Dislocation Twin Research Articles

In situ atomistic observation of the deformation mechanism of Au nanowires with twin–twin intersection

Twin–twin intersections are often observed in face-centered cubic (FCC) metallic nanostructures, which have important contributions to the plastic deformation and strengthening of FCC metals with low stacking fault energies. However, a deep insight into the underlying mechanism involved in the formation and evolution of twin–twin intersections remains largely lacking, especially in experiments. Here, by conducting the in situ straining experiments under high resolution transmission electron microscope (TEM), we directly visualize the dynamic evolution of a twin–twin intersection in Au nanowire at the nanoscale. It shows that dislocations in the incoming twin can either glide onto or transmit across the barrier twin via dislocation interaction with the twin boundary, resulting in the twin–twin intersection. Dynamic twinning and de-twinning of the twin–twin intersection govern the whole deformation of the nanowire. These findings reveal the dynamic behaviors of twin–twin intersection under mechanical loading, which benefits further exploration of FCC metals and engineering alloys with twin–twin intersection structures.

Effect of Twin Boundary Motion and Dislocation-Twin Interaction on Mechanical Behavior in Fcc Metals.

The interplay of interface and bulk dislocation nucleation and glide in determining the motion of twin boundaries, slip-twin interaction, and the mechanical (i.e., stress-strain) behavior of fcc metals is investigated in the current work with the help of molecular dynamics simulations. To this end, simulation cells containing twin boundaries are subject to loading in different directions relative to the twin boundary orientation. In particular, shear loading of the twin boundary results in significantly different behavior than in the other loading cases, and in particular to jerky stress flow. For example, twin boundary shear loading along results in translational normal twin boundary motion, twinning or detwinning, and net hardening. On the other hand, such loading along results in oscillatory normal twin boundary motion and no hardening. As shown here, this difference results from the different effect each type of loading has on lattice stacking order perpendicular to the twin boundary, and so on interface partial dislocation nucleation. In both cases, however, the observed stress fluctuation and “jerky flow” is due to fast partial dislocation nucleation and glide on the twin boundary. This is supported by the determination of the velocity and energy barriers to glide for twin boundary partials. In particular, twin boundary partial edge dislocations are significantly faster than corresponding screws as well as their bulk counterparts. In the last part of the work, the effect of variable twin boundary orientation in relation to the loading direction is investigated. In particular, a change away from pure normal loading to the twin plane toward mixed shear-normal loading results in a transition of dominant deformation mechanism from bulk dislocation nucleation/slip, to twin boundary motion.

Dislocation dissociation induces secondary twinning in titanium

The twin–twin interaction phenomenon in Ti is studied by incorporating experimental observation and theoretical analysis. Secondary $$ \{ {10\bar{1}2} \} $$ twins are stimulated in primary $$ \{ {11\bar{2}2} \} $$ and $$ \{ {11\bar{2}4} \} $$ via twinning dislocation dissociation. Primary twin variants are found either share the same $$ \langle {10\bar{1}0} \rangle $$ zone axis or with different zone axes. Schmid law, dislocation theory and crystallographic analysis are used to investigate the formation processes and physical mechanisms of the twin–twin interaction phenomenon. Results show that non-Schmid secondary twins are stimulated under the effect of primary twinning interactions. Besides to secondary twinning dislocations, glissile and sessile dislocations generate as a result of primary dislocation dissociation. Glissile dislocation glide on the easy slip prismatic plane, while sessile dislocation accumulates and leads to the increase of local strain.

Improvement in damping capacity of extruded pure magnesium through precompression and subsequent annealing: Effects of dislocation and twin boundary motions

Abstract This study demonstrates that a combined precompression and subsequent annealing (PCA) treatment considerably improves the damping capacity of extruded pure Mg. Variations in the microstructure and damping capacity with the annealing temperature of PCA are investigated. Precompression along the extrusion direction induces an increase in the dislocation density and the formation of numerous {10–12} twins, the latter of which results in significant lattice reorientation and an increased total boundary length. Subsequent annealing at 200 °C causes barely any change in the formed twin structure, whereas annealing above 250 °C induces extensive grain growth through strain-induced grain boundary migration; this consequently reduces the twin fraction and dislocation density and increases the grain size. The samples subjected to PCA show higher damping capacities than the initial sample in both low- and high-strain-amplitude regions. The improved damping capacity in the low-strain-amplitude region is due primarily to the increase in the dislocation density and Schmid factor for basal slip; namely, more abundant dislocations move more readily, which leads to dissipation of a larger amount of energy via the vibration and sweeping motion of dislocations. In the high-strain-amplitude region, a large sweeping motion of dislocations within grown grains and additional sweeping motion of twin boundaries within twinned grains significantly improve the damping capacity. However, PCA treatment with annealing at 200 °C causes a relatively smaller increment in the damping capacity in the high-strain-amplitude region because the excessive dislocations and twin boundaries in this sample hinder their motions.

