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

Magnesium–lithium alloys have recently attracted more interest for both scientific research and industrial applications because of their interesting properties, such as super-lightweight, relatively high specific strength, stiffness and good formability [ 1 , 2 , 3 , 4 ]. The magnesium–lithium alloys can reduce their density down to 1.35–1.65 g/cm 3 because of the addition of the lightest metal, lithium (Li), which is nearly half of the density of aluminum-based alloys. It has been shown that the addition of Li can lower the c/a ratio of the Mg crystal structure and effectively decreases the critical resolved shear stress (CRSS) required to activate additional slip systems, thus improving their ductility [ 1 , 2 ]. The addition of Li to Mg alloys can transform the hexagonal close packed (hcp) structure to the body centered cubic (bcc) structure, substantially increasing the ductility of the Mg–Li alloys and reducing their density [ 2 , 3 , 4 , 5 ]. The strength of dual phase alloys is lower than that for other Mg based alloys; therefore many works on ternary and quaternary additions have been performed to find the solution [ 6 , 7 , 8 , 9 , 10 ]. It has been found that Al is one of the most potential strengtheners of the Mg–Li alloys due to the formation of coherent, metastable MgAlLi 2 phase [ 3 ]. The precipitation process in Mg–Li–Al alloys depends upon aging parameters. Ma et al. [ 4 ] investigated the aging behavior of Mg–8Li–1Al and Mg–11Li–3Al and suggested that the increase in Li and Al contents improves the aging effect. The precipitates are mainly of the MgAlLi 2 phase that is likely to coarsen during aging of the α + β two-phase alloy. In the dual phase Mg–Li-Al alloys containing up to 5% Al, it has been observed that Hall–Petch strengthening in ultra-fine grained alloys superimposes with strengthening as a result of alloying with Al (solution hardening, particulate strengthening) [ 11 ]. The superposition of these strengthening mechanisms becomes stronger with the increase of Al content, presumably due to the enhanced grain refining effect. A similar observation was reported in the squeeze cast β phase Mg–Li-Al alloys, in which the strength increased with the Al content up to 5 wt% accompanied with a substantial decrease of grain size and the precipitation of AlLi phase [ 6 ]. A dual-phase Mg–Li–Zn alloy has been processed by a severe plastic deformation (SPD) method which is a combination of extrusion and rolling processes and enables the production of a very fine grain structure and reveals a high superplastic deformation [ 7 , 12 ]. The effect of various quaternary additions like zinc [ 8 , 9 ], yttrium [ 13 , 14 ], mischmetal [ 15 ], strontium [ 16 ], neodymium [ 17 ], silicon [ 18 ], indium [ 19 ], tin [ 20 ] and scandium [ 21 , 22 , 23 ] to the ternary Mg–Li–Al alloys were also studied. In most cases, except for the Si addition, their positive effect on mechanical properties was reported and the mechanisms of improvement of properties were related to grain refinement and second phase strengthening.