Deformation mechanisms and differential work hardening behavior of AZ31 magnesium alloy during biaxial deformation

Magnesium alloys are frequently subjected to biaxial stress during manufacturing process,however,the work hardening behavior under such circumstance are not well understood.In this study,the deformation mechanisms and differential work hardening behavior of rolled AZ31 magnesium alloy sheets under biaxial loading are investigated.The change of plastic work contours with increasing plastic strain indicates the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state,resulting in higher macroscopic work hardening rates of biaxial loading than uniaxial loading,with the elastic-plastic transition part of work hardening extended and stage Ⅲ hardly emerged.Electron backscatter diffraction and Schmid factor analysis confirm the low activation of non-basalslip during biaxial loading tests.While the thickness strain is primarily accommodated by pyramidal<c+a>slip at the initial stage of biaxial deformation,{10–11}contraction twinning is activated at larger plastic strain.The low activation of non-basalslip also retards the dynamic recovery and cross-slip of basal and prismaticslips,leading to the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state.

Biaxial tensile behavior of CoCrFeNi high-entropy alloy under dynamic and proportional loadings

The dynamic failure behavior of CoCrFeNi High-Entropy Alloy(HEA)under plane biaxial stress was investigated in detail.The dynamic biaxial tensile tests were conducted using an Electromagnetic Biaxial Split Hopkinson Tensile Bar(EBSHTB)system.For comparison,the quasi-static uniaxial and biaxial tensile tests,as well as dynamic uniaxial tensile tests,were per-formed respectively.A cruciform specimen suitable for large plastic deformation was designed and employed in the experiments.The Finite Element Method(FEM)verified that the improved cruciform specimen could satisfy the basic requirements.The feasibility of the proposed specimen was further confirmed through loading tests.Finally,the quasi-static and dynamic yield loci of the HEA in the first quadrant of the principal stress space were plotted.The results indicate that the alloy exhibits obvious strain hardening effect and strain rate strengthening effect,the yield locus and plastic work contours can be accurately described by Hill'48 criterion.

Biaxial tensile behavior of Ti-6Al-4V under proportional loading

The biaxial tensile behavior of isotropic Ti-6Al-4 V is characterized in this paper.A novel cruciform specimen was designed and optimized to achieve uniform stress and strain distribution within the gauge area.Biaxial tensile tests were conducted at three different loading ratios by the biaxial testing machine.The Digital Image Correlation(DIC)technique was applied to determine strain distribution,and a high-speed camera was employed to record the fracture process.An Inverse Analysis(IA)approach with a combined experimental and numerical method was proposed to determine the true stresses at the gauge section of the specimen during biaxial tensile tests.The results indicate that the initial yield locus can be described by the Cazacu criterion accurately,whereas the Mises criterion can predict better the strengthening behavior of Ti-6Al-4 V in the first quadrant in the principal stress space.

Effect of Plastic Anisotropy on the Formability of Aluminum 6016-T4 Sheet Material

Sheet metal formed of lightweight materials such as aluminum sheeting has received great attention related to the reduction of vehicle emissions. This paper evaluates the anisotropic yield locus using Kuwabara＇s biaxial tensile tester and stretches formability using Hecker＇s hemispheri- cal punch stretching test for aluminum 6016-T4 sheet material. The anisotropic yield locus of the A16016-T4 sheet measured is fitted well by the modified Drucker yield func- tion. Moreover the best fitting to the experimental stress- strain curve from the tensile test was obtained by taking an appropriate hardening model. Analytical study to predict the stretch formability by using Hora＇s Modified Maximum Force Criterion （MMFC） was performed. The predicted forming limit curves （FLC） based on various yield functions were compared with the experiments and discussed.

