Research On Microstructure And Mechanical Properties Of ECAP-Processed Pure Magnesium At Ice-water Temperature

Magnesium and magnesium alloys are the lightest green metal engineering materials,which have been widely recognized in the fields of aerospace,automobile and electronics owing to excellent properties.However,Application of magnesium and magnesium alloys is restricted because of the poor formability.Grain refinement is a most effective way to improve the strength and ductility of magnesium and magnesium alloys.As a most application potential large plastic deformation technology,equal channel angular pressing(ECAP)has the ability to prepare bulk submicron or even nanoscale fine grain structure.Magnesium and magnesium alloys by ECAP are often carried out at high temperatures due to the poor room-temperature plasticity.However,the grain growth is easily at high temperature,so the refinement effect is weakened.A kind of sheath like core-shell structure has been used by the research group with which magnesium has been carried out multi-pass ECAP processed at room temperature.On this basis,in order to more refine the grain effectively.In this paper,the ECAP deformation at lower temperature was explored by further optimally modified iron sheath method and extrusion process.Magnesium was prepared by multi-pass ECAP processing at ice-water temperature.Subsequently,the changes of microstructure and mechanical properties of three kinds of initial microstructure of pure magnesium(as-cast with annealing,as-extruded without and with annealing Mg rods)after ECAP processing at ice-water temperature were investigated;further increase of ECAP passes,a more systematic study of grain refinement,microstructure evolution and mechanical behavior of as-extruded without annealing magnesium after ECAP at ice-water temperature was performed;At the same time,the medium and high temperature tensile test was implemented at different temperatures and strain rates for four passes ECAP processed as-extruded without annealing magnesium,the deformation mechanism,the evolution of microstructure and the relationship between microstructure and mechanical properties were discussed.The results are showed as followed:Grains are refined after ECAP processing at ice-water temperature for the three kinds ofinitial state of pure magnesium.After one pass,the grain refinement is made mainly by the mechanical shear fragmentation and partial dynamic recrystallization.The pure magnesium of as-cast with annealing exhibits the coexistence of coarse and fine grains.The microstructure of the unannealed and annealed as-extruded pure magnesium is similar.The grains are refined but not uniform,short and narrow,and there are slender equiaxed fine grain bands.After four passes,the largest grain refinement degree and the most homogeneous microstructure is formed for the pure magnesium of as-cast with annealing,whose average grain size is 2.283μm,and the twining hardly occurs inside refined grains.With the increase of ECAP passes,the texture strength is weakened and the components of the basal texture increases.After ECAP of three kinds of initial microstructure of pure magnesium,the whole trend of microhardness is resemble,the distribution of microhardness increases first and then decreases with the increase of ECAP passes.After four passes,the comprehensive mechanical properties of as-extruded without annealing of pure magnesium is the best,and its yielding strength,compression strength and maximum strain reach 101MPa,131MPa and 0.194,respectively.The more ECAP passes increase,the smaller and more uniform of microstructure as-extruded without annealing pure magnesium is.The average grain size is about 1 μm after eight passes.The proportion change of tensile twinning boundaries with {1012}86°<1210>is the most significant,and gradually decreases.After two and more passes,the new texture components of P and Y appear on the {0001} pole figure,which correspond to<a>{1010}<1120>prismatic slip and<a>{1101}<1120>pyramidal slip,respectively.Yield strength after ECAPed as-extruded without annealing pure magnesium increases first and then decreases with smaller grain sizes;the elongation also increases first and then decreases,exhibiting anti-Hall-Petch phenomenon.The tensile tests of pure magnesium after different ECAP passes were carried out,When the average grain size is larger than the critical size,twins is dominant deformation In the early stage of deformation;~When the average grain size is smaller than the critical size,dislocation slip dominates deformation.Pure magnesium of as-extruded without annealing after four passes ECAP was subject to tensile test.When the strain rate is 3.33×10-4s-1 and the temperature is 200°C,the maximum elongation to fracture reaches 98.3%.Besides,with the increase of tensile temperature and strain rate,the basal texture does not disappear,but its intensity enhances.The orientation of the inclined basal texture is also gradually parallel to the tensile direction.It is found that the dominant mechanism of high temperature deformation of fine pure magnesium is dislocation motion in the study of constitutive relation of tensile deformation at moderate high temperature.