The Study Of Strain Path Dependency In The Plastic Deformation Of Pure Magnesium

Magnesium and magnesium alloys belong to the close-packed hexagonal structure,and it is easy to form a strong texture after rolling,extrusion and other processes,with strong anisotropy and tension-compression asymmetry,and the plastic behavior under the proportional loading path is very complex.At the same time,the plastic deformation itself is closely related to the strain path,and the plastic behavior varies with different strain histories.Therefore,magnesium and magnesium alloys exhibit more complex plastic deformation behaviors under non-proportional loading strain paths.At present,scholars from various countries have carried out systematic research on the anisotropic yield behavior and hardening behavior of magnesium and magnesium alloys under proportional loading,and established anisotropic yield criteria and parameter identification methods for magnesium and magnesium alloys.The corresponding physical mechanism of plastic deformation is clarified.For non-proportional loading paths,due to the too complex plastic deformation of magnesium and magnesium alloys,the corresponding research work is relatively small,which is mainly manifested in: First,the research on plastic deformation behavior under non-proportional loading paths focuses more on yield stress or plastic strain There is a lack of comprehensive quantitative characterization of the system,especially the evolution law of the anisotropic yield trajectory under non-proportional loading paths;secondly,the research on the plastic deformation mechanism under non-proportional loading is relatively immature.Based on this,this paper mainly carries out the following research work on the extruded pure magnesium square bar:1)Six different non-proportional strain paths are designed,which are followed by single-pass,Two passes...Six passes of pre-compression and then uniaxial tensile and compression experiments are carried out along different orientations（0°,45° and 90° with respect to the ED direction）,and the corresponding yield stress and plastic strain ratio R value are measured;2)The corresponding yield trajectories and subsequent yield trajectories were constructed by the second-order Bezier curve interpolation method,and the anisotropic yielding behavior,tension-compression asymmetry and anisotropic hardening behavior of pure magnesium under different non-proportional strain paths were analyzed;3)redesigned5 different non-proportional strain paths,one pre-compression along ND/ED followed by uniaxial tensile and compression experiments along ED,and ND+TD/ND+ED/ED+ND After two passes of pre-compression,uniaxial tensile and compressive experiments were carried out along the ED to study the differences in acoustic emission characteristics under different non-proportional strain paths.and the microscopic deformation mechanism of the non-S-shaped curve;4)The evolution mechanism of the microstructure is further explained by the crystal plastic finite element simulation of the tensile deformation of the five-pass pre-compressed pure magnesium sample.The main findings are as follows:（1）The anisotropic yielding behavior of pure magnesium during plastic deformation is related to the strain path.With the change of the strain path,the shape and size of the yield trajectory and subsequent yield trajectories have changed to varying degrees.Both the shape and size of subsequent yield trajectories change significantly during the hardening process under non-proportional loading conditions,indicating the complex anisotropic hardening behavior of pure magnesium under non-proportional loading.However,with the increase of pre-compression passes with different orientations,pure magnesium tends to be more isotropic yielding behavior and uniform isotropic hardening behavior.（2）The tension-compression asymmetry of pure magnesium during plastic deformation is related to the strain path.Under different non-proportional strain paths,the T/C of the samples at yield does not decrease monotonically with the change of the strain path,but increases and decreases.Symmetry is greatly reduced.At the same time,with the change of the strain path,the tensile-compression asymmetry of the specimen during the deformation process also tends to weaken,and the strain path has the greatest influence on the tensile-compression asymmetry in the ED direction.（3）Anisotropic hardening behavior of pure magnesium under non-proportional strain path.Under the non-proportional strain path,the shape of the yield trajectory of the specimen changes greatly,showing anisotropic hardening behavior,but compared with the proportional loading case,the degree of hardening of the non-proportional strain path with90° orientation conversion is not significant.The EBSD results indicate that the macroscopically different anisotropic hardening behavior is due to the strain path dependence of the internal microstructure of the specimen.（4）The acoustic emission characteristic parameters of pure magnesium during plastic deformation have strain path correlation.The acoustic emission characteristics of the samples under different non-proportional strain paths are different.Compression deformation along the ND direction produces characteristic parameter values with long duration but low peak value,while compression along the ED direction shows the opposite phenomenon.The duration and peak value are respectively It is related to the nucleation time and number of twins inside the sample;when the tensile deformation direction is opposite to the pre-compression direction,the acoustic emission characteristic parameters will appear multiple peaks,which is caused by the de-twining mechanism.（5）The deformation mechanism of pure magnesium during plastic deformation has strain path dependence.When the directions of uniaxial tensile deformation and pre-compression deformation are not in a straight line,the deformation mechanism of the specimen is mainly twinning and slip;when the directions of uniaxial tensile deformation and pre-compression deformation are opposite,the deformation mechanism of the specimen is mainly de-twining;When the deformation mode after pre-compression is uniaxial compression,the deformation mechanism of the specimen is mainly slip.