Effect Of Equal Channelangular Pressing On Microstructure And Properties Of Mg-xSi Alloys

Magnesium and its alloys,which have a lot of advantages,such as the low density,high specific strength,high specific stiffness,great shock absorption,good damping performance,excellent electromagnetic shielding effects and environment-friendly properties,are the most promising light materials,attracting the industries' wide attention.So far,high-performance magnesium alloy has been widely used in aerospace,motor vehicles,electronic information materials and other fields.Because of the high cost of high performance magnesium alloy,which limits the further popularization and application of magnesium alloy,the development of a magnesium alloy with low cost and excellent performance is a hot topic in the field of materials research.As the second crust in the content of non-metallic elements,Element silicon?Si?is of low cost and rich in content,and can react with magnesium to produce the Mg₂ Si phase with high melting point,high hardness and high elastic modulus.Mg₂ Si phase is a high temperature enhanced phase.The mechanical properties of magnesium alloy can be improved remarkably by controlling the morphology and distribution of Mg₂ Si phase in magnesium alloy.Based on theprevious research,Zn,Al and Mn are removed and only Si is added in pure magnesium.The Mg-xSi binary alloys are prepared by smelting and casting process and deformed by equal channel angular pressing?ECAP?.The mass fraction of Si in Mg-xSi alloys are 1%,2% and 3%,respectively.This paper mainly study the effect of Si content and ECAP deformation on the microstructure and properties of the alloy.With the different content of Si in Mg-xSi binary alloy,the morphology of the Mg₂ Si phase is also different.In the Mg-1Si alloy with Si content of 1wt%,the Mg₂ Si phase is dendritic and rod-shaped in the grain boundary.In the Mg-2Si alloy with Si content of 2wt.%,there is not only the form of dendritic and rod-like Mg₂ Si phase,but also the coarse Mg₂ Si phase in the alloy structure.The edge of the coarse Mg₂ Si phase is sharp,and has a strong fragmentation effect to the alloy matrix.When the Si content is 3wt% in the Mg-3Si alloy,the coarse Mg₂ Si phase has a larger volume and a larger number in the alloy structure,and some of the Mg₂ Si phase are continuously distributed.ECAP deformation can refine the grains and Mg₂ Si phase of the Mg-xSi alloys.The mechanisms of refinement of alloy grains through ECAP deformation are mechanical shear crushing and continuous dynamic recrystallization.In the early stage of deformation,the mechanical shear occupies the main position,and the dynamic recrystallization dominates the deformation later.Because of the repeated mechanical shear and dynamic recrystallization,the grains tends to be even fine with the increase ofdeformation passes.The dendritic and rod-like Mg₂ Si phase can be broken by mechanical shear through the ECAP deformation.With the increase of deformation passes,the dendritic and rod-shaped Mg₂ Si phases gradually refined and tend to be distribute evenly.However,the coarse Mg₂ Si phase can not be refined through the ECAP deformation.Some coarse Mg₂ Si phases' edge is broken and passivated with the increase of deformation passes,and the cleavage of the alloy matrix is weakened.The Mg-xSi alloys are subjected to room temperature tensile test.In the as-cast alloys,the mechanical properties of the Mg-1Si alloy is the best,and its yield strength,tensile strength and elongation are 30 MPa,93MPa and6%,respectively.In the ECAPed Mg-xSi alloys,The yield strength,tensile strength and elongation are 80 MPa,171MPa and 13%,respectively.The deformation of ECAP could significantly improve the plasticity of the Mg-xSi alloys.For the ECAPed Mg-xSi alloys,with the deformation passes increasing,the elongation of the alloys increases,and the plasticity of the alloys is improved.The content of Si also have a certain impact on alloys' plasticity.Both of the as-cast and ECAPed Mg-xSi alloys,the higher the Si content,the lower the elongation of the alloy,and the worse the plasticity.The Mg-xSi alloys are subjected to high temperature tensile test at 423 K.In the as-cast alloy,the mechanical properties of Mg-1Si alloy is the best,and its yield strength,tensile strength and elongation are 23 MPa,68MPa and 14%,respectively.In the ECAPed alloys,the mechanical properties of the Mg-3Sialloy deformed by 2 passes is the best,and its yield strength,tensile strength and elongation are 48 MPa,82MPa and 15%,respectively.Compared with the as-cast alloy,the yield strength and tensile strength of the alloys extruded after1,2 passes are improved.The more the deformation passes,the lower the yield strength and the tensile strength of the alloy.But the elongation of the alloys is significantly improved with the number of the deformation passes increased.In the range of 40⁶0MPa and the conditions of 448 K,473K and 498 K,the creep stress exponent of the as-cast Mg-2Si alloy are 3.27,2.47 and 3.38,respectively.Under the conditions of 448 K,473K and 498 K and the load of40⁶0MPa,The creep activation energy of the as-cast Mg-2Si alloy are49.54139KJ·mol-1,37.39015KJ·mol-1 and 44.99609KJ·mol-1,respectively.Under the condition of 448 K,473Kand 498 K,the creep deformation mechanism of the alloy is dominated by the grain boundary slip mechanism.