Study On The Microstructures And Mechanical Properties Of Mg And Its Alloy By Equal Channel Angular Precessing At Moderate And Low Temperatures

Magnesium and its alloys have high potential as structural materials for engineering application owing to their low density, high strength-to weight-ratio and high stiffness ratio. Especially in21th century, the energy and environment problems have greatly stimulated the application of Mg and its alloys in automotive industry. However, as a result of hexagonal colos-packed (HCP) crystal structure and limited slip systems, Mg and its alloys have poor mechanical properties with high anisotropy during plastic deformation, and conventional techniques have not improved their mechanical properties significantly. Some investigations reported that controlling microstructure (grain size and its distribution and texture) is an effective way to improve the mechanical properties of materials. In resent years, equal channel angular pressing (ECAP) has been an effective means of refining the grain size of metallic materials to submeter or nanometer scales by accumulating high plastic deformation, which significantly improves the mechanical properties of materials after ECAP. At present, the ECAP of Mg and its alloys has to be carried out at the temperature higher than200℃due to poor plasticity, which leads to the refined grain size still larger than2μm. Up to date, very limited investigations have been reported in the microstructure evolutions and mechanical properties after ECAP at temperatures lower than200℃for Mg and its alloys.As the parameters of die corner affects on the homogeneous microstructre during ECAP, the deformation behaviors of materials are simulated by using3D simulation software in order to design an optimal die. Subsequently, the deformation of pure Mg and AZ80Mg alloy is carried out by ECAP assisted with back pressure in order to investigate the microstructre evolution at low temperatures. Accordingly, the mechiancal behaivors are investigated at room temperature for ECAP processed materials. In this paper, the major results are as the follows:(1) A3D finite element simulation software has been employed to simulate the deformation behaviors during ECAP for pure Ti and pure Mg with an optimized die, and back pressure, equivalent stress, strain and strain rate distribution have been systematically analyzed. The results show that the stress distribution on the upper and bottom surface of a specimen is balanced by the modification of the innter and outer corner of the die, which can improve the homogeneou deformation distribution of materials. The different materials (Ti and Mg) do not affect the homogeneous deformation distribution with the optimized die. On the other hand, the blacncd stress on the upper and bottom surface of specimens are not changed by applying different back pressures. Also, the high back pressure is favor to increase the plastic deformation level and extend the homogeneous deformation area. The strain rate in the shearing deforation area can be significantly reduced through modification of die structures and the application of back pressure, which can avoid cracking on the surface of specimens.(2) The deformation of pure cast Mg is carried out in the moderate temperature range between100to200℃by direct ECAP or two steps ECAP, in order to inversitate the effect of temperature, processing pass and routes on microstructure evolution and textures by using X-ray diffraction (XRD) and electronic backscatter diffrication (EBSD). Compressive testing has been carried out at room temperature for analyzing the effects of different conditions on the mechanical properties of the material. The results show that it is easy for cracking to occur in pure cast Mg by using route Be in direct ECAP. However, routes A, Be and C have been successfully processed by two steps ECAP for pure Mg at different temperatures (100to200℃) with4passes. In this case, the effect of grain refinement is arranged as Bc> A> C. As a results of different slip systems for different routes, slanting texture are achieved for both routes Be and C in which the preferred orientation generally decreases with the temperature decreasing, and basal texture is obtained by route A. For route A, the high fraction of high angle grain boundaries (HAGBs) is achieved, and the high HAGBs and basal texture have not significantly changed at different temperatures. During compressive testing, due to the different textures and grain distribution, the effect of improving the mechanical properties of materials is ranged as A> Be> C.(3) The deformation of pure Mg is carried out at room temperature by two step ECAP, in order to investigate the effects of routes, processing pass and back pressure on the microstructure evolutions and their mechanical properties. The process of microstructure evolutions and grain refinement mechanisms are characterized by using EBSD. Compression testing is carried out for analyzing the influence on the microstructure mechanical properties in the room temperature ECAP. The results show that the higher yield strength combination with larger plastic deformation is achieved by secondary ECAP with route A, which is attributed to the multi-scale grain size distribution and basal texture. For route Be, as a result of homogeneous grain distribution, the plastic deformation significantly increases with the number of passes increasing, but the yield strength decreases a little bit due to slanting texture. For route C, owing to many residual deformation twins and slanting texture, the small plastic deformation combined with low yield strength is tailored even after processed with8passes. Back pressure is an effective means to refine grain size. With high back pressure, the grain size is refined to submeter with homogeneous distribution even in a single pass by ECAP. The higher back pressure contributes to larger plastic deformation and preferred orientation.(4) The deformation of AZ80Mg alloy is carried out by direct ECAP or two step ECAP, respectively, in order to investigate the microsturcure evolution and its effects on mechanical properties of the material. The results show that the effects of both routes A and Be are similar to grain refinement at temperature of320℃, and the grain size is decreased firstly and then increased after4passes. At low temperature of200℃, many deformation twins with higih density dislocations are obtained, and some precipitated particles are refined to100nm, respectively. The increase in hardness is attributed to an increase in twins and dislocation density, which causes more chances of mutual interaction of dislocations within grains and blocking of dislocation motion by the presence of subgrain boundaries and homogeneous distribution of precipitated particles. The type of textures are mainly operated by route but not temperature. Basal texture and slanting texture are tailored by route A and route Bc, respectively.