Tertiary Twins Research Articles

Strength-ductility synergy in twinned titanium fabricated via dynamic equal channel angular pressing and heat treatment

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σy) and total elongation (ɛt) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σyɛt product is ∼45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment.

Triple and double twin interfaces in magnesium—the role of disconnections and facets

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.

Twinning susceptibility of CrCoNi medium entropy alloy to cryogenic temperature during asymmetric rolling

In this study, the CrCoNi medium-entropy alloys (MEA) were processed by asymmetric rolling (298 K) and asymmetric cryorolling (ACR) (173 K and 83 K), respectively. During the rolling process, as the strain increased (from 50% to 80% reduction), the dislocation distribution inhomogeneity intensified. Meanwhile, the variation of twinning in different systems was more prominent. The cryogenic temperature promoted the inhomogeneity of the structure in the CrCoNi MEA, allowing dislocation cells and dislocation tangling zones to generate at lower strains. And the twinning susceptibility to temperature was more notable than that to strain. Secondary twins, and high-density stacking faults were found in the 173 K ACR CrCoNi MEA at low strain. And tertiary twins were found in the 83 K ACR CrCoNi MEA. It can be found from the experimental results that the improvement effect of the cryogenic environment during rolling on the mechanical properties of the CrCoNi MEA was greater than that of the increase in reduction (50%–80%). Asymmetric cryorolling shows a more efficient strengthening influence on CrCoNi MEA subjected to smaller deformation.

Characterization of primary, secondary and tertiary {20[formula omitted]1} successive internal twins in the D019 ordered hexagonal α2-Ti3Al phase

Due to the lack of independent slip systems and the difficulty in mechanical twinning, D019 ordered α2-Ti3Al phase is known to be of substantial mechanical anisotropy, which partly accounts for the low ductility of two-phase (α2+γ) TiAl alloys. However, recent studies have shown that in Al-rich α2 phase compression deformation twins can be activated, which could be beneficial for the ductility of the alloys. In the present study, for the first time, we have revealed a novel internal twinning mechanism of α2 phase in a TiAl alloy deformed at high temperature. Three twin generations of a common type {202¯1}<1¯014> were revealed: Secondary twins formed within the primary twin, and tertiary twins formed in the secondary twin. It is important to note that this internal twin structure can only be observed in the <21¯1¯6> direction but not the commonly used <112¯0> direction. The shear plane P of all twin variants is of type {12¯10}. Due to the variation of the twinning elements between the twin variants, the twinning shear is blunted. The interfaces between the twin variants are characterized by high-resolution electron microscopy. Special attention was paid to the structure of the coherent and incoherent twin boundaries. The effect of chemical ordering is discussed. Our results also shed light on the complex twinning mechanisms in hexagonal structures.

Twinning characteristics in rolled AZ31B magnesium alloy under three stress states

Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR.

Twinning in rolled AZ31B magnesium alloy under free-end torsion

The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stress-shear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains; all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain.

Size-dependent formation and thermal stability of high-order twins in hierarchical nanotwinned metals

Introducing hierarchical twins into nanotwinned (NT) materials is regarded as an effective way to further improve their mechanical properties. It can be imagined that, with the increase of the order of hierarchical twins, it is insufficient to solely take single twin spacing into consideration. For example, the effect of the spacings of primary and secondary twins should be considered together for tertiary twinning. By virtue of theoretical modelling and atomistic simulations, we investigate the influence of low-order twin spacings on high-order twinning. The optimization strategy of high-order twin density and spacings with respect to low-order twin spacings are proposed. It is demonstrated that there exists a trade-off between high-order twin density and twin spacing which can be tuned by the low-order twin spacings. In addition, the atomistic deformation mechanisms related to low-order twin spacings are discussed. Different size-dependent propagation behaviors of partial dislocations are unveiled, relying on the combination of low-order twin spacings. At last, the great thermal stability of high-order twins is also verified, which is attributed to a strong pinning effect of partial dislocations onto low-order twins, leading to a special stress partitioning phenomenon. Our findings may provide a theoretical benchmark for the fabrication of high-order hierarchical nanotwinned (HNT) structures and thus, assisting the design of high-performance mechanical materials.

