Schmid Factor Research Articles

In-situ investigation of coordinated deformation behavior of Ti-6242S alloy with duplex microstructure

In this work, the coordinated deformation behavior related to slip activation, slip transfer and microcrack growth of Ti-6242S alloy with duplex microstructure was investigated through the in-situ tensile testing. It was found that the deformation process of duplex microstructure can be divided into three stages: slip deformation, the formation of slip band and microcrack nucleation, crack propagation and fracture. And single slip was the dominant slip mode, and multiple slip was the auxiliary slip mode. During the initial tensile stage, the slip lines appearing within colony α were shallower than those within primary α owing to the hindering of α/β interface. The slips penetrated across the entire colony region by straight line. This was because there existed a Burgers orientation relationship between lamellar α and β layer, and the orientation of lamellar α was similar. Meanwhile, the common crystal plane between adjacent grains can still provide good condition for slip transfer when the grain boundary was greater than 15°. Moreover, the slip transfer between adjacent slip systems was more prone to occur when there existed good geometric compatibility and high Schmid factor of exit slip system, which provided coordinated deformation between adjacent grains and avoided stress concentration. However, due to the existence of incompatible deformation, the cracks were generated at the stress concentration areas and then grew along the grain boundary or localized slip band.

Microstructure evolvement and wear properties of Ni60-based coating by ultrasonic power-assisted laser cladding

Ultrasonic vibration is known to cause significant refinement of microstructure, the effect of ultrasonic power on grain size, grain orientation and Schmid factor by laser cladding in the Ni60/WC composite coating was studied systematically. Meanwhile, the effect of microstructure on wear properties and hardness was elucidated. The results show that the performance of the cladding layer is the best when the ultrasonic power is 600 W, which is due to the combined effect of ultrasonic cavitation and acoustic flow. The average grain diameter is refined from 28.36 μm to 23.02 μm, and the refinement range is 18.9%. The formation of low Angle grain boundaries (LAGBs) is promoted by ultrasound, and the dislocation density is increased. The texture strength of the coating is distributed over {100}, {110} and {111} surfaces, and the strength decreases from 12.307 to 5.447 The microhardness is increased by 16% (from 768.85HV1 to 896.95HV1), the wear mechanism gradually from adhesive to abrasive wear and the average friction coefficient decreases from 0.47 to 0.216 with the increase of ultrasonic power, which improve the wear resistance of the coatings. Based on the understanding of microstructure evolvement, the grain refinement mechanism effected by ultrasonic power was explained from the aspect of cavitation processes.

Operative slip systems and their critical resolved shear stresses in η-Fe2Al5 investigated by micropillar compression at room temperature

The plastic deformation behavior of single crystals of orthorhombic η-Fe2Al5 has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. Plastic flow is observed even at room temperature by the operation of six slip systems; (001)<010>, (001)<110>, (001)<130>, {22‾3}<110>, {311}<1‾03> and {301}<1‾03>. The CRSS values for the six identified slip systems are very high all in the range of 1.1∼1.5 GPa and do not vary much with specimen size. In the middle of the stereographic projection, the (001)<010>, (001)<110>, (001)<130> and {22‾3}[110] slip systems operate according to the relative Schmid factors with the similar CRSS values in the range of 1.08∼1.23 GPa. In orientations close to [001], the {311}<1‾03> slip system as well as the {301}<1‾03> slip system operate with a much higher CRSS values around 1.5 GPa, producing wavy slip traces due to the occurrence of frequent cross-slip among these slip planes. In orientations close to the [100]-[110]-[010] symmetry line, on the other hand, premature failure occurs without the operation of any slip systems, although, the Schmidt factor-wise, the {311}<1‾03> and {301}<1‾03> slip systems could operate. The selection of slip systems, their CRSS values and the possible dislocation dissociation modes are discussed based on the overlapped atomic volume that occurs during shear along the slip direction on the slip plane, taking into account the partial occupancies of Al atoms in the linear atomic chain along the orthorhombic c-axis direction.

