Extension Twinning Research Articles

Elucidating the extreme anisotropy in the J-integral value of commercially pure titanium

Extensive multivariant twinning and higher GND-density in TD-RD than RD-ND orientation cause extreme J-integral anisotropy in CP-Ti. Furthermore, c-axis nearly parallel to crack propagation exhibits dominating fluted voids by extensive prismatic slip, while c-axis perpendicular to crack propagation shows flat and fluted voids by dominating multivariant twinning and slip in determining CP-Ti fracture toughness.

The improvement on mechanical anisotropy of AZ31 magnesium alloy sheets by multi cross-rolling process

The rolled AZ31 Mg alloy sheets were manufactured by unidirectional rolling and cross-rolling with two passes and five passes in this study. The improvement of multi cross-rolling on the anisotropy of mechanical properties, microstructure and texture of AZ31 Mg alloys was investigated by tensile fracture experiments, optical microscopy (OM) and electron backscatter diffraction (EBSD). The results indicate that the samples after rolling which were rotated by 90° showed weaker anisotropy than the samples rolled by 0° path. The ultimate tensile strength of the samples after 0° direction rolling increased from 236 MPa to 338 MPa on the transverse direction (TD) compared to as-rolled samples. The ultimate tensile strength of unidirectional rolling samples with two and five passes paralleled to TD is increased by 7% and 14% than that of cross-rolling samples. The enhanced mechanical properties are attributed to fine-grained strengthening, twin strengthening and dislocation strengthening. The {10−12} extension twin, {10−11} contraction twin and {10−12}-{10−11} double twin were investigated in the microstructure of samples by multi cross-rolling. All the samples showed the basal texture on the {0001} < 11–20 > . The basal texture of cross-rolling samples was apparently weakened for comparison to unidirectional rolling samples and the basal texture of five-pass cross-rolling samples was weaker than that of two passes. The improvement of anisotropy after cross-rolling can be attributed to grain refinement, the activation of non-basal slip and the weakening of basal texture.

The annealing strengthening effects in a high-ductile ZG205 alloy sheet

A high-ductile Mg-1.70Zn-0.47Gd (wt.%) alloy sheet was pre-tensile strained to 0.2% and 4% (0.2S & 4S) in the transverse direction with an initial strain rate of 0.001s−1, followed by annealing at temperatures ranging from 100 to 250 °C. The effect of annealing temperature on yield strength (σys) was investigated. Generally, the σys increased up to 200 °C annealing temperature, and beyond this temperature annealing softening was observed. In pre-tensile strained specimens, the more the amount of strain, the higher was the σys. Thus, in 0.2S and 4S specimens, the maximum increase in σys by annealing strengthening alone was 5 and 18 MPa, respectively. After straining at room temperature, different variants of {101‾2} extension twins were observed, and dominant deformation mode was established in un-twinned and severely twinned grains separately by Schmid factor analysis considering the relative activity of slip and twinning. It was concluded that the annealing strengthening mechanism was precipitation strengthening, as revealed by the similar microstructural features in TEM micrographs of 0.2S and 4S specimens.

Suppressed transformation of compression twins to double twins in Mg by activation of non-basal slip

The transformation of compression twins (CT) to double twins (DT) is studied in a dual-textured Mg-6.5%Zn(wt.) alloy during deformation along the extrusion axis. After 7.3% compression, 85% of CT are transformed to DT. This ratio drops to 22% and 36% during tension although the applied stresses and strains in tension were much higher. The Schmid factor of the actual DT variants was very low (and often negative) in tension and compression and could not explain the differences in DT activity. It is shown that the suppressed CT→DT transformation during tension is accompanied by the activation of 〈a〉 non-basal slip -rather than 〈a〉 basal slip- which presumably hinder the transformation because the dissociation of 〈a〉 basal dislocations is necessary to nucleate and grow extension twins in primary CT. These findings point out an effective strategy to suppress DT by activating 〈a〉 non-basal slip, which may be useful to design Mg alloys with high ductility.

