Study On Evolution Of Deformation Mechanism And Strain Deformation Inhomogeneity Of Mg-2Zn-0.1Ca Alloy Sheet With Weak Basal Texture

The poor formability of magnesium alloy sheet has restrained its wide use for light-weight applications.Fortunately,the development of advanced magnesium alloy sheet with weak basal texture makes it possible to handle this issue.However,the evolution of the deformation mechanisms,including dislocation slip and twinning,during plastic deformation has not been fully investigated.In this thesis,an Mg-2Zn-0.1Ca alloy was used as starting material,quasi-in-situ scanning electron microscopy（SEM）,electron backscatter diffraction（EBSD）and digital image correlation（DIC）technique combined with slip trace analysis were combined to examine the mechanical properties,microstructure,distribution of strain and deformation mechanism.The effects of the texture,grain size and strain rate on the evolution of deformation mechanism and inhomogeneity were analyzed.The main conclusions are summarized as follows:（1）For all studied samples,specifically the RD,45°and TD samples,a deformation mechanism transition is observed,i.e.basal slip（ε=0.5%）→basal slip+non-basal slip（ε=1.5%）→basal slip+non-basal slip+tension twinning（ε=7.5%）.The deformation mechanism has an obvious dependence on the texture,i.e.atε=0.5%,the basal slip trace occurred mainly in the grains with high SF of the TD sample,while the distribution of SFbasal slip is wide for the RD sample;atε=7.5%,tension twinning plays a more important role in the TD sample than the 45°and RD samples.There is a yield strength-texture dependence phenomenon due to a higher proportion of basal slip in the RD sample with“hard”orientation.With the strain from 1.5%increases to 7.5%,the work hardening behavior of the RD and TD samples has a“trade off”phenomenon,which is related to the change of deformation mechanism.（2）The non-basal slip is more easily activated and the twins is more easily inhibited with the grain size decrease from 55μm to 13μm.An obvious increasing of yield strength of the TD sample from 72MPa to 113MPa with the grain refinement,and an increasing from 125.7MPa to 135.7MPa for the RD sample,grain refinement reduces the anisotropy of yield strength of RD sample,showing a texture effect of grain refinement on the yield strength obviously.A“V”shape of work hardening rate curve was observed in the FG sample at the initial deform stage with grain refinement,which was caused by the lack of movable dislocation source.（3）Grain refinement has an obvious texture effect on the strain inhomogeneity.At low strain（1%）,theε_xx strain in the coarse grain sample has a high orientation correlation with the SF_{basal slip},while theε_xx strain in the fine grain sample has no obvious dependence with the SF_{basal slip}.That is to say,grain refinement significantly reduces the correlation betweenε_xx strain and SF_{basal slip}.Twins also play an important role in bearing theε_xx strain in coarse grain samples.In addition,theε_xx strain of the twins is higher,and the morphology of the twins in the cloud pattern of strain distribution is similar with the twin morphology.（4）The yield strengths of the low-strain-rate(1×10^-5 s^-1)and high-strain-rate(1×10^-1s^-1)tested samples were 102MPa and 139MPa and their elongations were 24.3%and 13.5%,respectively,which illustrates the significant influence of strain rate on tensile properties,particularly ductility.For the samples deformed at different strain rates,a deformation-mode transition was observed during straining,namely,basal slip（ε=2%）→basal slip+non-basal slip+twinning（ε=7%）.It’s evident that high strain rate promoted the activity of prismatic and pyramidal slip systems and non-Schmid behavior of basal slip.This results from the increased applied stress,which is required by high strain rate.（5）The pronounced increase in the average dislocation velocity were largely responsible for the substantial increase in strain rate.Dramatic increase in applied stress is required to increase the strain rate owing to the high average dislocation velocity and the positive correlation between dislocation velocity and applied stress.It reveals the reason why the sample subjected to high-strain-rate loading showed higher flow stress than the counterpart deformed at low strain rate.High strain rate promoted the non-Schmid behavior of strain concentration,which can be ascribed to the enhanced activity of both basal and non-basal slip systems in the grains with hard orientations for basal slip.A higher resolved shear stress is necessary for the onset of slip in adjacent grains deformed at high strain rate,leading to the observed reduction in slip transferability.High strain rate did not magnify strain heterogeneity,but gave rise to more significant stress concentration.The intensified stress concentration,as well as reduced slip transferability,accounted for the reduction in ductility at high strain rate.