Microstructure, texture, and mechanical properties of the extruded and multi-directionally forged Mg–xGd alloys

Abstract

The microstructural and textural evolutions as well as room-temperature mechanical properties of three binary Mg–Gd alloys with 1, 2, and 3 wt% of Gd were studied in the as-extruded condition and after two, four, and six passes of multi-directional forging (MDF). Microstructural observations of the extruded samples were indicative of partial dynamic recrystallization resulting in bimodal grain structures comprised of elongated un-recrystallized patches and fine recrystallized grains. After six passes of the MDF process, the fraction of the un-recrystallized regions decreased because of dynamic recrystallization, and thus, relatively uniform microstructures were achieved in all alloys. The greater fraction of the un-recrystallized patches and the finer grains in the recrystallized regions of the Mg–3Gd alloy confirmed the more pronounced retardation of recrystallization via solute drag mechanism at the higher Gd concentrations. Evaluation of crystallographic texture revealed that by applying the MDF process, the conventional fiber texture of the extruded condition was replaced with a new randomized texture, in which the basal planes had a tendency to rotate at angles of lower than 90° to the transverse direction. Mechanical properties were evaluated using shear punch testing (SPT) and the results showed that with increasing the number of MDF passes in each alloy, the achieved finer grain structure was responsible for improving the shear strength. In addition, increasing the Gd content of the alloys resulted in finer grain size, more solute atoms, and higher fraction of Mg 5 Gd second phase particles, all of which led to the enhancement in shear strength.