Study On Microstructural Evolution And Mechanical Properties Of Fine-grained Puremg Processed By ECAP

Equal channel angular pressing(ECAP)is a severe plastic deformation technology for preparing fine-grained bulk materials,while the ECAP processing of Mg and its alloys is mostly carried out at a constant temperature,and the degree of microstructure refinement is limited.In this study,pure Mg was used as the research object,and the ultrafine grained pure Mg was prepared by ECAP and a combinational extrusion-equal channel angular pressing(EX-ECAP).The microstructures evolutions of pure Mg were observed by OM,SEM,EBSD and TEM.The hardness and tensile tests were carried out at room temperature.The effects of pressing temperature and pass on the microstructure and properties were investigated.The research results provided theoretical basis and experimental support for the preparation of high performance ultra-fine grained magnesium alloys.The results show that the grain size of as-cast pure Mg is refined obviously after one pass ECAP at temperature range of 200-300 ℃.Fine strip twins parallel or intersecting with each other are found.The dynamic recrystallization(DRX)mechanism is comprehensive effect of discontinuous DRX and twin DRX at 200-275 ℃,however,twin DRX is the predominant mechanism at 300 ℃.The minimum grain size of 25.6 μm is obtained at 200 ℃.After 1/2 pass of ECAP process at 225 ℃,the microstructure prior to shear deformation is consisted of coarse grains and twins.The microstructure in the shear zone is mainly shear bands and a large number of fine grains surrounding the coarse crystal.After shear deformation,the grain structure is mainly dynamic recrystallized grains and a small amount of long strips of coarse crystals.The types of dislocations are non-base dislocations with b = 1/3<112?0> and edge dislocations on the base with b = 1/3<112?0>.After 2-8 passes of ECAP process at 250 ℃,a minimum grain size of 21.9 μm is obtained at 8 passes.The maximum hardness is 38.6 HV.The yield strength and ultimate tensile strength first increase and then decrease with the decrease of grain size.Elongation increases first,then decreases and then rises.The yield strength,ultimate tensile strength and elongation reach 56 MPa,143.5 MPa and 8.6 %,respectively.The tensile fracture of pure Mg at room temperature is transformed from brittle fracture to ductile fracture.The degree of grain refinement by 8-pass EX-ECAP(constant temperature)decreases with the increase pressing pass.The average hardness of as-extruded pure Mg is 60.6 HV,and the EX-ECAP(constant temperature)deformation hardness value is reduced.The multi-pass ECAP at decreasing temperature is superior to the multi-pass ECAP in the grain refinement effect.The average grain size of pure Mg by 8 passes ECAP at decreasing temperature(50 ℃)is 1.65 μm,and after 8 passes by ECAP at decreasing temperature(25 ℃),ultra-fine grain structure with a grain size of ～200 nm is obtained.The formation of ultra-fine grains is due to the fact that the multi-pass ECAP at decreasing temperature hinders grain growth on the premise of ensuring the integrity of samples.The hardness of pure Mg by 8 passes ECAP at decreasing temperature(25 ℃)is peaked at 130 HV.