Effect of prolonged isothermal exposure on elevated-temperature, time-dependent fatigue-crack propagation in INCONEL alloy 783

The effect of isothermal exposure on the elevated-temperature, time-dependent fatigue-crack propagation (FCP) in INCONEL Alloy 783 is investigated. Commercially produced Alloy 783 was annealed and aged following the standard heat-treatment procedure. One set of specimens was then isothermally exposed at 500 degreesC for 3000 hours. All specimens were subjected to FCP tests with various hold-time periods and sustained-loading crack-growth tests at 538 degreesC and 650 T in a laboratory-air environment. Without a hold time, the as-produced and isothermally exposed materials had comparable FCP rates at both test temperatures. With hold times of 100 and 300 seconds, the as-produced and isothermally exposed specimens had comparable FCP rates at 538 T. Hold-time testing of the as-produced material at 650 degreesC showed abnormal time-dependent FCP and sustained-loading crack-growth retardation. However, hold-time testing of isothermally exposed material at 650 degreesC showed the steady sustained loading crack growth and fully time-dependent FCP typically observed in many superalloys. Comparison with Alloy 718 data from the literature shows that FCP rates of as-produced Alloy 718 and isothermally exposed Alloy 783 are comparable at 650 degreesC. A fully time-dependent FCP model based on the damage-zone concept and a thermal-activation equation is proposed to characterize the FCP behaviors.