Twin-twin Interactions Research Articles

Experimental determination of latent hardening coefficients in FeMnNiCoCr

The equi-atomic FeMnNiCoCr high entropy alloy is attracting unprecedented attention due to its exceptional strain hardening behavior extending to large strains and to low temperatures (77K). In this paper, we analyze the nano- to macroscale deformation response of FeMnNiCoCr single crystals and explain variations in strain hardening based on the activation of different twin and slip systems and their interactions. We experimentally determine the latent and the self hardening moduli upon twin-twin, slip-twin, twin-slip and slip-slip interactions. Choosing single crystal orientations that isolate these interactions enables the evaluation of the pertaining hardening moduli without ambiguity. Differing from the earlier experimental approaches employed, which necessitate sample reorientation to quantify the self and latent hardening coefficients, in this work, we demonstrate a novel framework where plastic straining is implemented in a monotonic fashion entailing the latent and primary systems to operate simultaneously. To extract the hardening moduli and to characterize the twin-twin, slip-twin, twin-slip and slip-slip interactions on experimental grounds, <111>tension and <001>compression single crystalline samples are studied by high resolution digital image correlation, electron backscatter diffraction and transmission electron microscopy techniques. The results demonstrate that the magnitude of residual Burgers vectors play a key role in explaining the experimental hardening trends.

Intragranular cross-level twin pairs in AZ31 Mg alloy after sequential biaxial compressions

Sequential double extension twinning in AZ31 Mg alloy happens easily during sequential biaxial compressions. Widespread twin-twin interaction leads to the formation of intragranular cross-level twin pairs, each of which can be defined by a primary twin variant PVi (i = 1–6) and its nearby secondary twin variant PVj-SVk (j, k = 1–6, j ≠ i, k) across the twin-twin boundary between them. Theoretically, 9 possible misorientations exist between the two variants. Misorientation between two crystals can be expressed by a pair of rotation axis (r) and the minimum rotation angle (θ). The crystallography of a cross-level twin pair can be described unanimously by the misorientations between PVi and PVj-SVk (i.e., r1-θ1), PVi and PVj (i.e., r2-θ2), and the host grain and PVj-SVk (i.e., r3-θ3), resulting in 13 possible orientation relationships (ORs) that can be expressed explicitly by the three misorientation angles (θ1, θ2, θ3). A cross-level twin pair is a key element of a crossing or crossed twin structure, which is common in the deformed microstructure. Statistical electron backscatter diffraction analysis reveals that OR2 of (49.7°, 60°, 60°) appears the most frequently. Twinning shear transmission is almost irrelevant to the formation of a cross-level twin pair. Although the majority of the identified twins are high Schmid factor (SF) ones with SF > 0.3 and SF ratio > 0.8, SF analysis fails to explain the highly preference of OR2 and the abnormal absence of OR8 of (44.0°, 60°, 60.4°), probably due to the complexity of the formation of a cross-level twin pair.

Determination of latent hardening response for FeNiCoCrMn for twin-twin interactions

Twin migration stresses and the associated latent hardening response in high entropy fcc alloys are derived corresponding to the dual interactions between all plausible twin systems. Employing an atomistically-informed, anisotropic analysis; the hardening matrix coefficients are calculated for the promising equiatomic FeNiCoCrMn high entropy alloy. The results point that the self-hardening coefficients (diagonal matrix components) are greater in magnitude than the latent hardening coefficients (off-diagonal components). This finding is a deviation from previous views in the literature which incorrectly attributed most of the hardening to latent terms. The predicted migration stresses and the strain hardening behavior exhibit a strong deviation from the Schmid law. The theoretical predictions are demonstrated to be in well agreement with the experimental trends. Furthermore, the local deviations from the stoichiometric chemical composition are shown to be of cardinal importance in the atomistic scale fault energy barrier levels.

