Compression Twins Research Articles

Microstructure and mechanical properties of large-scale Mg-Gd-Y-Zn-Mn alloys prepared through semi-continuous casting

Large-scale Mg-8Gd-4Y-1Zn-Mn (wt.%) alloy ingot with a diameter of 315 mm and a length of 2410 mm was prepared through semi-continuous casting. Chemical composition, microstructure and mechanical properties at different locations of the samples with as-cast, T4 and T6 heat-treated states, respectively, were investigated. No obvious macro segregation has been detected in the high-quality alloy ingot. The main eutectic structures at all different locations are composed of α-Mg, Mg3RE-type, Mg5RE-type and LPSO phases. At the edge of ingot, the unusual casting twins including {101-2} extension twins and {101-1} compression twins were observed due to the intensive internal stress. In T4 heat-treated alloy, the micro segregation was eliminated. The remained phases were α-Mg and LPSO phase. Combined with the remarkable age-hardening response, T6 samples exhibits improved mechanical properties at ambient temperature, which derives from the dense prismatic β' precipitates and profuse basal γ' precipitates.

On the slip and twinning mechanisms on first order pyramidal plane of magnesium: Molecular dynamics simulations and first principal studies

Molecular dynamics simulations and first-principles calculations are carried out on first order pyramidal plane (π1) of magnesium to study both compression twinning (CTW) and dislocation slip. To this end, a generalized stacking fault energy analysis is employed on dense (π1D) and loose spaced (π1L) π1 planes. The crystal shearing resistance is extracted by using a minimum-energy path (MEP) finder called the nudged elastic band (NEB) method. The MEP regarding 1¯012101¯1π1L slip system shows that the unfaulted crystal structure is recovered in the middle of the path with non-straight and pronounced curved slip trajectories. Besides, it will be revealed that metastable configurations on the half of the MEP for 1¯012101¯1π1L slip system are indeed related to a dissociated 〈c + a〉 extended dislocation of loose pyramidal-I 〈c + a〉 slip, 1/31¯1¯23101¯1π1L. Also, after extracting the dissociation mechanism related to this dislocation, it will be shown that loose pyramidal-I 〈c + a〉 slip can involve shuffling. Moreover, the MEP for dense pyramidal-I 〈a〉 slip, 1/31¯21¯0101¯1π1D, shows transmutation of this slip into CTW in the middle of the path. This transmutation process will be further examined on CTW growth, and it will be demonstrated that this CTW mechanism is energetically more favorable compared to other twinning mechanisms.

The strain accommodation of grain boundary and deformation mechanism of directionally solidified Mg alloy

The Mg93.72Gd5.85Y0.43 alloy with columnar grains and preferred orientation of [224(_)5] was selected in this paper, and its deformation mechanism was investigated by room-temperature tensile test, quasi-in-situ observation and Electron Back Scatter Diffraction (EBSD). The results showed that all the columnar grains had the “soft” orientation ([224(_)5]) for both basal <a> slip system and {101(_)2} tensile twinning system. At the initial deformation stage (ε ≤ 3%), basal <a> dislocations started to move and crossed grain boundary, forming the basal-basal slip system pairs (B/B). As the deformation increased (ε ≤ 50%), {101(_)2} tensile twins selectively nucleated in single columnar grain and then expanded and merged rapidly, forming the bands of tensile twins and the basal slip system + tensile twinning-basal slip system pairs (BT/B). At the later deformation stage (ε ≤ 74.5%), the orientation transition brought by tensile twinning triggered the activation of {101(_)1} compression twins, leading to severe stress concentration and crack nucleation near the large-angle grain boundary and twinning boundary. The evolution of B/B pairs to BT/B pairs driven by grain boundary compatibility stress resulted in relaxation of the stress concentration at grain boundaries and guaranteed the excellent plasticity of the experimental alloy. A modified grain boundary compatibility stress calculation was proposed in this paper and was proved to be more effective in predicating the nucleation of tensile twins than the geometric compatibility factor.

