Occurrence Of Deformation Twinning Research Articles

Dislocation behavior and strengthening mechanisms of Cu–Te alloy drawn at room and cryogenic temperatures

In the work, the cryogenic and room temperature drawing behaviors of Cu–Te alloy with a wide range of accumulative drawing true strain up to 1.1 are systematically investigated. For the first time, the storage of geometrically necessary dislocations (GND) and statically stored dislocations (SSD), occurrence of deformation twinning, as well as their respective roles in strain hardening in the drawn Cu–Te alloy at cryogenic and room temperature have been studied. The results show that the liquid nitrogen temperature drawn (LNT-D) Cu–Te alloy presents higher yield strength (YS), ultimate tensile strength than the room temperature drawn (RT-D) Cu–Te alloy. Microstructural investigation reveals that cryogenic wire drawing of Cu–Te alloy promotes the formation of simultaneous twinning and shear bands, accompanied by the fragmentation of Cu2Te phase. The LNT-D Cu–Te alloy exhibits planar slip characteristics, while RT-D Cu–Te alloy presents wave slip characteristics. Compared with RT-D Cu–Te alloy, the higher YS of LNT-D Cu–Te alloy can be mainly attributed to dislocation strengthening and twinning strengthening, which account for 76.25–80.45% and 4.79–4.91% of the total YS respectively. During room temperature drawing, both GND and SSD play an important role in the hardening of Cu–Te alloy. During cryogenic temperature drawing, work hardening is predominantly through rapid multiplication of SSDs.

In-situ high energy X-ray diffraction study of deformation behavior in Nb nanoparticle-martensitic NiTi composite wire

This study investigated the deformation behavior in Nb nanoparticle-martensitic NiTi composite wire during tensile loading by means of in-situ high-energy X-ray diffraction. Compared to Nb nanowires, the dispersedly distributed Nb nanoparticles have little effect on the transformation and deformation behaviors of NiTi matrix, causing the absence of R phase transformation and the occurrence of deformation twinning in NiTi matrix. Meanwhile, the occurrence of deformation twinning in NiTi matrix can reduce the shape strain to be coordinated by the unfavorable orientated martensite variants, and thus decreases the internal stress among NiTi martensite variants generated during martensite reorientation process. In addition, TEM result proved that Nb nanoparticles can achieve exceptionally large elastic strain in B19′-NiTi matrix (∼4%), just as Nb nanowires. However, this large elastic strain (stress) will be released with the occurrence of NiTi deformation twinning.

Partial melting and reaction along deformation features in plagioclase

AbstractGeological processes involving deformation and/or reactions are highly influenced by the rock grain size, especially if diffusion‐controlled processes take place such as metamorphic reactions and diffusion creep. Although many processes, inducing grain‐size reduction, are documented and understood at relatively high stresses and low temperatures (e.g., cataclasis) as well as at lower stress and higher temperature conditions (e.g., bulging and subgrain rotation), deformation twinning, a plastic deformation mechanism active in various minerals at lower temperatures, has been neglected as nucleation site for melting and reaction and thus as a cause for grain‐size reduction so far. We conducted experiments on natural plagioclase‐bearing aggregates at 2.5 to 3 GPa confining pressure and temperatures of 700°C to 950°C using two different deformation apparatus, a deformation multianvil apparatus (DDIA) and a Griggs press, as well as a piston‐cylinder apparatus. Regardless of the apparatus type, we observe the breakdown of plagioclase into an eclogite‐facies paragenesis, which is associated with partial melting in the high temperature domain of the eclogite facies. Partial melting mostly takes place along the grain and interphase boundaries. However, several melt patches or plagioclase decomposition products coincide with the occurrence of deformation twins and grain‐scale microcracking in plagioclase indicating intracrystalline melting and reaction in addition to melting and reaction along grain and interphase boundaries. In the present study, we demonstrate how the interplay between brittle microcracking and plastic deformation twinning can cause intracrystalline melting and/or reaction, which has the potential to lower the effective grain size of plagioclase‐rich rocks and thus impacts their reactivity and deformation behaviour.

