Deformation Twinning Mechanism Research Articles

Twin-assisted precipitation during hot compression of an Mg-Gd-Zn-Zr magnesium alloy

Dynamic precipitation in an Mg-Gd-Zn-Zr alloy during hot compression at 350°C and 450°C and at strain rates of 0.001-0.1s−1 is investigated. The deformation conditions were designed such as to promote twinning or slip of dislocations. The results indicated that both twinned and non-twinned grains were present at 350°C, with the extent being a function of the applied strain rate. The formation and propagation of {101̅2}〈1̅011〉 primary tensile twins within the twinned grains was identified as being at root of the twin-assisted precipitation phenomenon. The precipitates had spherical-morphology in the twinned grains whereas parallel arrays of rod-shaped precipitates formed in the non-twinned grains due to dynamic precipitation on the slip bands and the stacking faults. Twin-assisted precipitates were suppressed by increases in the strain rate at 350°C because of the shorter time required for solute element diffusion towards the nucleated precipitates at that temperature. Increasing the deformation temperature to 450°C changed the dominant deformation mechanism from twinning to slip of dislocations, resulting in the formation of rod-shaped precipitates similar to those observed in non-twinned grains at 350°C. An EBSD analysis was used to study the dominant deformation mechanism as a function of the Schmid factor, and the twinning deformation mechanism was found to be predominant in the grains having a lower Schmid factor.

Twinning-governed plastic deformation in a thin film of body-centred cubic nanocrystalline ternary alloys at low temperature

The plastic deformation of FeCuNi ternary alloys of BCC nanocrystalline subjected to compressive or tensile loading is considered via molecular dynamics (MD) simulations at low temperature, to reveal the solid solution effect on BCC metals. Deformation mechanisms at nanoscale have been well revealed for FCC nanocrystalline structure, however, it remains unanswered for BCC nanocrystalline structure. The increase of solute concentration promotes the nucleation of partial dislocation due to the reduction of stacking fault energy, and suppresses the motion of dislocation owning to the severe lattice distortion, which is beneficial to improving the strength and ductility of FeCuNi alloys. There is a critical solute concentration to determine the strengthening or the softening of FeCuNi alloys. Interestingly, the deformation twinning mechanism, instead of traditional dislocation mechanism, dominates the plastic deformation of BCC ternary alloys. In addition, it is clear to observe the phase transformation from BCC to HCP, full dislocation slipping, cross-slip, grain growth, and grain boundary (GB)/twinning boundary (TB) motion. Fracture behaviors of FeCuNi alloys reveal that the addition of Cu and Ni elements results in the larger fracture toughness.

Dislocation interaction and twinning-induced plasticity in face-centered cubic Fe-Mn-C micro-pillars

Deformation twinning contributes to a high work-hardening rate through modification of the dislocation structure and a dynamic Hall-Petch effect in polycrystalline steel. Due to the well-defined compression axis and limited deformation volume of micro-pillars, micro-compression testing is a suitable method to investigate the mechanisms of deformation twinning and the interactions of dislocations with twin boundaries. The material investigated is an austenitic Fe-22 wt%Mn-0.6 wt%C twining-induced plasticity steel. Micro-pillars oriented preferentially for deformation twinning and dislocation glide are compressed and the activated deformation systems are characterized. We observe that deformation twinning induces higher flow stresses and a more unstable work-hardening behavior than dislocation glide, while dislocation glide dominated deformation results in a stable work-hardening behavior. The higher flow stresses and unstable work-hardening behavior in micro-pillars oriented for deformation twinning are assumed to be caused by the activation of secondary slip systems and accumulated plastic deformation.

Mechanical behavior of homogeneous and nearly homogeneous Ti3Sn: Role of composition and microstructure

The impact of composition and grain size on mechanical behavior of as-cast Ti3Sn and near Ti3Sn alloys is investigated at room temperature in uni-axial compression and three-point bending tests. The concentration dependencies of both yield strength and bending failure strain show extreme behavior in the room temperature homogeneity range with minimum (about 90MPa) and maximum (15%), respectively, for the Ti75.6Sn24.4 composition. The failure strain in adjacent two-phase regions does not depend on Ti/Sn ratio and is about 2–3%. The yield strength of all the alloys is shown to increase according to the Hall-Petch type relation. A defect structure evolution during the process of deformation, studied with optical and electron microscopy, is indicative of the activation of a twinning deformation mechanism in a matrix phase. Inter-grain and inter-phase boundaries impede this deformation and affect nucleation and propagation of cracks.

