The microstructural features and strengthening mechanisms of Mg-Al-Zn-Ca-(RE) extruded alloys having high-volume fractions of second-phase particles

The microstructures and tensile properties of Mg-4∼6Al-2∼4Zn-2∼3Ca-0∼1RE (in wt.%) extruded alloys have been investigated. C15 and C36 Laves phases, Ca2Mg6Zn3, Al2REZn2, and Al11RE3 phases were formed as networks during solidification in different alloys. After extrusion, coarse dendritic were significantly refined due to dynamic recrystallization, second phases were fragmented into pieces arranged as stripes, and typical basal fiber texture {0002} <10-10> with the extrusive direction paralleled to (0001) were formed in all studied alloys. Laves phase particles were prone to be crushed into nanoparticles, which appeared more easily to hinder the recrystallized grain growth and could bring about more than 30 vol % ultra-fine grains after extrusion. During extrusion, twinning-induced dynamic recrystallization occurred at first, and both discontinuous and continuous dynamic recrystallization dominated the microstructural evolution. The volume fraction of second phases had critical roles in recrystallization process, and a nearly complete recrystallization degree could be obtained in alloys containing more than 9 vol% second phases. The strengthening mechanisms were evaluated based on classic strengthening models, which agreed well with the experimental results. It was found that grain boundary strengthening, second phase strengthening, and texture strengthening contributed around 60%, 30%, and 10%, respectively, to yield strengthening, except for Mg-4Al-4Zn-2Ca alloy because of the high percentage of unrecrystallized region. Moreover, the overestimate of second phase strengthening happened in alloys having more than 9 vol% second phases, which revealed that too many second phases had limited influence on microstructures and the resulting mechanical properties.