Ultralow Temperature Densification of a Titanium Alloy by Spark Plasma Sintering

Full densification of a Ti alloy (Ti6Al4V) is achieved at unprecedented low temperatures of 650 °C and 555 MPa by spark plasma sintering (SPS) without any sintering aid. The alloy demonstrates a globular equiaxed microstructure with comparable mechanical properties as that sintered at conventional high temperatures (950 °C, 60 MPa). In contrast with the diffusion‐driven sintering of the Ti alloy at conventional SPS conditions, the near‐full densification (99%) attained at low‐temperature/high‐pressure regimes is attributed to plastic deformation‐driven mass transport processes. Sintering pressures of 555 MPa result in a dislocation dense microstructure and the activation of compressive twins imparting lattice strains of up to 2.86 × 10−3, suggesting a 25% increase in lattice distortions as compared with those sintered at moderate pressures of 60 MPa. Depth‐sensing nanoscale indentation reveals the globular microstructure of the high‐pressure‐sintered alloy to retain its elastic modulus and hardness of 138 and 4.8 GPa, respectively, with &lt;2% deviation from those sintered at conventional SPS temperatures. This energy‐efficient technique proposes an alternative to thermally exhaustive routines and presents an advantage for engineering titanium‐matrix composites with nanofillers and controlled reaction products by reducing processing temperatures by up to 300 °C.

Influence of trace Ca addition on texture and stretch formability of AM50 magnesium alloy sheet

Abstract The AM50, AM50–0.1Ca, AM50–0.3Ca and AM50–0.5Ca (wt.%) alloys were hot-rolled and their mechanical properties were determined for the purpose of investigating the effect of trace Ca addition on the texture and stretch formability of AM50 alloy. The results show that the addition of trace Ca can effectively modify the basal texture, which is characterized by the split of basal poles deviated from the normal direction (ND) after the hot rolling, while a broad spread of the basal planes toward the transverse direction (TD) after the annealing. Such change of the basal texture is related to the prior formation of massive compression twins and the decrease of the c/a ratio. Erichsen value increases from 2.25 to 4.21 mm with the increase of Ca content. The enhancement of stretch formability is ascribed to the weakened basal texture, which results in the increase of n-value and the decrease of r-value.

Influence of twinning-induced recrystallization on texture evolution in a high strain rate compressed Mg-Zn alloy

Recrystallization texture evolution in a high stain rate compressed Mg-1Zn alloy was investigated using electron backscattered diffraction (EBSD). Besides of the 101¯2 tension twins (T), 101¯1 compression twins (C) and 101¯1-101¯2 double twins (CT), the 53°101¯0 tension-tension double twins (TT) and deviated 75°112¯0 tension twin boundaries (T′) were observed in the as-deformed sample. When annealed, twinning-induced recrystallization occurred in C and CT twins at the very beginning, which was proved to strongly weaken the basal texture due to a strong orientation scattering by the recrystallized grains. In contrast, the recrystallization of TT twins weakened the basal texture insignificantly. In addition, the lenticular T twins tended to de-twin rather than recrystallize, which remained after 24 h annealing.

Mechanism of Secondary Deformation of Extruded AZ31 Magnesium Alloy by Viscoplastic Self‐Consistent Model

The viscoplastic self‐consistent (VPSC) model is used to establish a combination of different deformation mechanisms. By using this model, axial tension and compression tests of extruded AZ31 magnesium alloy at room temperature are simulated. The influence of secondary deformation mechanism (prismatic &lt;a&gt; slip, pyramidal &lt;c + a&gt; slip, and compression twin) on mechanical response and texture evolution is expounded. Increased activity of the prismatic &lt;a&gt; slip is conducive for the improvement of flow stress in mechanical response during axial tension and for the splitting of pole densities in the {0002} pole figure during axial compression. However, increased activity of the pyramidal &lt;c + a&gt; slip causes the basal texture to transfer to the extrusion direction in the {0002} pole figure during axial compression. The compression twinning has a negligible influence on the plastic deformation and mechanical response of AZ31 magnesium alloy during axial tension and compression. However, the compression twinning should be included in VPSC modeling to predict the texture evolution accurately.

Temperature dependent evolution of anisotropy and asymmetry of α-Ti in thermomechanical working: Characterization and modeling

