Twin Bundles Research Articles

On the crystallography of plastic flow instabilities in FCC metals during impact loading: a study modelled on copper single crystals

Abstract The formation of shear bands (SBs) at high strain rates directly precedes the fracture of metals. Their appearance in the microstructure signals a reduction or complete loss of the load-bearing properties of a metallic parts. This paper analyses the crystallographic aspects of the mechanism responsible for the occurrence of this form of unstable metal flow under model conditions. The study examined Cu single crystals with unstable orientations close to C{112}<111> and S{123}<634>, which were deformed to logarithmic strains of 0.9 in a channel-die, with the punch driven by explosive energy to achieve a strain rate of 4 × 105 s−1. Microstructure and texture evolutions were characterized across a wide range of scales, primarily using scanning and transmission electron microscopy techniques. In both orientations analysed, the extremely high strain rate leads to intense deformation twinning, on all four {111} planes. The appearance of compact twin bundles of two generation, directly precedes the formation of SB. In each of the analysed cases, the rigid-body rotation of the crystal lattice in the band region, along with twinning in the reoriented matrix platelets, increases the density of texture components around the G{110}<001> orientation.

Improving the tensile ductility of the high-strength nanotwinned high manganese steel by a strategy combining cold rolling and warm rolling

While a high density of nano twins can effectively enhance alloys, it also leads to a significant reduction in ductility, similar to other nanostructures. In this work, a deformation strategy combining cold rolling and warm rolling has been employed on a nanotwinned Fe-25Mn-0.5C steel to improve ductility. A high density of nano twins was introduced in the coarse-grained matrix first by cold rolling for high strength. Subsequent warm rolling at 500 °C sheared the large nano twin bundles, resulting in a mixed microstructure consisting of nano twins and more randomly oriented refined grains. Planar slipping of dislocations, instead of twinning, dominates the tensile processes, which brings strong pile-up and continuous work-hardening. Compared with the cold-rolled counterpart with full nano twins, the one with a mixed microstructure exhibits three times higher tensile elongation (21.3 % vs. 5.6 %) while maintaining similar tensile strengths (about 1440 MPa). The evolution of microstructures and the underlying strengthening mechanisms of the mixed microstructure were extensively discussed. Additionally, a cold-rolled and statically annealed sample was also included for comparison.

Deformation and damage of equiatomic CoCrFeNi high-entropy alloy under plate impact loading

High-entropy alloys (HEAs) are considered as potential structural materials for aerospace and defense applications where impacts are recurrently encountered. The dynamic mechanical properties and the underlying deformation and damage mechanisms are significant for safety assessment and structural design optimization, but are underinvestigated. In this work, two types of plate impact experiments, i.e., shock compression and spallation, are performed on typical quaternary CoCrFeNi HEA (at%), to investigate its dynamic mechanical properties and microscopic deformation/damage mechanisms. Free-surface velocity histories are measured to evaluate the mechanical properties and damage processes, including the Hugoniot elastic limit (HEL; ∼0.8 GPa), spall strength (∼3.2 GPa) and pullback rates. The spall strength of the CoCrFeNi HEA is higher than those of most medium- and high-entropy alloys ever reported, except for the Al0.1CoCrFeNi HEA. The deformed samples are characterized with scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. Shock-induced dislocation slip and deformation twinning dominate plastic deformation. With increasing impact velocity, dislocation density increases significantly and twin bundles appear instead of individual twins. For incipient spallation, voids nucleate preferentially at grain boundaries, especially at grain boundary triple junctions. Damage in the CoCrFeNi HEA is ductile in nature.

