΔKth and high starting da/dN values with increasing temperature was related to the crack blunting at the tip of crack at 450 and 650 °C. To understand the reason for the change in m value with different R ratio and temperature, quantitative fractographic analysis was conducted on the FCP-tested IN706 specimens. The fraction of cleavage fracture varied in complex manner, depending on ΔK, testing temperature and R ratio. It was found that intergranular fracture, cleavage fracture competed with one another, determining the shape of da/dN vs. ΔK curves of IN706.