The strong anisotropy, asymmetry and distorted evolution of plasticity in thermomechanical processing of high performance metallic materials such as α-Ti make it difficult to identify the deformation mechanisms. In this research, a general characterization framework was established to determine the anisotropic and asymmetrical deformation behavior of α-Ti under different thermomechanical loading conditions. The temperature dependent discontinuous constitutive framework was developed for modeling of the distorted evolution of plasticity. The link between crystal plasticity simulation and continuum modeling considering the evolving yield surface of materials with temperature was then established. By taking the thermomechanical working of thin-walled α-Ti tube as a study case, the high temperature digital image correlation (HT-DIC) measurement approach, and the electron backscattered diffraction (EBSD) based physical measurement of arc and small-column samples were proposed to obtain the fundamental tension/compression properties and texture evolution of the alloy at different working temperatures. A viscoplastic self-consistent (VPSC) crystal plasticity based numerical simulation was conducted to determine the temperature dependent anisotropy and asymmetry in plastic deformation. A multi-objective optimization method was used to calibrate the VPSC model parameters and an interpolation approach was employed to smoothly identify the nonlinear evolution of yield loci with strain and temperature. By considering five main deformation mechanisms, viz., prismatic slip, basal slip, pyramidal <c+a> slip, {10–12}<10-1-1> tension twinning and {11–22}<11-2-3> compression twinning, the interactive relationships among flow stress, R-value, yield locus, deformation mechanism, texture evolution and working temperature were established and elaborated. The major findings include: 1) α-Ti tube behaves a significant anisotropy and asymmetry in terms of flow stress, R-value, yield locus, and deformed texture at room temperature, but the anisotropy and asymmetry are significantly reduced at elevated temperatures due to the decrease of twinning activity; 2) The temperature dependency of twinning activity in α-Ti depends on the decrease of the critical resolved shear stress (CRSS) value of pyramidal <c+a> slip with temperature; 3) For the materials with the c-axes of grains towards to normal direction (ND), the increasing activities of pyramidal <c+a> slip, tension twinning and compression twinning decrease the R-value; 4) The activation of twinning generates significant anisotropy and asymmetry and leads to an irregular evolution of yield loci. The developed theoretical framework of characterization and modeling is efficient for producing the above findings and provides a basis for generating a whole spectrum of knowledge of the anisotropy and asymmetry in plastic deformation of the high performance materials that are generally difficult to characterize.

Effects of temperature and alloying content on the phase transformation and [formula omitted] twinning in Zr during rolling

The effects of temperature and Ti content on the deformation mechanisms of pure Zr and Zr-Ti alloys were investigated by transmission electron microscopy. The results indicate the existence of a relation between deformation-induced phase transformation from a hexagonal close-packed structure to a face-centered cubic structure and {101¯1} deformation twinning. That is, when one is suppressed, the other will be promoted. The phase transformation was suppressed while the {101¯1} compressive twinning was promoted with increasing the rolling temperature and/or Ti content. This can be attributed to the activation of basal <a> dislocations at high temperature and the increased stacking fault energy with Ti content.

Role of Y on the microstructure, texture and mechanical properties of Mg–Zn–Zr alloys by powder metallurgy

Ultra-fined Mg-5.1 wt%Zn-0.18 wt%Zr-3wt.%Y alloys (denoted as MY) were successfully fabricated via powder metallurgy (PM). The grain refinement, texture evolution, and twinning behaviors were systematically studied. Relationship between texture evolution and microscopic deformation mechanism is also discussed herein. Rare element (Y)-containing alloys with uniformly equiaxed fine grains exhibited weaker texture intensity. Deformation twinning in compression was hardly activated and replaced by dislocation and prismatic slips. The combined effect of the added Y and grain refinement resulted in the simultaneous enhancement of strength and ductility, especially for the compressive yield strength of the extruded alloy. And the tension/compression asymmetry of the material was significantly reduced due to the changed deformation mechanism.

Hierarchical Twinning Induced Texture Weakening in Lean Magnesium Alloys

Rolled and annealed Mg-1wt. %Zn-1wt. %Gd-0.6wt. %Zr (ZEK110) alloys were subjected to room temperature in-plane compression along the rolling direction, followed by isochronal annealing treatments for 1 h. Despite a starting orientation favoring c-axis extension, the as-deformed microstructure revealed a hierarchical network of twins with profuse quantities of second and third generation twinning appearing within primary tension and compression twins. Complex twin-twin and twin–particle interactions were accompanied by the activation of both basal and non-basal dislocation slip in the neighborhood. While the population density of twins nucleated was significantly high, twin growth was severely retarded due to the presence of secondary phases. In terms of the overall distribution of grain orientations, the as-deformed texture displayed a rather weak basal component with a large portion of the basal poles aligned towards the longitudinal direction with an angular spread of ± 30°. Recrystallization commenced within twins, at twin-twin intersections at lower annealing temperatures and occurred additionally near secondary phases at higher annealing temperatures, giving rise to diverse orientations of both basal and off-basal character. Subsequent growth led to favorable coarsening of the off-basal orientations resulting in an overall texture weakening. The findings provide critical insights with respect to engineering high-strength, high-ductility lean magnesium alloys, comprising hierarchical microstructures that are not only associated with favorable crystallographic textures but additionally display a multi-scale strengthening behavior.