Dynamic strain aging and twin formation during warm deformation of a novel medium-entropy lightweight steel

The present study deals with the extraordinary twin formation during compressive deformation of a Fe-37.55Mn-8.5Al-0.6C (wt.%) low-density steel at warm temperature regime (200–600 °C). Despite the conventional opinion regarding the restrictive effect of increasing temperature on the occurrence of deformation twinning, not only the twin boundary fraction did not decrease, but also twinning was encouraged at 400 °C and even 600 °C. This was discussed relying on the occurrence of dynamic strain aging phenomenon, which caused a bump in corresponding hardening rate curves. The medium entropy of the alloy, which made short-range diffusion of solute atoms feasible and facilitated local pinning of the mobile dislocations, provided a proper condition for the occurrence of high temperature strain ageing. Finally, the unbalanced pinning of trailing and leading dislocations influenced on their velocity and caused widening the stacking faults which in-turn could significantly assist deformation twinning at such high deformation temperatures.

Effect of initial grain size on hot deformation behaviour of Cu-Cr-Zr-Ti alloy

Hot uniaxial compression tests were carried out on Cu-0.8Cr-0.05Zr-0.05Ti (wt%) alloy specimens with three different grain sizes: 30 μm, 150 μm and 450 μm to study the effect of initial grain size on hot workability of the alloy. The deformation temperature and applied strain rate ranges were 600–900 °C and 0.001–1 s−1 respectively. At higher deformation temperatures, fine grained specimens exhibited early onset of dynamic recrystallization (DRX) as compared to coarse-grained specimens. Conversely, the work hardening plots of coarse-grained specimens exhibited plateaux at higher strain rates indicating occurrence of deformation twinning. Microstructure examination revealed a decrease in the extent of DRX and an increase in deformation twinning with increase in initial grain size. The failure in coarse-grained specimens was in the form of wedge cracking observed at triple junctions at lower deformation temperatures. The higher stress values exhibited by coarse-grained specimens as compared to fine grained specimens was found to be a result of absence of DRX and the strain being accommodated by deformation twinning rather than grain boundary sliding (GBS) due to limited grain boundary area. Processing maps using the dynamic material model (DMM) were plotted for all three initial grain sizes and were validated with optical microstructures. The ‘unstable/unsafe’ deformation domain was found to be wider with increase in initial grain size. The present study explicates that there is an increase in difficulty of hot working and reduced control of microstructure in the alloy with increase in initial grain size.

Effects of Stacking Fault Energy on Deformation Mechanisms in Al-Added Medium Mn TWIP Steel

In this study, the effect of aluminum (Al) addition to a manganese (Mn) steel Fe-12Mn-0.5C in regard to the change in stacking fault energy (SFE) and the consequent evolution of deformation microstructure and texture were investigated during cold rolling. An analysis of the texture and microstructure was performed to understand the deformation micro-mechanisms. Deformation micro-mechanisms were substantiated by the estimation of dislocation density and the arrangement of dislocations in the deformed microstructure by X-ray line profile analysis, which revealed significant changes in the dislocation structure with the addition of Al. Three stages of deformation mechanism were observed in all Al-added compositions. In the early stages of deformation, slip as well as twinning prevailed. In the intermediate stage, twinning took over completely and at large strains, macroscopic shear bands became the dominant deformation mode. An increase in the propensity of nanometer-sized deformation twins was observed with rolling strain. However, the addition of Al decreased the overall twin fraction in the deformed microstructure. The theoretical twinning stress was calculated to explain the crucial role of SFE on the occurrence of deformation twins in these steels. The deformation texture was predominantly of the brass type for all the Al-added compositions; however, appreciable differences were seen with Al content. The 〈111〉//ND γ-fiber, which develops in Al-free Fe-12Mn-0.5C, completely disappeared in 3 wt pct Al-containing material.

Surface nanocrystallization of body-centered cubic beta phase in Ti–6Al–4V alloy subjected to ultrasonic surface rolling process

The nanocrystallization mechanism of a body-centered cubic β phase in Ti–6Al–4V alloy subjected to ultrasonic surface rolling process was investigated. A gradient nanostructure (thickness: ~400 μm) that the β grain size in thickness gradually changes from ~0.76 μm in the interior to ~36.5 nm at the topmost surface was formed in Ti–6Al–4V alloy surface layer. The gradient nanostructure of the β phase is formed primarily via dislocation activities without the occurrence of deformation twinning. Dislocations were first generated in β phase at the phase boundaries where a high density of dislocations occurred in α phase. The coarse β grains were then gradually transformed into equiaxed nano grains via longitudinal splitting and transverse breakdown, which are induced by dislocation glide, entanglement, accumulation, and rearrangement. Additionally, with increasing strain, the β nanograins will be further refined via dislocation activities.