Global understanding of deformation behavior in CoCrFeMnNi high entropy alloy under high-strain torsion deformation at a wide range of elevated temperatures

A number of recent studies have investigated deformation behavior of CoCrFeMnNi (Cantor) alloy at elevated temperatures by using plastic deformation to relatively small strains such as tensile testing. Therefore, little has been known about the deformation behavior of this typical FCC high-entropy alloy (HEA) in case that the material is subjected to ultra-high strains at various temperatures. In the present study, the equi-atomic CoCrFeMnNi HEA was successfully deformed over a wide range of strains (von Mises equivalent strains (ε) of 1∼5.5) by torsion at various temperatures ranging from 25 °C to 1100 °C. Deformation twinning was extensively activated at moderate to high strains (ε ≥ 1) and even found in the deformation at elevated temperatures as high as 600 °C where deformation twinning is not normally expected in Cantor alloy. The HEA showed outstanding deformability and the highest strains to fracture reached 4.0∼5.5 at low temperatures below 400 °C. The excellent deformability was attributed to the extensive twin activities including the formation of twin bundles and thin nanotwins as well as microbands formation. However, localized shear deformation that was promoted by the high straining at low temperatures could negatively affect the deformability. The heavy deformation led to a significant reduction of the grain sizes down to 50 nm∼150 nm. A sudden shortage of ductility occurred at intermediate temperatures, where small strains to fracture (1.2∼1.3) were realized at 600 °C∼700 °C. The embrittlement was accompanied by the formation of micro-voids at grain boundaries and intergranular fracture. The susceptibility to the embrittlement was caused by the precipitation of Cr-rich σ-phases at grain boundaries. Dynamic recrystallization (DRX) of the FCC matrix led to an accelerated softening at high temperatures above 600 °C. Nucleation and growth of DRX grains in Cantor alloy were not fundamentally different from those in conventional FCC alloys. This study gives an insight into the microstructure evolution and mechanical response in Cantor alloy under shear deformation over a wide range of strains and temperatures.

Formation mechanism of the eye-shaped nano-twin bundles in cold rolled 316L stainless steel

The microstructural evolution of a typical coarse-grained 316L stainless steel was explored with the rolling reduction changed from 0% to 80%. The formation mechanism of the novel eye-shaped nano-twin (NT) bundles was clearly revealed. The results show that numerous dislocations were propagated and accumulated around original grain/annealing-twin boundaries at low rolling strain level (20%), which led to a high stress concentration and induced the generation of deformation twin bundles. With the rolling reduction increase to 40%, the number of deformation twin bundles increased and the average thickness of deformation twins decreased. In addition, some micro shear bands were motivated to cut the deformation twin bundles, resulting in the formation of eye-shaped deformation twin bundles. With the further increase of the rolling reduction (60–80%), the average thickness of deformation twins was decreased to nano-scale. Many eye-shaped NT bundles were formed by the cutting activity from the micro shear bands.

Twinning behavior and hydrogen embrittlement of a pre-strained twinning-induced plasticity (TWIP) steel

The twinning behavior and hydrogen embrittlement (HE) of a twinning-induced plasticity (TWIP) steel were investigated for different pre-strains at different strain rates. The lower pre-strain rate led to thinner twins and higher twin volume fraction. The influence of the twin characteristics on HE sensitivity was analyzed from statistical data on the evolution of surface crack characteristics during secondary straining period after cathodic hydrogen charging. Based on the ratio of crack initiation sites caused by the intersection of twins and grain boundaries, it was found that thinner twins in twin bundles of similar thickness promoted the generation of hydrogen-induced cracks (HICs).

Microstructure and mechanical properties of novel CrCoNi–Al2O3P medium entropy alloy-matrix composites

In this study, novel CrCoNi–Al2O3 composites with 2.5–7.5 wt% Al2O3 were prepared by mechanical alloying (MA) plus rapid spark plasma sintering (SPS) method, and their microstructure and mechanical properties were systematically investigated. The results demonstrate that the Al2O3 nanoparticles are distributed homogeneously in the CrCoNi medium entropy alloy (MEA) matrix with ultra-fine grain size (≤0.37 μm). Moreover, the composites have clean, regular and well bonded matrix/α-Al2O3 interfaces. Interestingly, a large number of nano twin bundles, large-scale 9R phase and geometrically necessary dislocations (GNDs) were formed near the matrix/α-Al2O3 interfaces. The Vickers hardnesses of the 2.5–7.5 wt% Al2O3 composites reach 521–603 HV0.3, and the compressive yield strength is 1877–2359 MPa with fracture strains of 9.3–31.6%. The yield strength of the composites is by 65.4–107.8% higher than that of the pure CrCoNi matrix, meanwhile they still have considerable ductility, which indicates composite strengthening is a feasible method to improve mechanical properties of high/medium entropy alloys. The composite includes multiply strengthening mechanisms, namely grain refinement strengthening, twin strengthening, Orowan strengthening and dislocation strengthening. The contributions of various strengthening mechanisms were quantitatively analyzed, which were well consistent with the experimental values.

