Study On Microstructures And Mechanical Properties Of Mg-Zn Alloys Microaaloyed With Ca And Ce/La

Magnesium alloys have wide applications in the field of automobile, aerospace and electronic products due to the low density, high specific modulus and strength. However,Mg alloys exhibits poor strength compared with its competitor Al alloys. The novel Mg alloys with high strength, e.g. GW series, always have a high cost due to large amounts addition of expensive RE. Therefore, the development of low-cost high-strength Mg alloys is of significance to widen the application of Mg alloys. The present study developed low-cost high-strength Mg-Zn-Ca-Ce/La alloys by microalloying in Mg-Zn based alloys. Optical microscopy(OM), scanning electronic microscopy(SEM) and transimission electronic microscopy(TEM) were employed to investigate the influence of microalloying Ca and/or Ce/La on the microstructure of Mg-6Zn alloy. The texture of the as-extruded Mg-6Zn based alloys were investigated by X-ray diffraction(XRD) and EBSD. Influence of microalloying elements on texture evoulution was studied. The mechanical properties of the as-extruded alloys were analyzed and the relationship between strength and microstructure were discussed.Ca addition effectively promoted the dynamic precipitation of Mg4Zn7 during hot extrusion in Mg-Zn-Ca alloys, and the density of fine precipitates Mg4Zn7 increased with the increase of Ca addition. Ca-Zn atomic clusters due to the low mixing enthopy between Ca and Zn and the undissolved Ca2Mg6Zn3 in Ca containing alloys resulted in dynamic precipitation during extrusion. A bimodal microstructure with fine dynamic recrystallized(DRXed) grains and coarse deformation region was observed in Ca containing alloys, which was because of the pinning effect of fine precipitates and partice stimulated nucleation(PSN) of the undissolved Ca2Mg6Zn3 phases. Ca had an obvious influence on the texture of the as-extruded Mg-6Zn alloys. The recrystallization texture was weakened and the texture component was transferred from(27)1120(29) to(27)1010(29) after Ca addition. The texture weakening was due to the particle stimulated dynamcic recrystallization(PSN), whilst the pinning effect from Ca atoms or fine Mg4Zn7 resulted in the texture component transformation. The strength of the asextruded Mg-Zn-Ca alloys was increased with the increase of Ca content, while the ductility was decreased. The yield strength of as-extruded ZX608 alloy was increased from 136 MPa of the as-extruded Z6 alloy to 234 MPa. Such strength improvement was mainly attributed to grain boundary strengthening and precipitates strengthening.The second phase in the as-cast Mg-Zn-Ce/La alloys was Mg-Zn-Ce/La phase with orthorhombic structure, and the microstructure of the as-cast alloy was refined with the increase of Ce/La addition. The DRXed grains of the as-extruded Mg-Zn-Ce/La wererefined after Ce/La addition, but the contents of Ce/La in the range of 0.2wt.%-1.0wt.%had no obvious influence on the grain refinement after extrusion. The DRXed grains for Ce/La containing was refined from 15 ?m of Z6 alloy to about 10 ?m. Ce/La had large influence on the texture of the as-extruded Mg-Zn alloys. Texture weakening was detected and RE texture component was observed in Ce/La containing alloys after extrusion. The texture weakening was due to the twinning induced nucleation(TIN),shear induced nucleation(SIN) and particle stimulated nucleation(PSN). The RE texture component was only observed in the DRXed grains larger than 8 ?m. Such texture weakening and the formation of RE texture component resulted in the improvement of ductility. The elongation-to-fracture of the as-extruded ZCe602 and ZLa602 was higher than 35%.Combined addition of Ca and Ce/La was more effective to improve the mechanical properties of Mg-6Zn alloys compared with single addition of Ca, Ce or La. The main second phases in the as-cast Mg-Zn-Ca-Ce alloys were Mg-Zn-Ce-Ca phase with orthorhombic structure distributing in triple juctions and Ca2Mg6Zn3 distributing at grain boundaries. Ca and Ce double microalloying effectively prevented dynamic recrystallization and promoted dynamic precipitation of the Mg-6Zn alloy during extrusion. The dynamic precipitates in the as-extruded Mg-Zn-Ca-Ce alloys were Mg Zn2 with spherical shape, Mg4Zn7 with rod shape and Ca2Mg6Zn3 at grain boundaries. The strength of the as-extruded Mg-6Zn alloy was improved significantly after combined addition of Ca and Ce, which was attributed to fine DRXed grains,dense precipitates and high density dislocation in un DRXed region. The main second phases in the as-cast Mg-Zn-Ca-La alloys were Mg-Zn-La-Ca phase with orthorhombic structure distributing in triple juctions and Ca2Mg6Zn3 distributing at grain boundaries.The combined addition of Ca and La also enhanced the strength of the as-extruded Mg-6Zn alloys due to the refined microstructure and dynamic precipitation. The RE texture component was not detected in the as-extruded Mg-Zn-Ca-Ce/La alloys, which was related to the fine DRXed grains. Ca and Ce double microalloying was more effective than Ca and La on the improvement of strength of Mg-6Zn alloys.