In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4%(±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at-2% strain gradually increased from 26.3% to 43.5%;the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number;and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

Quasi in-situ EBSD analysis of twinning-detwinning and slip behaviors in textured AZ31 magnesium alloy subjected to compressive-tensile loading

Twinning and detwinning behavior,together with slip behavior,are studied in a textured AZ31 magnesium alloy under compressive and tensile strains along the rolling direction(RD)after each interrupted mechanical test via quasi in-situ electron backscattered diffraction technique.The results show that twinning firstly takes place under the compressive strain along the RD.With the increasing compressive strain,{1012}tensile twins firstly nucleate,then propagate,and finally thicken.While under a reversed tensile strain along the RD,detwinning occurs.No nucleation happens during detwinning.Thus,tensile twins can detwin at lower tensile strain,followed by thinning,shortening,and vanishing.Slips are also activated to accommodate the plastic deformation.In the matrix,prismatic slip can only dominate at relatively high strains.Otherwise,basal slip dominates.While in the twins,prismatic slip can activate at lower strains,which is ascribed to the texture reorientation.

Constitutive Modeling of Slip, Twinning and Detwinning for Mg Alloy and Inhomogeneous Evolution of Microstructure

To investigate the relationship between macro-plastic behavior and meso-deformation mechanism of Mg alloy AZ31, the mathematical models for various deformation mechanisms of slip, twinning and detwinning are established, respectively. Furthermore, in order to capture the Bauschinger effect under cyclic loading, the back stress is introduced into the three independent deformation mechanisms, respectively. Finally, using the above-mentioned model, a new cyclic plastic constitutive model based on the constitutive theory of crystal deformation for magnesium alloy is established. On this basis, the numerical simulation for AZ31 under cyclic loading with the axial strain amplitude of 1.2% is carried out in accordance with the aforementioned crystal plas- ticity theory associated with the representative volume element model. The comparison between the stress-strain curves obtained from the simulation and the experiments shows that the macro- scopic mechanical responses predicted using the proposed model are in good agreement with the experimental results. In particular, the unique characteristics of cyclic macro-plastic behavior observed in the experiments can be satisfactorily captured by the presented crystal plasticity model. At the mesoscale, these features are caused by the alternate occurrence of twinning and detwinning mechanisms. The further analysis of meso-plastic behavior shows that there are het- erogeneous distributions of twinning, stress-strain and stress triaxiality in polycrystal under cyclic loading.

Twinning and detwinning during compression–tension loading measured by quasi in situ electron backscatter diffraction tracing in Mg–3Al–Zn rolled sheet

Twinning and detwinning are the important deformation modes in magnesium alloys during cyclic loading at room temperature. To analyze these two deformation mechanism, cyclic compression–tension experiments were performed on Mg–3Al–1Zn rolled sheet along the rolling direction. In these tests, the microstructure evolutions of a series of grains during deformation were traced by using quasi in situ electron backscatter diffraction(EBSD). Important quantities like the Schmid factors of twinning system, the fraction of twinning during compression, and the fraction of twinning after reverse loading were calculated on the basis of measured quantities. The influence of Schmid factor of twinning variants on detwinning upon reverse loading was analyzed. Detwinning would prefer to proceed during reverse loading if the Schmid factor of twinning in the twinning area before reverse loading is sufficiently large.

Effect of γ phase on mechanical behavior and detwinning evolution of directionally solidified Ni-Mn-Ga alloys under uniaxial compression

Specimens with single-phase martensite, net-like γ/martensite mixed structure and lamella-likeγ/martensite mixed structure were designed to investigate the effect of the γ phase on the mechanical behavior and the detwinning of non-modulated(NM) martensitic variant in Ni-rich Ni-Mn-Ga alloys under uniaxial compression. It can be found the existence of the γ phase significantly enhances the compressive stresses, and the net-like γ phase specimen presents a higher value of compressive strain than that of the lamella-like γ phase specimen. Especially, the detwinning plateau of the lamella-like γ phase specimen is almost invisible due to the martensite colonies with low Schmid factors. Finally, according to the calculation of deformation gradient tensor, we found that the tensors along compression direction(εzz) of net-like γ phase/martensite mixed structure specimen and single-phase martensite specimen are lower than that of the specimen with lamella-like γ phase/martensite mixed structure, which well explained the detwinning strain for these specimens. The present study not only highlights the role ofγ phase on the mechanical behavior, but also provides more guidelines for the mechanical training of Ni-Mn-Ga shape memory alloys.