Deformation Induced Hierarchical Twinning Coupled with Omega Transformation in a Metastable \u03b2-Ti Alloy

Hierarchical twinning, at multiple length scales, was noted in a metastable body-centered cubic (bcc) β-titanium alloy on tensile deformation. Site-specific characterization within the deformation bands, carried out using EBSD and TEM, revealed {332} <113> type primary bcc twins, containing different variants of secondary and tertiary twins, as well as the formation of stress-induced martensite (α”). Within the primary {332} <113> type twin, “destruction” of the prior quenched-in athermal ω phase was observed, while a stress-induced ω phase reforms within the tertiary twins, revealing the intricate nature of coupling between deformation twinning and displacive ω transformation.

Tilt boundaries and associated solute segregation in a Mg–Gd alloy

Microstructures of a Mg–Gd solid solution single phase alloy that has been compressed at room temperature and subsequently annealed have been examined by bright-field and high-angle annular dark-field scanning transmission electron microscopy. It is found that the deformed microstructure contains many nano-sized grains. These nanograins exhibit strong texture: most of them have their 〈12¯10〉 parallel to each other. A range of tilt boundaries are detected between these nanograins, and they can be produced by basal-plane tilt or prismatic-plane tilt about the 〈12¯10〉 axis. Crystallographic analysis indicates that some of these tilt boundaries can be regarded as being generated by impingements of 〈12¯10〉 co-zone variants of {101¯1},{101¯2} and {101¯3} primary twins or secondary or tertiary twins. Segregation of Gd atoms occurs in the tilt boundaries after annealing of the cold deformed sample. The larger size Gd atoms segregate preferentially to dilated atomic sites within, or adjacent to, each tilt boundary to reduce local elastic strain. Consequently, the segregated Gd atoms form a range of unique, chemically ordered patterns specific to tilt boundaries.

Tension-compression-tension tertiary twins in coarse-grained polycrystalline pure magnesium at room temperature

Using electron backscatter diffraction, the microstructural features of tension–compression–tension (T–C–T) tertiary twins are studied in coarse-grained pure polycrystalline magnesium subjected to monotonic compression along the extrusion direction in ambient air. T–C–T tertiary twins are developed due to the formation of a compression–tension double twin inside a primary tension twin. All the observed T–C–T twin variants are of TiCjTj type. TiCi+1Ti+1 (or TiCi−1Ti−1) variants are observed more frequently than TiCi+2Ti+2 (or TiCi−2Ti−2) variants. The number of tertiary twin lamellae increases with the applied compressive strain.

A novel method for predicting variant selection during primary, secondary and tertiary twinning in titanium

The present paper proposes a novel method based on Schmid׳s law by introducing a calculation of “Normalized Schmid factor” as an indispensable criterion for predicting twin variant selection in commercially pure titanium. Through comparing the calculation with experimental results, this novel method is proved to be a more effective criterion than Schmid׳s law and the accuracy is about 90% during primary, secondary and tertiary twinning variant selection.

Mechanisms of exceptional ductility of magnesium single crystal during deformation at room temperature: Multiple twinning and dynamic recrystallization

Specially oriented magnesium single crystals were subjected to plane strain compression along the 〈112¯0〉 direction in c-axis extension at ambient temperature. The samples deformed up to a logarithmic final strain of −1, illustrating exceptionally high formability, even though basal and prismatic slip were initially inhibited due to the loading orientation with respect to the deformation geometry. Macroscopic {101¯2} extension twins forming massively during early stages of deformation completely consumed the whole sample, resulting in new soft orientations for slip. Additional twinning events took place in the form of secondary and tertiary twinning. At somewhat advanced stages of deformation, newly formed (recrystallized) grains were observed within numerous bands associated with former {101¯1} contraction twins within the primary extension twinned matrix. Recrystallized grains were rotated around the c-axis of their parent twin by an average angle of 30°, resulting in a shift of orientations from the second type (〈112¯0〉) prismatic fiber of ideal twin orientations towards the first type (〈101¯0〉) prismatic fiber of recrystallized grains. This led to a substantial weakening of the texture intensity at the final strain, as well as a high frequency peak of 30° grain boundaries in the misorientation distribution. The obtained results are discussed with respect to the texture evolution during multiple twinning in conjunction with continuous dynamic recrystallization at room temperature.