Investigation of defect structure in homoepitaxial (2¯01) β-Ga2O3 layers prepared by plasma-assisted molecular beam epitaxy

Abstract We report growth and comprehensive study on structural properties of homoepitaxial ( 2 ¯ 01 ) β-Ga2O3 layers grown by plasma-assisted molecular epitaxy. Effects of growth temperature on surface morphology, crystal quality of homoepitaxial β-Ga2O3 layers were investigated in detail. Characterization indicates that the growth temperature plays an important role on both of surface morphology and the crystal quality of the homoepitaxial β-Ga2O3 layers. Characteristic of defects in homoepitaxial ( 2 ¯ 01 ) β-Ga2O3 layers was investigated comprehensively. Two types of defects were found out including threading dislocation and twin boundary. Origin and formation mechanism of these defects were clarified. The densities of the twin boundary and dislocation based on growth temperature are investigated and discussed. As the growth temperatures changed from 750 to 850 °C, the densities of the threading dislocation etch pit changed from 1.1 × 106–6.6 × 105 cm−2, while those of the twin boundary defect changed from 7.5 × 103–2.1 × 102 cm−1. It is clear that while the densities of the twin boundary were dramatically reduced by 35 times, the densities of the threading dislocation were reduced by 1.7 times.

Comparison of dislocation density, twin fault probability, and stacking fault energy between CrCoNi and CrCoNiFe medium entropy alloys deformed at 293 and 140K

To elucidate deformation behavior behind the exceptional mechanical properties of CrCoNi based medium entropy alloys, the deformation related microstructural parameters were determined by using in situ neutron diffraction and peaks profile analysis methods. Superior tensile strength and elongation of the CrCoNi alloy is relevant to higher twin fault probability (Ptw, up to 3.8%) and dislocation density (ρ, up to 9.7 x 1015 m−2) compared to those (1.3% and 3.4 x 1015 m−2, respectively) of the CrCoNiFe at 293K. Meanwhile, at 140K, the Ptw of the CrCoNiFe significantly increased up to 4.4% with the stable ρ of ~5.0 x 1015 m−2 and its mechanical properties overwhelm those of the CrCoNi at 273K. Such twinning dominant deformation mechanism at low temperature is also assured by lower stacking fault energy (SFE) of the CrCoNiFe (7.4 mJ/m2) at 140K compared to those of the CrCoNi (16.1 mJ/m2) and CrCoNiFe (32.1 mJ/m2) alloys at 293K.

Reasonable utilization of {10-12} twin for optimizing microstructure and improving mechanical property in a Mg-Gd-Y alloy

During the deformation of Mg alloys, {10-12} extension twin, as one of the most active deformation mechanisms, contributed much to the formation of basal texture, yet little to dynamic recrystallization (DRX) grains, i.e., effective grain refinement. In this work, {10-12} extension twin had been creatively utilized to achieve grain refinement and property improvement in a Mg-Gd-Y alloy by multi-directional impact forging (MDIF). Firstly, flourishing {10-12} extension twins divided the parent grain into many fine independent regions (~10 μm). Meanwhile, a large number of twin boundaries were pinning mainly by interaction with neighboring dislocations in twin or matrix. Secondly, those pinned twin boundaries can effectively accumulate dislocation slips and promote the formation of extensive low angle grain boundaries (LAGBs). These high densities of LAGBs can further subdivide the independent regions provided by extension twins into micro-regions (~5 μm). Thirdly, those fine micro-regions developed to be DRX after a dynamic recovery. Finally, we obtained a yield isotropy high-strength Mg alloy with an ultimate tensile strength of 354 MPa after MDIF of 110 passes. This work proposed a brand new insight to prepare wrought Mg alloy with fine grain, yet without strong texture, which is beneficial to better formability and mechanical property isotropy.

