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. Ex aminations of samples prior to and after testing using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (T EM) were also performed to develop a better understanding of the microstruc tural mechanisms governing this behavior. These observations combined with analyses of the test data indicate that annealed 7075 Al under present test ing conditions exhibits characteristics of dislocation creep with a concomi tant contribution from grain boundary sliding (GBS). By contrast, the resul ts for overheated 7075 Al suggest that GBS is suppressed. This hypothesis i s supported by observations of large particles at grain boundaries in the o verheated microstructure and few or no particles at boundaries in the annea led microstructures. Rupture times for the different stress states were als o compared with respect to four multiaxial stress parameters, each of which is linked to a particular physical mechanism that can facilitate creep rup ture. It was found that creep rupture in annealed 7075 Al (regardless of sa mple 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 i n the matrix. Thus, the rupture findings also indicate that GBS is prevente d in the overheated microstructure, while it gives rise to significant stre ss redistribution in the annealed microstructure.