Deformation Twinning of Molecular Dynamics Simulation in a Ternary Titanium Alloy under Nanoindentation

Nanotwinned (nt) metals exhibit excellent mechanical, electrical and thermal properties, and therefore attract much attentions. To fabricate large area nt surface, the fundamental mechanisms of deformation twinning induced by molecular dynamics (MD) are necessary to be explored. Nevertheless, MD of nt metals currently focus mainly on nt copper (Cu) and other single element metals with face-centered cubic (fcc) structure. In addition, MD simulations are usually performed on a built nt model, rather than from a single crystal, due to the difficulty of forming nanotwins. In this study, a single crystal is constructed in a ternary titanium (Ti) alloy with hexagonal closed-packed (hcp) lattice cell. Deformation twinning of MD simulation is performed in a ternary Ti alloy under nanoindentation from the built single crystal. Zonal structure is found during loading under nanoindentation, and nanograins transforms into nanotwins. Deformation twinning is significant to understanding the formation of nanotwins, as well as fabricating large area nt surface on a Ti alloy.

Twinning interactions induced amorphisation in ultrafine silicon grains

Detailed transmission electron microscopy analysis on a severely deformed Al-Si composite material has revealed that partial dislocation slips and deformation twinning are the major plastic deformation carriers in ultrafine silicon grains. This resembles the deformation twinning activities and mechanisms observed in nano-crystalline face-centred-cubic metallic materials. While deformation twinning and amorphisation in Si were thought unlikely to co-exist, it is observed for the first time that excessive twinning and partial dislocation interactions can lead to localised solid state amorphisation inside ultrafine silicon grains.

Coupled Effects of Strain Rate and Temperature on Deformation Twinning in Cu-Zn Alloy

Cu-Zn alloy is an advanced material with deformation twinning mechanism, which is complicated by coupled effects of temperature and strain rate. In this paper, a theoretical model of Cu-Zn alloy is proposed, which accounts for the coupled effects of strain rate and temperature. The model can accurately predict the experimentally observed tendency of the spacing evolution of twin boundary (TB), and it confirms that low temperature and high strain rate promote deformation twinning. Moreover, it is shown that deformation twining is more susceptible to low temperature than to high strain rate, while TB spacing and twin layer thickness values decrease at high strain rates and low temperatures.

Strain rate dependence of deformation-induced transformation and twinning in a metastable titanium alloy

A metastable β Ti–10V–3Fe–3Al–0.27O (wt.%) alloy was subjected to thermo-mechanical processing to induce α and ω phase formation, so that the alloy can exhibit the features responsible for both transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) behaviour during deformation. The alloy thereafter was deformed at different strain rates (10−3, 10−1, 101, 102 s−1) at ambient temperature. At slow strain rate (≤10−3 s−1), in addition to slip the alloy displayed a dominant deformation mechanism with α′′ martensite formation, where mechanical {332}〈113〉β twinning and deformation-induced ω phase were also activated. At an intermediate strain rate, 10−1 s−1, there was a competition between stress-induced phase transformations and stress-induced twinning deformation mechanisms. With increasing strain rate to 101 s−1 or higher; it was found that the dominant deformation mode was twinning. These results have been correlated with the β phase stability of the samples.

Twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s −1 . These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading.

ICME for Crashworthiness of TWIP Steels: From Ab Initio to the Crash Performance

During the last decade, integrated computational materials engineering (ICME) emerged as a field which aims to promote synergetic usage of formerly isolated simulation models, data and knowledge in materials science and engineering, in order to solve complex engineering problems. In our work, we applied the ICME approach to a crash box, a common automobile component crucial to passenger safety. A newly developed high manganese steel was selected as the material of the component and its crashworthiness was assessed by simulated and real drop tower tests. The crashworthiness of twinning-induced plasticity (TWIP) steel is intrinsically related to the strain hardening behavior caused by the combination of dislocation glide and deformation twinning. The relative contributions of those to the overall hardening behavior depend on the stacking fault energy (SFE) of the selected material. Both the deformation twinning mechanism and the stacking fault energy are individually well-researched topics, but especially for high-manganese steels, the determination of the stacking-fault energy and the occurrence of deformation twinning as a function of the SFE are crucial to understand the strain hardening behavior. We applied ab initio methods to calculate the stacking fault energy of the selected steel composition as an input to a recently developed strain hardening model which models deformation twinning based on the SFE-dependent dislocation mechanisms. This physically based material model is then applied to simulate a drop tower test in order to calculate the energy absorption capacity of the designed component. The results are in good agreement with experiments. The model chain links the crash performance to the SFE and hence to the chemical composition, which paves the way for computational materials design for crashworthiness.