Influence of wire rolling on Zircalloy-2: tensile behaviour and microstructural investigation

In the present work, the microstructure, tensile and texture properties of Zircalloy-2 (Zr-2) processed by the room temperature wire rolling (RTWR) have been investigated. The detail microstructural investigation was performed with the help of optical microscopy, Transmission Electron Microscopy (TEM) and Electron Back Scattered Diffraction (EBSD). Dislocation density was calculated by using X-ray diffraction (XRD) through modified Williamson Hall technique. The dislocation density increased as the true strain increased from 0.69 to 1.32, but at the same time when true strain increased from 1.32 to 2.77, dislocations density start decreasing. It was occurred due to the formation of large volume fraction of dynamic recrystallization grains (DRX) after inducing true strain of 2.77. The maximum yield strength of 750 MPa was achieved after true strain of 1.32, but as the induced true strain increased to 2.77, the yield strength decreased to 620 MPa. It was due to the formation of high volume fraction of DRX grain and grain coarsening led by the dislocations annihilation mechanism. The highest ductility was achieved after true strain of 2.77 is attributed to domination of dislocations annihilation assisted mechanism. The EBSD and tensile test investigation further confirmed the presence of Extension twinning ({101¯2}<101¯1>) after inducing a true strain of 1.32 and more, which signify the severe deformation through the RTWR. Further, deformation mechanism of Zr-2 alloy has been proposed through processing by RTWR with the help of experimental investigation.

Deformation mechanisms of as-extruded Mg–3Bi–1Ca (wt.%) alloy during room-temperature tension

In this work, the deformation mechanisms of the as-extruded Mg–3Bi–1Ca (BX31, wt.%) alloy with strength-ductility synergy during room-temperature tension were studied in detail by in-situ electron backscattered diffraction (EBSD), slip trace analysis, visco-plastic self-consistent (VPSC) modeling and transmission electron microscopy (TEM). The quantitative results of in-situ EBSD and slip trace analysis showed that basal <a> slips offered most of the deformation at the early stage, and with increasing strains the proportions of non-basal slips and {10-12} extension twinning gradually increased. At the medium and later stages, each deformation mode approached to be stable and the macroscopic strain became uniform. The critical resolved shear stresses (CRSSs) of different dislocation slip systems were estimated to establish the VPSC modeling. The VPSC-simulated results were highly consistent with the experimental ones. On the other hand, the microcosmic deformation incompatibility gave rise to the hetero-deformation induced (HDI) stress, caused by the distinct schmid factor (SF) and the low geometrical compatibility factor (m’) between adjacent grains. Geometrically necessary dislocations (GNDs, especially for the pyramidal <c+a> edge dislocations) near grain boundaries would be generated to accommodate the deformation incompatibility. In addition, the basal <a> screw dislocations in {10-12} extension twin interiors were also observed, which provided the extra strain hardening capacity.

A mechanism-based multisurface plasticity model for hexagonal close-packed materials with detailed validation and assessment

We present a multi-surface model (MSM) of plasticity to simulate the anisotropic and asymmetric yield responses often observed in hexagonal close-packed (HCP) materials such as magnesium and its alloys. Motivated by the salient outcomes from high-resolution polycrystal plasticity simulations and experimental insights on magnesium alloys, we propose a three-surface representation incorporating two separate anisotropic, symmetric yield surfaces for the soft (basal) and hard (non-basal) glide, and an anisotropic, asymmetric yield surface for extension twinning. This reduced representation of the high-resolution crystal plasticity offers computational attractiveness to analyze large-scale engineering simulations while maintaining key micromechanical details of soft versus hard glide. The model is calibrated to a particular three-dimensional dataset for polycrystal magnesium alloy and is validated against a range of crystal plasticity results for different loading conditions. The capability of the model is then assessed for complex boundary-value problems at the macroscale (round notched bars) and at the microscale (void growth and coalescence). The validation and assessment demonstrate the ability of the proposed MSM to capture finer details of deformation such as identifying regions where soft and hard glide may occur around the void or in other heterogeneous loading states.

