Coherent Twin Boundaries Research Articles

Influence of thermal annealing on the microstructure evolution, fracture and fatigue behavior of nanocrystalline Cu films

In this study, the effect of thermal annealing on the microstructures, fatigue and fracture behavior of nanocrystalline Cu films on compliant substrates was investigated. Microstructure examinations revealed the void elimination, grain growth and twin formation after thermal annealing. Monotonic tensile results showed that the crack onset strain was dramatically improved from 3% to 13% after thermal annealing, while the yield strength was not reduced significantly, which were rationalized by the elimination of nanovoids and the formation of twins. In contrast, cyclic tests revealed that the fatigue tolerance was notably deteriorated by thermal annealing, which can be explained in terms of coherent twin boundaries and nanovoids. The coherent twin boundaries tend to trigger the fatigue cracks due to the elastic modulus mismatch between parent and twin as well as the difference in the Schmid factors, while the nanovoids facilitate the dynamic grain growth and delays the formation of fatigue cracks.

On the migration of {10[formula omitted]1} and {10[formula omitted]3} coherent twin boundaries in magnesium

In this work, migrations of (101¯1) and (101¯3) coherent twin boundaries (CTB) in pure magnesium are studied by first-principles methods. The migration process is divided into nucleation and gliding of twinning disconnection (TD), with the TD nucleation being approximated by a rigid-shifting of the CTB. Full energetic maps obtained from this analysis reveal the necessity of applying shear strains simultaneously along the twinning direction η1 and along k1, which is normal to η1, to activate the 2-layer CTB migration in these two different twinning modes. The energetic map identifies the most efficient external shear strain direction for activating CTB migration. This direction lies between the Burgers vectors bTD-2 of the 2-layer-height TD and η1. The shear strain required to initiate TD gliding is smaller than that needed for TD nucleation, implying that the migrations of (101¯1) and (101¯3) CTBs are primarily governed by the TD nucleation.

The role of incoherent twin boundaries on the plasticity of Cu micropillars

The role of a ∑3{112} incoherent twin boundary (ITB) on the shear stress of Cu at the micron scale has been investigated through microcompression of bi-crystalline pillars containing ITB, as well as single-crystalline pillars, in two different compression directions. The Cu sample containing ITBs was synthesized using magnetron sputtering on a sapphire substrate. Firstly, pillars along [111] compression direction were milled on the film surface. As multiple slip systems were activated upon loading, the dislocation-ITB interaction in this direction was dominated by the dislocation–dislocation interactions. Another set of pillars was milled from the side of the film (in the thickness of the film) in a nominally [134¯] compression direction. Compression in this direction activated a single slip in each grain, which facilitated the investigation of the interaction between dislocations and ITBs. Post-mortem images showed that slip traces were not distinctly connected at the boundary unlike ideal slip transmission in pillars containing a coherent twin boundary. Moreover, bi-crystalline pillars in the single slip direction are stronger than single-crystalline pillars. The observations indicate that ITBs are not impenetrable for dislocations, but the boundary demonstrates some resistance to transmission.

Elastic energy and interactions between twin boundaries in nanotwinned gold

Nanotwinned metals exhibit a surprising combination of ultrahigh strength and high ductility. We have performed atomistic simulations in gold to study interactions between coherent twin boundaries (CTBs) and between incoherent twin boundaries (ITBs) for two extreme cases: with close surfaces and without any surface, in order to simulate a relaxed or a constrained system. The estimated displacement fields give the extension of interactions between twin boundaries which are 6 atomic planes for the CTB and in the range of 30–42 atomic planes for the Σ3{112} ITB in the relaxed system. The displacement field in the constrained system is, to a very good approximation, the difference of the displacement field in the relaxed system and a homogeneous quantity which depends on the excess volumes. The elastic strain introduced by the twin boundaries in the constrained system and the associated elastic energy γexcess are deduced from the excess volumes. Our study highlights how the mechanical behaviour (e.g. the yield strength) of samples with TBs separated by distances less than a critical distance whose order of magnitude is (103−105)δTB≈(0.5−360)nm can be affected.