The effect of three-dimensional loading and texture on deformation mechanism of Zircaloy-4 alloy: Using space Schmid factor model

The effect of three-dimensional loading and texture on deformation behavior (including basal <a>, prismatic <a>, pyramidal <a>, first and second order pyramidal <a+c> slips, tensile and compression twins) of Zircaloy-4 alloy sheet was systematically investigated using a new model:space Schmid factor model (SSFM). The SSFM was correlated with the results as calculated by EBSD technique. The results of compression tests and EBSD showed that yield strength was depended upon the orientation of the crystal and decreased in the sequence: TD > RD-35° > RD. Then, analysis of relationship between the loading direction and yielding anisotropy of the Zr-4 sheet revealed that the predominant deformation mode of the Zr-4 sheet was pyramidal <a+c> slip when compressed along the transverse direction (TD) and prismatic <a> slip when compressed either along the rolling direction (RD) or 35° away from the rolling direction (RD-35°). Besides this, a few {101‾2}<1‾011> tensile twins (TT) were exclusively observed in each case. However, the TT get activated easily in RD compression as compared to the other two directions. In general, the SSFM provided a well-substantiated explanation of the deformation mechanisms of the Zr-4 alloy sheet under different loading directions.

High density stacking faults of [formula omitted] compression twin in magnesium alloys

101¯1 compression twins with high density stacking faults were studied at atomic scale using Cs-correction transmission electron microscopy. On one side of the 101¯1 twin boundary, there were many steps arranged alternately with the coherent twin boundaries. Most of the steps were linked with stacking faults inside twins. Burgers vector of twinning dislocations and the mismatch strain at steps were characterized. Due to the compressive mismatch strain at steps, the high density stacking faults inside twins were formed at twin tips during twinning process. The localized strain at the steps would be related to the crack nucleation in magnesium alloys.

Exploring the mechanism of \u201cRare Earth\u201d texture evolution in a lean Mg\u2013Zn\u2013Ca alloy

The entire recrystallisation sequence and associated crystallographic texture evolution of Mg-0.8Zn-0.2Ca (wt.%) alloy was tracked using a quasi-in-situ electron backscatter diffraction (EBSD) method. Characteristic “Rare Earth” (RE) texture was formed, originating mainly from double twins and twinning-related shear bands consisting of compression and double twins. The RE textures appeared during the nucleation stage and were preserved during the relative uniform grain growth period because of solute segregation and concurrent precipitation although the alloying element content was very low. Ca and Zn co-segregated along grain boundaries with no evidence that segregation was preferred along special types of grain boundaries. The interactions between deformation microstructures, concurrent precipitation, solute drag, grain growth and texture evolution are discussed in detail. All the results indicate that Ca performs a similar role to that of RE elements in forming RE texture.

Role of microstructure on the tension/compression asymmetry in a two-phase Ti-5Al-3Mo-1.5V titanium alloy

An investigation was conducted to understand the role of microstructure on the asymmetry of mechanical properties in quasi-static tension and compression in a two-phase titanium alloy Ti-5Al-3Mo-1.5V. Different microstructures were obtained (lamellar colony, basketweave and martensitic) on adopting different cooling rates after solution treatment in single phase β region. The microstructure with martensite exhibited higher strain to failure and lower yield strength than basketweave. Detailed TEM microstructural analysis helped reveal the existence of compression twins and strain-induced phase transformation in compression and tension tests respectively, both of which are relatively rare to be observed in two-phase titanium alloys.

Deformation Twinning in Polycrystalline Mg Microstructures at High Strain Rates at the Atomic Scales

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates. A new defect characterization algorithm (extended-common neighbor analysis (E-CNA)) is developed that allows for an efficient identification of various types of twins in HCP microstructures. Unlike other local orientation analysis methods, the E-CNA method allows for atomic scale characterization of the structure of different types of twin boundaries in HCP microstructures. The MD simulations suggest that the local orientation of individual grains with the loading axis plays a critical role in determining the ability of grains to nucleate either compression twins or tension twins. The twinning behavior is observed through nucleation of a pair of planar faults and lateral growth of the twins occurs through nucleation of steps along the planar faults. The kinetics of migration of steps that determine the rate of growth of twins are investigated at the atomic scales. The twin tip velocity computed at high strain rates compares well with the experimentally reported values in the literature.

