Construction of an Arrhenius constitutive model for Mg-Y-Nd-Zr-Gd rare earth magnesium alloy based on the Zener-Hollomon parameter and objective evaluation of its accuracy in the twinning-rich intervals

According to a high-temperature compression test of rare earth magnesium alloy(WE43),a strain-compensated constitutive model of the Arrhenius equation based on Zener-Hollomon parameters was established,and the rheological behaviors were predicted.The model exhibited relatively serious prediction distortion in the low-temperature and high-strain rate parameter interval,and its accuracy was still unsatisfactory even after modification by a correction operator considering the coupling of temperature and strain rate.The microstructure characterization and statistical analysis showed that a large number of twinning occurred in the parameter intervals with prediction deviation.The occurrence of twinning complicated the local internal stress distribution by drastically changing the crystal orientation and led to significant fluctuations in the macroscopic strain-stress and hardening curves relative to the rheological processes dominated by the dislocation and softening mechanisms,making the logarithm of the strain rate and stress deviate from the linear relationship.This twinning phenomenon was greatly influenced by the temperature and strain rate.Herein,the influence mechanism on twinning behavior was analyzed from the perspective of the interaction of dislocation and twinning.

Effect of rolling reduction on deformation mechanism and twinning behavior of WE43 magnesium alloy

The effect of rolling reduction of the last pass on the dislocation slip and twinning behavior during direct hot rolling of a cast WE43 magnesium alloy at 480℃ was investigated.The results showed that prismatic<á>slip was always the main deformation mode during rolling at 480℃.In addition,the activated twinning type was associated with rolling reduction.The{1012}extension twinning was activated at a slight rolling reduction(2%),while{1011}compression twinning and{1011}−{1012}double twinning were activated at larger rolling reduction(12%and 20%).Schmid factor calculation showed that the activation of{1012}extension twin variants followed the Schmid Law,whereas the activation of{1011}compression twin variants did not follow it.Even if the rolling reduction reached 20%,almost no dynamic recrystallization(DRX)grains were found,presumably because the amount of deformation required for DRX to occur was not reached.

Compressive Deformation of Semi-Solid AZ91D Magnesium Alloy

The compression tests of semi-solid AZ91D Mg alloy have been conducted on a parallel-plate viscometer. The results are as follows. With increasing the compression temperature, the deformation rate or the strain rate of the specimens rises, but the compressive stress continuously decreases; the deformation strain is obviously linear with the compressive stress and independent on compression temperature under a given compression load. In the wake of the compression load being added, the compressive strain increases but the compressive stress decreases clearly; the deformation strain is obviously linear with the compressive stress under different compression load. The mathematical apparent viscosity model about the semi-solid compressed AZ91D Mg alloy has been established, i.e ηapp =2004.2exp（15.61 fs）γ^1.317.fn^-1.3511

High temperature compressive deformation behaviors of extruded AZ91 magnesium alloy

The compressive deformation behaviors of the extruded AZ91 at temperatures around the solidus were investigated. The compressive stress—strain curves were measured, and the hot deformation microstructures were observed. Dynamic recovery (DRV) and dynamic recrystallization (DRX) take place during compression. The main deformation mechanism is climbing of dislocations controlled by the lattice diffusion. The presence of the liquid relaxes the stress concentration which facilitates the deformation. Higher activation energy is attributed to the non-basal sliding in addition to the presence of liquid phases.

Achieving high ductility and strength in magnesium alloy through cryogenic-hot forming

Magnesium alloys are the lightest structural alloys and have attracted substantial research attention in the past two decades. However, their mechanical properties, including ductility and strength, are limited after forming due to the formation of coarse grains and strong texture. This study proposes and proves a new cryogenic-hot forming process concept. Cryogenic deformation is imposed before the hot deformation. The effect of the cryogenic step has been compared with a conventional direct hot deformation process. The mechanical properties, microstructure,and texture of both the novel and conventional process routes have been compared. The cryogenic-hot deformed sample exhibits the highest ductility and fracture strength(ultimate tensile strength: 321 MPa, ductility: 21%) due to effective grain refinement and texture weakening by cryogenically formed twin-twin interaction induced recrystallisation. The proposed cryogenic-hot forming process can be a potential innovative manufacturing method for producing high-performance magnesium components.