Effects of solute segregation on the mobility and mechanical properties of {10[formula omitted]2} coherent twin boundary in magnesium

The effects of solute segregation on the mobility and mechanical properties of {101¯2} coherent twin boundary (CTB) are studied using first-principles calculation methods. 16 alloying elements are chosen to test the effects. The mobility of {101¯2} CTB is limited by the pinning effect led by the solute segregation. The pinning limit is shown to be highly dependent on the solute element segregating at the CTB, and the pinning efficiency is revealed to be only associated with the position that the solute atoms segregate in. Solute segregation affects the toughness and strength of the {101¯2} CTB. Ba and Li are CTB embrittlement elements while Ca, Si, Zn, Al, Ag, La, Ce, Nd, Gd, Tb, Y, Lu, and Zr are CTB toughening elements. Ba, Li, together with Sr, Ca, are found to be CTB weakening elements, and La, Ce, Nd, Al, Zn, Gd, Tb, Y, Lu, Ag, Si, and Zr are CTB strengthening elements. The pinning effect, and the effects on the CTB toughness and strength are relative to the solute segregation propensity in the CTB. Higher segregation propensity at the compression site of {101¯2} CTB is revealed to result in stronger pinning effects, stronger CTB toughening and strengthening effects, while for the segregations at the extension site, the trend is opposite.

High-density twin boundaries in transition metal nitride coating with boron doping

The formation mechanism of twin boundaries in transition metal nitrides has remained largely unexplored owing to their high stacking fault energy that hinders twin boundary formation. Herein, we fabricated TiB0.11N1.16 coatings with high-density twin boundaries, including coherent twin boundaries and incoherent twin boundaries. The experimental results revealed that boron doping facilitates the coating orientation transition from [200] to [111], which provides an orientation advantage and indirectly promoting the formation of twin boundaries. Theoretical analyses indicated that boron segregation leads to the formation and stabilization of incoherent twin boundaries via two aspects: i) it reduces incoherent twin boundary energy and ii) forms large internal stress, which stabilizes the reduced incoherent twin boundary energy and prevents detwinning. The coupling of the two effects not only forms and stabilizes the incoherent twin boundaries, but also provides a possible precursor structure for the formation of coherent twin boundaries. Owing to the stress field change caused by the nonuniform distribution of boron, some twinned columns transform into the matrix orientation driven by system energy minimization, thus forming coherent twin boundaries. Moreover, the hardness and toughness of the TiB0.11N1.16 coatings were superior to those of twin-free ceramic coatings. The proposed mechanisms will help to introduce twins in materials with high stacking fault energy.

A different type of intergranular corrosion facilitated by hydrogen in austenitic stainless steel

This research aims to investigate the distribution of hydrogen and hydrogen-facilitated intergranular corrosion (IGC) in an austenitic stainless steel, which comprises mainly Σ3 and random grain boundaries. The coherent twin boundaries Σ3, which routinely exhibited excellent corrosion resistance, were observed to be prone to IGC after hydrogen charging. Furthermore, the results demonstrated that the inhomogeneous distribution of absorbed hydrogen finally induced IGC at grain boundaries in both the solution-treated and sensitized specimens. The mechanism of hydrogen-facilitated IGC differs from the conventional chromium-depleted theory. A model was established to describe this different type of IGC induced by hydrogen.