Effect of surface high-density twinned structure on the fatigue crack initiation of Ti–6Al–4V

Fatigue crack initiation sites would transfer from surface to subsurface of metal specimens through shot peening (SP) or laser shock surface strengthening. This phenomenon is typically attributed to the introduction of compressive residual stress on the surface of the specimen during the surface strengthening process. In this study, Ti–6Al–4V specimens featuring a high-density twinned structure on their surfaces, which eliminated compressive residual stress, were fabricated. Fatigue test result revealed that fatigue crack initiation sites in specimens with twinned structures were predominantly located in the subsurface, near the junction area between the twinned structure and the matrix. This phenomenon was accompanied by an increase in fatigue life. The microstructural features in the fatigue crack source were analyzed using focused ion beam (FIB) coupled with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). It was observed that primary crack formed in badly oriented grains that are difficult to initiate dislocation slip according to the Schmid factor (SF). Nonetheless, TEM observations revealed the activation of basal <a> dislocations in these grains, potentially leading to crack nucleation along basal slip bands and subsequent propagation along the basal plane. Our results suggest that a high-density twin structure on the surface can enhance the fatigue strength of Ti–6Al–4V by transferring the fatigue crack source to the subsurface.

Effects of intergranular deformation incompatibility on stress state and fracture initiation at grain boundary: Experiments and crystal plasticity simulations

The heterogeneous deformation of polycrystalline metals inherently originates from the intergranular deformation incompatibility. This paper proposes physical parameters related to the crystal orientations, the Schmid factor of the most activated slip system, and the misorientation angle to characterize the deformation incompatibility between the adjacent grain couple. A comprehensive multiscale investigation is conducted to reveal the mechanism from intergranular deformation incompatibility to fracture initiation at grain boundaries. At the specimen scale, experimental and numerical uniaxial tensile tests are performed on smooth and pre-notched dog-bone specimens to achieve different loading paths on the materials. The heterogeneous fields of stress triaxiality explains the heterogeneous size of the dimples observed in fractography. At the grain scale, electron backscatter diffraction analysis is conducted to characterize the microstructural properties around the nucleated voids within the materials. Voids are captured at the grain boundaries with directions parallel to the loading direction and intergranular deformation incompatibility is characterized using the proposed parameters. Simulations on the plastic deformation of realistic microstructures are performed to clarify the phenomenon. The results reveal that the fluctuation in stress triaxiality at grain boundaries is ascribed to intergranular deformation incompatibility, leading to fracture initiation at these sites. The relationships between the proposed physical parameters of intergranular deformation incompatibility and fluctuation in stress triaxiality are summarized in all circumstances. Finally, the ductile damage at the grain scale is predicted by the Rice–Tracey model, and the results show that the effects of microstructures on heterogeneous plastic deformation and stress state can be well considered.

Activation of contraction twins during compression and related texture characteristics in α-titanium

This work demonstrates that axial compression along 0002and101¯0 in individual grains of α-Titanium can effectively activate up to six variants of 112¯2112¯3¯ contraction twins (CT). Compression parallel to 101¯0 primarily introduces 101¯2101¯1¯ type extension twins (ET), orienting the c-axis almost along the compression axis. Further compression activates up to six 112¯2112¯3¯ CT variants, each with a specific orientation having Euler angles of (0°−60°+n60°, 63.5°, 30°), where n is the sequence number of the CT variant. All six CT variants may not be activated during uniaxial compression due to the deviation of the parent grains' c-axis from the compression direction (CD) and deformation heterogeneities. The nucleation and interaction of CT variants lead to the formation of coincident site lattice boundaries with characteristics (θ−uvtw) angle axis pair of 63.5°101¯0−∑7b, 51.2°11¯00−∑23a, 60°112¯1−∑19b, and 77.3°18¯70−∑17b. The activation of multi CT variants and their interaction can contribute to grain fragmentation and texture weakening. From statistical analyses, the nucleation and growth of CT variants appeared to follow the Schmid law. However, deviations from the Schmid behavior occurred by the activation of low Schmid factor CT variants and could be attributed to slip and twin activities in neighbor grains.

Mechanical properties and microstructure evolution of Sn–Bi-based solder joints by microalloying regulation mechanism