Simulation of mechanical behavior of AZ31 magnesium alloy during twin-dominated large plastic deformation

Experiments and visco-plastic self-consistent (VPSC) simulations were used to quantify the amount of twinning and the relationship to stress?strain behavior in a textured Mg?3Al?1Zn plate. Two different compression directions were utilized to favor{1012} extension or{1011} compression twinning.{1012} twins nucleate at the beginning of plastic deformation and grow to consume the parent grains completely. During compression along the normal direction,{1011} twinning and{1011}?{1012} double twinning start at strain of 0.05, and the number of twins increases until rupture, above strain of 0.15.{1011} and{1011}?{1012} twinning also occur during compression along the transverse direction, start at strain of 0.06 and then multiply in grains totally reoriented by{1012} twins. Using suitable parameters, the VPSC model can accurately predict the occurrence of extension, compression and double-twinning as well as the flow stresses and deformed textures. According to VPSC simulations, twinning and slip have the same latent hardening parameters.

Modeling of hot deformation behavior and prediction of flow stress in a magnesium alloy using constitutive equation and artificial neural network(ANN)model

The aim of the present study was to investigate the modeling and prediction of the high temperature flow characteristics of a cast magnesium(Mg-Al-Ca)alloy by both constitutive equation and ANN model.Toward this end,hot compression experiments were performed in 250-450℃and in strain rates of 0.001-1 s^(−1).The true stress of alloy was first and foremost described by the hyperbolic sine function in an Arrhenius-type of constitutive equation taking the effects of strain,strain rate and temperature into account.Predictions indicated that unlike low strain rates and high temperature with dominant DRX activation,in relatively high strain rate and low temperature values,the precision of the models become decreased due to activation of twinning phenomenon.At that moment and for a better evaluation of twinning effect during deformation,a feed-forward back propagation ANN was developed to study the flow behavior of the investigated alloy.Then,the performance of the two suggested models has been assessed using a statistical criterion.The comparative assessment of the gained results specifies that the well-trained ANN is much more precise and accurate than the constitutive equations in predicting the hot flow behavior.

Deformation behavior of AZ31 magnesium alloy at different strain rates and temperatures

The deformation behavior of AZ31 was examined by compression and tension testes over a wide strain rate and temperature range,strain rate from 10-3 to 103 s-1,temperature from 300 to 623 K. Analysis of flow behavior and microstructural observations indicate that in tension tests dislocation glide is the most important deformation mechanism in the test strain rate and temperature range,while in compression tests twinning deformation mechanism is important at lower temperature when the strain rate ranges from 10-3 to 10 s-1. At 103 s-1 strain rate,dislocation glide and twinning are present at the same time. At the strain rate of 2 964 s-1,adiabatic shear band can be found easily,even at the strain rate of 1 537 s-1 adiabatic shear localization zone can be found. In adiabatic shear localization zone,there are fine recrystallization grains. But in adiabatic shear band,the grains cannot be identified by optical microscopy.

Hot compression deformation behavior of MB26 magnesium alloy

The flow stress features of MB26 magnesium alloy were studied by isothermal compression at 300-450 ℃ and strain rate of 0.001-1 s-1 with Gleeble 1500 thermal simulator. In addition,the deformation activation energy Q was calculated. The results show that the strain rate and deformation temperature have obvious effect on the true stress. The peak value of flow stress becomes larger with increasing strain rate at the same temperature,and gets smaller with the increasing deformation temperature at the same strain rate. The alloy shows partial dynamic recrystallization. The flow stress of MB26 magnesium alloy during high temperature deformation can be represented by Zener-Hollomon parameter including the Arrhemius term. The temperature range of 350-400 ℃ is suggested for hot-forming of this alloy.

