Study On Microstructure And Properties Of Magnesium Alloy Processed By High Pressure Torsion

Magnesium alloys are extremely attractive in industrial fields such as aerospace,automotive,and 3C due to their low density,high specific stiffness and specific strength,and abundant reserves,and are called"21st century green engineering materials".However,poor formability and low strength of Mg alloy at room temperature limit their wide application as an engineering structural material.In recent years,the high pressure torsion?HPT?process has been widely studied due to the significant grain refining effect.However,there are relatively few literatures on how to regulate the microstructure of alloys through heat treatment to further improve the strength of nanostructured Mg alloys,and there are few reports on the aging behavior and thermal stability of nanocrystalline Mg alloys prepared by HPT processing.In this paper,the microstructure and mechanical properties of Mg-Zn-Y alloy and Mg-Gd-Y-Zn-Zr alloy are studied by combining HPT processing and aging treatment.After aging treatment of Mg-Zn-Y alloy,a large amount of nano-sized MgZn and MgZn₂ precipitated phases are generated on the matrix.With the increase of the number of torsional turns,the W phase was broken,and a large number of dislocation cells eventually evolved into nanocrystalline grains of high-angle grain boundaries with an average grain size of 53 nm.During the HPT deformation,the nano-sized MgZn and MgZn2 phase were dissolved due to the severe shear strain.The hardness of nanocrystalline Mg-Zn-Y alloy was increased by 24 HV after aging at 90oC for 14 h.After aging at 200oC,the alloy exhibited accelerated and enhanced aging hardening response.After annealing at 300oC for 2 h,the microstructure of the alloy still remained within the UFG range with an average grain size of 1.2?m,showing good thermal stability.After the solution-treated Mg-Y-Nd-Gd-Zr alloy was subjected to HPT for 5 turns,a nanostructured supersaturated solid solution with an average grain size of 65 nm was obtained.After aging at 120oC for 22 h,the peak hardness of the alloy was 138 HV,and a large amount of nanoscale solute segregation was observed at the grain boundaries.After over-aging at 200oC,this solute segregation phenomenon basically disappeared,and the Mg₂₄Y₅ phase was preferentially precipitated at the grain boundaries.By establishing a reasonable strengthening model,the contribution of various strengthening mechanisms to the mechanical properties of the alloy after over-aging was quantitatively evaluated.