Solving the strength-ductility tradeoff in the medium-entropy NiCoCr alloy via interstitial strengthening of carbon

Interstitial solid strengthening is an effective strategy to harden metallic materials, however, it usually deteriorates the ductility. Here, we report that addition of carbon into the medium-entropy NiCoCr alloy successfully enhances the strength at no expense of ductility. It was found that up to 0.75 at.% carbon was completely solid-solutionized in (NiCoCr) 100-xCx (x = 0, 0.10, 0.25, 0.50 and 0.75 at.%) without formation of any carbides. With the increase of carbon content from 0 to 0.75 at.%, the yield and fracture strength were increased from 242 to 347 MPa to 727 and 862 MPa, respectively, whilst the ductility kept as high as about 75%. It is noteworthy that the integral of the stress over strain for the alloy with 0.75 at.% carbon reaches a value of 59 GPa %, surmounting the level of many reported multi-principal elements alloys. Our analysis indicates that carbon addition increases stacking fault energy, thus delaying occurrence of twinning and decreasing the thickness of twin lamellas. At the early deformation stage, carbon decreases the stress localization and stimulates dislocation multiplication. After occurrence of deformation twinning, finer twinning structure in the alloys added with carbon not only can obstacle and trigger more dislocations, but also transfer plastic deformation more efficiently, thus promoting the twinning process, postponing the plastic instability and eventually giving rise to a more pronounced work-hardening. Our results not only have important implications for understanding the solid solution strengthening mechanism in medium-entropy alloys, but also shed lights on developing advanced metallic alloys with a unique combination of strength and ductility.

Gradient nanostructure evolution and phase transformation of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process

The gradient nanostructure evolution and the mechanism governing this evolution of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process were investigated. A gradient nanostructure consisting of a roughly equiaxed nanograin layer, an elongated nano-lamellar layer, an elongated ultrafine lamellar layer, a refined grain layer, and a low-strain coarse-grained layer was formed with a thickness of more than 400 µm. The formation of gradient nanostructure of α phase was dominated by complex dislocation activities in hcp grains without twins occurring, supplemented by hexagonal close-packed (hcp) titanium (Ti) to face-centered cubic (fcc) Ti phase transformation. During the microstructural evolution, the coarse hcp-Ti grains were first elongated into lamellae. Then, these sub-micron lamellae were gradually transformed into roughly equiaxed nanograins via two deformation modes of longitudinal splitting and transverse breakdown, accompanied by dynamic recovery. The fcc-Ti grains were deformed mainly via twin-twin intersections and twin-dislocation interactions, accompanied by longitudinal splitting and transverse breakdown, resulted in refinement of the micron-scale fcc-Ti grains to roughly equiaxed nanograins. The interaction of hcp and fcc phases influenced and synergistically promoted the microstructural evolution process. In addition, the microhardness improvement in the surface layer of Ti-6Al-4V alloy was attributed to the increase of dislocation density, grain refinement and the occurrence of deformation twinning in fcc-Ti grains.

Stress-induced twinning of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V

Twinning behavior of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V subjected to severe plastic deformation was investigated. High-resolution transmission electron microscopy observations first showed that the deformation twinning of nanocrystalline hexagonal close-packed titanium was induced. The occurrence of deformation twinning was attributed to the extremely high stress due to specific grain orientations and the large strain rate. The twinning mechanism of nanocrystalline hexagonal close-packed titanium in Ti-6Al-4V was the overlapping of two stacking fault ribbons. This study provided a new thinking that it might be possible to design nanocrystalline titanium and its alloys to extensively activate twinning for further improving their properties.

Non-twinning deformation mechanism of pure copper under high speed electromagnetic forming

Abstract The conventional high strain rate forming usually induces the occurrence of deformation twins in face-centered-cubic metals such as copper with medium stacking fault energy. In order to investigate the possible mechanical twinning of copper under a new type of high speed processing technique—electromagnetic forming, the pure copper was electromagnetically bulged followed by microstructural characterization through electron backscattered diffraction and transmission electron microscopy. The results revealed a reduction of twin boundaries with increasing plastic strain, indicates a non-twinning deformation mechanism which is different from the deformation twinning observed in other high speed forming process. The physical origin of the present phenomenon is discussed in detail according to the energy barrier and the critical shear stress that are required for the nucleation of perfect and partial dislocations that determine the formation of twins.