Sn–Bi based solders are used in electronic packaging for interconnection processes. However, the rate of research on the comprehensive performance of solders is difficult to match the rapid development of advanced manufacturing of integrated circuits, resulting in the inability to obtain interconnect structures with excellent reliability for electronic devices. To demand a more effective modification method, we chose to dope 1.0 wt % In element in Sn58Bi–1Sb alloy. The strength, micromechanical properties and creep resistance of the solder were improved due to the combined effect of solid solution strengthening and diffusely distributed second phase strengthening. Furthermore, the addition of In element dramatically improved the thermal properties and wettability due to the generation of Bi3In5 intermetallic compounds (88.9 °C) and the activation energy of the solder wettability reaction was reduced to 247.36 J/mol. Notably, the addition of In element increased the amount of β-Sn phase deviation and decreased the Schmid factor value of β-Sn phase, resulting in a significant increase in the strength and micro-zone creep resistance. Under the action of current, a large amount of uniform Bi particle deviations and sub-crystalline structures persist in the β-Sn phase of the Sn58Bi–1Sb1In solder matrix. In the Cu/Sn58Bi–1Sb1In/Cu joints, many Bi particles are staggered in the β-Sn phase. Since the resistivity of the β-Sn phase is smaller than that of the Bi phase, the energization process leads to a possible further increase of the local currents at certain locations in the β-Sn phase, which reduces the electromigration resistance of the β-Sn phase. After energization, the biphasic twin structure with excellent electromigration resistance starts to degrade. The results show that the doping of In element comprehensively improves the performance of Sn58Bi–1Sb solder. It opens up a new idea for the design of alloying modification of tin-bismuth based solder.

Parameter optimization and anisotropy mechanism in different build directions of the microstructures and mechanical properties for laser directed energy deposited Ti6Al4V alloy

The effects of laser power, scanning speed, and build directions (BDs) on the microstructural evolution and mechanical properties of laser directed energy deposited (LDEDed) Ti6Al4V alloy were systematically investigated in this study. All the finished LDED specimens were not heat treated and the tensile properties were not influenced by surface roughness. Particularly, the differences in grain size, grain boundary distribution, geometrically necessary dislocation (GND) density, grain orientation spread (GOS), and Schmid factor (SF) of the specimens in different BDs were investigated by scanning electron microscopy, electron backscattering diffraction, transmission electron microscopy, and tensile test. The results showed that the aspect ratio, dilution rate, and deposition angle all tended to increase with increasing laser power and scanning speed. The yield strength and the ultimate tensile strength decreased with the increase of laser power, while the elongation was the opposite. The parent grain reconstruction results indicated that the high-temperature transition phase (β phase) of the 0° LDEDed specimen was equiaxial, whereas that of the 90° LDEDed specimen was columnar. The differences in the β phase of the specimens in different BDs were directly inherited to the corresponding low-temperature sub-phase (α phase), resulting in finer grain sizes, greater GND density, larger GOS, and smaller SF in the 0° LDEDed specimen, thereby contributing to higher strength and hardness. In contrast, these values were reversed for the 90° LDEDed specimen, resulting in better plasticity and toughness. Finally, the anisotropy mechanism of the microstructures and mechanical properties of the LDEDed Ti6Al4V alloy in different BDs were revealed. This work could provide more systematic and comprehensive guidance in parameter optimization and offer valuable insights into the anisotropy mechanism of the microstructures and mechanical properties of LDEDed Ti6Al4V alloy.

Effect of grain structure on fatigue crack propagation behavior of 2024 aluminum alloy under different stress ratios

The fatigue load that a material experiences and its microstructure are important factors influencing fatigue crack propagation behavior. This study employed laser scanning microscopy and electron backscatter diffraction (EBSD) technology, along with fatigue crack propagation experiments, to investigate the fatigue crack propagation behavior of 2024 aluminum alloy under varying stress ratios (R). The results showed that the stress amplitude (σp) was the main factor controlling the fatigue crack propagation life (N). Additionally, detailed characterization of the fatigue crack propagation path was conducted using crystal models and EBSD. A fatigue crack propagation model for 2024 aluminum alloy under different R-values was established based on the grain twist angle and Schmid factor. Finally, the impact of R on the crack tip shielding (Ks) was systematically analyzed, elucidating the intrinsic mapping relationship between R, microstructure, and crack propagation characteristics.

The abnormal deformation behaviors in commercial pure Ti during high speed compression

The effect of high speed compression on the deformation behaviors of pure titanium (Ti) was investigated in this paper. The results indicated that deformation twinning dominated the entire deformation process. In addition to the commonly observed twinning modes 101̅2<101̅1>,112̅1<112̅6>and 112̅2<112̅3>twinning, the uncommon twinning modes 101̅1<101̅2>, 112̅4<224̅3> twinning, and a rare twinning mode 112̅3<112̅2> twinning which has not been reported in previous literature, were observed. The 112̅3<112̅2> twinning presented an orientation relationship of 87° <101̅0>. Moreover, deviations from the ideal twinning relationship were observed in some twins which might due to the deformation induced dislocation-twin interactions. Alongside twinning, high angle kink bands with a misorientation of 61°<71017̅3> were locally activated to accommodate the deformation. These abnormal deformation behaviors, including the formation of unusual twins and high angle kink bands, occurred in grains that were less favorable for triggering common dislocation and twinning modes, based on the Schmid factor calculations. The high strain rate during high speed compression limited heat dissipation time and generated additional energy, thereby enhancing the flow stress and raising the temperature, which facilitated the occurrence of these abnormal deformation behaviors.

