Microstructure and mechanical properties of an as-cast AZ91 magnesium alloy processed by equal channel angular pressing

An as-cast AZ91 magnesium alloy was processed by equal channel angular pressing (ECAP) at 593K and its microstructure and mechanical properties were studied using electron microscopy and room temperature tensile tests, respectively. It has been found that after the first pass of ECAP, the grain size of the alloy shows a bi-modal distribution, containing fine grains of about 14 mu m and large dendrite structure. The dendritic structure completely disappeared after two passes of ECAP. The average grain size of the alloy after six passes of ECAP becomes less than 10 mu m. The yield stress of the alloy has significantly increased from 65 MPa of the as-cast alloy to 135 MPa after the first pass of ECAP, but does not show much change with further ECAP. However, the elongation to failure measured from the alloy processed by the first pass of ECAP is similar to that measured from the as-cast alloy. A noticeable increase of the elongation to failure has been observed after the second pass of ECAP, which then remains at the similar level with further ECAP process. The fractography of the tensile tested samples have been studied using scanning electron microscope (SEM) and focused ion beam (FIB) microscope. The facture surface of the as-cast alloy is predominated by cleavages. Although not predominantly, cleavage has also been frequently observed in the alloy processed by one pass of ECAP. With further ECAP process, the facture surface becomes profuse in dimples, characteristic of ductile facture, consistent with the ductility change observed. FIB observation suggests that the cracking is mainly initiated at the blocky particles.