Deformation nanotwins in coarse-grained aluminum alloy at ambient temperature and low strain rate

In order to reveal the possible occurrence of deformation twins in coarse-grained aluminum/aluminum alloy at normal experimental conditions, a 5A02-O aluminum alloy with coarse grains was compressed quasi-statically to various plastic strains at ambient temperature, followed by high-resolution transmission electron analysis. The results revealed some long streaks produced by the thin plate-like structure with 2 atomic planes thick in the specimen undergoing a large strain, while under a relatively small plastic strain, the striped characteristics disappeared. The fast Fourier transform and theoretical analysis have shown that these long streaks are nanotwins, derived from the overlapping of stacking fault ribbons formed by Shockley partial dislocation on adjacent slip planes, which are triggered by the large plastic strain.

Effects of Temperature and Strain Rate on Stress-Strain Relationship and Deformation Twinning in Fe-5%Si Alloy

The effects of temperature and strain rate on the stress–strain relationship and deformation twinning, during tensile tests, in polycrystalline Fe–5%Si alloy were studied. We performed tensile tests over a wide range of temperatures from 173 to 273 K, and strain rates ranging from 0.0001 to 0.1 /s, to clarify the relationship between total elongation and ratio of deformation twins, and the dependence of deformation twinning on grain orientation. All tensile specimens were fractured in a completely brittle manner. The total elongation decreased as the temperature decreased and the strain rate increased. The presence of deformation twins, in all fractured specimens, was confirmed by scanning electron microscope–electron backscattering diffraction analyses. The area fraction of the deformation twins increased as the total elongation decreased. However, a strong influence from the grain orientation on twinning activity was not observed for all temperatures and strain rates. A previous study on Fe–Si alloy single crystals showed that deformation twins form easily in <001>-oriented single crystals, but not in <111>-oriented single crystals. Our observations, on the dependence of deformation twinning on grain orientation in polycrystalline Fe–5%Si alloy, did not agree with those from single crystals. The present findings suggest that grain orientation does not play an important role in determining the occurrence of deformation twinning; not even in polycrystals. It is believed that the stress concentration, due to piled-up dislocations, during tensile deformation, cannot be relieved by the slip at low temperatures or high strain rates, and thus significantly affects deformation twinning.

Development of Very Tough Ferritic Stainless Steels by Optimizing the Recrystallized Microstructure

In this work, the conventional rolling (CR) and warm rolling (WR) have been carried out with an ultra purified ferritic stainless steel. After different rolling processes and subsequent recrystallization annealing, different recrystallized microstructure characteristics had been obtained. It was observed that as compared to the conventional process, the warm rolling and subsequent recrystallization annealing could obviously refine the recrystallized microstructure of the hot band. The ductile-to-brittle transition for the ultra purified ferritic stainless steel can be closely related to the occurrence of deformation twinning. The refined microstructure can decrease the temperature at which twinning occurs and, consequently, the ductile-brittle transition temperature for the ultra purified ferritic stainless steel was lowered to be below -40°C, and its notch toughness was significantly improved when the fine grain route can be applied.

Appearance of dislocation-mediated and twinning-induced plasticity in an engineering-grade FeMnNiCr alloy

By comparing the microstructural and texture evolution with tensile stress–strain response of an Fe–24Mn–7Ni–8Cr (mass%) alloy, a slip-dominated deformation process and, at a later stage of deformation, twinning-induced plasticity are observed. The occurrence of deformation twinning is texture sensitive and occurs only in the 〈111〉 fibre texture component. Based on these experimental observations, a model is presented, which reflects an orientational and configurational peculiarity of face-centred cubic stacking faults bordered by two Shockley partials. With this model, the onset point of stacking fault growth, i.e. movement of the leading partial and stopping of the trailing partial, is evaluated. This point reflects the formation of twins in the sense that a twin is regarded as an arrangement of stacking faults on every consecutive slip plane. Furthermore, based on the tensile test results, a model-compatible description of the mechanical behaviour is shown and a reasonable stacking fault energy of about 8mJm−2 is calculated for the onset of partial dislocation breakaway, i.e. the onset of deformation twinning.

