Study On The Deformation Behavior Of Magnesium Alloy Under Biaxial Stress State At Room Temperature

Magnesium alloys have high specific strength,specific stiffness,shock absorption and other advantages,which give them great potential in energy saving,weight reduction and shock reduction.Their excellent electromagnetic shielding performance and high specific energy also infer broad application prospects in the field of 3C and new energy industry.Hence,magnesium alloys are also known as "green materials in the 21 st century".However,due to the closed-pack hexagonal(HCP)structure,the lack of easy deformation mechanisms at room temperature makes the mechanical properties of magnesium alloys far less than those of steel or aluminum alloys;for the strong asymmetry and anisotropy caused by the special deformation mechanisms,the deformation behaviors and the corresponding mechanical properties of magnesium alloys are quite different and unpredictable under different stress states.Hence,compared with steel and aluminum alloys,it needs to face more complex problems during desiging the performance indexes or developing the new machining processes of magnesium alloys.In this paper,the deformation responses of magnesium alloys under complex stress states(e.g.biaxial tensile or compressive stress states)were systematically studied by using a series of self-developed experimental observation equipment and devices.Scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),focused ion beam scanning electron microscopy(FIB)and digital image processing technology are involved to analyze the evolution of microstructures as well.The biaxial stress state is selected since it is a common stress state during the processing of magnesium alloys.It is also easy to deduce the influence of other stress states on the deformation of magnesium alloys based on the effect of the stress state changing from uniaxial stress state to biaxial.Based on the above results and combining with the improvement of the existing theories and models,a new initial yield criterion is established.It can consider not only the load conditions and the orientation of the textures,but also the effect of micro-deformation behavior on macro-yield behavior of magnesium alloys.The good application of this new criterion on two kinds of magnesium alloys proves its reliability.The main work and results are as follows:(1)A series of experimental devices had been developed,which could be used for in-situ observation of microstructures,adjustment of microstructures and textures,and improvement of mechanical properties of magnesium alloys during deformation under various stress states.These devices were simple in structure.Some of them could be combined with SEM,EBSD and FIB to greatly expand their functions.Others could apply biaxial loading with variable proportion,which offered more control on the adjustment of the microstructures of magnesium alloys.Hence,these devices could provide great convenience for the follow-up research work.(2)Based on Mises criterion,the stress strength was used to normalize the stress tensor,so the imported energe was kept the same during the calculation of global Schmid factor(GSF).The modified GSF for the same mechanism under different stress states had the same comparability.The study based on the new GSF showed that the activation of different deformation mechanisms would change obviously under different stress states.This inferred that the main deformation mechanisms under different loading conditions or processing conditions might be significantly different.(3)By comparing the in situ microstructural evolution of magnesium alloys under uniaxial and biaxial tensile stresses,the activations of prismatic slip and extension twinning were proved significantly inhibited under biaxial tensile stresses.The appearance of a small amount of extension twins further hindered the activation of prismatic slip.However,the appearance of extension twins under uniaxial tensile stress would not have a significant impact on the activation of other deformation mechanisms.The main reason for these phenomena was the comprehensive effect of the stress state and the orientation of the strong basal texture.(4)The evolution of the micro-strain in magnesium alloys during room temperature forming were studied by etching micro-grids on the surface of magnesium alloys via FIB.Digital image processing technology was used to calculate the evolution of micro-strain during the formation as well.The results showed that the distribution of the micro-strain in magnesium alloy was significantly inhomogeneous with increasing biaxial strain.The high strain regions had been determined at the early stage of deformation.With the increase of macro-strain,the micro-strain in the regions with high strain increased rapidly and was much higher than the average strain.The main reason for this phenomenon was that the biaxial tensile stress state and the strong basal texture,which led to a serious lack of easy deformation mechanisms in magnesium alloy during the formation at room temperature.(5)The in-situ experimental results of uniaxial compression and biaxial compression tests showed that the GSF for prismatic slip of magnesium alloy under uniaxial compressive stress state(UC)along the rolling direction was high.However,the activation of tension twins gave a limit of the activation of prismatic slip.In the later stage of deformation,the reorientation of the texture made it harder for prismatic slip to activate.The pyramidal < a+c> slip was therefore activated to coordinate the plastic deformation.On the contrary,under biaxial compressive stress state(BC),prismatic slip could not be activated at the initial stage of deformation due to the geometrical hard orientation of the texture.However,with the increase of deformation,the reorientation caused by extension twinning was beneficial to the initiation of prismatic slip.This also resulted a lower flow stress in the later stage of deformation under BC than that under UC.(6)Based on the above conclusions and combined with the improvement of existing methods for calculating the intergranular strain compatibility in magnesium alloys,a new initial yield criterion was established.The critical resolved shear stress of different deformation mechanisms obtained via VPSC and the texture components with specific gravity were also introduced into the criterion.Hence,this new initial criterion could take into account the micro-deformation coordination mechanism of magnesium alloys.The experimental results showed that the new criterion had high accuracy and could reproduce the experimental results well.