Twin-twin interactions and contraction twin formation in an extruded magnesium alloy subjected to an alteration of compressive direction

Twinning and detwinning represent major deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to identify twin-twin interactions and contraction twin formation in an AZ31 magnesium alloy when the compressive direction was changed from the extrusion direction (ED) to the normal direction (ND) via electron backscatter diffraction (EBSD) and Schmid factor analysis. {101¯2} extension twins of multiple variants were observed after compressive deformation of 4.3% along ED. The detwinning of {101¯2} extension twins occurred along with the formation of {101¯1} and {101¯3} contraction twins, when the compressive direction was changed to ND. The extension twins in some grains almost fully vanished, making the grains back to a twin-free state. A new twin-twin interaction mechanism being different from double twinning (defined as twins within a twin) was identified, due to the impingement of {101¯1} contraction twins nucleated in the matrix grain on a pre-existing {101¯2} extension twin.

Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes

Here, we report a comprehensive study that combines in situ scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.

Single and double twin nucleation, growth, and interaction in an extruded magnesium alloy

The objective of this study was to identify twinning characteristics and mechanisms in an extruded AZ31 magnesium alloy under favorable conditions of profuse 101¯2 extension twinning using in-situ optical microscopy, electron backscatter diffraction, and X-ray diffraction analysis. The propagation of a single twin variant led to a relatively fast saturation of twin nucleation after which the increase in strain was predominantly accommodated by the growth of existing twin lamellas. For distinct twin variants, the intersecting twins led to the confinement of the spaces constrained by the fine twin lamellas. Embryonic twin structures acknowledged theoretically or through atomistic simulations were experimentally observed, including the vanishing of primary112¯1 embryonic twin via the nucleation and growth of either single or multiple 101¯2 secondary extension twin variants during deformation. Newly identified twin-twin interaction scenarios included the ladder-like and branching-like twinning structures which occurred depending on the applied strain, incoming twin paths, and their impingement on the pre-existing twin boundaries. The formation of the branching-like structures involved three or more non-co-zone 101¯2 extension twin variants where the activation and growth of one variant among them were found to be favored by the external stresses via Schmid factor analysis.

Abnormal migration of twin boundaries in rolled AZ31 alloy containing intersecting [formula omitted] extension twins

Twin boundary (TB) migration under the condition of the interaction between different {10–12} twin variants with a high misorientation of 60.4° was investigated by using in-situ electron backscattered diffraction technique combined with Schmid factor analysis. It was found that TBs would continue to migrate when the sides of intersecting twin bands enclose an acute angle, but cease to migrate on the sides making an obtuse angle. Besides, due to the twin-twin interaction, half locked and entirely locked twin boundaries occurred as deformation progressed. Moreover, strain accommodated by double twin and new active twin was found at further imposed strain in current study.

Improving the mechanical properties of a hot-extruded AZ31 alloy by [formula omitted] twinning lamella

The strengthening mechanisms of hot-extruded AZ31 magnesium alloy by {101¯2} extension twinning lamellas during multi-pass compression are investigated in present study. The numbers of {101¯2} twinning lamellas increased dramatically with the ED-TD orthogonal compression. Grain refinement induced by {101¯2} twinning lamellas greatly improved the mechanical properties of reloading along ED and TD direction. In addition, the primary {101¯2} twinning lamella can be an effective way to strengthen material via twin-twin interaction during secondary twin growing and propagating.

Deformation twinning in hexagonal materials

Abstract

Role of volume fraction of second phase particles, dislocation-twin and twin-twin interactions in the reduced tension-compression yield asymmetry

In the present study, TZA521 alloy and TZA1021 alloy were designed and subjected to extrusion process as well as extrusion followed by rolling process to uncover the role of volume fraction of second phase particles (Mg2Sn phase in this work), dislocation-twin and twin-twin interactions in the reduced tension-compression yield asymmetry. Our results show that yield asymmetry is only weakly dependent on the volume fraction of Mg2Sn phase, owing to the presence of some coarse Mg2Sn phases and relatively large inter-particle spacing. Grain subdivision by 101̅2 twins and dislocation-twin interactions are found to reduce the tension-compression yield asymmetry of a slip predominant deformation, i.e. tension along the rolling direction in the extrusion followed by rolling condition, while twin-twin interactions mainly account for the reduced tension-compression yield asymmetry during compression along the rolling direction for both alloys in the rolling following extrusion condition. The relevant mechanisms are discussed.