Unusual shear induced unconventional [formula omitted] twinning in Mg

Conventional (1¯011) twinning in hexagonal metals has the twin plane K1 = (1¯011) and the corresponding twinning direction η1=[1¯012¯]. Twinning shear is generally accomplished through the gliding of two-layer and/or four-layer twinning dislocations (TDs) along η1 associated with atomic shuffles. Using molecular dynamics simulations, we reveal that an unusual shear along −η1 associated with atomic shuffles on the twin plane K1 is resulted by gliding of a one-layer TD. Correspondingly, an unconventional {1¯011} twinning occurs through successive gliding of one-layer TDs. Such a twinning mechanism is found to be correlated to high stress/strain concentration, such as at crack tip.

A novel nanostructure to achieve ultrahigh strength and good tensile ductility of a CoCrFeNiMn high entropy alloy.

A CoCrFeNiMn high entropy alloy with a novel nanostructure consisting of ultrafine grains, TiO(C) nanoparticles and nanotwins has been fabricated. It achieves an ultrahigh tensile yield strength of 1507 MPa by coupling multiple strengthening mechanisms, including grain boundary strengthening, twin boundary strengthening, nanoparticle strengthening and dislocation strengthening. The work hardening ability is also improved by coupling the interactions of dislocations with nanoparticles and nanotwins during plastic deformation, leading to a good tensile ductility with a uniform elongation of 4.7%.

The low-temperature mechanical properties of the Fe40Mn40Co10Cr10 high-entropy alloy, the ductility of which is induced by twinning

In this work, we studied the low-temperature plasticity, elastic and dissipative characteristics, as well as the microstructural evolution of the Fe40Mn40Co10Cr10 high-entropy alloy in a wide temperature range of 300 - 0.5 K. The temperature dependences of yield strength, strain hardening, strength and ductility, as well as acoustic absorption and dynamic Young's modulus are obtained. It was found that the structure of Fe40Mn40Co10Cr10 alloy in the initial state is single-phase with fcc lattice, and in the deformed state at low temperatures it becomes two-phase due to the deformation-induced phase transition. In addition, EBSD analysis of the alloy structure revealed a change in grain morphology and the appearance of twin dislocations after plastic deformation at all investigatedtemperatures. Acoustic studies showed that the transition from the initial to the deformed state changes the character of the temperature dependence of the dynamic Young's modulus from almost linear to exponential, and reduces the absolute values. The Fe40Mn40Co10Cr10 alloy has excellent strength and ductility at a high strain hardening rate, which is explained by the significant contribution of the twinning process. With a decrease in temperature from 300 to 4.2 K, a strong temperature dependence of the yield strength is observed, which indicates the thermal activation of the nature of the plastic deformation of the alloys in this temperature range. In the temperature range of 0.5-4.2 K, an anomaly of the yield strength was observed, namely a decrease in the value of the yield strength with decreasing temperature from 4.2 K to 0.5 K. The anomalous dependence of the yield strength is due to a change in the mechanism of overcoming local barriers from thermoactivated to inertial one, when part of the local obstacles is overcome by dislocations without activation. This leads to a decrease in yield strength with decreasing temperature. At a temperature of 4.2 K and below, the smooth nature of the plastic deformation changes from smooth to serrated. The jumps begin immediately after the yield strength and gradually increase from 40 MPa to ~ 160 MPa. The results obtained are important for practical applications of high-entropy alloys at low temperatures.

Effect of twin boundary spacing on the deformation behaviour of Au nanowire

The deformation behaviour of Au twinned nanowire is investigated by molecular dynamic simulation. It is found that the relationship of yield strength with twin boundary spacing (TBS) can be divided into three ranges. The deformation mechanism of Au twinned nanowire is revealed according to the dislocation–twin interactions. For those with large and medium softening TBS ranges, some Shockley partial dislocations and stair-rod dislocations are formed, and a small step is left on the TBs during the dislocation–twin interactions. But the amount of dislocations in the nanowire with medium TBS is remarkably larger than that with large TBS, and more different {111} slip planes are activated. For that with small strengthening TBS range, lots of Shockley partial dislocations are formed, and they form highly-localized shear zone at the middle of nanowire. TBs undergo a destruction and following recovery process during the dislocation–twin interactions, and the TBS is enlarged during this process.