Crystal anisotropy of AZ31 magnesium alloy under uniaxial tension and compression

To investigate the deformation twinning and the plastic anisotropy of the hexagonal-close-packed（HCP） single crystal, the crystal plastic constitutive model including slip and twinning deformation was established with finite element method based on crystal plasticity theory. The model was verified by test data. Newton-Raphson iteration method was developed with the stress components directly as the basic variables of iteration. The plastic deformation behavior of single crystal AZ31 alloy was analyzed numerically under monotonic tension and compression, respectively, in four different strain paths（i.e. along 〈2110〉, 〈 0110〉, 〈0001〉 and 〈0111〉） with this model. The stress-strain curves were obtained in the above paths. The numerical calculation results show that this crystal model is feasible to predict the activity of slip/twinning system and to describe the number of active twin variants, the types of dominant twin variants and twin intersection. Due to the polar nature of mechanical twinning in inelastic deformation of the material, the plastic behavior of the single crystal material is demonstrated to be notably anisotropic and high asymmetry.

Flow behavior and microstructure of ZK60 magnesium alloy compressed at high strain rate

Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dynamic recrystallization （DRX） developed mainly at grain boundaries at lower strain rate （0.1-1 s^-1）, while in the case of higher strain rate （10-50 s^-1）, DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 ℃, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10-50 s^-1 and in the temperature range of 250-350 ℃ was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.

Microstructure and hot compression behavior of twin-roll-casting AZ41M magnesium alloy

The relationship of true stress and true strain of AZ41M magnesium alloy under twin-roll-cast （TRC） and hot compression was analyzed us- ing a Gleeble 1500 machine. Microstructural evolutions of the TRC magnesium alloy under different deformation conditions （strain, sWain rate and deformation temperature） were examined using optical microscopy and discussed. The relationship of true stress and true sWain pre- dicted that lower deformation temperature and higher sWain rate caused sharp strain hardening. Meanwhile, the flow stress curve turned into a steady state at high temperature and lower strain rate. The intermediate temperature and strain rate （623 K and 0.01 s^-1） is appropriate.

Texture evaluation in warm deformation of an extruded Mg-6Al-3Zn alloy

To assess the effect of strain and strain rate on texture evolution of an extruded Mg–6Al–3Zn alloy,compression tests were carried out.Samples were prepared in the extrusion direction(ED)and normal direction(ND).The compression tests were performed at 250℃ and with different strain rates of 0.01 sec−1 and 1 sec−1 and different strains.Microstructural observation and texture investigation show that at early stages of deformation,extension twins lead to the development of strong basal texture intensity along rolling direction(RD)in ED samples and contraction twins result in texture evolution along transverse direction(TD)in ND samples.Also,microstructural investigation at high strains reveals that dynamic recrystallization occurs in both samples and consequently the basal texture intensity has been decreased.

Effect of twinning on flow stress during initial stage of hot compression of twin roll cast Mg-5.51Zn-0.49Zr alloy

A Thermecmastor-Z hot deformation simulator,optical microscopy,XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 ℃ and 400 ℃ and a given strain rate of 10-2s-1.The results suggest that flow stress drop during initial stage of hot compression at 300℃,generally led by dynamic recrystallization,is attributed to twinning,correspondingly to dynamic recrystallization as deformation temperature is raised to 400 ℃.

Microstructure evolution of casting Mg alloy AM60B subjected to compression deformation

In order to research the microstructure evolution of casting Mg alloy AM60B after compression,the isothermally compressive deformation of different compression ratios followed by metallographic observation was performed.The influence of grain boundaries and second phases on the deformation and recrystallization behavior of the alloy was investigated with optical microscopy,followed by transmission electron microscopy(TEM) to gain an insight into the interplay between the dislocations and microstructure features.The investigation results show that the deformation structure featured by refined grains forms first at as-cast grain boundary when the compression ratio is low,and then spreads throughout the whole cross-section of the casting when the deformation ratio approaches 70%.TEM observation indicates that,dislocations preferentially distribute in the region next to the grain boundaries and second phases,which leads first to the recrystallization occurring there and bounds the recrystallization process in later deformation.Therefore,the grain boundaries and second phases are beneficial to keeping the recrystallized microstructurc with fine grains,and may contribute to the formation of an inhomogeneous grain size distribution on the cross-section of the alloy.