Engineering twin boundaries for enhancing strength and ductility of thermoelectric semiconductor PbTe

Twin boundary engineering is a potential strategy for achieving robust mechanical properties of materials. Our previous molecular dynamics simulations indicated that the nanotwin could significantly enhance the ductility of thermoelectric (TE) semiconductors PbTe due to coherent twin boundary (CTB) migration accompanied by the ‘catching bond’ at room temperature. To further improve the mechanical strength or ductility of PbTe, we investigated the role of the shear direction, the CTB orientation and the temperature on mechanical properties of nanotwinned PbTe. Under the shear stress along [11̅0] loading direction, the partial dislocations with a/6 [12̅1] and a/6 [21̅1̅] Burgers vectors are preferentially activated on (111) twin plane with higher yield strength and ultimate shear strength than that of the (111)[112̅] slip system. The nanotwinned PbTe with CTB orientation ranging from 125° to 161° has both higher fracture strain and larger ultimate shear strength than 0° CTB orientation. This is attributed to the motion of the twinning partial dislocation significantly enhancing the ductility while the blocking of dislocations by CTBs further improving the shear strength and deformability of PbTe. Moreover, the low temperature (below 100 K) energetically enables the partial dislocation to nucleate and glide on the strong Te-CTB plane, which induces successive CTB migration along Pb- and Te-CTB planes, resulting in enhanced ductility of nanotwinned PbTe.

Twin BiSbO4 nanoseeds with {0 1 0}-exposed facets: Facile synthesis and excellent photocatalytic degradation of DDT in highly and long-term contaminated soil

Twin engineering is one of the most recent perspective photoactivity enhancing approaches with its ability to provide highly ordered atomic arrangements that accelerate both the transport of free charge carriers and the electron-hole separation, but is still limited to a few photocatalysts, however. In this research, we report the controllable template-free synthesis and UV-photocatalytic activity of twin BiSbO4 nanoseeds with {010}-exposed facets for dichlorodiphenyltrichloroethane (DDT) degradation extracted from highly and long-term contaminated soils for the first time. The results revealed that the formation of the desirable BiSbO4 was strongly affected by precursors and hydrothermal time. Astonishingly, at optimized synthesis conditions, the as-synthesized BiSbO4 nanobundles containing twin nanoseeds enclosed by exposed {010} facets with an average length of 60 nm and width of 20 nm were resulted purely. The greatly enhanced photodegradation efficiency of the optimized sample with 98.5 ± 0.83 % removal of total DDT after 4 h of UV irradiation was achieved, significantly higher than that of the bulk sample (62.5 ± 0.77 %). The existence of both the exposed high-energy {010} facets and the coherent twin boundaries was suggested to contribute to the boosting of the DDT photocatalytic activity of the twin BiSbO4 nanoseeds with {010}-exposed facets. Additionally, a twin-dependent photocatalytic mechanism was also proposed for the twin BiSbO4.

Mapping hierarchical and heterogeneous micromechanics of a transformative high entropy alloy by nanoindentation and machine learning augmented clustering

Conventional macromechanical tests provide limited insights into complex hierarchical deformation behavior of a transformative high entropy alloy (HEA). In this work, a high throughput microstructure-micromechanical correlative study is presented by combining high-resolution nanoindentation, site-specific microscopy, and Gaussian mixture model (GMM) clustering. The investigated HEA has a heterogenous microstructure consisting of austenite and martensite phases. Comparison of elastoplastic and microstructural maps illustrate dependency of phase, crystal orientation, and interfacial constraints on the micromechanical response. The disproportionately high hardness found in martensite-rich area is attributed to its higher lattice stability to shear, creation of coherent twin boundaries, and copious dislocation activities in the twin interfaces formed in martensite phase during nanoindentation. The hierarchy in twinning behavior depends on the relative direction of loading with the c-axis of h.c.p. martensitic phase. Deformation in f.c.c. austenitic grains is slip-dominated and demonstrates orientation dependency during incipient plasticity and phase transformation. GMM based classification of hardness to modulus ratio intuitively correlates work-hardening with phase distribution due to the distinctive deformation micromechanical responses of austenite and martensite phases.

