Study On The Tension-Compression Yield Asymmetry Of Magnesium Alloy At Room Temperature

Tension-Compression yield asymmetry is an interesting mechanical phenomenonand exists in many metals, including magnesium and magnesium alloys. Thisasymmetry mechanical property needs to be considered in wide application, especiallywhen the production is in reverse load situation, such as cars. It means that it isunscientific to evaluate whether the material is up to application standard or not. On theother hand, magnesium and magnesium alloy have poor plastic formability andrestricted ductility, owing primarily to its hexagonal close packed (HCP) crystalstructure and consequent limitation on number of available slip systems. Therefore, theinvestigation on tension and compression deformation and its tension-compressionasymmetry have been the most key research focuses in this Mg alloys field.In this work, the tension-compression yield asymmetry was investigated from thestandpoint of heat treatment and Al content. Commercial Mg-Al-Zn (AZ series)magnesium alloys present an excellent opportunity to study this effect since it containsdifferent content of Al and a relatively large density of precipitates can be produced byaging. Thus, the extruded AZ31、AZ61、AZ80magnesium alloys were selected as thematerial for study. The effect of heat treatment and Al content on tension-compressionyield asymmetry was investigated, by means of Optical microscopy (OM), X-raydiffraction (XRD) and electron backscatter diffraction (EBSD). Moreover, the atomicmotion of {10-12} twinning in Mg alloys was established to tentatively discuss how Alelement affect {10-12} twinning. The main results are as follows:First, on the basis of {10-12} twinning atomic model, the movement of atoms wasdescribed. By the calculation of displacement vector, the relationship between the axialratio and twinning was established. This is an innovation point in this study which canbe used to analyze the influence of the change in crystal parameter by the addition ofalloying element on twinning behavior.Next, extruded samples of AZ31, AZ61, and AZ80were tensed and compressed atroom temperature. The CYS/TYS ratios of three materials were0.48,0.56and0.65,respectively, suggesting the tension-compression yield asymmetry was weakened.Further studies by OM, EBSD and XRD showed that the tension-compression yieldasymmetry of extruded AZ series Mg alloys was attributed to the difference of twins’amount between tension and compression deformation. With the increasing of Al content,{10-12} twinning was inhibited and then this difference was decreased, therebyimproving the tension-compression yield asymmetry. Besides, the influence of Al ontwinning can be analyzed from three aspects. Firstly, the crystal parameter was changedby the addition of Al element. According to the relationship between the axial ratio andtwinning, the process of twinning during the yield deformation became harder as Alcontent increases，which is a innovation point of this study. Secondly, the {10-12}twinning can be suppressed by the decreasing grain size of AZ61and AZ80comparedwith AZ31. Finally, the precipitations at grain boundary had influence on twinning tosome extent.Finally, the extruded AZ80alloy was applied to solution heat treatment whichresults in almost complete dissolution of the small dark second phase at grainboundaries into the matrix and provides an equiaxed microstructure. However,precipitations can be seen at grain boundaries and within grains by aging treatment. Byusing mechanical test, it was found that aging treatment was much more effective inreducing the asymmetry than solution treatment. The asymmetry ratio increases from0.65to0.67in extruded and solution-treated samples to0.78in aged sample. In order tounderstand the tiny effect of solution treatment on the asymmetry, quadrangularprism-shaped atomic group unit model was used to show that alloying elements insolution suppressed the {10-12} twinning. The smaller grain size also inhibitedtwinning during deformation. Moreover, finer twins and lower twin volume fraction inaged alloy in response to3%compression were observed. Based on the character oftwins in aged condition, the macroscopical interaction between the precipitation andtwin was proposed to illustrate the significant impact of aging treatment on theasymmetry.