Research On Microstructural Evolution And Mechanical Properties Of Fe-canned Pure Mg After Room-temperature ECAP

Magnesium(Mg)and its alloys are potential materials in industry application.As they are characterized with hexagonal close packed(HCP)crystal structure,only limited slip systems can be activated when deformed at room temperature.As a result,they are poor in plasticity and are easy to crack when processed at room temperature,thereby restricting their applications in many fields.In this thesis,the as-extruded pure Mg bars were canned with Fe and room-temperature equal channel angular pressing(ECAP)on Mg was achieved.Four-pass ECAP-processed samples were prepared and then were annealed.The microstructure and texture evolution of as-ECAPed and as-annealed pure Mg samples were measured by SEM-EBSD,and local microstructures and deformation mechanism of the deformed samples were also analyzed by SEM-EBSD and TEM.Finally,compressive mechanical properties of the as-ECAPed and as-annealed pure Mg samples were measured by using an electromechanical universal testing machine at room temperature.This paper revealed the evolution of microstructures and mechanical properties and deformation mechanism of the as-ECAPed pure Mg with fine grains,and the stability of the as-annealed microstructures and mechanical properties.The paper can also establish a good technology foundation to prepare Mg and alloys with ultra-fine/nano grains and supply a theoretical basis to develop and process bulk Mg and alloys with large size and ultra-fine/nano grains.Grain size was obviously reduced and microstructure was homogeneous after direct extrusion.Basal texture is nearly parallel to ED and prismatic and pyramidal textures are weaker.compressive yield strength(CYS),ultimate compressive strength(UCS)and compressive strain(CS)were improved,especially UCS,which was caused by second strain hardening due to tension twins during hot deformation.Grain size was further reduced and microstructure was more homogeneous after four-pass ECAP.With increasing ECAP pass,grain orientation was transformed from prismatic to basal orientation and most of grains were basal-orientated with their c-axes parallel to TD after four-pass ECAP.CYS,UCS and CS were decreased first and then increased.The compressive mechanical properties show the best after four-pass ECAP.After annealing at 200?×30 min,grain size of each-pass sample was increased due to static recrystallization(SRX)and a large number of equiaxed grains formed.The as-annealed textures were weakened first and then strengthened from one pass to four passes,and the one-pass annealed textures were still the strongest.CYS,UCS and CS of the two-pass annealed sample were the worst.CYS of the one-pass annealed sample was the best and CS of the four-pass annealed sample was the best.After the one-pass ECAPed sample was annealed at 150?×60 min,the distribution of grain size was extremely inhomogeneous.After annealing at200?×30 min,the average grain size was the finest and the intensity of texture was the weakest due to significant SRX in the as-annealed microstructure.Withincreasingannealingtemperature,textureswere strengthened due to preferred grain growth,textures were the strongest after annealing at 300?×15 min and were more inclined to ED.The CYS,UCS and CS of one-pass sample after annealing at 200?×30 min were the best due to the most refined grains and the most homogeneous microstructure.After the four-pass ECAPed sample was annealed at 150?×60 min,the average grain size was the finest.Grain size became larger with increasing annealing temperature.The intensity of annealed textures was higher than that of the four-pass deformed samples,but the annealed textures were still weaker as a whole.After annealing at 150?×60 min,the CYS,UCS and CS of four-pass sample were the best due to the most refined grains.The CYS after annealing at 250?×30 min is higher than that after annealing at 200?×30min due to the existence of tension twins.The UCS and CS decreased gradually with increasing annealing temperature.When pure Mg was processed by room-temperature ECAP,(0001)basal slip was the main deformation mode.During the two-pass ECAP,grains were further refined and non-basal slipping systems were activated,including(10~—10)and(11~—20)prismatic slip,(10~—11),(11~—21)and(11~—22)pyramidal slip.After being processed by one-pass ECAP,severe plastic strains induced by ECAP caused elongated grains,a large number of subgrains,DRX grains and local shear bands.With increasing strains,the shear bands disappeared progressively.The grains were further refined due to DRX after two-pass ECAP.After four-pass ECAP,the grains showed refined and equiaxed morphology and the microstructure was the most homogeneous.After being processed by one-pass ECAP,DRX textures were similar to the deformed textures,that was CDRX.After four-pass ECAP,new DRX texture component formed,which was different from the deformed textures.DRX mechanism included CDRX and DDRX.During four-pass ECAP,(10~—12)tension twinning participated in the deformation.TDRX is also one of the DRX mechanism during the room-temperature ECAP on pure Mg.