Development of texture and grain size during extrusion of ZA63 alloy containing stable quasicrystalline i-phase and its effect on tensile and compression strength

Abstract

We have focused on dispersion of quasicrystalline i-phase in a “Rare Earth-free” magnesium alloy Mg–6Zn–3Al (ZA63). The alloy was chill-cast to produce orthorhombic φ phase interdendritically and quasicrystalline i-phase particles with strong pinning effect at grain boundaries, giving the boundaries a very curved shape. The cast alloy was subjected to a two-step direct extrusion process to study phase (particle) distribution, grain structure, microtexture and related parameters using a host of techniques associated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the resulting mechanical properties. The first extrusion (at 563 K) with a small extrusion ratio of 6.25:1 produced a dispersion of φ phase along the extrusion direction and i-phase decorating the boundaries of grains of size of about 30 μm with a mild basal texture. The second extrusion (at 483 K) with an extrusion ratio of 25:1 reduced the average grain size to ∼2μm, with comparable yield strength 287.0 ± 4.1 MPa in tension and 299.0 MPa in compression (elongations ∼ 21%). The i-phase was found to be stable by differential scanning calorimetry (DSC) and the φ phase appeared to transform to the i-phase.