Experimental investigation on uniaxial cyclic plasticity of cast AZ91 magnesium alloy

The uniaxial cyclic plasticity of cast AZ91 magnesium(Mg) alloy was investigated by conducting a series of cyclic straining and stressing tests at room temperature, and a unique cyclic plasticity(especially for ratchetting) and its physical nature were revealed. The experimental results demonstrate that the cast AZ91 Mg alloy behaviors tension-compression symmetry, because the dislocation slipping and twinning occur during both the tensile and compressive deformations;although the cast AZ91 alloy presents a certain pseudo-elastic behavior during unloading due to the detwinning, there is no obvious S-shaped asymmetric hysteresis loop like that of wrought Mg alloy in the cyclic tensile-compressive tests, and an obvious cyclic hardening is observed;moreover, the ratchetting of the cast AZ91 alloy presented in the cyclic stressing tests depends remarkably on the prescribed mean stress and stress amplitude, but slightly changes with the stress rate, and the evolution of responding peak/valley strain greatly differs from that of wrought Mg alloys and stainless steels. This work provides rich experimental data for establishing the constitutive model of cast Mg alloys.

Unloading behaviors of the rare-earth magnesium alloy ZE10 sheet

Due to their low symmetry in crystal structure,low elastic modulus(~45 GPa)and low yielding stress,magnesium(Mg)alloys exhibit strong inelastic behaviors during unloading.As more and more Mg alloys are developed,their unloading behaviors were less investigated,especially for rare-earth(RE)Mg alloys.In the current work,the unloading behaviors of the RE Mg alloy ZE10 sheet is carefully studied by both mechanical tests and crystal plasticity modeling.In terms of the stress-strain curves,the inelastic strain,the chord modulus,and the active deformation mechanisms,the substantial anisotropy and the loading path dependency of the unloading behaviors of ZE10 sheets are characterized.The inelastic strains are generally larger under compressive Loading-Un Loading(L-UL)than under tensile L-UL,along the transverse direction(TD)than along the rolling direction(RD)under tensile L-UL,and along RD than along TD under compressive L-UL.The basal slip,twinning and de-twinning are found to be responsible for the unloading behaviors of ZE10 sheets.

One Dimensional Modeling of the Shape Memory Effect

This paper aims to build a constitutive model intended to describe the thermomechanical behavior of shape memory alloys. This behavior presents many facets, among them we have considered the simple way of shape memory, which is one of most important properties of shape memory alloys. Because of numerous stages of this effect, the subject was divided into three independent parts. For each part, we built the corresponding thermodynamic potential and we deduced the constitutive equations. To make this model workable, we have developed an algorithm. The simulation was performed using the NiTi as shape memory alloy.

Deformation Mechanism of AZ31B Magnesium Alloy Under Different Loading Paths

Deformation and texture evolution of AZ31 B magnesium（Mg） alloy sheet under uniaxial tension, compression, and reverse loading after different pre-strain（compression and tension） were investigated experimentally. The results indicate that the pre-compressive strain remarkably affects the reverse tensile yield stress and the width of the detwinning-dominant stage during inverse tension process. Similar to stress–strain curve of the uniaxial compression, the curve of reverse tensile yield value also has ‘S＇ shape, and its minimum value is only 38 MPa. The relationship between pre-compressive strain and the width of detwinning-dominant stage presents a linear growth, and the greater the precompressive strain is, the smaller the strain hardening rate of the detwinning-slip-dominant stage is. Compared with the reverse tension under pre-compression, the influence of the pre-tension deformation on the deformation mechanism of subsequent compression is relatively simple. With the increase in pre-tension strain, the yield stress of the reverse loading is rising.

Significant annealing-induced hardening effect in nanolaminate d-nanotwinne d(CrCoNi)_(97.4)Al_(0.8)Ti_(1.8)me dium-entropy alloy by severe cold rolling

Due to the easy coarsening caused by poor thermal stability,the verified annealing-induced hardening in nanograined metals can only maintain at a relatively low-temperature range.In this study,a nanolam-inated(CrCoNi)_(97.4)Al_(0.8)Ti_(1.8)medium-entropy alloy with an average lamellae thickness of∼20 nm embedded by thinner nanotwins was fabricated by severe cold rolling to achieve superior thermal stability.Compared with the conventional nanotwinned CrCoNi with nanotwins inside ultra-fined grains,the hier-archical nanolaminated-nanotwinned(CrCoNi)_(97.4)Al_(0.8)Ti_(1.8) exhibits a significant annealing-induced hard-ening effect,i.e.,hardness increasing from∼250 HV in the original specimen to∼500 HV in the cold-rolled status and finally∼630 HV after annealing at 600℃for 1 h.Detailed microstructure characterizations reveal that the reduced dislocation density and formation of L1_(2)ordered domain are mainly responsible for such hardening effect,which is facilitated by the effectively suppressed coarsening with annealing temperature,i.e.,slow detwinning process and well-retained low-angle nanolamellar structure.The coarsening mechanisms from the cold-rolled nanolamellae to the fully recrystallized micro-equiaxed structures under the annealing temperatures ranging from 400 to 800℃ were also elucidated by atomic observations.