Study On Microstructure And Mechanical Properties Of Heterogeneous Mg-Gd Alloy Laminates Prepared By Accumulative Extrusion Bonding

Mg alloys have low density,high specific strength,excellent shock absorption and electromagnetic shielding properties,and have a great prospect as a lightweight and functional material.However,since Mg alloys have a close-packed hexagonal crystal structure,there are fewer slip systems that can be started at room temperature and poor plasticity.It is difficult to achieve high strength and high plasticity of Mg alloys through conventional processes.Heterostructure provides a new approach to attain a simultaneous improvement of strength and ductility for Mg alloys.Thus,based on poor ductility of Mg alloys at room temperature and the basic scientific problems of imbalance of plasticity and strength,in this paper,heterogeneous Mg-1Gd /Mg-13 Gd laminates were prepared via accumulate extrusion bonding(AEB)because different contents of Gd elements have different effects on the recrystallization behavior of magnesium alloys.Utilizing optical micrograph(OM),electron backscatter diffraction(EBSD),scanning electron microscope(SEM)and universal testing machine,investigated the impact of extruded passes and temperature on microstructure and mechanical properties of heterogeneous Mg-Gd alloy laminate,and revealed its mechanism of strengthening and toughening.The main research contens and results are obtained as follows:Heterogeneous Mg-Gd alloy laminate were obtained in 3 passes by AEB,and the microstructure evolution and its influence on mechanical properties under different extrusion passes were studied.The results showed that 1 pass(N1)and 2 pass(N2)have obvious heterogeneous lamella structure,meanwhile,the coarse-and fine-grained layers exhibted different texture components and texture indensity,while the microstructure of3 pass(N3)was uniform,and the bimodal texture deflected in the ED direction was presented.The ultimate tensile strength(291 MPa)of N2 reached 95% of a single 13 Gd,and the elongation(20.6%)reached 93% of a single 1Gd.Moreover,it had the highest additional strength and extra elongation.In order to further regulate the heterogeneous structure and optimize the process parameters,the microstructure evolution and the effect on the mechanical properties of N2 were studied at extrusion temperatures of 330 ?,380 ?,and 430 ?.The results showed that the grain size in the coarse-and fine-grained layers hardly changed from330 ? to 380 ?,and the coarse-and fine-grained layers exhibted different texture components and texture indensity.However,the grains in the coarse-and fine-grains layers obviously grown up and retained similar texture component at 430 ?,moreover,it has maximum texture intensity.With the increase of extrusion temperature,the growth rate of recrystallized grains in the fine-grained layers was faster than coarse-grained layers and the precipitates in the fine-grained layers gradually increased and coarsened.Furthermore,there was no distinct bulk diffusion of the Gd element during this temperature range.The N2 had excellent match of strength and ductility at an extrusion temperature of 380 ?.Based on the excellent mechanical properties of N2 at 380 ?,the effects of Schmid factor and twins on the mechanical properties were analyzed.And also,the geometrically necessary dislocation(GND)density distribution characteristics of N2 under different strains and the heterogeneous deformation induction(HDI)force in different extrusions were also quantitatively studied,revealing the mechanism of HDI strengthening and work hardening.The results showed that tensile twins in different extrusion passes have a smaller coordinated plastic contribution rate.However,they can adjust the orientation of the crystal and release the stress concentration,and stimulate further slip,which can obtain more plastic deformation.The synergistic effect of the coarse-and the fine-grained layers in N2 and the high Schmid factor are conducive to the improvement of ducility.In addition,with the increase of the tensile strain,HDI strengthening and work hardening played a vital role in improving strength and ducility of N2.Moreover,N2 had a suitable layer thickness for GNDs to emit,slip and accumulate,which optimized the influence of interface-affected zones(IAZs)on the mechanical properties,enhancing the HDI strengthening and work hardening effect.