Study On The Micro-meso Scale Evolution Characteristics And Macro Scale Mechanical Properties Correlation Of CVCDE Magnesium Alloy Deformation Microstructure

As a kind of metal engineering material,magnesium alloy has the physical properties such as low density,high specific strength and high elastic modulus,and shows excellent comprehensive properties in the application process.It has been widely used in aviation,aerospace,chemical engineering and other industries.At room temperature,the formability of magnesium alloys is poor,and the materials processed by traditional extrusion and rolling processes often have strong basal texture,which increases the difficulty of subsequent processing.A large number of experimental results have shown that refining grain size is one of the most effective methods to improve the strength of magnesium alloys.The continuous variable channel direct extrusion method retains the three-dimensional compressive stress distribution characteristics.By setting up the nonlinear cavity channel structure,the cumulative deformation is increased,the degree of grain refinement and homogenization is effectively improved,and the dual goals of forming and modification are realized.Magnesium alloy undergoes upsetting extrusion deformation during the process of flowing through the cavity channel.Under the action of shear strain,the texture changes to some extent,which is helpful to reduce the influence of texture on the plastic deformation of magnesium alloy and realize the active control of grain size and orientation.Magnesium alloys show non-uniform deformation behavior on micro and macro scales,which leads to asymmetric and anisotropic mechanical properties,thus leading to poor formability and fracture toughness.In order to study the influence of different technological conditions on the macroscopic deformation behavior and fracture characteristics of magnesium alloys,this paper selects magnesium alloys directly extruded with continuous variable channel extrusion(CVCDE)as the research object.By introducing strength difference parameter,the anisotropic mechanical properties under the influence of technological conditions are revealed,and a quantitative evaluation of tension-compression asymmetry of CVCDE magnesium alloys is completed.By analyzing the macro and micro fracture characteristics,twin behavior and the evolution of orientation difference of CVCDE magnesium alloy,the internal mechanism of improving the macro mechanical properties of the material by continuous variable channel direct extrusion and the micro mechanism of anisotropy are clarified.By studying the tensile plastic deformation behavior under different textures,the synergistic changes of yield strength and grain size of CVCDE magnesium alloy was determined,and the internal relationship between texture and mechanical properties was discussed.The results show that the variation law of Hall-Petch of magnesium alloys with different shapes is different,that is,the yield strength and average grain size of CVCDE magnesium alloy profiles formed under different technological conditions do not follow the Hall-Petch relationship,but show a roughly opposite trend.For CVCDE magnesium alloy rods with different loading directions,the yield strength increases with the decrease of grain size,which accords with Hall-Petch law.The Hal-Petch constant k shows a decreasing and then increasing trend during the tensile process in different loading directions(0~°,45~°,and 90~°).Based on the mesoscopic crystal plasticity finite element framework,a continuous variable channel direct extrusion texture evolution prediction model coupled with slip and twinning mechanisms was established,and the dominant twin orientation criterion was introduced to achieve reliable prediction of grain orientation.Texture simulation is used to clarify the transformation of micro-deformation mechanisms and the influence of shear strain on texture changes in the process of magnesium alloys deformation,and to clarify the stress-strain distribution and microstructure characteristics of CVCDE magnesium alloys.The results indicate that there are differences in the stress states of characteristic units located at different positions in the cavity channel,and the characteristic units at each position exhibit different strain distribution patterns,that is,the accumulated strain of magnesium alloys at each position is different.During plastic deformation,the magnesium alloy near the characteristic unit located at the edge is subjected to significant shear strain,promoting the opening of the non-basal slip<c+a>system,causing grain orientation to deviate,which is significantly different from the texture of the magnesium alloy near the characteristic unit located at the center.In order to study the interrelationship between the recrystallization behavior of true plastic deformation and mechanical properties(such as yield strength and hardness)of materials,CVCDE magnesium alloys were selected as the research object to complete the analysis of the coordination of thermoplastic deformation and texture evolution process of magnesium alloys with characteristic structures.The critical condition of dynamic recrystallization of CVCDE magnesium alloy was determined by Avrami formula and relative softening coefficient.It was found that the nucleation mechanism of the second phase particles in CVCDE magnesium alloys was promoted,and the influence of morphological changes such as crushing and peeling of the second phase particles on recrystallization behavior was expounded.