Unique damage process in micro-sized copper single crystal with double-slip orientation in response to near-[112] tension-compression fatigue

In order to clarify the fatigue process for a micro-sized copper (Cu) single crystal with a double-slip orientation, a tension-compression cyclic loading test was carried out with in situ FE-SEM observation. Although the primary slip system B4 finally brought about a crack from eminent intrusions, the process was complex due to characteristic interactions among multiple slip systems. The process was approximately divided into three stages; early slipping, cave-in and damaging. In the early stage, two slip systems with high Schmid factor (SF), B4 and C1, worked at first, however both immediately stopped working. Extrusions/intrusions were caused by the survived slip system B5 (third-highest SF). In the second stage, a triangular depression was brought about by the cross-slip from B5 to C5 and localized wavy undulation was formed by the continuous cross slips. In the final stage, though both of B4 and B5 worked, the slip system with the highest SF, B4, dominated to form the eminent extrusions/intrusions, resulting in the fatigue cracking. The applied resolved shear stress amplitude was a few megapascals, indicating significantly lower fatigue strength than that of the bulk counterpart while it was in same range for that of micro-sized Cu with a single slip orientation. The complex fatigue damage morphology of the specimen was explained by accounting for the interaction of dislocations between the slip systems.

Optimizing recrystallization behavior induced by Mn micro-alloying for reduced anisotropy in rolled Al–Cu–Li alloys

The application of rolled Al–Cu–Li alloy is significantly limited by its anisotropy of mechanical properties, which is dominated by microstructures including grain morphology and textures. In this work, the Mn micro-alloying effect with different additions of 0∼0.8 wt% on recrystallization behavior, microstructural evolution, and mechanical anisotropy in rolled Al–Cu–Li alloys is investigated. The results reveal that Mn-free alloy exhibit a high volume fraction (53.4 %) of the deformation textures, including Brass ({110}<1 1‾ 2>), S ({123}<634‾ >), and Copper ({112}<111‾ >), resulting in a large anisotropy in mechanical property. With the addition of Mn, the number density of Al–Cu–Mn precipitates is increased in the as-rolled alloy, which promotes the recrystallization tendencies during solution treatment and decreases the anisotropy. Among the experimental alloys, the 0.4 wt% Mn-containing sample exhibits a large fraction (55.6 %) of grains with no preferred orientation and a evident recrystallization textures including Cube ({100}<001>) and Goss ({110}<001>). This contributes to good comprehensive mechanical properties (YS = 551 MPa, UTS = 583 MPa, EL = 12.0 %) with a much lower anisotropy. However, an excessive addition of 0.8 wt% Mn leads to the grain coarsening and deteriorative mechanical properties. Based on the above results, the relationship between the texture and anisotropy is established using the effective Schmid factor and the equivalent slip system number.

Subsurface damage evolution of β-Ga2O3 (010) substrates during lapping and chemical mechanical polishing

For semiconductor single crystal substrates, subsurface damage (SSD) is an important factor for device failure. For β-Ga2O3 with two cleavage planes, the SSD is the key issue that needs to be addressed. In this work, β-Ga2O3 (010) substrates were lapped and polished precisely. The formation and evolution mechanisms of SSD were analyzed by cross-section transmission electron microscopy and wet chemical etching techniques. The linear distribution of scratches was found on the substrates after lapping. After chemical mechanical polishing (CMP), the surface scratches were eliminated. However, the SSD layer may still exist under the surface. Chemical etching was developed to identify the "invisible damage" to the CMP-finished (010) substrate. The SSD including nanocrystalline, tilt boundary, twist boundary, and high-density dislocations in β-Ga2O3 was first reported. Distinctively, there was no slip occurrence in the SSD of the (010) plane different from the (100) and (001) planes. The Schmid factor of multiple slip systems was calculated to analyze the cause of no slip on the subsurface. The formation mechanism of SSD was believed that when the slip system could not be activated, the stress was released through rotation and distortion of the crystal plane. The accumulation of grain boundary dislocations at twist grain boundary junctions provided a driving force for the formation of nanovoids. The relationship of substrate damage evolution and crystal strain with different machining parameters was established by high-resolution X-ray diffraction and Raman spectra. A non-damaging β-Ga2O3 (010) substrate with a surface roughness of less than 0.2 nm was obtained by the optimized CMP process. The research results had guiding significance for damage control of β-Ga2O3 and other brittle crystals in ultra-precision machining.