Quasicontinuum Analysis of Dislocation Propagation during Nanocontact

Multiscale simulations using the quasicontinuum (QC) method with the embedded-atom method (EAM) potential are performed to investigate the process of nanocontact including sliding and subsequent withdrawal between Ni tip and Au substrate. The multiscale model reveals that deformation twinning in Au substrate is induced not only by the sheer stress but also by the adhesive stress. Combining with the generalized planar fault energy (GPF) curve of Au, the underlying formation mechanism of deformation twinning is studied in detail. During the withdrawal process, the dislocation degeneration and the vacancy evolution are observed.

A mechanism-based model for deformation twinning in polycrystalline FCC steel

Deformation twinning, a common and important plastic deformation mechanism, is the key contributor to the excellent combination of strength and ductility in twinning-induced plasticity (TWIP) steel. In the open literature, a significant amount of research has been reported on the microstructural characteristics of deformation twinning and its influence on the overall deformation behavior of TWIP steel. In this study, we examine the feasibility of a mechanism-based crystal plasticity model in simulating the microstructural level deformation characteristics of TWIP steel. To this end, a model considering both double-slip and double-twin is developed to investigate the stress–strain behavior and local microstructural features related to the formation and growth of micro-twins in low stacking fault energy (SFE) TWIP steel. The twin systems are described as pseudo-slips that can be activated when their resolved shear stress reaches the corresponding critical value. A hardening law that accounts for the interaction among the slip and twin systems is also developed. Numerical simulations for different mesh sizes and single crystal patch tests under different loading modes are carried out to verify the modeling procedure. Our simulation results reveal that, despite its simple nature, the double-slip/double-twin model can capture the key deformation features of TWIP steel, including twin volume fraction evolution, continuous strain hardening, and the final fracture in the form of strain localization.

Texture evolution during annealing of AZ31 Mg alloy rolled sheet and its effect on ductility

Magnesium sheet alloys have huge potential in automotive closure applications due to their light weight. However, their limited room temperature formability restricts their widespread applications. Microstructure (both geometric and crystallographic) plays a key role in dictating the formability of an alloy. In this research, a systematic study of the microstructure evolution of a hot-rolled AZ31 sheet during thermal annealing was carried out, and the effect of microstructure on room-temperature deformation was characterized. The initial hot-rolled sheet had a dominant basal texture with equiaxed grains and shear bands. The annealing treatment imparted to the sheet alloy triggered an anomalous, inhomogeneous grain growth in grains with a large deviation from the basal orientation. At 350°C annealing temperature, a bimodal grain size distribution of grains with basal and non-basal orientations was observed, and this combination was shown to significantly enhance ductility resulting from a complex interplay between slip and twinning deformation mechanisms. This key finding is expected to contribute to the development of engineered microstructures for enhanced room temperature formability in this alloy.

Deformation twinning and textural evolution of pure zirconium during rolling at low temperature

Industry pure zirconium sheets with a strong c-axis fiber texture were rolled to different strains at 77 K to investigate the twinning behavior and deformation mechanism. The microstructure and texture of the rolled specimens were characterized by scanning electron microscopy (SEM) together with electron backscatter diffraction (EBSD) techniques. The results show that the ▪ compression twinning mode is the dominant deformation twin at low strains loaded along the c-axis, and the ▪ tensile twinning generates as the second twin in ▪ twins. The selection of twinning modes is governed by Schmid factor (SF) due to the calculating of SF and the EBSD simulating of twinning distribution. The evolution of texture during rolling affected by twins with increase of the strain was explained.

A practical constitutive model for AZ31B Mg alloy sheets with unusual stress–strain response

Magnesium (Mg) and its alloys have been increasingly used in the automotive industry owning to their superior mechanical compared to other metals. Sheet forming of Mg alloys requires not only a good understanding of the materials but also an appropriate material model. The hardening behavior of Mg alloy sheets during plastic deformation is the main concern in tool design and product manufacturing processes. A simple but computationally effective constitutive model is suggested to predict the mechanical behavior of Mg alloy sheets. The proposed material model is implemented as a user-defined material subroutine in the commercial finite element code PAM-STAMP®, then calibrated and validated using in-plane cyclic loading tests with different values of prestrain. The continuous tension–compression–tension and compression–tension–compression tests are conducted on a newly developed testing system. After calibration, the model can accurately predict the yielding asymmetry behavior and the stress–strain curve featured by the unusual sigmoidal shape which is the result of the activation of twinning and untwinning deformation mechanisms.