Variant selection during secondary and tertiary twinning in pure titanium

Three generations of twins were identified by electron backscatter diffraction techniques in pure titanium subjected to uniaxial compression at room temperature. Many primary contraction twins were observed as the initial texture was favourable for their formation. Numerous secondary extension twins formed within the primary contraction twins and some tertiary contraction twins within the secondary extension twins. The orientations of these three generations of twins were determined and their associated Schmid factors (SFs) were calculated. The formation of the twin variants selected in each generation and the absence of certain potential variants are explained by rotating the twinning displacement gradient tensor expressed in the twin system reference frame into the crystal reference frame of the relevant neighbouring grain. The presence of the observed secondary and tertiary twins is accounted for in terms of the ease of imposing the required accommodation strains on their neighbours. The results show that secondary twins can form even with low SFs as long as their accommodation takes place by prismatic or basal glide. However, the formation of certain second and third generation potential high SF twins was impeded when this would have required accommodation by the most difficult deformation mode: pyramidal glide.

Twin boundary spacing effects on shock response and spall behaviors of hierarchically nanotwinned fcc metals

Atomistic deformation mechanisms of hierarchically nano-twinned (NT) Ag under shock conditions have been investigated using a series of large-scale molecular dynamics simulations. For the same grain size d and the same spacing of primary twins λ1, the average flow stress behind the shock front in hierarchically NT Ag first increases with decreasing spacing of secondary twins λ2, achieving a maximum at a critical λ2, and then drops as λ2 decreases further. Above the critical λ2, the deformation mechanisms are dominated by three type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other boundaries; (b) partial dislocations emitted from twin boundaries (TBs) travel across other TBs; (c) formation of tertiary twins. Below the critical λ2, the deformation mechanism are dominated by two softening mechanisms: (a) detwinning of secondary twins; (b) formation of new grains by cross slip of partial dislocations. Moreover, the twin-free nanocrystalline (NC) Ag is found to have lower average flow stress behind the shock front than those of all hierarchically NT Ag samples except the one with the smallest λ2 of 0.71 nm. No apparent correlation between the spall strength and λ2 is observed in hierarchically NT Ag, since voids always nucleate at both GBs and boundaries of the primary twins. However, twin-free NC Ag is found to have higher spall strength than hierarchically NT Ag. Voids can only nucleate from GBs for twin-free NC Ag, therefore, twin-free NC Ag has less nucleation sources along the shock direction when compared to hierarchically NT Ag, which requiring higher tensile stress to create spallation. These findings should contribute to the understandings of deformation mechanisms of hierarchically NT fcc metals under extreme deformation conditions.

Observation and Schmid factor analysis of multiple twins in a warm-rolled Mg–3Al–1Zn alloy

This study aims to understand the features of twinning that occurred during warm-rolling of Mg–3Al–1Zn alloys. The rolling was performed at 150°C with the c-axis of most grains nearly parallel to the transverse direction. Electron backscatter diffraction analysis was conducted to examine microstructural evolution of the rolled sample. Multiple twins including {10−12} extension twin, {10−11} contraction twin, {10−11}–{10−12} and {10−12}–{10−11} double twins and {10−12}–{10−11}–{10−12} tertiary twins were observed in the rolled sample. These twins were present even in the same grain orientation. Based on the applied rolling deformation, an effective Schmid factor was calculated to understand the activation of twinning. Calculated Schmid factor maps were obtained as a function of (0001) pole. The activation of {10−12}–{10−11} double twin was explained by the statistical analysis of Schmid factors for primary {10−12} twin and secondary {10−11} twin.

Variant selection of primary, secondary and tertiary twins in a deformed Mg alloy

Samples of magnesium alloy AZ31 were deformed in plane strain compression in a channel die at 100°C and a strain rate of 5×10−3s−1. The initial texture was favorably oriented for extension twinning. At a true strain of ε=−0.11, many primary extension twins were observed to consume their parent grains completely. Furthermore, numerous secondary contraction twins formed within the primary extension twins and some tertiary extension twins grew within the secondary contraction twins. The orientations of the parent grains and all three generations of twins were measured. The twin variants selected during each of the three stages of twinning were determined by electron backscatter diffraction techniques and the absent potential twin variants were also identified. The way in which the selected primary extension twins grow so as to consume the parent grains and contact all the neighboring grains is explained in terms of the accommodation strains imposed on the neighboring grains. The analysis shows that the primary twin selected is not necessarily the variant with the highest Schmid factor but the one that requires the least accommodation work in most of the neighboring grains. The same principle was found to hold for the secondary and tertiary twins. By contrast, potential high Schmid factor twins that required the consumption of appreciable accommodation energy did not form. A Taylor simulation produced similar results and indicates that the accommodation strain concept is consistent with the principle of the minimization of plastic work.