On the work hardening of titanium: new insights from nanoindentation

Nanoindentation was used to probe the local slip resistance in CP-Ti deformed in compression to different extents. Changes in hardness in the deformed grains and twins were compared with the change in flow stress measured during deformation, with the aim to elucidate the relative contribution of slip and twinning to the work hardening of Ti alloys. The hardness values were calibrated with measurements on binary Ti–Al alloys. The hardness increased only slightly with deformation and cannot explain the observed work hardening. Although twinned regions were found to be harder than the parent grains, this increase was found to be small once the effect of crystal orientation was accounted for. The increase in hardness in the twins was slightly higher for compressive twins than for tensile twins. It is proposed that this modest hardness increase in the twins is more consistent with the presence of twinning stresses than with a change in the local flow stress caused by dislocation interactions. The implications of these findings to the work hardening of CP titanium are discussed.

Crystal plasticity modeling of strain rate and temperature sensitivities in magnesium

This paper develops a single-crystal plasticity model with Johnson–Cook-type hardening laws to examine the strain rate and temperature sensitivities of magnesium (Mg). Slip, twinning and their interactions are deemed the dominant plastic mechanisms. The twinning-induced lattice reorientation is implemented by taking the initial grain after reorientation as a “new” grain with an updated orientation. Distinct strain rate and temperature dependences are considered for different slip and twinning modes. Non-basal slip is believed strain rate and temperature dependent, while compression twinning (CT) is assumed to exhibit temperature sensitivity in a certain temperature range. To validate the proposed model, plane-strain compression tests of Mg crystals under different strain rates and temperatures are simulated. The experimental data available in the literature are compared with the predicted results, and the tendencies of stress–strain curves are analyzed based on the corresponding evolution of slip and twinning. It is found that twinning-induced lattice reorientation significantly influences the mechanical behavior of Mg, especially when tension twinning (TT) dominates the plastic deformation. High temperatures and low strain rates enhance the activity of non-basal slip, and CT becomes easily activated as the temperature is increased beyond $$150\,^{\circ }\hbox {C}$$ , which coincides well with experimental observations. The strain rate sensitivity rising with temperature is also predicted by the model.

Tailoring the Rolling Texture of AZ31 Mg Alloy with Calcium and Tin Addition

A small amount of Ca and Sn is added into AZ31 alloy separately to modify its deformation behavior and optimized rolling texture. Results show that addition of Sn increases the activation of twinning in AZ31. Some unusual double twins, inducing and double twins, are activated in AZ31–0.5Sn alloy. Refined grains with similar orientation as their parents are produced by continuous static recrystallization, which limits texture modification in AZT310. Ca gives rise to a remarkable grain refinement during hot rolling. Recrystallized grains in AZX310 exhibit a weaker double peak texture with a wide orientation spread. It is found that Al2Ca particles simulate recrystallized nucleation and some of these refined grains exhibit non‐basal orientations. Primary compression twins are favorable nucleation sites for recrystallization and they are more effective in texture modification compared to other double twins.

Slip, twinning and twin-twin interaction in a gradient structured titanium

Quantitative analysis on the deformation mechanisms of a rotationally accelerated shot peening (RASP) processed gradient structured Ti was studied by using electron back-scattering diffraction technique. A 2000 μm thick gradient structure was introduced from the strain-free center to the treated surface. The low angle grain boundary fraction evolution showed that, with decreasing distance from the surface, dislocation generated continuously in the core region, then became saturated in the twin transition region, and finally decreased in the nano-structured region. Three twinning systems were identified: 112¯1, 101¯2 extension twin (ET) and 112¯2 compression twin (CT). The volume fraction of 101¯2 ET increased firstly and then decreased when approached to the surface, while 112¯2 CT showed an opposite tendency. The volume fraction of 112¯1 ET decreased gradually form the center to the surface. Both ET ⟷ ET and CT ⟷ CT twin-twin interactions were found to accommodate the strain and result in grain refinement. Glissile or sessile dislocations could be generated as a result of twin-twin interactions. With twin-twin interactions proceeding continuously, twin lamellae became narrower and even de-twinning was triggered. Due to different grain refinement levels, a hardness gradient was also found in RASP-processed Ti.