Twin Nucleation Research Articles

Grain size dependence of twin nucleation in magnesium alloys

Twinning plays a prominent role in the plastic deformation of low-symmetry crystals such as magnesium alloys. However, the capture of twin nucleation has been challenging since the process is rapid and difficult to predict, making it impossible to observe directly. Because of this, the understanding of the fundamental twinning mechanism through experiments and simulation techniques incorporating twinning has progressed slowly. In this study, a quantitative analysis integrating an in-situ three-point bending experiment and crystal plasticity modeling was conducted on the twin nucleation in AZ31 magnesium alloys with different grain sizes. The threshold for triggering twin nucleation has been quantified. The prediction ability of three widely used criteria for twin nucleation, i.e., geometrically necessary dislocation (GND) density, von Mises stress, and stored energy density, is evaluated. The results suggest that only the stored energy density is able to capture the twin nucleation location accurately in all the considered grain sizes. The critical value exhibits an exponential reduction with the increase in grain size. Based on the quantitative analysis, the mechanism of the grain size effect on twin nucleation is proposed.

Study on the plasticity improvement mechanism and grain refinement of AZ80 Mg alloy under cryogenic multi-directional forging

The introduction of twin lamellae and dislocation interaction through Multi-directional forging (MDF) at room temperature (RT) has been reported to significantly improve the mechanical properties of Mg alloys. However, the Mg alloys typically exhibited poor plasticity at RT, making it highly susceptible to cracking. This study further enhanced the deformation capacity of AZ80 Mg alloy by adding a pre-cryogenic step before MDF. Compared to direct MDF at RT, MDF at cryogenic temperature (CT, −196 °C) further increased the alloy's deformation capacity from 0.72 (4P) to 1.08 (6P), with the tensile yield strength (TYS) and ultimate tensile strength (UTS) increasing to ∼420 MPa and ∼512 MPa. The alloy subjected to cryogenic MDF exhibited denser twin lamellae and a higher proportion of {10–12}-{10–12} twin interactions. The more frequent twin interactions under CT inhibited excessive twin growth and further increased the dislocation density of the matrix, facilitating the nucleation of new twins. Additionally, with increasing deformation passes, the texture of the RT specimens became more single, while the CT specimen showed a trend toward texture dispersion. The study elucidated that poor forgeability of the alloy under MDF at RT was hindered by unfavorable texture and limited twin activation. However, this issue was mitigated under CT, where the activation of more atypical twins with low Schmid factors (SFs) significantly enhanced the deformation coordination ability. Furthermore, the activation of atypical twins and extensive twin interactions promoted texture dispersion in CT specimens, which facilitated the activation of various slips/twins and suppressed excessive twinning-induced hardening, thereby improving forgeability.

Asymmetrical plastic deformation during spherical micro-indentation of magnesium

The complex deformation of magnesium (Mg) and its alloys has been the focus of many studies in lightweight technologies. In this paper, spherical micro-indentation tests followed by post-test electron microscopy were carried out on large grain pure Mg to isolate the effects of crystal orientation on the activation of deformation along different slip or twinning systems. Both pre- and post-indentation crystal orientations were measured using electron backscatter diffraction (EBSD). The pre-indentation orientations were mapped into a crystal plasticity finite element (CPFE) model to further analyze the results. It is shown that the resulting deformation twinning and the degree of indentation-induced misorientation were strongly correlated with the crystal orientation in the region of the indentation. Depending on the crystal orientation, multiple waves of basal slip were observed to form asymmetrically around the indents. These slip bands lead to more than 12° lattice rotations that are captured by CPFE modeling. For the first time, it is shown that indentation can lead to significant out-of-plane displacement field that can induce twin nucleation at the interface of far-field (>100 μm) neighbouring grains. CPFE simulations indicate that maintaining far-field strain compatibility leads to the nucleation of twins rather than a slip transfer or slip-induced twinning mechanism.

Interplay Between Nucleation and Kinetics in Dynamic Twinning

Abstract In this work, we apply a phase-field modeling framework to elucidate the interplay between nucleation and kinetics in the dynamic evolution of twinning interfaces. The key feature of this phase-field approach is the ability to transparently and explicitly specify nucleation and kinetic behavior in the model, in contrast to other regularized interface models. We use this to study two distinct problems where it is essential to explicitly specify the kinetic and nucleation behavior governing twin evolution. First, we study twinning interfaces in 2D. When these interfaces are driven to move, we find that significant levels of twin nucleation occur ahead of the moving interface. Essentially, the finite interface velocity and the relaxation time of the stresses ahead of the interface allow for nucleation to occur before the interface is able to propagate to that point. Second, we study the growth of needle twins in antiplane elasticity. We show that both nucleation and anisotropic kinetics are essential to obtain predictions of needle twins. While standard regularized interface approaches do not permit the transparent specification of anisotropic kinetics, this is readily possible with the phase-field approach that we have used here.