Dynamic recrystallization kinetics of as-cast AZ91D alloy

The flow behavior and dynamic recrystallization（DRX） behavior of an as-cast AZ91 D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220-380 ℃ and strain rate range of 0.001-1 s^-1.The effect of temperature and strain rate on the DRX behavior was discussed.The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates.To evaluate the evolution of dynamic recrystallization,the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves.It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves.A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91 D alloy.

Twin recrystallization mechanisms in a high strain rate compressed Mg-Zn alloy

Static recrystallization of a high strain rate compressed Mg-1 Zn(wt.%)alloy was investigated using electron backscattered diffraction(EBSD).A novel 53°1010 structure was observed in the as-deformed alloy,which showed a{1012}-{1012}double twin relationship with the matrix.When the as-deformed alloy was annealed at 200°C,the{1011}compression twins and{1011}-{1012}double twins showed a higher priority to recrystallize.In addition,the coarse{1012}tension twins and their inner double twins were preferentially to recrystallize,while the lenticular tension twins had little impact on the recrystallization.Therefore,obtaining more compression twins or coarse twins instead of lenticular tension twins can be an effective approach to manipulate recrystallization process in deformed Mg alloys.

Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr稀土镁合金室温包套压缩变形行为及变形机制

在室温条件下对固溶处理的热挤压态Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr稀土镁合金分别进行单轴压缩与包套压缩实验,采用光学显微(OM)、电子背散射衍射(EBSD)、X射线衍射(XRD)和透射电子显微(TEM)等研究了该合金在变形后的微观组织。合金在单轴压缩条件下的最大变形量为18%,而在包套压缩条件下的最大变形量高达32%。结果表明:该合金在两种变形条件下的微观组织演化规律类似,即均会因协调塑性变形的需要而产生{1012}拉伸孪晶和{1121}拉伸孪晶,且前者的数量多于后者。同时,在变形过程中晶粒c轴向压缩方向的聚集程度逐渐增大。在18%变形量条件下,包套压缩试样的位错密度(0.2728 nm^(−2))明显高于单轴压缩试样的位错密度(0.1796 nm^(−2));在18%变形量条件下的单轴压缩试样中,主要激活的位错类型为〈a〉位错,而〈c+a〉位错数量较少;而在包套压缩试样中,〈c+a〉位错被大量激活来承载塑性应变,进而导致Mg-8.5Gd-4.5Y-0.8Zn-0.4Zr稀土镁合金的室温塑性显著提升。

Influence of free-end torsion on compressive behavior of extruded AZ31 rod at various temperatures

The influence of free-end torsion on compressive behavior of an extruded AZ31 rod at various temperatures was studied.Pre-torsion generates a high density of dislocations and a large number of{1012}twins in the matrix,which can largely enhance the compressive yield strength at RT and 100℃.However,with increasing temperature,hardening effect via pre-torsion gradually decreases.When the compressive temperature reaches 300℃,pre-torsion reduces the compressive yield strength.Moreover,initial dislocations and twins via torsion help to refine the sub-structure and accelerate the continuous dynamic recrystallization during compression at 200℃.Thus,twisted sample exhibits more rapid flow softening behavior than the as-extruded sample at 200℃.When compressed at 300℃,the twins and dislocations via torsion were largely eliminated during the holding time,and the discontinuous dynamic recrystallization was enhanced.It is found that the compression curves of twisted sample and as-extruded sample tended to be coincident at 300℃.Related mechanisms were discussed in detail.

Effect of Frequency on Fatigue Lifetime of Extruded Mg-3%Al-1%Zn Alloy

Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1 Hz and 10 Hz in air. The microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）.When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10 Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at these two frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.