Deformation behaviors and microstructure evolution of Ti6321 alloy under air blast loadings

The response of the Ti–6Al–3Nb–2Zr–1Mo alloy plate under air blast loading was studied. The deformation of the target plate under explosion increased with the increase of charge. The failure features included inelastic deformation, partial tearing, and tensile tearing under different degrees of loadings. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) were used to characterize the evolution of microstructure. The result showed that the density of dislocations has a positive correlation with the blast loadings, meaning the major role of the slip. The twinning would help the slip of dislocations in the deformation. The first type of twins generated under blast loadings were {10–12} extension twins. A high-resolution transmission electron microscope (HRTEM) was employed to study the special boundaries of the {11–24} contraction twins and {11–22} contraction twins. Due to the blast loadings, they were not exactly parallel to the theoretical twinning plane. The twin boundary was parallel to the base plane of the matrix.

A mesoscopic damage model for the low-cycle fatigue of an extruded magnesium alloy

This paper presents a novel mesoscopic damage model to characterize the low-cycle fatigue damage evolution of an extruded AZ31 magnesium (Mg) alloy, taking into account the effect of twinning. The damage caused by the slip bands (SBs)–twin boundaries (TBs) and SBs–grain boundaries (GBs) interactions is treated based on the Tanaka–Mura model and the Eshelby inclusion theory. Strain energy values at the TBs and GBs are defined as the TB and GB damage variables, respectively. Explicit formulae for the TB and GB damage evolution are derived from the characteristics of the {101¯2} extension twin and the basal texture. A fracture energy-based crack initiation criterion is established, and the proposed damage model is validated against the existing experimental results. The model demonstrates its ability to reproduce the damage evolution processes, and the predictions of the crack initiation life are within the twice error band.

Identifying the effect of coherent precipitates on the deformation mechanisms by in situ neutron diffraction in an extruded magnesium alloy under low-cycle fatigue conditions

As a new class of heat-treatable magnesium alloys, the low-alloyed Mg-Al-Ca-Mn alloy has great engineering potential because of its excellent extrudability and high strength by the dispersion of Guinier-Preston (G.P.) zones. The complex deformation mechanisms associated with hexagonal crystals, the interactions between defects with such G.P. zones, as well as their impacts on fatigue lifetime, are all critical issues before the actual deployment of such materials. In this study, this type of Mg alloy with and without G.P. zone dispersion during cyclic deformation was investigated by in situ neutron diffraction measurements. The relationship between the macroscopic cyclic deformation behavior and the microscopic response (particularly twinning and detwinning) at the grain level was established. The general deformation mechanism evolution in samples under the solution-treated (S.T.) state was generally similar to that in samples under the peak-aged (P.A.) state. Both samples plastically deformed by extension twinning during compression, and by a sequential process of detwinning and dislocation motion under reverse tension. Results suggest that the precipitates provided the greatest strengthening against the prismatic slip (∼ 45% increase), the moderate to the {101¯2} extension twinning (∼ 27 %), and the least to the basal slip (∼ 11% increase). The P.A. samples have shorter fatigue lifetime primarily due to the excessive dislocation pileups and the higher backstress that promote crack initiation.