Near-α titanium alloy dwell load-induced deformation twinning to coordinate the deformation mechanism associated with crack initiation

In this study, we identified a specific phenomenon of coordinated deformation of twins in near-α titanium alloys during dwell fatigue (DF). The main crack source regions and internal cracks in the micro-texture region (MTR) and no-MTR samples with inconsistent orientation characteristics were characterized. The results demonstrate that the main cracks in all specimens are aligned with the (0001) basal plane, which is associated with basal slip. Notably, twins are found and confirmed to be involved in the DF crack initiation process in high Al content near-α Ti60 alloys where deformation twins are rare, and tend to nucleate at the DF basal cracks. Further in-situ dwell investigation reveals and proposes that the dislocation pile-up and prismatic-basal (PB) interfacial features between soft/hard grains lead to deformation twin nucleation and growth from hard grain boundaries. Concurrently, the pyramidal 〈c+a〉 dislocation slip is observed to be in concurrent operation with the induced twins. These findings suggest that deformation twins not only coordinate basal grain deformation but also hinder the initiation and propagation of basal cracks caused by basal 〈a〉 slip. Moreover, a high density of pyramidal 〈c+a〉 dislocations within the twin and their dissociation to form basal stacking faults (SFTs) with a specific nanometric spacing make the twinning process accompanied by significant lattice distortions inside the twin. These newly formed nanotwin boundaries and SFTs act as barriers to dislocation motion, enhancing the strength and DF lifetime of near-α Ti60 alloys and effectively reducing the dwell sensitivity of no-MTR alloys. Our findings extend the understanding of the coordinated roles of dislocation slip, twin nucleation and formation of basal SFTs in near-α titanium alloys in dwell fatigue.

Hierarchical nanolayered structures-enabled record-high fracture resistant zircaloy

As a widely used nuclear structural material, zirconium (Zr) alloys are susceptible to brittle hydride-induced cracking. The intrinsic inability to arrest this cracking tendency in Zr poses a significant threat to the service safety of Zr cladding tubes. Here, we propose a microstructural design strategy, by introducing a hierarchical nanolayered duplex-phase structure in the Zr-2.5Nb alloy, to effectively inhibit the propagation of cracks. An unprecedented fracture toughness of KJIc ∼ 165 MPa·m1/2, corresponding to JIc ∼ 256 kJ/m2, is achieved due to the uniform and dense plastic deformation that develops ahead of the crack tip, which is uncharacteristic of Zr-based materials. Our analysis reveals that the effective crack tip blunting occurs because the high density of multiply oriented α/β interfaces in the hierarchical Zr-2.5Nb alloy stimulates ample <c+a> dislocations and facilitates deformation twin nucleation, both of which are challenging to activate in conventional Zr-based materials at room temperature. The exceptional fracture toughness enabled by the hierarchical nanolayered structure provides a new pathway to designing damage-tolerant hexagonal metals for safety-critical applications.

Rate-independent mechanism of deformation twinning in single crystal magnesium

The dynamics of {101‾2}〈101‾1〉 extension twins in a single crystal of pure magnesium are studied through uniaxial compression experiments coupled with direct optical imaging. The experiments covered a seven orders of magnitude strain rate, between 10−5−102s−1, covering all rates of engineering applications of magnesium and its alloys. Under uniaxial compression along a direction perpendicular to prismatic planes, only two variants of extension twins are activated, facilitating in-situ tracking of individual twin boundaries throughout the deformation process. Across the entire strain rate range, the twinning transformation evolves in a self-similar pattern of twin nucleation and thickening. Deformation fronts initiate at the edges of the sample and propagate toward the center, creating parallel twins with characteristic thicknesses and distances from each other. The velocity of the deformation fronts scales linearly with the imposed external strain rate and a nearly uniform resolved shear stress is measured over the entire strain rate range. We show that this unique process is attributed to the constant ratio between the nucleation rate and sideways velocity of individual twin boundaries, where both processes are proportional to the imposed strain rate. These findings indicate that, within the large strain rate range investigated in this study, the evolution mechanism of extension twinning is practically rate-independent.