Influence of uniaxial tensile pre-strain on forming limit curve by using biaxial tensile test

This paper focused on the effect of pre-strain on forming limit curves(FLC)of 5754-O aluminum alloy sheet through utilizing biaxial tensile approach.Based on Swift model and Yld2000-2 d yield criterion,the dimensions of cruciform specimen was optimized through applying finite element method for increasing the strain at specimen center.After that,with the recommended specimen size,the cruciform specimen was tested under various stroke ratios to experimentally characterize the limit strains under different pre-strain levels.Subsequently,the biaxial tensile tests were simulated by Abaqus to obtain the limit strains and validate the material models.It can be observed in both experiments and simulations that the pre-strained uniaxial tension followed by plane tension or equi-biaxial tension can improve the formability of sheet metals.Besides,the strain path change affects the trend of first derivative of strain rate difference between neighboring points with respect to time.An early increase occurred and then fell back to the stable value,the steady evolution continued until to a new increase reaching the critical value.The M–K prediction approach was simulated to verify the influence of pre-strain on FLC.It can be found that the early increase peaks of the major strain incremental ratio rose with the amplitude of pre-strain.Finally,the phenomenon of pseudolocalization caused by the strain path change was explained through evolution of stress state inside the groove.

Investigation on Yield Behavior of 7075‑T6 Aluminum Alloy at Elevated Temperatures

Aluminum alloys have drawn considerable attention in the area of automotive lightweight.High strength aluminum alloys are usually deformed at elevated temperatures due to their poor formability at room temperature.In this work,the yield behavior of 7075 aluminum alloy in T6 temper(AA7075-T6)within the temperature ranging from 25°C to 230°C was investigated.Uniaxial and biaxial tensile tests with the aid of induction heating system were performed to determine the stress vs.strain curves and the yield loci of AA7075-T6 at elevated temperatures,respectively.Von Mises,Hill48 and Yld2000-2d yield criteria were applied to predicting yield loci which were compared with experimentally measured yield loci of the AA7075-T6.Results show that yield stress corresponding to the same equivalent plastic strain decreases with increasing temperature within the investigated temperature range and the shape of yield loci evolves nearly negligibly.The experimental yield locus expands with an increase of equivalent plastic strain at the same temperature and the work hardening rate of AA7075-T6 exhibits obvious stress-state-dependency.The nonquadratic Yld2000-2d yield criterion describes the yield surfaces of AA7075-T6 more accurately than the quadratic von Mises and Hill48 yield criteria,and an exponent of 14 in the Yld2000-2d yield function gives the optimal predictions for the AA7075-T6 at all investigated temperatures.

Biaxial tensile behavior and yield loci of dual-phase steel sheets

The biaxial tensile tests were carried out to investigate the deformation behavior and yield loci of dual-phase （DP） steel sheets under biaxial tensile conditions. The true stress-true strain curves of DP steel sheets for different loading ratios were obtained, and the experimental yield loci were determined based on the equivalent plastic work principle. The experimental yield loci were compared to the theoretical yield loci based on Hi1148, Hi1190, Hi1193, Hosford, Barlat89, Yld2000-2d and Gotoh yield criteria. It is found that Yld2000-2d yield criterion can describe the general trends of experimental yield loci of DP steel sheets with comparably higher accuracy. Hosford criterion has the maximum error for DP590, whereas Hi1148 and Barlat89 have the maximum errors for DP780 and DP980.

Deformation mechanism of commercially pure titanium under biaxial loading at ambient and elevated temperatures

Commercially pure(CP)titanium is thermally processed and subjected to biaxial stress.However,the evolution of the microstructural deformation mechanisms under such circumstances is not adequately understood.In this study,the mechanical responses and microstructural deformation mechanisms of TA2 CP titanium sheets under equi-biaxial loading at room temperature(RT),300℃,and 400℃were studied.The activated slip and twinning systems were investigated by transmission electron microscopy(TEM)after polished cruciform specimens were biaxially tensile-tested at RT and elevated temperatures.The results show that{11¯22}contraction twinning and{10¯12}extension twinning are the main deforma-tion mechanisms of RT biaxial deformation,while dislocation glide is predominant in biaxial deformation at 300℃and 400℃.This difference yields varied work-hardening behaviors at RT and elevated tem-peratures.In biaxial deformation at 400℃,the main slip trace type is multiple slip.The interaction of different slip systems in multiple slip created shear deformation concentration areas and further induced cross-slip.However,in biaxial deformation at 300℃,the amounts of simplex and multiple slip were significantly reduced compared to those at 400℃because the lower temperature increased the critical resolved shear stress and insufficient activated slip systems were available for grain deformation.There-fore,several stress-concentration areas were generated with the activation of cross-slip.