Effect of long-period ordered stacking on twinning behavior and mechanical properties of Mg-Al-Y alloy during uniaxial hot compression

Two alloys with (T4-4h) and without (T4-8h) long-period stacking ordered (LPSO) phase were obtained by solution treatment of as-cast Mg-1Al-12Y alloy at 540 ℃ for 4 h and 8 h, respectively. The compressive tests (300 – 450 ℃, ε=0.01s−1) showed that the strengthening effect of LPSO in T4-4h alloy gradually disappeared with temperature increase. In addition, the corresponding compression deformation behavior of the two alloys was also investigated by electron backscatter diffraction (EBSD). It was found that the twinning of the two alloys was dominated by {101¯2} extension twins, and the large amount of lamellar LPSO in T4-4 h alloy inhibited the activation and growth of twins. The [0001] // compression direction (CD) texture formed after deformation is mostly attributed to the activation and growth of extension twins. By analyzing the activated twin variants, it was found that the activation of twin variants mainly depended on Schmid factor (SF). For twin variants with smaller SF, they may be activated due to local stress concentration. Based on the analysis results of in-grain misorientation axis (IGMA) and SF, the deformation mechanism of the alloys at elevated temperature is dominated by basal slip and non-basal slip. The stronger mechanical properties of T4-4h alloy than those of T4-8h are attributed to the obstruction of non-basal slip movement by LPSO.

On the Plasticity and Deformation Mechanisms in Magnesium Crystals

This work presents an overview of the mechanical response and microstructure evolution of specifically oriented pure magnesium single crystals under plane strain compression at room temperature. Crystals of ‘hard’ orientations compressed along the c-axis exhibited limited room temperature ductility, although pyramidal ⟨c + a⟩ slip was readily activated, fracturing along crystallographic 112¯4 planes as a result of highly localized shear. Profuse 101¯2 extension twinning was the primary mode of incipient deformation in the case of orientations favorably aligned for c-axis extension. In both cases of compression along ⟨112¯0⟩ and ⟨101¯0⟩ directions, 101¯2 extension twins completely converted the starting orientations into twin orientations; the subsequent deformation behavior of the differently oriented crystals, however, was remarkably different. The formation of 101¯2 extension twins could not be prevented by the channel-die constraints when c-axis extension was confined. The presence of high angle grain boundaries and, in particular, 101¯2 twin boundaries was found to be a prerequisite for the activation of 101¯1 contraction twinning by providing nucleation sites for the latter. Prismatic slip was not found to operate at room temperature in the case of starting orientations most favorably aligned for prismatic slip; instead, cooperative 101¯2 extension and 101¯1 contraction twinning was activated. A two-stage work hardening behavior was observed in ‘soft’ Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was attributed to changes in the microstructure rather than the interaction of primary dislocations with forest dislocations.

Fe-Mn-Al-C high-entropy steels with superior mechanical properties at 4.2 K

There exist urgent demands to develop structural materials with superior mechanical properties at 4.2 K. Some high-entropy steels (HESs) show potentials as cryogenic materials, but their deformation behaviors and mechanical properties at 4.2 K have been rarely investigated. Moreover, the aging-induced embrittlement of HESs also severely restricts their applications. In this work, the Fe-Mn-Al-C HESs with different Al contents (0, 4, 10 or 15 at.%) and grain sizes were fabricated, and their deformation behaviors and mechanical properties at 4.2 K were systematically studied. With increasing the Al content, (Fe,Mn)23C6 carbides are effectively inhibited in 10%Al HES after aging at 923 K for 10 days. Therefore, the premature fracture of 10%Al HES is avoided, leading to the excellent combination of high strength (∼1.5 GPa) and high fracture toughness (255 MPa·m1/2) at 4.2 K. The deformation mechanisms shift from the extensive twinning and deformation bands in 0Al HES to the lesser twinning, Taylor lattices and deformation bands in 10%Al HES, which contribute to the high strength and ductility of the 10%Al HES. Furthermore, the 10%Al HES with larger grains displays a much higher fracture toughness at 4.2 K, and this inverse size effect on the cryogenic toughness was elaborately revealed.

