Microstructural Evolution And Mechanical Properties Of Magnesium-bismuth Based Alloy By Caliber Rolling

Magnesium and its alloys are the lightest metal avaliable nowadays in the world.Due to their inherent merits,such as high specific strength,good impact resistance,excellent damping and shock absorption,great heat dissipation and electromagnetic shielding performance,Mg alloys almost meet specific requirements for lightweight of aerospace,automotive as well as 3C electronic products.They are easy recycling and can reduce energy consumption and environmental pollution,so well known as"Eco-friendly green materials in 21^st century".Thus,Mg and its alloy attact more and more attention and become one of the fastest growing materials for industrial applications in developed countries and regions.However,compared with other metals,the mechanical properties of mojority magnesium alloys are still not satisfacted.How to improve the mechanical properties of mg alloys has been a hot topic world widly.Therefore,the researchers are eager to develop magnesium alloy with both low cost and high performance in order to broad prospective application.Provious studies have shown that multipass caliber rolling is a novel severe plastic deformation（SPD）method and can be utilized to fabricate high-performance ultrafine-grained steel and titanium alloys.However,only rare literatures about magnesium alloys using this method are currently reported.In general,cumulative strain and deformation heat can promote the occurrence of dynamic recrystallization（DRX）and weaken the texture;at the same time,the broken of the second phase as well as dynamic precipitation can pin the dislocations and DRXed grain boundaries,which in turn,enhanced mechanical properties can be obtained.In this study,we developed a new Mg-Bi based alloy.And Mg-3Bi（in wt.%,B3）alloy is choosen and trace amount of Ca（0.5wt.%）was added called BX30 alloy based on composition optimization from hot compression.The flow stress-strain curve and constitutive equation of BX30 alloy are systematically studied.The results of processing map show that the strain play a significant role in hot deformation behavior.In addition,the optimal processing conditions of BX30 alloy was determined and verified by experiments.For the sake of prediction of processing parameter during mluti-pass caliber rolling,the FEM simulation of the rolling process was carried out using DEFORM^TM software,and the stress,strain,temperature,and damage distribution during the rolling process were analyzed comprehensively.Consquently,Mg-3Bi and Mg-6Bi alloy were selected for multi-pass rolling experiments.The increase in the number of passes resulted in much finer grain size,which improves the elongation and strength simultaneously.Then,the multi-pass caliber rolling was carried out using B3 alloy with trace amount of Ca addition（0.5wt%,namely BX30）.The tensile strength and elongation of as-rolled BX30 alloy were 211MPa for 7passes and 16%in case of 9 passes.For further improvement of mechanical properties of BX30 alloy,a pre-extrusion processing with extrusion radio of 25 was introduced,and eventally the tensile strength of 216MPa and elongation of 23%was achevied.According to microstructural evolution analysis,the following reason for mechanical properties enhancement can be drawn.（1）Grain refinement,dynamic recrystallization occurs during caliber rolling,so the refined grains are obtained;（2）Second phase strengthening,during SPD process,the second phase can fully broken and uniformly distributed along the rolling direction;Moreover,there will be nano-size precipition generated at the same time.Both micro-size and nano-scale second partiles given synergistic strengthening effect somehow.Furthermore,the DRXed grain boundary can be pinned and suppressed for coarsen.