Microstructures and Mechanical Properties of Extruded Mg-Gd-Al Alloys: Influence of Initial Second-Phase Morphology Before Extrusion

Abstract

The effect of the second-phase morphology on dynamic recrystallisation in a Mg–9Gd–0.8Al alloy during hot extrusion was investigated at a temperature of 400 ℃. Microstructure analysis of the as-cast Mg–9Gd–0.8Al alloy revealed a predominant composition comprising an α-Mg matrix, a petal-like (Mg,Al)₃Gd phase, and a lamellar Mg₅Gd phase along the grain boundary, along with a small proportion of a square Al₂Gd phase within the grain. Upon subjecting the alloy to a solid-solution treatment at 540 ℃, the lamellar phase underwent precipitation, eventually dissolving into the matrix as the treatment time increased. The recrystallisation volume fraction of the alloy showed a positive correlation with the solution time following hot extrusion at 400 ℃. The mechanical properties of the extruded alloy were tested, and the results revealed that the as-cast extruded alloy had the highest tensile strength of 317 MPa among the tested samples, mainly owing to its bimodal structure. Moreover, the broken (Mg,Al)₃Gd phase initiated particle-stimulated nucleation. Here, the extruded alloy subjected to 10 h of solution treatment exhibited the highest yield strength of 240 MPa. The increased yield strength was attributed to the presence of the broken (Mg,Al)₃Gd phase and the dispersion of lamellar (Mg,Al)₂Gd phases at the recrystallised grain boundaries. Notably, the alloy extruded with a treated solution for 50 h exhibited the most favourable plasticity compared to the others, achieving an elongation of 29.7%. These results underscore the significance of the study in understanding the relationship between the second-phase morphology and alloy behaviour after hot extrusion.

Graphical Abstract