Effect of 90° route on microstructure of AZ31 magnesium alloy sheets by forging-bending repeated deformation

In this paper, a novel method of severe plastic deformation (SPD), i.e., forging-bending repeated deformation process, was proposed to prepare AZ31 magnesium alloy sheets. The effects of the new process on the equivalent strain, flow rate, microstructure, and hardness at different passes were investigated by finite element method (FEM), electron backscatter diffraction (EBSD) and Zwick/roll hardness tester. The results showed that the effective strain and flow velocity increased with the increase of the pass. Due to the transition path, the shear deformation was sufficient at each position and the uniformity was improved. After 4 deformation passes, the refined grain size was 5.02 µm on average. The dynamic recrystallization (DRX) mechanism in the first pass was discontinuous dynamic recrystallization (DDRX), and the interaction of twins- induced DRX and continuous dynamic recrystallization (CDRX) completed the microstructure evolution in the second pass. Meanwhile, {10−12} extension twins promoted the subsequent CDRX process by dividing coarse grains and changing grain orientation. In addition, slip-dominated pyramidal<c+a> slip was also committed to grain refinement, which can achieve high plasticity. Ultimately, the hardness of AZ31 magnesium alloy can reach 77.6HV after processing and deformation, and the grain size and high-density dislocation had significant effects on the distribution of hardness.

Twinning-induced plasticity with multiple twinning modes and disclinations in Mg alloys

Twinning-induced plasticity plays a critical role in determining the deformation behaviors of hexagonal close-packed metals (e.g., Mg and Ti), due to the lack of five independent slip systems required for a general deformation. In particular, twinning modes that can accommodate c-axis strain (tensile or compressive) are especially important, which are necessary complementary deformation modes to <a>-type dislocations. However, only limited types of extension twins, e.g., {101¯2} type twin, have been unambiguously identified in the literature, which is theoretically inadequate to accommodate a complex deformation with c-axis extension. Using topological defect theory, here we show that another extension twinning mode, {112¯6} twin, can be formed through reactions between disclinations and {101¯2} twins in Mg-alloys. Based on symmetry of the deformation space, we demonstrate that the {112¯6} twin originates from the inter-connection of correlated deformation paths, which can provide a tensile strain of 5.2% along c-axis and a compressive strain of 4.5% along 〈112¯0〉. {112¯6} twins can be formed through three formation mechanisms, i.e., direct formation, twin-twin intersection, and double twinning, which are validated by a combination of experimental characterizations and phase field simulations. Our work not only suggests a symmetry-based method to analyze twinning-induced plasticity, but also provides a new insight into the deformation mechanisms and mechanical properties of Mg-alloys.

Effect of Re and Al additions on the microstructure and mechanical properties of Nb-18Ti-12W alloy

Microstructure and mechanical properties at 25–1200 °C of new refractory alloys, Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA), all compositions are in atomic percent, are reported. The alloys are developed as an alternative to a denser, more expensive and difficult-to-process refractory alloy WC-3009. After production by arc melting and hot isostatic pressing at 1400 °C, 207 MPa for 3 h, the alloys have a BCC crystal structure with the average grain size of ∼150–200 μm and contain low volume fraction of (Ti,N)-rich precipitates near grain boundaries. NTW shows excellent malleability at all studied temperatures. Its yield stress at 25 °C, 1000 °C and 1200 °C is 874 MPa, 353 MPa and 202 MPa, respectively. The NTW-R and NTW-RA alloys are stronger at all the studied temperatures; however, they lack in compression ductility at 25 °C. Extensive deformation twinning and formation of intergranular voids precede the fracture of these two alloys at 25 °C.

Evolution of microstructure and texture of AZ80 magnesium alloy under hot torsion with constant decreasing temperature rate