The structure dependence of oxidation behavior of high-angle grain boundaries of alloy 600 in simulated pressurized water reactor primary water

The intergranular degradation of seven different types of high-angle grain boundaries (HAGBs) were investigated on Alloy 600 after exposure to simulated pressurized water reactor primary water. All boundaries are susceptible to preferential intergranular oxidation (PIO) except for ideal coherent twin boundary. Diffusion induced grain boundary migration (DIGM) normally occurs and its depth is positively correlated with the PIO extent. Interestingly, the PIO susceptibility is independent on the grain boundary misorientation angle or Σ value, but related to the grain boundary atom packing density (GBAPD). Grain boundaries with higher GBAPD values show higher PIO resistance as the element diffusion is slower.

High temperature plasticity at twin boundary in Al: An in-situ TEM perspective

Coherent twin boundaries are of special importance in structural materials due to their strengthening ability by impeding dislocation motion while maintaining possible glide along their plane. Interactions of lattice dislocations with twin boundaries have been extensively studied experimentally and numerically but relaxation and deformation processes at high temperature are not fully understood. The reason is related to complex mechanisms of dislocation decompositions into disconnections, their further motion and possible reactions. Moreover, as in shear-coupled grain boundary (GB) mechanisms, motion of disconnections in the interface plane is expected to lead to twin motion perpendicular to its plane. Here, shear-coupled motion of a coherent twin in pure Al is explored during in situ straining in a transmission electron microscope (TEM) in a favorable geometrical configuration. Surprisingly, the twin boundary does not couple to shear but slightly migrates by propagating nanoscale incoherent facets. Although, disconnections of Burgers vector 1/6〈112〉 have been extensively observed moving in the interface plane, their motion did not lead to migration as expected but presumably to GB sliding. This was interpreted by the motion of disconnections with opposite single and double steps in the [111] direction. Extensive dislocation/GB interactions were observed and reactions following absorption are interpreted by contrast analysis and motion observations. Incoming dislocations do not always decompose but often react with the GB disconnection microstructure which may lead to intergranular motions or dislocation emissions in the adjacent grain. As these processes usually involve disconnection climb in the interface plane, direct transmission, i.e. spatially and temporally correlated absorption/emission processes, is not observed as revealed by large scale observations. Finally, the internal disconnection microstructure of the twin boundary often forms networks which although flexible are found to slow down intergranular plasticity. Such networks should strongly control stress induced mechanisms in realistic GBs.

Short crack propagation near coherent twin boundaries in nickel-based superalloy

Short crack propagation near a coherent twin boundary in polycrystal nickel alloy is investigated with 3D crystal plasticity extended finite element modelling (CP-XFEM), utilising reconstructed experimentally characterised microstructures and crack path observations. Short 3D cracks at coherent twin boundaries are shown to grow on parallel (111) slip planes at very high rate so as to be an intrinsic part of the nucleation process. This is found to occur predominantly because of the stress states established by twin/parent constraint driven by the local elastic anisotropy. If elastic isotropy is modelled, crack growth is found to be non-planar and inclined to the twin boundary. The twin/parent twist angle is also found to influence local stress state, such that the actual 60° twist angle gives rise to the highest local stresses, preferential crack nucleation and rapid growth parallel to the twin boundary, and the deviations from 60° change the crack morphology and rate of growth.

Trans-twin dislocations in nanotwinned metals

The strength-controlling dislocation mechanism of a material can be illuminated by partition of the flow stress into its effective stress and back stress components. Recent experiments report a nearly constant saturated effective stress of about 100 MPa for nanotwinned Cu with a range of nanotwin thicknesses less than 100 nm [Z. Cheng et.al., Proc. Natl. Acad. Sci. U.S.A. 119 (2022) e2116808119]. This surprising result implies that the effective stress is controlled by the long dislocations spanning multiple nanotwin lamellae, termed trans-twin dislocations, rather than the commonly thought threading dislocations confined within individual nanotwin lamellae. Here we use molecular dynamics to simulate the formation and motion of trans-twin dislocations across multiple nanotwin lamellae. We show that the interconnected segments of a trans-twin dislocation can glide concertedly on the corrugated {111} slip planes in consecutive nanotwin lamellae with little resistance from coherent twin boundaries. Our results indicate that the finite resistance to slip transmission across coherent twin boundaries can set a lower limit of the effective obstacle spacing to hinder dislocation glide and thus dictate the upper limit of the saturated effective stress of nanotwinned metals. This work provides new understanding of the strength-controlling mechanism in nanotwinned metals.