Shear response of 〈110〉 asymmetric tilt grain boundaries in BCC-Fe

The present study investigated the effect of shear load on the 110 asymmetric tilt grain boundaries of body-centered cubic iron with molecular dynamics simulations. Various mechanisms like grain boundary (GB) sliding, shear-coupled migration, GB migration with rotation, GB decomposition, and GB deformation with dislocations emission were observed. Low-angle tilt GBs migrated with the help of two dislocation mechanisms, gliding and climbing. Two types of twin nucleation mechanisms were noticed in high-angle tilt GBs with varying tilt angles (η). For 16° ≤ η ≤ 112° and η ≥ 130° cases, dynamic self-adjustment of the atoms at the GBs caused GBs decomposition with the formation of twin boundaries (TBs). Newly nucleated TBs migrated under further applied shear load. For 112° < η < 130° case, TBs nucleated from the GB on either side of the misfit dislocations due to their localized core structure on the GB plane.

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

A novel Al-Zn-Mg-Cu-Si-Zr-Er alloy fabricated by laser powder bed fusion

A novel Al-Zn-Mg-Cu-Si-Zr-Er as-printed alloy with a tensile strength of 450 MPa was successfully prepared by laser powder bed fusion (LPBF). Solidification cracks were effectively suppressed by the synergistic effect of Si, Zr and rare earth Er. Many cellular eutectic structures composed of α-Al, Mg2Si and Al2CuMg phases were observed in the alloy, with Al3(Er, Zr) and MgZn2 as the fine precipitates inside the cells. In addition, twins were observed for the first time in Al-Zn-Mg-Cu alloy prepared by LPBF due to the high rapid solidification inherent to LPBF. Twin boundaries acted as nucleation point for the precipitation of Mg2Si and Al2CuMg, which is called “twin nucleation” theory. This study could provide a scientific theoretical reference and new approach for the preparation of high-strength aluminum alloys by LPBF.

In-situ deformation mechanisms of a novel Ti-5Mo-4Cr-1V-1Zr metastable β titanium alloy

To overcome the strength-plasticity trade-off in the structural titanium alloys, a novel metastable β titanium alloy Ti-5Mo-4Cr-1V-1Zr (Ti-5411) with high strength and high plasticity was designed by the d-electrons theory, average electron-to-atom ratio (e/α‾) and atomic radius difference (Δr‾) theory. Combined in-situ scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), the deformation mechanisms of the novel Ti-5411 metastable β titanium alloy were systematically investigated. The results show that the Ti-5411 alloy exhibits excellent yield strength (∼689 MPa), tensile strength (∼930 MPa) and total elongation (∼39%). The in-situ tension indicates that slip activities, crystal rotation, stress induced martensite (SIM) α″ transformation and {332}<113> deformation twin are the major deformation mechanisms of Ti-5411 alloy. Besides, with the increase of strain degree (0–0.5 mm displacement), deformation twins increase, widen and interlace. At 0.35 mm tensile displacement, the orientation of the β grains rotates ∼6.65° to accommodate the increased macrostrain. Additionally, martensite α″ also assists the nucleation of twins. Some {332}<113> twins grow and merge by consuming martensite α″ during deformation, and the residual martensite α″ remains in the merged twins.

Multi-scale annealing twins generate superior ductility in an additively manufactured high-strength medium entropy alloy

Coherent twin boundaries (CTBs) are internal planar defects that offer a promising pathway for designing advanced metallic materials with superior strength-ductility synergy. However, incorporating nanoscale CTBs into additive manufacturing (AM) microstructures is highly challenging without severe plastic deformation. Here, by utilizing the intrinsic cellular structures in AM alloys, we for the first time achieved a high density of multi-scale annealing twins in a laser powder bed fusion (LPBF) Ni35Co35Cr25Ti3Al2 medium-entropy alloy. These multi-scale annealing twins, together with nanoprecipitates and dislocations, resulted in gigapascal strength (∼1.4 GPa) and substantial tensile ductility (∼25 %). We reveal that the AM-induced cellular structures, decorated with entangled dislocations and Ti segregation at the cellular boundaries, facilitate the abundant nucleation of multi-scale annealing twins through interactions with migrating recrystallization boundaries. Additionally, the cellular precipitation networks enhance the thermal stability of nanoscale annealing twins. Frequent dislocation-TB interactions during deformation contribute to superior strain hardenability and thus good ductility. Synergized multiple strengthening mechanisms, i.e., boundary strengthening, precipitation strengthening, and dislocation strengthening, are responsible for the excellent strength. Our present findings advance the design of AM microstructures by harnessing the beneficial effects of cellular structures and provide valuable guidance for developing alloys with exceptional mechanical properties.

