Metallurgical Mechanisms upon Stress Relaxation Annealing of the AD730TM Superalloy

Fine microstructural analyses have been performed to identify the microstructural mechanisms controlling stress relaxation during aging heat treatment of AD730TM disk superalloy. Morphological evolution of the hardening γ′ precipitates and plastic activity occur during relaxation tests. For a 500 MPa initial stress, the relaxation test shows atypical behavior with sluggish relaxation in the first hours and then a faster one. To understand this atypical behavior, isothermal dilatometry tests were used to decouple the effects of stress and temperature. The latter revealed a contraction of the specimen when subjected to a constant temperature. This contraction induces a tendency for an increase in stress during the relaxation test to meet the imposed condition of constant total deformation. Relaxation is then controlled by the competition between the classical relaxation mechanisms (vacancy diffusion and/or dislocation gliding) which tend to lower the stress and the contraction of the specimen which tends to increase the stress during the test.