The effect of kink-like defects on the twin boundaries of nanotwinned Ta under nanoindentation

To figure out the effect of kink-like defects on twin boundaries, BCC Ta with three different kinds of microstructures: single crystal (SC), perfect twin boundaries (PTB) and defective twin boundaries (DTB), are studied under nanoindentation. Both indenter load and hardness in PTB are enhanced by twin boundaries but reduced in DTB due to intrinsic kink-like defects. According to atomic surface morphology, the relative sink-in depth in DTB is lower than that in PTB but higher than that in SC. By comparing atomic images of prismatic loops in three different stages, it can be found that kink-like defects in DTB perform two opposite functions: acceleration and delay. In initial plastic stage, defective twin boundaries can accelerate pinching-off event and nucleation. In transmission stage, dislocation backward emission is advanced by defective twin boundaries. In obstruction stage, the symmetry of coherent twin boundaries will be broken and asymmetrical shear loops are formed, which will delay prismatic loops moving across twin boundaries. Dislocations of 12〈111〉 are the main factor of nanoindentation in [1¯1¯2] direction. Kink-like defects will lead to less drops in DTB on dislocation densities than that in PTB. These findings are helpful to reveal the influence of intrinsic kink-like defects in BCC nanotwinned materials.

Atomic scale simulations of [formula omitted] symmetric incoherent twin boundaries in gold

The crystalline defects may play an important role in the improvement of the properties of materials. This is the case of twin boundaries which improve the mechanical properties. Σ3{111} coherent twin boundaries (CTBs) and Σ3{112} symmetric incoherent twin boundaries (ITBs) are two defects studied in this work. Atomistic simulations with four interatomic potentials commonly used in the literature and density functional theory have been used to characterize in detail these two defects in gold. The results show that the excess volumes introduced by twin boundaries strongly depend on the potential. After the ITB relaxation, a portion of 9R phase is formed. This phase is rotated from the face centred cubic (fcc) phase through an angle α which has been determined. The 9R phase formation energy γform has been quantified through atomic scale simulations. We put forward a model to calculate this formation energy γform as a function of the intrinsic stacking fault energy γISF and the excess volume. This model shows that the interaction energy between ISFs is much larger for the Ackland and Baskes potentials than for the Foiles and Grochola potentials. Furthermore, the elastic deformation ɛxx produced in the system to ensure consistency between the 9R phase and the fcc phase is computed. It allows us to discuss the width of the 9R phase in terms of γform and elastic energy and not uniquely in terms of γISF, as previously done in the literature.

Nanoscale chemical segregation to twin interfaces in τ-MnAl-C and resulting effects on the magnetic properties

• The atomistic structure and chemical composition of true twin and order boundaries in the ferromagnetic τ-MnAl-C were studied. • The distribution of carbon is inhomogeneous across the twin boundary and carbon clusters were found. • Micromagnetic simulations were conducted to study the effect of chemical segregation at twin interfaces on the magnetic properties. • Targeted doping of interfaces in τ-MnAl may be a promising strategy to increase the coercivity of the material for applications. In this study, aberration-corrected scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) was used to investigate the atomistic structure and chemical composition of true twin and order twin boundaries in ferromagnetic τ-MnAl-C. True twins and order twins were distinguished based on the diffraction patterns using TEM. No elemental segregation was observed at the coherent true twin boundary but some Mn enrichment within a region of about 1.5-2 nm was found at the incoherent true twin boundary. A transition region with Mn enrichment about 4-6 nm wide was found at the order twin boundary. A carbon cluster with a size of around 5 nm was also found at the twin boundary. Micromagnetic simulations were conducted to study the effect of this chemical segregation at twin interfaces on the magnetic properties. The results showed that the coercivity tends to increase with increasing structural and chemical disorder at the interface.