Acoustic emission study of the mechanical anisotropy of the extruded AZ31 alloy

Abstract The mechanical anisotropy behaviour of the extruded AZ31 magnesium alloy was investigated by in-situ measurements of acoustic emission. Specimens taken in the extrusion direction were uniaxially loaded in tension and compression at room temperature. The acoustic emission count rates were correlated with stress – strain curves and their values show local maxima at the yield point, which is linked with cooperative dislocation processes and/or with the occurrence of deformation twins, especially in compression tests. During compression tests, another acoustic emission maximum occurs after the yield point which indicates the activation of further deformation mechanisms. In this paper the acoustic emission results are presented and discussed in terms of texture evolution during plastic deformation.

Origin of deformation twinning from grain boundary in copper

The occurrence of deformation twinning in face-centered-cubic (fcc) crystals with medium to high stacking fault energy is normally very difficult. Through specially designing the orientation of Cu single crystal, the authors do not only observe profuse nanoscale deformation twins with a tower shape originating from grain boundary but also find some of them terminated in the interior of the grain. The observations clearly prove that the deformation twinning can nucleate through successively emitting partials from grain boundaries, indicating the ubiquity of deformation twinning even in those fcc single crystals deformed at suitable conditions.

High Strength Ti of Substantial Interstitial Contents Produced by Back Pressure Equal Channel Angular Consolidation of Dehydrided Particles

Back pressure equal channel angular (BP-ECA) processing was utilised to consolidate a dehydrided (DH) Ti powder of high interstitial content (1.15 wt.% O, 0.09 wt.% N) at 630°C into fully dense bulk ultrafine-grained (UFG) Ti. The consolidated samples showed an increase in the contents of oxygen (1.34 wt.%) and nitrogen (0.3 wt.%). The measured densities of 4.53 g/cm3 for the consolidated samples after 1 and 3 passes were very close to the theoretical value of pure Ti. TEM revealed the formation of a bimodal microstructure in the one-pass sample, comprising equiaxed grains of several micrometers in size with ultrafine grains of the order of 100 nm uniformly distributed at the triple grain junctions. Most grains had high-angle grain boundaries with some boundaries exhibiting non-equilibrium characteristics. Upon further BP-ECA processing to three passes, the micrometer-sized grains were refined down to the ultrafine level and copious nanoscale deformation twins were introduced by severe plastic strain into those ultrafine grains of the order of 100 nm. As a result of high interstitial contents and refined grains, the sample after processing for 3 passes exhibited remarkably enhanced true yield and ultimate strengths of 1510 and 2050 MPa, respectively. Significantly, a noticeable compressive ductility was simultaneously attained despite such a high interstitial content, thanks probably to the non-equilibrium grain boundaries, bimodal grain structure and the occurrence of deformation twinning.

Implementation of Twin Reorientation and Softening Schemes in a Polycrystal Plasticity Model for Mg Alloys

Uniaxial compression tests on hot-rolled AZ31 Mg alloy were carried out at a temperature of 300°C. In order to investigate work hardening and texture evolution during plastic deformation, cylindrical specimens were compressed to the rolling direction. Experimental investigation reveals that flow curves are strongly dependent on microstructure evolution such as deformation twinning and softening phenomenon. The occurrence of deformation twinning and softening phenomenon was revealed by the observation of microtexture using electron backscatter diffraction (EBSD). A visco-plastic self-consistent (VPSC) polycrystal model was used to simulate the work hardening, softening and texture evolution during the uniaxial compression. In order to calculate orientation of deformation twins, predominant twin reorientation (PTR) scheme was implemented into the polycrystal model. A softening scheme was also implemented in the polycrystal model to predict softening phenomenon and texture evolution after a peak stress.

Etch Pit Observation of Dislocations in Early Deformation of Cu-Al Alloy and Pure Mg Polycrystals

In order to make clear the micro-yielding mechanisms of polycrystalline metals including twins, the movement of dislocations in the surface grains of Cu-6.8at%Al alloy and pure Mg polycrystals during the early stages of deformation was directly observed by using etch pit technique. The fresh dislocations multiply from the Frank-Read sources within the grains, and pile up against the twin and grain boundaries of two kinds of specimens. The pile-up dislocations on the primary and/or secondary slip planes are also confirmed in Cu-6.8at%Al alloys. Especially during the compressive loading for pure Mg, the occurrence of deformation twins is remarkable with an increase of strain rate, whereas the distribution of fresh dislocations tends to decrease in the surface grains. The present results suggest that the effect of twin boundaries on micro-yielding is almost equivalent for that of grain boundaries, which act as barriers to moving dislocations even in the pre-yield deformation.