The Study On Grain-scale Deformation Mechanism Of Mg?Y Alloys By Crystal Plastic Finite Element Method

Magnesium has a hexagonal close-packed?HCP?crystal structure with a large number of slip systems,but there are few independent slip systems that can be activated at room temperature,resulting in poor plasticity and poor room temperature processability.The addition of rare earth elements?RE?can effectively reduce the critical shear stress?CRSS?ratio of the basal slip and non-basal slip of the magnesium alloys and reduce the energy barrier of the non-basal slip system to improve its plasticity and room temperature processability.Among them,the addition of yttrium?Y?has a good strengthening effect on the plasticity,strength and ductility of the magnesium alloys.However,the way in which the Y element improves the properties of the magnesium alloy,and the specific internal mechanism of action is still to be considered and solved.In order to study the strengthening mechanism of Y in the plastic deformation process of magnesium alloys,the in-situ tensile test of rolled Mg-0.8wt.%Y alloy was carried out by crystal plastic finite element method?CPFEM?.The grain-scale simulation successfully predicted the activation of the basal slip and non-basal slip in the grain,and explained the strengthening mechanism of Y.CPFEM combines the crystal plasticity theory with the finite element method.It has important applications and exploration significance in the study of stress and strain distribution,grain orientation,grain torsion and texture change during deformation.In this work,the macroscopic stress-strain curve of Mg-0.8wt.%Y alloy can be obtained by in-situ tensile test installed in electron backscatter diffraction?EBSD?system.At the same time,the surface morphology and grain orientation of the sample before and after deformation can be obtained by EBSD.Using the grain orientation and position information provided by EBSD,the finite element crystal model can be established in the finite element calculation software DAMASK,and the phenomenological law is used as the plastic deformation constitutive relation of the material,and the slip and twinning change at the grain size is simulated.Through the parameter debugging,the influence of various parameters in the phenomenological law on the macroscopic stress-strain curve was studied.The critical resolved shear stress?₀?,the ratio of the critical resolved shear stress to the saturated shear stress(₀/),and the hardening modulus?H?were found that have the most obvious influence on the mechanical properties of the material.In this paper,the simulated stress-strain curve with high degree of fitting to the in-situ tensile mechanical curve is obtained on the macroscopic scale by simulating the stretching of the grains in the selected area.9 grains with obvious surface slip traces are the main research objects.The simulation results inside the grain are compared with the SEM images obtained in the in-situ tensile test,and the slip trace analysis is used to identify the type of slip system.According to the simulation and experimental results,it is found that the basal slip is the main deformation mode of the magnesium alloys during the plastic deformation stage,and the deformation caused by it is about 50%of the total strain.The prismatic slip and the pyramidal?a?slips accounted for approximately 25%and 23%of the total strain,respectively.The start of the basal slip can rely on Schmidt's law as the main basis for judgment;for the start of non-basal slip,Schmidt factor is no longer the main judgment condition,and stress concentration and boundary conditions will effect their activation;the non-basal surface?c+a?slip and the twinning have almost no activation,contributing little to the plastic deformation of the Mg-0.8wt.%Y alloy.