Effects Of Different Compression-torsion Paths On Grain Refinement And Texture Evolution Of Rare Earth Magnesium Alloy

Mg-Gd-Y-Zn-Zr alloy containing unique long period stacking ordered(LPSO)phase has broad application prospects in aerospace,defense and military industry,automotive electronics and other fields due to its excellent mechanical properties at room temperature and high temperature.Studying the microstructure and grain refinement mechanism of alloy deformation under complex stress state is the theoretical basis for realizing the overall forming of components under complex loading conditions.As a thermal simulation experiment,compression-torsion can realize the analysis of alloy forming under complex stress loading and provide powerful help for the design of experimental parameters of rotary extrusion forming of subsequent large components.Among them,there is still a lack of research on the influence of the path of compression-torsion deformation on the microstructure and grain refinement mechanism of the alloy.In this paper,Mg-9Gd-4Y-2Zn-0.5Zr alloy was taken as the research object,and the compression-torsion and cyclic compression-torsion experiments were carried out by Gleeble-3500 thermal simulator.The shear stress and compressive stress conditions were applied to the research material at the same time.The deformation behavior and grain refinement mechanism of the alloy under different compression-torsion paths were analyzed,which provided a basis for the study of the complex stress state of rotary extrusion.Compression-torsion and cyclic compression-torsion deformation are non-uniform deformation.The strain is closely related to the radius,and the strain at each position is different.Therefore,multiple sets of experimental data can be collected inside a sample.The cyclic compression torsion test samples are divided into two types :(1)C-CT1(Cyclic compression torsion,No.1)alloy changes the torsion direction twice during the compression torsion process;(2)The torsion direction of C-CT2(Cyclic compression torsion,No.2)alloy changed once during the compression-torsion deformation.Based on the previous experiments,three alloys with different compression-torsion paths were deformed at 440 °C and strain rate of 0.01 s-1,and the microstructure evolution,grain refinement mechanism,microhardness and texture evolution were compared and analyzed.The temperature sensitivity of C-CT2 alloy with the best microstructure was further analyzed(deformation temperature was 380 °C,410 °C,440 °C and 470 °C).The results show that :During the deformation process,the compressive-torsional and cyclic compressivetorsional alloys show a gradient structure from the center to the edge of the sample,but the gradient structure of the compressive-torsional alloy is more obvious than that of the cyclic compressive-torsional alloy.At the same position in the TD(transverse direction)direction,the dynamic recrystallization grains in the cyclic compression-torsion alloy are more while the LPSO phase is more twisted,broken and dissociated.A small amount of dynamic recrystallization is found in the lamellar LPSO phase with a large internal spacing in the CCT2 alloy.The volume fraction of the dynamic precipitated phase is large.With the increase of the distance between the observation position and the center,the cluster shape is transformed into a dispersed shape,and the number of edge positions decreases,and the dissolution mechanism occurs.The phenomenon of C-CT2 cyclic compression-torsion alloy is more obvious.The grain refinement mechanisms at different locations of different compressiontorsion paths are also different.The dynamic recrystallization volume fraction of the compression-torsion alloy shows a significant increase outward along the radius direction,and the grain size decreases.The overall grain refinement mechanism is a continuous dynamic recrystallization mechanism.The recrystallization volume fraction and grain size of the cyclic compression-torsion deformed alloy have similar trends,but the overall structure is finer and more uniform.The microstructure of C-CT2 alloy is the best,and the average grain size and fine dynamic recrystallization grains are better than those of C-CT1 alloy.The discontinuous dynamic recrystallization mechanism appears in both cyclic compression-torsion deformed alloys as the radius increases.The analysis of the temperature sensitivity of C-CT2 alloy shows that with the increase of C-CT2 cyclic compression-torsion deformation temperature,the grain refinement gradually evolves from the continuous dynamic recrystallization mechanism to the discontinuous dynamic recrystallization mechanism,and the recrystallization volume fraction increases with the increase of grain size.The hardness change trend of the alloy with different compression-torsion paths is similar at different positions.The hardness difference of the compression-torsion alloy from the center to the edge is slightly higher than that of the other two cyclic compression-torsion alloys,and the hardness difference at different positions is relatively large.With the increase of temperature,the hardness change rate and hardness value of the alloy gradually decrease.The deformation texture is greatly affected by different compression-torsion paths.The texture strength of compression-torsion alloy is the highest,and the texture strength of CCT1 alloy is slightly higher than that of C-CT2 alloy.The deformation texture under different compression-torsion deformation conditions is significantly affected by strain :with the increase of the radius of the TD direction,the degree of texture weakening increases,and the texture component will develop from a single basal texture to a non-basal texture.With the increase of deformation temperature of C-CT2 alloy,the texture strength increases.