Automated Twin Identification Technique for Use with Electron Backscatter Diffraction

Historically, twinning classification has been obtained by optical microscopy, bulk x-ray and neutron diffraction, and transmission electron microscopy (TEM). Recent research has shown that automated electron backscatter diffraction (EBSD) can be used to quantify twin content and thereby greatly improve the reliability of twinning statistics. An automated twin identification technique for use with EBSD has facilitated a greater understanding of deformation twinning in materials. The key features of this automated framework are the use of the crystallographic definition of twin relationships, and the correct identification of the parent orientation in a parent/twin pair. The complex nature of the parent/twin interactions required the use of a voting scheme to correctly identify parent orientations. In those few cases where the voting scheme was unable to determine parent orientation (&lt; 2%) the algorithm allows for manual selection. Twin area fractions are categorized by operative twin systems along with secondary and tertiary twinning. These statistics are reported for deformation and annealing twin populations in deformed a-zirconium and asannealed 316L stainless steel, respectively. These improved twin statistics can help provide insight into the effect of deformation processes on microstructural evolution, as well as provide validation of plasticity models for materials that exhibit deformation twinning.

Effect of deformation twinning on microstructure and texture evolution during cold rolling of CP-titanium

The evolution of microstructure and texture during cold rolling of commercial-purity titanium (CP-Ti) was studied with particular reference to deformation twinning and dislocation slip. For low to intermediate deformation up to 40% in thickness reduction, the external strain was accommodated by slip and deformation twinning. In this stage, both compressive ({1 1 2¯ 2}〈1 1 2¯ 3¯〉) and tensile ({1 0 1¯ 2}〈1 0 1¯ 1¯〉) twins, as well as, secondary twins and tertiary twins were activated in the grains of favorable orientation, and this resulted in a heterogeneous microstructure in which grains were refined in local areas. For heavy deformation, between 60 and 90%, slip overrode twinning and shear bands developed. The crystal texture of deformed specimens was weakened by twinning but was strengthened by slip, resulting in a split-basal texture in heavily deformed specimens.

Transmission electron microscopy study of twins in melt-textured YBa2Cu3Oxsuperconductors subjected to deformation

Abstract Melt-textured YBa2Cu3O x (123) superconductors have been shown to possess large critical current densities J c when the current is flowing along the strongly superconducting a-b planes. In order to further enhance J c, the number of effective flux pinning centres has to be increased. Whether or not twin boundaries are effective pinning sites is of particular interest since twins are inherent defects found in 123. In this investigation, the twin structure of 123 have been modified by mechanical deformation. It is found that the twin density has been increased by deformation processing, and secondary as well as tertiary twins have been generated to relieve the higher level of stresses. In addition, complex twin features and extensive twin interactions are seen in the deformed superconductors. Transport J c measurements, however, revealed that J c does not increase monotonically with decreasing twin spacing, thereby indicating that twin boundaries are not dominant pinning centres at moderate magneti...

Structural characterization of gallium arsenide epitaxial layers grown on Si(001)

The nucleation and growth of undoped gallium arsenide (GaAs) epitaxial layers, grown by metallo-organic vapour-phase epitaxy (MOVPE) on Si (001) substrates, were investigated by transmission electron microscopy (TEM). The influence of growth time and thermal annealing on the initial stages of growth was mainly considered. TEM studies indicated that GaAs nucleates in the form of three-dimensional epitaxial islands on Si (001) and eventually coalesced into a continuous film as growth proceeds. Dark-field TEM studies showed that high densities of primary and tertiary twins are present in the epitaxial (001) GaAs buffer layers. These microtwins, present in the epitaxial buffer layers, are able to extend to the top surface of the subsequently grown layers and were found to be the main reason for the degradation in the surface morphology of GaAs-on-Si layers.