Influence of Extrusion on the Microstructure and Mechanical Behavior of Mg-9Li-3Al-xSr Alloys

Mg-9Li-3Al-xSr (LA93-xSr, x = 0, 1.5, 2.5, and 3.5 wt pct) alloys were cast and extruded at 533 K (260 °C) with an extrusion ratio of 28. The microstructure and mechanical response are reported and discussed paying particular attention to the influence of extrusion and Sr content on phase composition, strength, and ductility. The results of the current study show that LA93-xSr alloys contain both α-Mg (hcp) and β-Li (bcc) matrix phases. Moreover, the addition of Sr refines the grain size in the as-cast alloys and leads to the formation of the intermetallic compound (Al4Sr). Our results show significant grain refinement during extrusion and almost no influence of Sr content on the grain size of the extruded alloys. The microstructure evolution during extrusion is governed by continuous dynamic recrystallization (CDRX) in the α-Mg phase, whereas discontinuous dynamic recrystallization (DDRX) occurs in the β-Li phase. The mechanical behavior of the extruded LA93-xSr alloy is discussed in terms of grain refinement and dislocation strengthening. The tensile strength of the extruded alloys first increases and then decreases, whereas the elongation decreases monotonically with increasing Sr; in contrast, hardness increases for all Sr compositions studied herein. Specifically, when Sr content is 2.5 wt pct, the extruded Mg-9Li-3Al-2.5Sr (LAJ932) alloy exhibits a favorable combination of strength and ductility with an ultimate tensile strength of 235 MPa, yield strength of 221 MPa, and an elongation of 19.4 pct.