Stress-state dependency of ductile fracture in an extruded magnesium alloy and its underlying mechanisms

A comprehensive investigation into the ductile fracture of an extruded magnesium alloy at various stress states is conducted. Tension tests of the smooth and differently notched round specimens, plane strain tension as well as shear tests are carried out to obtain the medium to high stress triaxialities. Additionally, new types of notched round specimens are proposed for compression tests, by which the negative stress triaxialities are realized. In all notched round specimens of the magnesium alloy, ductile fracture is found to initiate from the notch root rather than the inner center of specimen, which is different from some relevant reports on aluminum alloy and steel. Based on a hybrid experimental-numerical procedure, fracture strains and the corresponding loading paths in all tests are obtained. It is found that the fracture strain of magnesium alloy decreases monotonically with increasing stress triaxiality in the range covered by the smooth round and plane strain specimens, while it is less sensitive to the stress states covered by the notched compressive specimens. Besides, the fracture occurring at various compressive states is dominated by shear mode, and the ductility is slightly better than that at the shear state. Despite that the fracture strain of magnesium alloy varies unconventionally in a wide range of stress triaxialities, it is accurately described by the two-component DF2014 fracture model. Furthermore, the fracture mechanisms of magnesium alloy in various deformations are explored by fractographic analysis, and the important roles played by second phase particles and deformation twins in the variation of fracture strain are revealed.