Basic Investigation On The Influence Of Alloying Elements On Strain Hardening Behaviors Of Magnesium Alloys

As the lightest metal structure material,magnesium alloy has many excellent properties such as high specific properties and low density and is widely used in transportation,electronic communication,aerospace,aviation.Compared with traditional materials such as iron,aluminum,copper,zinc,magnesium ore with substantial reserve,complete types and wide distributions,has broad application prospects.Therefore,as an excellent environmentally friendly material,magnesium alloy has become the third type of metal material after steel and aluminum,and is widely used in various fields.Strain hardening refers to the phenomenon that material hardness and strength increase,but plasticity and toughness decreases in deformation process.Strain hardening is one of the most effective ways to enhance mechanical properties of metallic materials by plastic deformations.There is a lot of research showing that strain hardening characteristics have an important influence on mechanical properties of metallic materials.Further deformation could be resisted by strain hardening behavior of metallic materials and tensile instabilities would be limited,hence the ductility and deformability are controlled.It is one of the most important phenomena in the mechanical behavior of engineering materials and an important basis for the widespread use of metals.Studying the influencing factors and mechanisms of strain hardening behavior can promote the development of high-performance magnesium alloys.The uniaxial tensile and compressive tests were carried out to investigate strain hardening behaviors of magnesium alloys at various deformation conditions.Effect of alloying elements on strain hardening behaviors of binary and LPSO-containg alloy were studied;effect of strain rate and deformation temperatues on strain hardening behaviors of various magnesium alloys were investigated;design scheme of high stress and well strain hardening performance of magnesium alloys was put forward.Finally,we get the following main results:(1)The strain hardening behavior of Mg-X binary magnesium alloy was studied,and it was found that with the increase of alloying element content,the work hardening ability of Mg-Al alloy first increased and then decreased,reaching the peak in Mg-3Al alloy.The strain hardening ability of Mg-Zn alloy increased and that of Mg-Sn,Mg-Y and Mg-Gd reduced with increasing alloying elements addition.In the early stage of deformation,the most important factor that affected the strain hardening ability of the alloy was the grain size.As the deformation continues,the factors that affected the difficulty of opening the non-basal surface slip(such as grain orientation,solid solution atoms)had more obvious effects.The texture type had a significant effect on the strain hardening behavior of magnesium alloys.In alloys with strong basal texture,the strain hardening rate decreased with the increase of applied stress.Rare-earth texture alloy had a low strain hardening rate due to basal slip and twinning easy to start at the beginning of deformation.But with the increase of strain,the strain hardening rate curve would have a horizontal or rising stage.(2)Under the influence of dynamic recovery,the strain hardening ability of all Mg-3X alloys decreased with increasing temperature.However,Mg-Y and Mg-Gd alloys were subjected to dynamic strain aging under high temperature,which leaded to the periodic serrated character of their stress-strain curves and strain hardening rate curves.Moreover,their mechanical properties and strain hardening capacity were less affected by temperature.In the temperature ranged from room temperature to 150°C,the difference in strain hardening ability was mainly due to the grain size and texture type.When the temperature was 250?,the difference between grain boundary slip and dynamic recovery had a greater influence on the strain hardening behavior.(3)The strain rate sensitivity coefficients m of Mg-1Al,Mg-1Zn,Mg-1Sn,Mg-1Y and Mg-1Gd alloys were 0.025,0.042,0.020,0.006 and 0.002,respectively.Rare earth-containing alloys such as Mg-1Y and Mg-1Gd were less sensitive to strain rate.The value of m was mainly related to the twin deformation,the increase amount of tensile twins would reduce the value of m,while the increase amount of compression twins will increase it.As the strain rate increases,the strain hardening ability of Mg-1X alloy increased,because more twins in the alloy participate in plastic deformation at high strain rates.(4)With the increasing Zn content,when(?-?_0.2)=40 MPa,the strain hardening rate increased from 1267 MPa for GZ61 alloy to 1628 MPa for GZ65 alloy during tensile process.The existence of LPSO phase increased the strain hardening rate.During the compressive process,with the increase amount of LPSO phase,when(?-?_0.2)=160 MPa,the strain hardening rate decreased from 3019 MPa for GZ61 alloy to 2624MPa for GZ65 alloy.When(?-?_0.2)=280 MPa,the strain hardening rate increased from2711 MPa for GZ61 alloy to 3249 MPa for GZ63 alloy and then dropped to 3019 MPa for GZ65 alloy.The LPSO phase underwent significant kinking deformation during compression,which would increase the strain hardening rate of these alloys.With increasing deformation temperature,the strength and strain hardening rate of LPSO-containing magnesium alloys decreased.The elongation of GZ61 and GZ63alloys did not increase significantly,but the increase in temperature increased elongation of GZ65 alloys.In addition,the change of strain rate has almost no influence on mechanical properties and strain hardening behavior of LPSO-containing alloys.The results of the thermal deformation experiment showed that the kinking deformation of the LPSO phase would make it easier to accumulate dislocations,and promote dynamically recrystallize during high temperature deformation,then results in the formation of significantly recrystallized grains.