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 degrees C. Several points were notable, including the increase in near-threshold OK, OKth, 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 OKth and high starting da/dN values with increasing temperature was related to the crack blunting at the tip of crack at 450 and 650 degrees C. To understand the reason for the change in m value with different R ratio and temperature, quantitative fractographic analysis was conducted on the FCPtested IN706 specimens. The fraction of cleavage fracture varied in complex manner, depending on OK, testing temperature and R ratio. It was found that intergranular fracture, cleavage fracture competed with one another, determining the shape of da/dN vs. OK curves of IN706.