Microstructure Evolution Mechanism And Deformation Behavior Of Mg-Zn-Ca Alloys Processed By Hot Extrusion And ECAP

In order to improve the strength, ductility and creep resistance of Mg alloys, Mg-Zn-Ca alloy was developed in the current study, and the as-cast alloys were extruded at different conditions, the influence of extrusion temperature, ram speed, and extrusion ratio on the microstructure, texture and mechanical properties of Mg-Zn-Ca alloy were investigated. Ultra-fine grained (UFG) Mg-Zn-Ca alloy with significantly improved mechanical properties was processed by equal channel anglur pressing (ECAP) with different passes and routes. The relation between microstructure, texture and mechanical properties of Mg-Zn-Ca alloy was investigated systematically, and the influence of grain size on the tension-compression yield strength symmetry and compressive deformation behavior of Mg-Zn-Ca alloys were also studied.The as-cast Mg-Zn-Ca alloy containg a large number of coarse Ca₂Mg₆Zn₃ phase had a grain size of about 150μm, exhibited poor strength and ductility. In the as-extruded alloy, the typical basal texture (with basal plane parallel to extrusion direction) was observed, the grain size was refined, and the mechanical properties were improved. With the increasing extrusion temperature and ram speed, the average recrystallized grain size and the volume fraction of recrystallization were increased, but the texture was departed from ideal basal texture, thus the Schmid factor for basal slip was increased when the tensile test was carried out along the extrusion direction. Both the grain coarsening and basal plane texture weakening resulted in a decrease of tensile yield strength of the extruded alloy. Moreover, the increase of the recrystallized grain size resulted in a higher work hardening rate, which increased the elongation of Mg-Zn-Ca alloys. From the analysis of microstruture, texture and mechanical properties in the as-extruded alloys with different extrusion ratio, it was proved that the yield strength of Mg-Zn-Ca alloy was dominated by the average grain size and texture of the recrystallized grains, the plastic deformation was difficult to activate in the unrecrystallized grains due to their intensive basal texture.With increasing ECAP passes, the average grain size of the as-extruded Mg-Zn-Ca alloy was decreased dramatically, and the microstructure became more homogeneous. Route Bc was most effective for grain refinement. The secondary phases were crashed after extrusion and ECAP, and some nano-scale precipitates were also observed, which had a definite orientation relationship with Mg matrix and restricted the grain growth during ECAP. The textures were different in the Mg-Zn-Ca alloys processed by ECAP using different routes, which had remarkable influence on the mechanical properties. A basal texture (with < 0001 > parallel to normal direction) was observed in the as-ECAPed alloy for 4 passes using route A, which improved the tensile yield strength. The texture with {0 001} plane inclining 45o to ED was obtained in the as-ECAPed alloy for 4 passes using route C, the basal slip was favorably activated, which decreased the tensile yield strength of alloy. The intensive basal texture in the as-ECAPed alloy using route A hindered the activation of basal slip, which resulted in a higher work hardening rate in elastoplastic transition stage during tensile test at room temperature. With the increasing strain, this texture was favorable for cross-slip, resulting in a lower work hardening rate after yield deformation. The tension-compression yield strength symmetry in the as-ECAPed Mg-Zn-Ca alloy was improved, because the grain refinement restricted the activation of tensile twin. The microstructure variation of the UFG Mg-Zn-Ca alloy during the compression at room temperature was not obvious, the main deformation mechanism was basal slip, the yield strength was high, but the work hardening rate was very low due to the grain refinement, which led to the low ductility. While for the compressive deformation of the coarse Mg-Zn-Ca alloy at room temperature, the compressive yield strength was lower due to the activation of tensile twins, the work hardening rate was higher, the ductility was improved, and the recrystallization was observed in the final stage of deformation. The strain rate sensitivity of Mg-Zn-Ca alloy was increased with increasing deformation temperature or decreasing grain size during the compression at elevated temperatures, the main deformation mechanism was dislocation creep controlled by grain boundary diffusion. There were a large number of non-equilibrium grain boundaries in the as-ECAPed alloy, which provided the extra driving force for the deformation process, resulting in a decrease of the deformation activation energy.