Hot torsion tests for AZ80 magnesium alloy were carried out in the temperature range of 380 °C-260 °C, with a constant decreasing temperature rate of 10 °C/s in order to weaken the basal texture and refine the grains. The results indicated that the average grain sizes were refined forming gradient structure with increasing specimen radial position from center (12.2–5.4 μm), and that the initial basal texture intensity of the extruded magnesium alloy was weakened from 46.2 to 8.3. Furthermore, the extension twins (ETs) could be disintegrated from the twins forming separated twins with smaller sizes. Interestingly, ETs with the same twin variant intersecting with each other could be coalesced forming grains with similar orientation, while ETs with different twin variants were separated by twins boundaries contributing to grain refinement. Moreover, in addition to the conventional continuous dynamic recrystallized (CDRX) grains with 30˚ orientation rotated around C-axis of the parent grains, CDRXed grains with 30˚ rotation around a-axis and random rotation axis were also discerned. Besides, the CDRX evolution induced twins were also elaborated, exhibiting the complex competition between CDRX and twining. Hot torsion deformation with constant decreasing temperatures rate is an effective way of grain refinement and texture modification.

Precipitate characteristics and precipitation-strengthening mechanism of Mg-13Gd-4Y–2Zn-0.6Zr alloy

In this study, the precipitate characteristics and precipitation-strengthening mechanism of solution-treated Mg-13Gd-4Y–2Zn-0.6Zr alloy after the upsetting and extrusion were investigated at different aging treatment temperatures. The prismatic plate-shaped nano-sized β′ precipitates and the prismatic rod-shaped micro-sized β precipitates were precipitated in low-temperature (e.g. 170 °C) and high-temperature (e.g. 330 °C) aging treatment, respectively. The critical nucleation resolved shear stresses (CRSSN) for the observed {10–12} and {11–21} extension twins and the twin number fractions of the two-type twins in the total twin number were unaffected by the precipitates. However, the greater total twin number in the alloy aged at 330 °C indicates that the strengthening effect of β precipitates on twinning was greater than that of β′ precipitates. The calculation results of Orowan equation showed that the strengthening effect of β′ precipitates on the pyramidal slip was greater than that of β precipitates. The greater hardness for the alloy aged at 170 °C was attributed to the greater strengthening effect of β′ precipitates on the pyramidal slip.

Significant strain hardening ability of AZ91 magnesium alloy fabricated by spark plasma sintering

The strain hardening behavior of AZ91 alloys fabricated by spark plasma sintering (SPS) under different sintering temperatures was investigated by compression at room temperature. The sintered AZ91 alloys with random texture exhibit high yield strength and significant strain hardening ability. The hardening abilities (including strain hardening rate, strain hardening exponent and hardening capacity) increase with increasing sintering temperature. The strain hardening of sintered AZ91 alloys is dominated by dislocation slip and accompanied with extension twinning. The increment of hardening abilities is mainly due to the increasing of dislocation induced strain hardening because the rate of dislocation propagation is enhanced by coarse-grain. Additionally, the strain hardening induced by twinning is suppressed by fine-grain and intragranular lamellar Mg17Al12 precipitate.

Mechanism of Plastic Deformation of As-Extruded AZ31 Mg Alloy during Uniaxial Compression

The deformation mechanism and texture evolution of AZ31 Mg alloy compressed in three different directions at room temperature were studied, and the relationship between the two was compared through experiments and viscoplastic self-consistent (VPSC) modeling. Setting up only one specific deformation mode was the predominant mechanism by changing the CRSS ratio for the different deformation modes. The following conclusions were drawn: (1) It was demonstrated that basal slip causes a slow and continuous deflection of the grain toward the transverse direction (TD). When the sample is compressed in the extruded direction (ED), prismatic slip leads to grains being deflected toward the ED in the initial stages of compression, and when the sample is compressed 45° to the extrusion direction (45ED) and perpendicular to the extrusion direction (PED), prismatic &lt;a&gt; slip contributes little to the texture evolution. (2) When the sample is compressed along three different directions, pyramidal &lt;c+a&gt; slip leads to the grain being deflected toward the normal direction (ND), and the {10-12} extension twin deflects the grain at a large angle. (3) When only the {10-11} compression twin is activated, the grain will be deflected in the ND while the sample is compressed along the ED and 45ED, but when the sample is compressed in the PED, the grains are concentrated from both sides of the ED to the center.