Thermal stability of nanogradient microstructure produced by surface mechanical rolling treatment in Zircaloy-4

A nano-/ultrafine grain gradient microstructure, which is composed of high-angle grain boundaries (HAGBs) and low-angle grain boundaries or subgrains of dislocation–twin, was fabricated in Zircaloy-4 using surface mechanical rolling treatment (SMRT). Thermal stability of gradient microstructure has been investigated through characterizing the evolution of microstructure during post-SMRT annealing treatment from 400 to 600 °C using optical microscopy and transmission electron microscopy. Experimental results show that the gradient microstructure exhibits a good thermal stability at 400 °C, since the overall grain size remains similar, except a decrease in dislocation density due to recovery. In comparison, a hierarchical microstructure is formed after annealing at 600 °C. An obvious grain growth was observed at the depth of 50 μm. The activation energy for grain growth of nanograined Zircaloy-4 is estimated to be ~ 161 kJ/mol between 400 and 600 °C. Nano-/ultrafine grains predominantly consisting of HAGBs have the highest thermal stability. Both yield strength and ultimate tensile strength of Zircaloy-4 decrease due to anneal, specifically at 600 °C.

Unusual Shear Induced Unconventional {1011} Twinning in Mg

Conventional (1011) twinning in hexagonal metals has the twin plane K1 = (1011) and the corresponding twinning direction η1=[1012]. Twinning shear is generally accomplished through the gliding of two-layer and/or four-layer twinning dislocations (TDs) along η1 associated with atomic shuffles. Using molecular dynamics simulations, we reveal that an unusual shear along -η1 associated with atomic shuffles on the twin plane K1 is resulted by gliding of a one-layer TD. Correspondingly, an unconventional {1011} twinning occurs through successive gliding of one-layer TDs. Such a twinning mechanism is found to be correlated to high stress/strain concentration, such as at crack tip.

Impacts of pre-strain on twin boundary mobility of magnesium

Twin boundary mobility (TBM) has significant influences on the mechanical properties of magnesium (Mg). In the current study, the effects of pre-strain on TBM in pure Mg are systematically investigated by evaluating the change of yield stress during compression and tension, and microstructure observations by in-situ electron back-scattered diffraction. Owing to lower dislocation density and less restriction from other boundaries, smaller twins in pure Mg subjected to 2% compressive strain exhibit higher TBM than larger twins in pre-4% and pre-8% compressed pure Mg. Decreased TBM with increasing pre-strain is primarily because of the larger number of dislocations, which impede twin boundary motion. These findings indicate that altering twin size and dislocation density by pre-strain is an effective way to tailor TBM, providing a basis for developing new industrial Mg alloys.

Characterization of Twin Transmission Structure of High Manganese Austenitic Steel by High-Resolution Transmission Electron Microscopy

Atomic-scale twin transmission structures were clearly characterized by high-resolution transmission electron microscopy. Two different twin transmission structures are found during another variant twin transmitting across pre-existing twins: dislocation twin transmission structure and local crystal reorientation twin transmission structure. In addition, twin-twin interactions can induce the formation of other structures, such as double twins, stacking faults and few e-martensite, and coherent twin boundaries of two variants twins can simultaneously migrate, appearing as if a twin partly penetrates the other one.

Effects of twin boundary orientation on plasticity of bicrystalline copper micropillars: A discrete dislocation dynamics simulation study

Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work, we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar cross-section. The simulations show significant strengthening effects for TB orientation angles less than 45°, where the interaction between mixed dislocations and TBs plays an important role. For angles larger than 45°, the interaction between screw dislocations and TB dominates, leading to weak strengthening. At 45°, dislocations on the dominant slip systems glide parallel to the TB and no strengthening is observed.

Fracture behavior of twin induced ultra-fine grained ZK61 magnesium alloy under high strain rate compression

In this study ultra-fine grained single pass extruded ZK61 magnesium alloy sheet is processed at temperature 350 °C with area reduction ratio ˜30. The complex dynamic mechanical behavior is studied experimentally with Split Hopkinson pressure bar over wide ranges of strain rates (1000–4000 s−1) along the normal direction. Electron backscattered diffraction, scanning electron microscopy and optical microscopy analysis are employed to reveal the changes in texture and fracture analysis. Positive strain rate sensitivity is observed up to strain rate 3000 s−1 that is signature of basal slip and extension twinning although the grain size was very small. This finding is contradiction to experimental and theoretical predictions suggesting the elimination of twinning induced deformation in magnesium alloys in very fine small grains <3 μm. Flow stress and a fraction of extension twinning are apparently decreased as the applied strain rate increased from 3000 to 4000 s−1; that is attributed to adiabatic rise in temperature, early reorientation of crystal. Extensive grain reorientation causes the nucleation of extension twinning that changes fiber texture to strong double peak basal texture under high strain rate compression. Nucleation of principle crack was attributed to twin–dislocation interaction that provides dynamic recrystallized grains followed by the adiabatic shear band at the boundary of coarse grains. Contrary, secondary cracks are nucleated and propagated due to high local stresses at the triple junction of grain boundaries. Besides, scanning electron microscopy revealed ductile fracture under high strain rate compression.

