Effect of KOBO Extrusion and Following Cyclic Forging on Grain Refinement of Mg–9Li–2Al–0.5Sc Alloy

KOBO type extrusion and following cyclic forging in two perpendicular directions at temperatures 150–200 °C were applied to new Mg–9Li–2Al–0.5Sc alloy to study its effect on strength and microstructure evolution. KOBO extrusion of as cast ingot caused significant grain refinement of (α + β) dual phase Mg–9Li–2Al–0.5Sc alloys down to range 1–10 μm for hcp α phase and 1–5 μm for the β phase as resulted from EBSD studies. At that stage ultimate tensile strength UTS of 168 MPa and elongation of 36% were attained. Additional deformation by cyclic forging alternatively from 2 perpendicular directions caused formation of much finer grains of average size of few hundred nm observed using TEM and close to 1 μm applying EBSD technique. Tensile strength increased after that operation above 200 MPa and elongation above 40%, which were the highest observed in the dual phase Mg–Li based alloys. Deformation by cyclic forging induced formation of strong texture with 0001 plane parallel to forging directions. Additional precipitates of size below 1 μm at α/β interfaces or within β phase were identified as hcp AlSc2 phase. Fine particles of hcp α phase were observed after severe plastic deformation SPD within β phase grains, as effect of diffusion assisted deformation. The amount of hcp α phase increased after SPD up to 70% from initial 40%. High density of defects at α/β interfaces on 0001 planes observed in HRTEM mode was responsible for promotion of hcp phase formation during SPD.