Hardness and microstructural evolution of CoCrFeNi high-entropy alloys during severe plastic deformation

In this study, the evolution of hardness and microstructure in a CoCrFeNi high-entropy alloy processed by high-pressure torsion was investigated. An initial increase in hardness was attributed to the rapid increase in low-angle grain boundaries (LAGBs), accompanied by a high density of dislocations. With continuous straining, some LAGBs evolved into high-angle grain boundaries and completed the grain boundary strengthening. A remarkable increase in hardness was achieved in the final stage of hardness enhancement, primarily due to the formation of numerous twins within nanograins, which were mainly formed by the emission of Shockley partial dislocations emerging from grain boundaries. Furthermore, a twin trailing the stacking fault was found in the nanograin, and a critical stress for twin nucleation was estimated to be about 2.16 GPa. Defects with low-energy configurations are closely correlated with the thermal stability of nanocrystalline CoCrFeNi alloys. The defects within nanograins yielded an average strain of about 1.1 % and the corresponding strain energy was identified to about 17 J g−1.

Atomic insights of grain boundary behaviors in TaWNbMo refractory high entropy alloys

Refractory high entropy alloys (RHEAs), such as TaWNbMo, demonstrate promising potential for high-temperature applications owing to their excellent mechanical properties. Given the pivotal role of grain boundaries (GBs) in determining the mechanical behavior of RHEAs, it is imperative to investigate their characteristics. In this work, the mechanical response of TaWNbMo RHEA and its components (Ta, W, Nb, and Mo) bicrystal samples with Σ3{112‾}<110> and Σ3{111‾}<110> GB under shear were performed using molecular dynamics simulations to investigate the motion characteristics of GBs and their influence on mechanical properties, as well as the variations brought by high entropy GBs. Both types of GBs were observed to migrate coupled with shear deformation, and the migration mechanism was analyzed based on the theory of interface defects. Additionally, twin nucleation and growth were observed in the samples with Σ3{111‾}<110> GB influenced by the high GB energy, and its mechanism was analyzed. Furthermore, a comparison was made between TaWNbMo and its component element metals, and the influence of high entropy GBs was analyzed. The multicomponent complexity invoked the local interfacial disconnection nucleation in HEA for both Σ3{112‾}<110> and Σ3{111‾}<110> GBs, the inhomogeneity of HEA also influenced the twin growth process from Σ3{112‾}<110> GB to proceed and thicken in a localized and non-uniform manner. These results contribute to a deeper understanding of the GB characteristics of TaWNbMo RHEA.

The influence of alien dislocations on the plasticity of [formula omitted]-axis Mg single crystals under tension and compression

Uniaxial tension and compression tests were conducted on as-grown [0001] Mg single crystals and on [0001] single crystals containing an alien distribution of basal dislocations to elucidate their effect on the deformation mechanisms and strain hardening. During tensile deformation, the basal dislocations induce elasto-plastic transition and micro-yielding that precede the gross deformation by twinning. They decrease the stress for twin nucleation, the tensile strength, and the failure strain compared to the virgin single crystals. The weakening of the microstructure and early failure are attributed to the higher density of micro cracks formed in the crystals with alien basal dislocations. The extraneous basal dislocations alter entirely the deformation mechanisms and strain hardening characteristic of [0001] single crystals under compression. They suppress the pyramidal slip as a dominant deformation mode and promote the gross deformation by cooperative basal slip and twinning during extended deformation plateau. The deformation twinning is of anomalous type, nucleating and propagating with the assistance of extraneous basal dislocations at the negative Schmid Factor and against the imposed compressive strain. τtw≈2.1±0.2MPa is found as the basal slip-assisted twinning stress in Mg.

Enhanced grain boundary cohesion mediated by solute segregation in a dilute Mg alloy with improved crack tolerance and strength

Effects of solute segregation at grain boundaries (GBs) on the deformation mechanism and fracture behavior remain obscure for magnesium (Mg) alloys. Here, by introducing Zn segregation at GBs, we obtained an Mg-0.5Al-0.4Mn-0.2Ce-0.4Zn (wt.%) alloy achieving fracture elongation (FEL) of ∼33.6 %, with a remarkable FEL improvement by 100 % in comparison to Zn-free counterpart. Meanwhile, the tensile yield strength (TYS, 195.5 MPa) is increased by ∼27.5 MPa after trace Zn addition. Although trace Zn addition improves the fraction of GBs with high misorientation, it reconciles crack tolerance with enhanced strength. The introduction of Zn not only promotes pyramidal 〈c + a〉 slips and inhibits twinning nucleation, but also enhances the GB cohesion by Zn segregation. Based on in-situ microstructure observation, we found that the enhanced GB cohesion enables the segregation-inspired hierarchical crack buffering, as well as deflecting and branching. Enhanced GBs can also facilitate the continuous emission of 〈c + a〉 dislocations in neighboring grains irrespective of the onset of microcracks, forming a plastic zone to retard local strain concentration, thus avoiding microcrack percolation and attaining a crack-mediated elongation reserve of above 15 %. Besides, the higher TYS in the Zn-containing alloy mainly stems from the enhanced solid-solution strengthening of Zn solutes, thus achieving strength and crack tolerance synergy.

