Microstructure and mechanical properties of extruded and ECAPed AZ31 Mg alloy, grain refined with Al-Ti-C master alloy

Abstract

Different amounts of Al-3Ti-0.15C master alloy (TiCAl), as grain refiner, were added to the AZ31 magnesium alloy (Mg-3Al-1Zn-0.3Mn) and the resulting microstructure, grain size distributions, texture, and mechanical properties were studied after extrusion and equal channel angular pressing (ECAP). Results showed that the addition of 1.0 wt.% TiCAl had the strongest grain refinement effect, reducing the grain sizes by 51.2 and 38.4% in the extruded and ECAPed conditions, respectively. The observed grain refinement was in part due to the presence of the thermally stable micron- and submicron-sized particles in the melt which act as nucleation sites during solidification. During the high-temperature extrusion and ECAP processes, dynamic recrystallization (DRX) and grain growth are likely to occur. However, second phase particles will help in reducing the grain size by the particle stimulated nucleation (PSN) mechanism. Furthermore, the pinning effect of these particles can oppose grain growth by reducing the grain boundary migration. These two phenomena together with the partitioning of the grains imposed by the severe plastic deformation in the ECAP process have all contributed to the achieved fine-grained structure in the AZ31 alloy with enhanced mechanical properties. The enhancement in the shear yield stress (SYS) and ultimate shear strengths (USS) were, respectively, 11.2 and 6.1% in the extruded state, and 7.6 and 3.9% in the ECAPed conditions. The weaker strengthening effect of grain refinement in the ECAPed alloys can be attributed to the textural modifications which partly offset the achieved grain boundary strengthening.