Grain boundary orientation effects on deformation of Ta bicrystal nanopillars under high strain-rate compression

We investigate grain boundary (GB) orientation effects on deformation of Ta bicrystal nanopillars under high strain-rate, uniaxial compression with molecular dynamics simulations. The GB is of the ⟨110⟩90° twist grain boundary type. We vary the angle between the GB normal and the loading direction (θ) in the range of 0°–90° while keeping the GB type unchanged. The GB orientation has strong effects on deformation mechanism, yield stress, failure strain, and dynamics, due to the combined effects of Schmid factors in constituent crystals and resolved shear stress on the GB plane. Single crystal plasticity and GB deformation are competing factors, and the GB-initiated deformation mechanisms (stacking faults vs. twinning, and GB sliding) depend on the local stress level around the GB. The large Schmid factors in constituent single crystals for θ=0° lead to twinning in the single crystals and the lowest yield stress; the ensuing GB deformation is achieved via stacking fault formation due to premature stress relaxation. However, nanopillar deformation in the cases of higher angles is dominated by GB deformation largely in the form of twinning, driven by enhanced stress buildup. GB-initiated deformation in the high Schmid factor nanocrystal precedes and may drive that in the low Schmid factor nanocrystal. The details of twin/stacking fault nucleation and growth/shrinking, twin-twin interaction, and twin-GB interaction are also discussed.

Effects of Twin-Dislocation and Twin-Twin Interactions on the Strain Hardening Behavior of TWIP Steels

Abstract Tensile tests of Fe-30Mn-5Si-2Al steel were carried out for different strains of 0.05, 0.14, 0.26, and up to the strain-to-failure in order to observe the evolution of microstructure during deformation and investigating the strain hardening behavior. Three-stage strain hardening behavior was observed in this steel during tensile test. In stage I, planar dislocation structure was observed by TEM and regarded as main deformation mechanism, and low strain hardening rate exponent was exhibited. Primary deformation twinning occurred in stage II, and the strain hardening rate exponent increased due to the blockage of dislocations' motion by twin boundaries. In stage III, the strain hardening rate exponent had increased to a value higher than 0.5. The obstacle effect of twin boundaries and twin-twin interaction had been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations' motion, which led to the twinning induced plasticity effect in the later stage of deformation.

Numerical Simulation of Twin-Twin Interaction in Magnetic Shape-Memory Alloys

AbstractTwin boundary motion is the mechanism that drives the plastic deformation in magnetic shape memory alloys (MSMAs), and is largely dependent on the twin microstructure of the MSMA. The twin microstructure is established during the martensitic transformation, and can be influenced through thermo-magneto-mechanical training. For self-accommodated and ineffectively trained martensite, twin thickness and magnetic-field-induced strain (MFIS) are very small. For effectively trained crystals, a single crystallographic domain may comprise the entire sample and MFIS reaches the theoretical limit. In this paper, a numerical simulation is presented describing the twin microstructures and twin boundary motion of self-accommodated martensite using disclinations and disconnections (twinning dislocations). Disclinations are line defects such as dislocations, however with a rotational displacement field. A quadrupole solution was chosen to approximate the defect structure where two quadrupoles represent an elementary twin double layer unit. In the simulation, the twin boundary was inclined to the twinning plane which required the introduction of twinning disconnections, which are line defects with a stress field similar to dislocations. The shear stress - shear strain properties of self-accommodated martensite were analyzed numerically for different initial configurations of the twin boundary (i.e. for different initial positions of the disconnections). The shear stress - shear strain curve was found to be sensitive to the initial configuration of disconnections. If the disconnections are very close to boundaries of hierarchically higher twins – such as is the case for self-accommodated martensite, there is a threshold stress for twin-boundary motion. If the disconnections are spread out along the twin boundary, twinning occurs at much lower stress.

Between microscopic and mesoscopic descriptions of twin–twin interaction

On a microscopic scale, deformation twinning is carried by the motion of twinning disconnections. A disconnection is an interfacial line defect characterized by a Burgers vector, a line vector, and a step vector. The Burgers vector (dislocation component of the disconnection) carries the deformation while the step vector (ledge component) carries the transformation from one twin variant to the other. On a mesoscopic scale, the deformation produced by twinning is a simple shear. A moving disclination dipole provides a mesoscopic model accounting for the twinning shear. Twin-twin interaction processes including the intersection of twins, the formation of structured twins, and the nucleation of cracks, may feature very complex mechanisms when analyzed on a microscopic scale. It turns out, however, that these mechanisms are controlled by the properties of large disconnection groups containing up to 10000 disconnections and more. These properties are sufficiently well approximated in the disclination dipole model. The disclination model for twin-twin interaction predicts orientation and volume fractions of secondary twins. The model also predicts the nucleation of cracks and crack growth. The disclination model was used to analyze the ductile-to-brittle transition of austenitic steel deformed at low temperature. The mesoscopic disclination model for twinning is successful because it accounts for the properties and mechanisms of disconnection groups.

