Microscopic damage mechanism of nickel-based superalloy Inconel 738LC under creep-fatigue conditions

Creep-fatigue damage in Inconel 738LC was clarified by in-situ observation and a new creep-fatigue life prediction model was proposed based on the mechanism identified. Creep-fatigue tests on standard specimens show that the tensile hold creep-fatigue lives were reduced to 60 to 80 percent and those in the compressive hold condition were reduced to 20 to 40 percent of the fatigue life of the same total strain condition. In-situ creep-fatigue tests on miniature specimens show that grain boundary sliding could be observed under the compressive strain hold condition and under the tensile strain hold condition grain boundary cavity damage and grain boundary sliding were observed These mechanisms are regarded as the main cause of the damage acceleration under the creep-fatigue loading conditions. Therefore, the new creep-fatigue life prediction model, which is based on the nonlinear damage accumulation method, employed two damage acceleration parameters ''d(sl)'' and ''d(cr)'' which represent grain boundary sliding damage and grain boundary cavity damage, respectively. Creep-fatigue lives of the test results were well predicted by the proposed model. [S0094-4289(00)01203-2].