Creep rupture mechanisms in annealed and overheated 7075 Al under multiaxial stress states

The creep deformation and rupture behavior of annealed and overheated 7075 Al was investigated under uniaxial, biaxial, and triaxial stress states. Examinations of samples prior to and after testing using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were also performed to develop a better understanding of the microstructural mechanisms governing this behavior. These observations combined with analyses of the test data indicate that annealed 7075 Al under present testing conditions exhibits characteristics of dislocation creep with a concomitant contribution from grain boundary sliding (GBS). By contrast, the results for overheated 7075 Al suggest that GBS is suppressed. This hypothesis is supported by observations of large particles at grain boundaries in the overheated microstructure and few or no particles at boundaries in the annealed microstructures. Rupture times for the different stress states were also compared with respect to four multiaxial stress parameters, each of which is linked to a particular physical mechanism that can facilitate creep rupture. It was found that creep rupture in annealed 7075 Al (regardless of sample orientation) is dominated by cavitation coupled with GBS. By contrast, the rupture behavior of overheated 7075 Al is consistent with a model that describes cavitation constrained by relatively uniform creep deformation in the matrix. Thus, the rupture findings also indicate that GBS is prevented in the overheated microstructure, while it gives rise to significant stress redistribution in the annealed microstructure.