Effect of grain orientation distribution on the mechanical properties of Al-7.02Mg-1.78Zn alloys

PurposeDuring the forming process, aluminum alloy sheets develop various types of textures and are subjected to cyclic loading as structural components, resulting in fatigue damage. This study aims to develop polycrystalline models with different orientation distributions and incorporate suitable fatigue indicator parameters to investigate the effect of orientation distribution on the mechanical properties of Al-7.02Mg-1.78Zn alloys under cyclic loading.Design/methodology/approachIn this study, a two-dimensional polycrystalline model with 150 equiaxed grains was constructed based on optical microscope images. Subsequently, six different orientation distributions were assigned to this model. The fatigue indicator parameter of strain energy dissipation is utilized to analyze the stress response and fatigue crack driving force in polycrystalline models with different orientation distributions subjected to cyclic loading.FindingsThe study found that orientation distribution significantly influences fatigue crack initiation. Orientation distributions with a larger average Schmid factor exhibit reduced stress response and lower fatigue indicator parameters. Locations with a larger average Schmid factor experience greater plastic deformation and present a higher risk for fatigue crack initiation. RVE with a single orientation undergoes more rotation to reach cyclic steady state under cyclic loading due to the ease of deformation transfer.Originality/valueCurrently, there are no reports in the literature on the calculation of fatigue crack initiation for Al-Mg-Zn alloys using the crystal plasticity finite element method. This study presents a novel strategy for simulating the response of Al-7.02Mg-1.78Zn materials with different orientation distributions under symmetric strain cyclic loading, providing valuable references for future research.

Texture evolution behaviour of α + β titanium alloy during the hot compression in β region

A new dual-phase titanium alloy Ti-5Al-5Mo-2V-2Cr-2Sn-2Zr has shown good application potential in aerospace and shipbuilding industries. This study aims to investigate the texture evolution mechanism of Ti-5Al-5Mo-2V-2Cr-2Sn-2Zr alloy processed in β single-phase region. Thermal simulation, X-ray diffraction and electron back-scatter diffraction were used to test and analyse the microstructure and texture. The &lt;100&gt; // compression direction (CD) texture is enhanced with deformation. Schmid factor (SF) of &lt;111&gt; //CD and &lt;100&gt; //CD grains are calculated. &lt;100&gt; //CD-oriented grains with higher SF are soft oriented grains and more favourable for the deformation coordination. Finally, the texture strengthened by continuous and discontinuous dynamic recrystallization behaviours is analysed in depth.

Double-peak strain hardening behavior of Mg–1.2 wt.%Y alloy

In this study, the mechanical behavior and deformation mechanism of an extruded Mg–1.2 wt.%Y rod under tension and compression along the extrusion direction (ED) were systematically investigated through experiments and crystal plasticity simulations. A double-peak strain hardening behavior comprising five distinct stages was observed under compression along the ED. The five stages are as follows: a fast drop in the strain hardening rate (stage I), steady increase in the strain hardening rate (stage II), gradual decrease in the hardening rate (stage III), second increase in the strain hardening rate (stage IV), and rapid decrease in the strain hardening rate (stage V). This unique strain hardening behavior led to an ultimate compressive strength of up to 539 MPa at a high strain of 0.4. Crystal plastic simulations using an elastic viscoplastic self-consistent model revealed a high activity and a long process of {101¯2} twinning in a strain range of 0–0.35 under compression along the ED. The twinning behavior examined via electron backscattering diffraction indicated that the {101¯2} twinning was activated in both grains with relatively high and very low Schmid factors. Subsequently, the mechanism for the presence of this double-peak strain hardening was established and, finally, the significance of this double-peak strain hardening for strengthening Mg alloys was discussed.

