Microstructural Control And Mechanisms Of Enhanced Strength And Ductility Of Bimodal-grained Magnesium Alloys

Owing to the low density,high specific strength and high specific stiffness,magnesium（Mg）alloys are considered to have broad applications in the field of automotive,aerospace and communications.In recent years,bimodal-grained Mg alloys have attracted wide attention because of its unique microstructure and excellent mechanical properties.However,little attention was paid on the microstructure control and strengthening mechanisms of bimodal-grained Mg alloys.In this work,the microstructure regulation and strengthening mechanisms of bimodal-grained Mg alloys processed by hard-plate rolling（HPR）have been investigated through three aspects:（1）the microstructure characteristics and evolution of HPRed bimodal-grained Mg-Al-Zn alloys were investigated;（2）the effects of second phases with different scales on the recrystallization behavior,microstructure evolution and tensile properties of bimodal-grained Mg alloys were investigated and the strengthening mechanisms were revealed;（3）the plastic deformation behavior and the formation of ultra-fine grains of bimodal-grained Mg alloy were investigated.The novel bimodal-grained Mg alloys with both high strength and high ductility were fabricated,providing important support to the development of microstructure regulation in bimodal-grained Mg alloys.The main studies can be summarized as follows:（1）The microstructural evolution of the Mg-9Al-1Zn alloy processed by HPR was studied.It is found that grains with high basal Schmid factor accumulate more strains during the rolling process.The formation mechanism of bimodal-grained structure is ascribed to synergistic effects of heterogeneous recrystallization caused by different grain orientation and the limited growth of recrystallized grains caused by Mg₁₇Al₁₂ phase.The work laid the foundation for microstructure regulation of bimodal-grained Mg alloys.（2）The effect of various rolling thickness reduction on microstructural evolution of the HPRed Mg-9Al-1Zn alloy has been investigated.It is found that volume fraction of fine grains（<3μm）increases with the increment of rolling thickness reduction.The sample with～85%reduction shows higher strength and ductility.The enhanced strength is primarily due to the grain boundary strengthening which accounts for～65%of the overall strength.（3）The microstructural evolution of HPRed Mg-9Al-1Zn alloys with Y addition has been investigated.An optimized volume fraction of fine grains（～52 vol.%）and well-maintained fine grain size have been obtained with 1 wt.%Y addition.It reveals that the bimodal grain structure can be tailored by the co-regulating effect from coarse Al₂Y particles and dispersed submicron Mg₁₇Al₁₂ particles that influence the recrystallization behaviour.Meanwhile,the driving force for recrystallization and Zener pressure are calculated.It reveals that the tailored formation mechanisms of bimodal grain structure are dominated by cooperative effects from the PSN machanism and Zener pinning effect owing to second-phase prarticles with different scales.（4）The effect of Y contents on mechanical properties of HPRed Mg-9Al-1Zn alloys and the mechanisms of improved strength and ductility were revealed.A superior combination of a high ultimate tensile strength（～405 MPa）and elongation to fracture（～9.4%）is achieved in the present AZ91-1Y alloy.The high strength and ductility are mainly attributed to the increased volume fraction of fine grains.The strengthening effect is primarily from grain boundaries,accounting for over 70%of the overall strength,followed by second-phase strengthening and solid solution strengthening.（5）The evolution of bimodal grain structure in the extruded Mg-8Gd-4Y-1Zn-0.4Zr alloy undergone HPR processing was studied.A new phenomenon that fine grains（3-5μm）of the extruded Mg-8Gd-4Y-1Zn-0.4Zr alloy are refined to the submicron scale（≤1 mm）by continuous dynamic recrystallization is found.This study revealed the mechanisms of the microstructural evolution and the formation of ultrafine grains in the bimodal-grained Mg-8Gd-4Y-1Zn-0.4Zr alloy,which provides an important foundation for developing novel bimodal-grained Mg alloys containing profuse ultrafine grains.（6）The mechanical properties of the Mg-8Gd-4Y-1Zn-0.4Zr alloy processed by HPR have been investigated.A superior combination of a high ultimate tensile strength（～420 MPa）and ductility（～21%）is achieved in the present HPRed Mg-8Gd-4Y-1Zn-0.4Zr alloy,in which ultrafine grains are deformed in fine-grained regions.After aging treatment,a large number of nano-sizedβ’precipitates are formed in the Mg matrix,resulting in the ultimate tensile strength of～520 MPa and the ductility of～11%achieved.This study provided a reference for the development of high-strength Mg alloys.