Microstructure And Mechanical Properties Of Microalloying Mg-Al-Ca-Mn Alloy And Its Composite

In this study,Mg-x Al-(4.8-x)Ca-0.6Mn(x=4,3,2.4,2.0 wt.%)alloys were fabricated by keeping the total content of Ca and Al at 4.8 wt.%.On this basis,TiC_p/Mg-Al-Ca-Mn nanocomposites were fabricated by adding a small amount of TiC_p(0.5 wt.%),and then the as-cast alloys and nanocomposites were subjected to hot extrusion deformation.The effects of Ca/Al ratios and nano-TiC_p on the microstructure and mechanical properties of as-cast and extruded Mg-Al-Ca-Mn alloys and TiC_p/Mg-Al-Ca-Mn composites were systematically studied.The results show that there are dendritic?-Mg grains and Al₈Mn₅ phases in all the as-cast Mg-Al-Ca-Mn alloys.With the increase of the Ca/Al ratios,the grain size gradually decreases and the second phases distributed along the grain boundaries change from the massive Mg₁₇Al₁₂ phase to the semi-continuous network Al₂Ca phase and the continuous network(Mg,Al)₂Ca phase and Mg₂Ca phase.With the increase of the Ca/Al ratios,the tensile yield strength(TYS)of the as-cast alloys gradually increase,while the ultimate tensile strength(UTS)and elongation(EL)tend to decrease.After adding TiC_p,with the increase of the Ca/Al ratios,the grain size of the as-cast TiC_p/Mg-Al-Ca-Mn composites is significantly finer than that of the as-cast alloys.The TYS of the as-cast composites is higher than that of the as-cast alloys.The improvement of the TYS of as-cast composites is mainly due to the combined role of grain refinement strengthening,Orowan strengthening and second phases strengthening.The grains of Mg-2.4Al-2.4Ca-0.6Mn(AXM2206)alloy were significantly refined by hot extrusion with different processes and the nano-sized Al₂Ca precipitates are dynamically precipitated concurrently.As the extrusion rate or extrusion temperature increases,the volume fraction of dynamic recrystallization(V_DRX)and the grain size of dynamic recrystallization(d _DRX)gradually increase,the TYS and UTS of AXM2206 alloy gradually decrease while the EL and fracture toughness values gradually increase.The AXM2206 alloy extruded at270?&0.1 mm/s processes excellent mechanical properties,with the TYS of 330.9 MPa,the UTS of 356.0 MPa,the EL of 7.2%,and the fracture toughness value of 25.63 KJ/mm³.With the increase of the Ca/Al ratios,the V_DRX and d _DRX of the extruded Mg-Al-Ca-Mn alloys gradually decrease.At the same time,the alloys extruded with different Ca/Al ratios show typical basal texture characteristics.With the increase of the Ca/Al ratios,the preferred orientation of the grains changes from<11-20>to<10-10>,the TYS and UTS of the extruded Mg-Al-Ca-Mn alloys gradually increase,while the EL and fracture toughness values gradually decrease.The grains of TiC_p/Mg-2.4Al-2.4Ca-0.6Mn(TiC_p/AXM2206)nanocomposites were significantly refined by hot extrusion with different processes,at the same time,the nano-sized Al₂Ca precipitates are dynamically precipitated.As the extrusion rate or extrusion temperature increases,V_DRX and d _DRX gradually increase,the TYS and UTS gradually decrease while the EL gradually increase.The TiC_p/AXM2206 nanocomposite extruded at 270?&0.1 mm/s shows excellent mechanical properties with the TYS of 403.3 MPa and the UTS of 415.9 MPa.With the increase of the Ca/Al ratios,the V_DRX and d _DRX of the extruded TiC_p/Mg-Al-Ca-Mn nanocomposites gradually decrease.Simultaneously,the extruded nanocomposites also show typical basal texture characteristics.With the increase of the Ca/Al ratios,the(0002)basal texture presents a tendency of first strengthening and then weakening,the TYS and UTS of the extruded nanocomposites gradually increase,while the EL value gradually decreases.The enhancement of the TYS is mainly attributed to the synergistic effects of grain refinement strengthening,second phases strengthening,thermal mismatch strengthening and Orowan strengthening.This study is of great significance to the development,the regulation of microstructure and the improvement of mechanical properties of microalloying high strength magnesium alloys and magnesium matrix composites.