Atomic-scale study of the {[formula omitted]} twinning and {[formula omitted]}-{[formula omitted]} double twinning mechanisms in pure titanium

The {112¯1} twinning and {112¯2}-{112¯1} double twinning mechanisms in pure Ti are systematically investigated using molecular dynamics (MD) simulations. MD simulation results indicate that a {112¯1} twin embryo is composed of a group of pyramidal stacking faults (PSFs) with 6d{112¯1} or 8d{112¯1}, caused by the successive slip of partial dislocations with the Burgers vector of 136〈112¯6¯〉M. The migration of {112¯1} twin boundaries (TBs) is mainly carried out by the slip of twinning dislocations (TDs) b⇀1 = 16(4γ2+1)<1¯1¯26> ±12<11¯00>. Two {112¯2}-{112¯1} double twinning mechanisms are discovered. One is that a {112¯1} twin forms and grows at the {112¯2} twin tip through the dissociation of the TD b⇀1(112¯2) at the {112¯2} twin tip, forming the {112¯2}-{112¯1} double twin structure. The other is that a basal <a>T dislocation interacts with a {112¯2} TB, forming a twin embryo composing of a group of PSF with the 6d{112¯1¯} and leaving a step at the {112¯2} TB, eventually forming the {112¯2}-{112¯1} double twin structure. The interfacial structures of {112¯1} twins have the terraced character with short coherent (112¯2)||(112¯0) facets and coherent twin boundary (CTB) segments. In addition, (0001)||(112¯4) facets with a disclination dipoles feature can form at twin tips. The coherent twin interfaces form at their intersected interfaces during the {112¯2}-{112¯1} double twinning process. Our present findings may provide clues for designing Ti alloys with excellent strength–ductility properties through obtaining a high-volume fraction of coherent interfaces in terms of {112¯2}-{112¯1} double twins.

Hydrogen embrittlement of twinning-induced plasticity steels: Contribution of segregation to twin boundaries

Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrogen with twin boundaries in a model TWIP steel by using isotope-labelled atom probe tomography, using tritium to avoid overlap with residual hydrogen. We reveal co-segregation of tritium and, unexpectedly, oxygen to coherent twin boundaries, and discuss their combined role in the embrittlement of these promising steels.

Grain boundary segregation-induced strengthening-weakening transition and its ideal maximum strength in nanopolycrystalline FeNiCrCoCu high-entropy alloys

The combined Monte Carlo and Molecular Dynamics simulations were employed to explore the influence of elemental segregation at grain boundaries (GBs) on the mechanical properties of FeNiCrCoCu high entropy alloys under uniaxial load. The chemical potential difference with respect to Ni atoms was first obtained, and used to reach the equilibrium elemental distributions in sample models with different Cu contents, grain sizes and twin thicknesses. By comparing the random and segregated configurations, the role of GB segregation was analyzed in detail. It is found that the GB segregation trend is Ni<Fe<Co<Cr<Cu. The Cu atoms have no preference to segregate at coherent twin boundaries, and their dominant segregation at GBs prevents the detwinning that usually occurs in softening stage, further increasing twin strength. The segregation with low concentration at GBs can improve the strength by suppressing dislocation emission from GBs and further enhance the GB-mediated plastic deformation; while excessive segregation at GBs results in harmful weakening effect with GB cracking. The transition from strengthening to weakening is closely related to the GB segregated Cu concentration. To estimate the critical segregation amount in case of maximum strength, a theoretical model was established, where three regions (strengthening, transition and weakening) are distinguished. It is revealed that the strengthening region occupies a small area, and the addition of Cu contents should be rigorously tailored. This work deepens the understanding of the strengthening mechanisms arising from GB segregation and provides insights into the design of ultrahigh-strength materials.