Microstructure And Mechanical Properties Of Pure Mg After ECAP At Room Temperature

Magnesium(Mg), with the density of 1.74g/cm3, is one of the most prominent metal materials and also has promising application in the fields of aerospace industry, automotive and electronical communication. However, Mg usually possesses relatively poor plasticity and ductility and then hinders its wide production and application in many fields because of the finite active slip systems due to the hexagonal close packed(hcp) crystal structure. Therefore, are highly restricted by its limited strength and low formability.Equal channel angular pressing(ECAP), is the most efficient means to refine grains of Mg and its alloys and enhance their comprehensive properties without changing the dimensions of the samples. For a long time, studies of ECAP for pure Mg have been conducted at a higher temperature, which may result in recrystallization, the growth of grains and the decline of mechanical properties. So only the ECAP process for pure Mg is conducted in a lower temperature can ideal nanostructures be obtained. Recent studies showed that by some schemes such as a back pressure,lower pressing speed or enlarged channel angular can ECAP process be conducted under 200℃.However, it is still hard for pure Mg and its alloys to be ECAPed at room temperature because the HCP Mg usually induces to crack in ECAP process.In order to avoid crack for pure Mg in the ECAP process and realize the ECAP deformation at room temperature,in my this thesis,with a method of Fe-canned commercial pure Mg bar, the Mg bar with a diameter of 8 mm was pressed by ECAP with a speed of 4mm/min, benefiting from the strong bonding force of Fe against Mg in the process of ECAP,a deformed pure Mg bar was successfully prepared of which the surface is smooth without a crack, the grains are small and mechanical properties are relatively excellent. The microstructures of as ECAPed and as annealed pure Mg were observed by SEM-EBSD, and their mechanical properties were also measured by Vikers hardness tester and electromechanical universal testing machine.The microstructure and texture of as ECAPed pure Mg were investigated. The results showed that:compared with as extruded pure Mg, the as ECAPed one showed prolonged microstructure which was not well-distributed, but the grain size was obviously smaller than that of as extruded pure Mg. The deformed texture of pure Mg is typically(0001) basal plane fiber texture, while(10-10) cylindrical and(10-11) pyramidal textures were weaker than that. Meanwhile, the texture was inclined by about 15° from ED towards ND after one-pass ECAP, which attributed to dynamic shear forces in the ECAP process and the strongest density was dropped due to the formation of low angular boundaries.The compressive mechanical properties of as ECAPed pure Mg were investigated. The results showed that : after one-pass ECAP, compressive mechanical properties were decreased due to the nonuniform deformation in ECAP process, but the hardness of deformed pure Mg firstly increased then decreased because of the nonuniform microstructure in the sample.The microstructure and texture of as annealed pure Mg were investigated. On one hand, the well-defined boundaries and a large number of equiaxed grains displayed that static recrystallization took place in the annealing process,the microstructure was the most homogeneous after annealing at 200℃for 30 min and the mean size of grains showed to be minimum. On the other hand,(0001),(10-10),(10-11) texture distribution nearly unchanged but scattered and pole density began to decrease with the increase of annealing time. So texture density could be weaker but distribution could not be changed, a lot of high angular boundaries formed at the same time.The compressive mechanical properties of as annealed pure Mg were also investigated. The results showed that:after annealing, the remaining strain in the samples gradually disappeared and static recrystallization occurred, so the compressive mechanical properties were improved obviously, of which the compressive mechanical properties were maximum after annealing at 200℃for 30 min, because in this situation the microstructure was the most homogeneous and the mean size of grains was minimum. It is the best annealing time for pure Mg at 200℃. Meanwhile, the hardness of as annealed pure Mg was rapidly decreased because of the decline of dislocation density in the sample.