Quantitative evaluation of orientation effects in wear and friction characteristics of pure nickel using experiments and atomistic simulations

Nanoscale tribological behavior of Ni for its applications such as MEMS is an important area of study. The present study investigated the nanoscale friction and wear behavior of Ni on different crystallographic orientations using experiments and molecular dynamics simulations. Results showed an anisotropic friction response, with (100) [001] case showing high COF followed by (111) [1¯10] and (110) [1¯10] cases. Analysis showed that such a trend in COF is observed due to variation in activation of the number of slip systems in each case as determined by the Schmid factor calculations. Dislocation analysis showed that (100) [001] case developed the highest number of glissile dislocations which is attributed to high COF value. The wear volume followed an opposite trend of COF order, and the spatial distribution of displaced material is found to be different in each case, indicating an anisotropic ploughing mechanism. Such behavior is observed due to variations in the availability of the number of slip directions, their orientation on the scratch plane, and dislocation evolution during nanoscratch. Overall, this study provides insights into the complex deformation mechanism and wear behavior associated with scratch tests and has implications for the development of wear-resistant materials.

High cycle fatigue performance at 650 ℃ and corresponding fracture behaviors of GH4169 joint produced by linear friction welding

GH4169 joints manufactured by Linear Friction Welding (LFW) are subjected to tensile test and stair-case method to evaluate the High Cycle Fatigue (HCF) performance at 650 ℃. The yield and ultimate tensile strengths are 582 MPa and 820 MPa, respectively. The HCF strength of joint reaches 400 MPa, which is slightly lower than that of Base Metal (BM), indicating reliable quality of this type of joint. The microstructure observation results show that all cracks initiate at the inside of specimens and transfer into deeper region with decrease of external stress, and the crack initiation site is related with microhardness of matrix. The Electron Backscattered Diffraction (EBSD) results of the observed regions with different distances to fracture show that plastic deformation plays a key role in HCF, and the Schmid factor of most grains near fracture exceeds 0.4. In addition, the generation of twins plays a vital role in strain concentration release and coordinating plastic deformation among grains.

Regulating the microstructure and strength of the advancing side interface zone in AZ31 friction stir welded joint via localized selective remelting

Friction stir welding (FSW) is a solid-state welding technique widely used for various series of magnesium (Mg) alloys. However, in the process of transverse loading for FSW joints in Mg alloys, the ultimate fracture often occurs near the advancing side (AS) interface zone. To enhance the AS interface strength and overall joint strength, this research employed tungsten inert gas welding (TIG) arc to locally remelt the AS interface zone in the FSW-AZ31 joint. The microstructure characterization after remelting showed that two TIG melting zones were formed in the interface area between the AS and nugget zone (NZ). Within these zones, the grains exhibited significant coarsening and the texture intensity noticeably decreased. Conversely, the interface between the retreating side (RS) and NZ, being far from the TIG melting zone, experienced minimal changes in texture and grain size. Mechanical testing conducted at room temperature indicated that the joint ultimately fractured at the RS, resulting in an interface strength difference of ∼16 MPa between the AS and RS. The enhanced strength of the AS interface was attributed to the low Schmid factor of basal slip, reduced dislocation density, and pronounced twinning behavior.

Mechanical anisotropy induced by the competition between twinning and basal slip of AZ31 magnesium alloy under biaxial tension

The mechanical behavior of a hot-rolled Mg AZ31 plate was studied under biaxial tensions along two perpendicular loading axes of x and y in the ND-TD plane, with the x-axis aligned at a 45° counterclockwise angle from the TD (ND and TD refer to the normal direction and transverse direction). A complex strain partitioning in two loading axes is observed with the variation of stress ratio σxx:σyy. For σxx:σyy= 1:1 and 1:1.5, the strains along the x- and y-axes, εxx and εyy, are both positive. However, With the stress ratio of σxx:σyy= 1:3, εyy remains positive, while εxx becomes negative. Interestingly, for σxx:σyy = 1:2, εxx changes from negative during the early stage of loading to positive upon failure. Crystal plasticity simulations and theoretical calculations reveal that the observed strain transition. The reasons for the strain transition with stress ratio is related to the competition between {101¯2} twinning and basal slip. For {101¯2} twinning, the accommodated strains by most of the twins are εxx>0 and εyy>0 for all the stress ratios considered. For basal slip, most of the basal variants accommodate a positive y-axis strain, while for loading cases with low stress ratios, most of the basal variants result in a negative x-axis strain. As the stress ratio decreases, basal slip becomes more active, while the activity of {101¯2} twinning gradually decreases, leading to the observed strain transition. For σxx:σyy = 1:2, the transition from negative to positive is related to the increasing activity of {101¯2} twinning with strain. Theoretical calculations reveal that the competition of basal slip and {101¯2} twinning under biaxial tension is determined by the variation of their Schmid factors with σxx:σyy.