Fetal heart rate accelerations, fetal movement, and fetal behavior patterns in twin gestations

Previous studies have shown that twins, when monitored simultaneously, show a remarkably high incidence of coincident fetal heart rate accelerations (about 58%). The current study examines synchrony of behavior patterns in twins. We examined 37 fetal monitor strips from 15 sets of twins with simultaneous fetal heart rate and fetal movement recorded by means of Doppler techniques (Toitu MT-430 fetal actocardiograph). The strips were analyzed for coincidence of fetal heart rate accelerations and fetal movement episodes and then conceptually for synchrony of fetal behavior patterns on the basis of descriptions of behavioral states by Prechtl. Thirty-six percent of fetal heart rate accelerations were found to be simultaneous. Forty-three percent of movement epochs were considered simultaneous. We found that twins exhibited synchronous behavior patterns (basically sleep or awake state) 94.7% of the time. Twin-twin interactions are more consistently related than suspected when observation is limited to heart rate alone, and the synchrony and role of fetal behavior states must be considered when twin interactions and behavior are studied.

The martensitic transformation in silicon—III. comparison with other work

In the previous papers [P. Pirouz, R. Chaim, U. Dahmen and K.H. Westmacott, Acta metall. mater. 38, 313 (1990); U. Dahmen, K.H. Westmacott, P. Pirouz and R. Chaim, Acta metall. mater. 38, 323 (1990)] the experimental observations on the cubic→hexagonal phase transformation in silicon were presented and a crystallographic analysis was given which explained this transformation on the basis of lattice accommodation in twin-twin interactions or propagation of secondary twins into the matrix. In the present paper, the martensitic nature of the transformation is discussed and other recent observations of hexagonal silicon in ion-implanted material, heat treated Czochralski Si, and silicon deformed under hydrostatic pressure are considered in the light of the present work.

Twin-twin interaction in a Co-Cr-Mo alloy

Twin-twin interaction in a Co-Cr-Mo alloy

Slip, twinning, and fracture in hexagonal close-packed metals

The role of deformation twinning in fracture of hexagonal close-packed metals is reviewed from a theoretical point of view. Strength and ductility are correlated with the intrinsic physical and metallurgical variables. The importance of c + a slip and of both “tension” and “compression” twins as independent modes for a generalized polycrystalline deformation is emphasized. Effects of slip-twin and twin-twin interactions on crack initiation and high-order twinning are reviewed. The competitive role of twin nucleationvs crack initiation is discussed. Shortcomings of our current understanding of the role of twinning are indicated, and some futher studies are recommended.

The interaction between slip and twinning systems and the influence of twinning on the mechanical behavior of fcc metals and alloys

The analysis of twin-slip and twin-twin interactions in fcc crystals is reviewed using the method proposed by Sleeswyk and Verbraak for the interactions between bcc twins. Slip or twinning dislocations impinging a twin boundary can generally be incorporated into the obstacle twin through often unfavorable dislocation reactions. Transmission electron microscopy observations are in agreement with the predicted mechanisms of stress relaxation. The link with Bollmann’s concepts for more general boundaries as applied to these simple Σ = 3 boundaries is emphasized. However the limitations of such analyses are pointed out particularly their inability to predict the influence of stress conditions. From the analysis and observations of interactions at twin boundaries the influence of twinning on work-hardening and fracture in fcc metals and alloys is tentatively outlined. In particular twinning can give rise to an increase of the flow stress particularly in polycrystals and it is actually responsible of the high mechanical properties of a number of commercial compositions. Experimental evidences of the role of twinning in the fast fracture and the fatigue fracture of fcc materials are reported which are in agreement with the inferences made from the study of various interactions.

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