Toward unraveling the effects of intermetallic compounds on the microstructure and mechanical properties of Mg–Gd–Al–Zn magnesium alloys in the as-cast, homogenized, and extruded conditions

The effects of gadolinium and aluminum intermetallics on the microstructure and mechanical properties of Mg-Gd-Al-Zn alloys in the as-cast, homogenized and extruded conditions were studied. It was revealed that the partial substitution of Gd with Al in the GZ61 alloy enhances the as-cast mechanical properties due to the evolution of intermetallic compound types, their morphology, and grain refinement of the matrix. In this regard, the best alloy was Mg-3Gd-3Al-1Zn due to the magnificent grain refinement from 1250 to 36 µm as a result of the presence of the Al2Gd compound and there was respectively ~60% and 250% increase in tensile strength and elongation to failure when compared with the as-cast GZ61 alloy. Moreover, increasing Gd toward higher amount of (Mg, Al)3Gd phase with branched morphology impaired the mechanical properties. Subsequently, the homogenization heat treatment and extrusion process showed significant effects on the shape and size of intermetallics and grain size of the matrix. Regarding the extrusion process, the evolution of the (Mg, Al)3Gd compound in the Mg-4.8Gd-1.2Al-1Zn alloy combined with the recrystallization of the matrix resulted in the change in grain size from 670 µm for the as-cast alloy to 2.4 µm, which was found to be the key factor to enhance the properties and there was respectively ~34% and 135% increase in tensile strength and elongation to failure when compared with the as-cast alloy. However, it was also found that the microstructural refinement of hot extrusion process can effectively conceal the influences of intermetallic compound types and the chemical composition of the alloy on the tensile properties.