Mechanism of Deformation Twinning in Tantalum Driven by Extremely Dynamic Shear-Compression at Low Temperature

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Low-cycle fatigue behaviors of hot-rolled AZ91 magnesium alloy under asymmetrical stress-controlled cyclic loadings

Abstract The low-cycle fatigue behaviors of hot-rolled AZ91 magnesium alloy are investigated by the uniaxial asymmetric stress-controlled cyclic experiments at room temperature. The fatigue failure mechanisms are analyzed by the fracture morphology characteristics. The effects of stress ratio and peak stress on the low-cycle fatigue life are discussed. Considering the coupled effects of ratcheting damage and fatigue damage on the material failure, a phenomenological fatigue life model is developed to evaluate the low-cycle fatigue life of the studied magnesium alloy. Results show that: (1) The fatigue cracks initiate from the surface of specimens. The cleavage-like facet features appear in the crack propagation region, while the residual ductile dimples and tearing edges appear in the fracture region. (2) The fatigue life increases with the increase of stress ratio and decrease of peak stress, and the stress intensity factor range and twinning deformation mechanism are sensitive to the stress ratio and peak stress. (3) The amount of residual twins on the fracture surface of specimens are greatly affected by the stress ratio and peak stress. Increasing the stress ratio and decreasing the peak stress can obviously decrease the amount of residual twins. The residual twins kink with each other, which deteriorate the grain boundary. (4) Comparisons between the measured and predicted fatigue life confirm that the developed phenomenological fatigue life model can give an accurate estimate of the low-cycle fatigue life of hot-rolled AZ91 magnesium alloy.

Temperature dependent deformation of the B2 austenite phase of a NiTi shape memory alloy

Temperature dependent deformation of the B2 austenite phase of a polycrystalline Ni49.9Ti50.1 (at.%) shape memory alloy was studied through combined macroscopic and microstructural characterization efforts. The objective was to determine mechanisms responsible for the macroscopic inelastic strains during isothermal loading of NiTi to 18% strain at temperatures above which the austenite phase exists in the nominally unloaded or stress-free condition (i.e., above the austenite finish temperature, Af). This study included in situ time-of-flight (TOF) neutron diffraction experiments used to follow the evolution of the lattice strains, texture, and phase fractions during deformation, ex situ macroscopic tensile experiments, and hot stage transmission electron microscopy (TEM). It was found that stress-induced martensite (SIM) formed at temperatures up to 310°C, which is well above the stress-free Af of 105°C. However, the stress induced martensite formed concurrently with general <001> slip processes and twinning by {114}B2 compound deformation twins, and did not occur as a separate distinguishable mechanism. Above the temperature that martensite cannot form with stress or the martensite desist temperature, Md, deformation was governed by the same slip and deformation twinning mechanisms, in addition to diffusion-assisted deformation processes. The overall results were combined to generate a deformation map that contained limits over which each of the identified deformation mechanisms was dominant in this Ni49.9Ti50.1 alloy.

Deformation twinning mechanisms from bimetal interfaces as revealed by in situ straining in the TEM

During in situ transmission electron microscopy indentation of layered Cu/Nb composites fabricated by accumulative roll bonding, we observed the nucleation and growth of deformation twinning in Cu. Combining dislocation theory and interface defect structure analysis, we propose that the processes of nucleation and thickening of deformation twins proceed as follows: (1) nucleation and emission of a nanometer thick twin nucleus due to the dissociation of interfacial misfit dislocations or the interaction between a run in Nb slip dislocation and the interface or both; (2) emission of a second twin nucleus by the same mechanism, but at some distance along the interface from the first; (3) in the region in-between them, twin thickening occurs by relatively rapid emission of oppositely signed twinning dislocations in order to reduce the backstresses due to the twins in processes 1 and 2; and finally (4) the continuation of this “alternating emission” growth mechanism such that eventually the entire domain in-between the two initially separated fine twin nuclei transforms into a thick twin.

Deformation mechanisms and kinetics of time-dependent twinning in an α-titanium alloy

Within the past decade, many studies have shown that twinning can occur in α, β, and α+β titanium alloys at speeds many orders of magnitude lower than the speed of sound. Traditionally, twinning has been described as a shear process that can occur almost instantaneously; however, the twinning deformation mechanisms in titanium alloys can be controlled by the diffusion of oxygen away from annihilated interstitial sites at the twin boundaries. The most recent studies which include the strain-rate effects on twinning support the hypothesis that twinning in these materials is controlled by a slow diffusion process rather than a quick shear process. The effects of strain-rate on twinning in a α-Ti–1.6V alloy were systematically investigated by means of optical and transmission electron microscopy and a diffusion-based model has been applied which explains the unexpected twin sizes observed at different strain rates.