Microstructural Changes during Creep and Fractography Study of Friction Stir-Processed Commercially Pure Cu

Friction Stir Processing (FSP) makes it possible to obtain a stir zone with very fine grain size, yet using FSP makes it impossible to obtain a uniform cross-section as far as the microstructure and mechanical properties are concerned. In the current study, the material was processed on both sides, thus yielding a wider, rectangular and more homogenous stir zone. Transmission Electron Microscope study was applied to both specimens made of the aforementioned friction stir-processed material and to parent material, both underwent creep at 355 °C under different loads. A fractography study conducted on broken specimens made of FSP’ed and parent materials that underwent creep at 252 °C. The investigation reported here revealed that in both cases dislocation structures were formed during creep. In addition, the fractography study yielded a micro-void coalescence fracture mechanism in the case of the parent material, whose cross-section was found to be uniform and fully dimpled. Unlike the parent material, the material that underwent FSP was found to have a layered structure where the inner layers were found to be dimple free, contrary to the outer dimpled layers. The existence of different layers may indicate on different properties and different fracture mechanisms along the depth axis.