High performance computing simulations of spall phenomenon in a submicron thick nanocrystalline aluminum

The spall failure phenomenon is investigated at nano-scale in a plate-on-plate impact configuration using large-scale molecular dynamics (MD) high performance computing (HPC) simulationson three multi-billion 0.5 μm thick nanocrystalline aluminum (nc-Al) systems, each with an average grain size in the range of 18–100 nm and each under impact velocities in the range of 0.7–1.5 km s−1. Using a material conserving atom section-averaging process, distributions of several mechanical responses were obtained and the extremely transient spall failure phenomena were located in the interaction zones of the stress release waves released from the impacted and free ends of the atom systems. The spall zones’ thicknesses were estimated along with the spall strengths. For the nc-Al atom systems, the spall strength was observed to increase as the impact velocity increased from 0.7 to 1.5 km s−1, but only showed a slight decrease as the grain size increased from 18 to 100 nm. The spall strengths thus estimated from the stress release waves’ interaction zones were found to be conservative when compared to the traditional estimates obtained from the pullback velocity signatures in the free-end velocity evolutions. The MD results were further analyzed using a crystal analysis algorithm and a twin dislocation identification method to obtain the densities of the atomistic microstructures evolving under the interaction of the stress release waves. High-fidelity large-scale HPC simulation results showed that certain dislocation partials strongly influenced the spall response. The Stair-rod type dislocation partials increased by more than a factor of 10 during the interaction of the stress release waves as the spall failure commenced.

Strain controlled isothermal low cycle fatigue life, deformation and fracture characteristics of Superni 263 superalloy

The total axial strain controlled low-cycle fatigue (LCF) behaviour of a wrought nickel-base superalloy Superni 263 has been evaluated at 298 K, 673 K, and 923 K employing the strain amplitudes in the range ± 0.25% to ± 0.80%. All the tests were performed using a triangular wave form at a constant strain rate of 10−3 s−1. The Superni 263 alloy was subjected to a solutionizing treatment at 1373 K/1.5 h followed by water quenching. An ageing treatment of 1073 K/8 h was given to the solutionized samples. The microstructure of the alloy is composed of γʹ in the intragranular regions and discrete M23C6 precipitates on the grain boundaries. Few undissolved (Ti, Mo)C and TiN precipitates were also noticed in the matrix. The strain amplitude and temperature played a significant role in the evolution of macroscopic cyclic stress response and the development of deformation substructure. The alloy displayed serrated flow in the plastic portions of stress-strain hysteresis loops in the tests conducted at 673 K and 923 K, predominantly at strain amplitudes higher than ±0.40%. Several manifestations depicting the occurrence of dynamic strain ageing (DSA) were derived by analysing both the macroscopic and micro-mechanistic information generated. The comparative evaluation of monotonic and cyclic stress-strain curves was conducted and the factors responsible for rapid cyclic hardening during DSA were identified. The fatigue life decreased drastically with increasing temperature, particularly at low strain amplitudes. The strain-life data could be represented well by using Basquin and Coffin-Manson relationships. The slope in Coffin-Manson relationship revealed more negative values under the influence of DSA. The crack initiation and propagation modes at all the test conditions remained transgranular without the indications of interference from creep and oxidation effects. The deformation substructure at elevated temperatures was characterized by the occurrence of co-planar slip, stacking faults, micro twinning and high dislocation density in between the slip bands.

Effect of Twin Boundary Orientation on the Mechanical Properties of Bicrystalline Copper Micropillars

Twin boundaries (TBs) constitute a special type of symmetric grain boundary (GB). TBs influence plastic deformation in a complex manner. They not only act as dislocation obstacles but can also accommodate twinning dislocations (TDs) whose motion enables twin boundary migration as an alternative deformation mechanism. Exploiting this dual effect offers interesting perspectives in view of designing materials that combine high strength and ductility. In the present work,we propose a model for dislocation-TB interactions in the framework of discrete dislocation dynamics (DDD) simulations, which we use to investigate the mechanical properties of bicrystalline copper micropillars containing a TB. We systematically investigate how the compressive response depends on the orientation of the TB with respect to the micropillar cross section. The simulations show significant strengthening effects for TB orientation angles less than 45o, where the interaction between mixed dislocations and TBs plays an important role.For angles larger than 45o, the interaction between screw dislocations and TB dominates, leading to weak strengthening. At 45o, dislocations on the dominant slip systems glide parallel to the TB and no strengthening is observed.