Moiré pattern features-dominated the inception of plasticity and strain hardening in graphene/Al multilayers

This study presents a new insight on strengthening the mechanical properties of graphene-reinforced aluminum (graphene/Al) matrix composites through interface structure design, specifically the moiré pattern interface. Molecular dynamics (MD) simulations were performed to investigate the deformation behaviors in graphene/Al composites upon varying moiré pattern interface. The symmetry of the moiré pattern distribution about tensile axis and interfacial atomic shear strain are illustrated to be the dominant roles in dislocation nucleation behavior, resulting in a transition from dislocation-mediated deformation to deformation twinning. It is revealed that twins can be induced by adjusting the graphene/Al moiré pattern interface, which is achieved by rotating graphene 29.21° relative to Al. Consequently, twin nucleation, growth and twin-dislocation interaction contribute to a higher yield strength and enhanced strain hardening capability in graphene/Al system. The present study raises the possibility of producing twins in high stacking fault energy metal Al could by utilizing graphene/Al interfaces. Furthermore, the introduction of the deformation twinning strengthening mechanism in matrix Al serves to further strengthen the graphene/Al composites.

Shuffling pathway of anti-twinning in body-centered-cubic metals

In this study, anti-twinning and deformation twinning of body-centered-cubic (BCC) metals under nanoindentation were examined. Molecular dynamics (MD) simulations of BCC tantalum (Ta) showed both the existence of anti-twinning and a novel two-layer-by-two-layer anti-twinning pathway consisting of a staggered shuffle sliding of {112} atomic planes in the [111¯] and [1¯1¯1] directions, generating finite shear displacement along the anti-twin direction. The density functional theory (DFT) multi-dimensional interlayer slipping energy landscape of the newly observed shuffling anti-twinning process has a lower energy barrier for both twin nucleation and growth than the recently proposed layer-by-layer anti-twinning process.

Nanovoid formation mechanism in nanotwinned Cu

Nanotwinned metals have been intensely investigated due to their unique microstructures and superior properties. This work aims to investigate the nanovoid formation mechanism in sputter-deposited nanotwinned Cu. Three different types of epitaxial or polycrystalline Cu films are fabricated by magnetron sputtering deposition technique. In the epitaxial Cu (111) films deposited on Si (110) substrates, high fractions of nanovoids and nanotwins are formed. The void size and density can be tailored by varying deposition parameters, including argon pressure, deposition rate, and film thickness. Interestingly, nanovoids become absent in the polycrystalline Cu film deposited on Si (111) substrate, but they can be regained in the epitaxial nanotwinned Cu (111) when deposited on Si (111) substrate with an Ag seed layer. The nanovoid formation seems to be closely associated with twin nucleation and film texture. Based on the comparative studies between void-free polycrystalline Cu films and epitaxial nanotwinned Cu films with nanovoids, the underlying mechanisms for the formation of nanovoids are discussed within the framework of island coalescence model.Graphical abstract

Formation of intracrystalline deformation twins in austenitic Mn-steel at the early stage of plastic deformation

In austenitic Mn-steels with low stacking fault energy, the deformation twin is known to play an important role in plastic deformation. Usually, the position grain boundary is preferred to observe and investigate the formation process of the deformation twin, while other suitable formation positions may be neglected. Here, high-resolution transmission electron microscopy is used to observe and characterize the formation of intracrystalline twins in 120Mn13 steel at the early stage of tensile plastic deformation at the atomic scale. The result shows that intracrystalline twins nucleate through overlapping stacking faults generated by three different perfect dislocations dissociating on consecutive {111}-type planes, which is different from twin nucleation mechanisms that have been proposed. Subsequently, they become large-sized intracrystalline twins or twin bands throughout the whole grain by twin nuclei growing themselves and connecting to adjacent twin nuclei or existing twins in the same growth direction. Intracrystalline twins are mainly formed in three positions, namely near the grain boundary, at the end or side of existing twins. The stacking fault sources of intracrystalline twins at different positions are different.