Dislocation substructures in tensile deformed Fe–Mn–Al–C steel

Microstructure of a Fe–25Mn–2Al–0.1C steel studied under uniaxial tension revealed a gradually decreasing three-stage strain hardening behavior. Early deformation microstructure comprised of dislocations configurations; like Taylor lattice and stair-rod dislocations. Deformation twinning nucleated in regions lacking homogenous dislocation substructure. Dislocations cells and fine twin bundles were observed near failure strain. The strain hardening was a concomitant effect of deformation twins and dislocation substructure, while the contribution of dislocations seems to overwhelm the contribution from twinning.

Hydrogen-enhanced densified twinning (HEDT) in a twinning-induced plasticity (TWIP) steel

The effect of hydrogen on the twinning evolution in a twinning-induced plasticity (TWIP) austenitic steel was investigated at different strain levels. The presence of hydrogen decreased the twin thickness and increased the twin density in individual twin bundles, in comparison with the hydrogen-free condition. This mechanism, reported for the first time, was termed hydrogen-enhanced densified twinning (HEDT). Based on these experimental observations, the effect of hydrogen on twinning was analyzed from the three stages of nucleation, growth and stability. HEDT may elevate the local stress, and thus significantly affect the hydrogen embrittlement (HE) of TWIP steels.

Revealing orientation-dependent martensitic transformation in a medium Mn steel by micropillar compression

In this contribution, for the first time, the effect of crystal orientation on the critical stress for martensitic transformation in a medium Mn steel is explicitly revealed by using micropillar compression experiments. Although the martensitic transformation is observed in both [100] and [110] micropillars during compression tests, the average critical stress for martensitic transformation in [100] micropillars (∼483 MPa) is much smaller than that in [110] micropillars (∼700 MPa). The deformation twins can be found in the deformed [100] micropillars while the deformed [110] micropillars are free of deformation twins. The estimated critical stress for twin divergence in the [100] micropillars (∼465 MPa) is close to the corresponding critical stress for martensitic transformation. The theoretical critical stress for martensitic transformation in both [100] and [110] micropillars is estimated to be the same (650–750 MPa). The lower critical stress for martensitic transformations in the [100] micropillars could be ascribed to the formation of deformation twins, which is confirmed by the microstructural observation that the martensite nucleates at the intersections of deformation twin bundles. Two martensitic transformation pathways, namely twin-assisted martensitic transformation and free-standing martensitic transformation are proposed to explain the orientation dependent martensitic transformation observed in the present medium Mn steel.

Effect of pre-deformation mode on the microstructures and mechanical properties of Hadfield steel

The microstructures and tensile properties of a conventional Hadfield steel subjected to explosion and cold rolling deformation modes were investigated in the current study. The deformation mode significantly changed the microstructure and tensile properties of the Hadfield steel. The grains in cold rolling condition were largely elongated along the rolling direction; however most grains in the exploded sample retained their equiaxed morphologies. The strain concentration was localized at the grain boundaries, and the change in the population of ∑3 annealing twin and ∑9 boundaries was smaller in the exploded sample than in the rolled condition. Moreover, the deformation twins were formed uniformly along grain boundaries in the Hadfield steel under the explosion condition. However, they primarily appeared as twin bundles in the cold rolled state. The change in the microstructure characteristics resulted in higher yield strength but lower tensile strength and elongation in the exploded sample compared with the cold rolled condition.

Design for strength-ductility synergy of 316L stainless steel with heterogeneous lamella structure through medium cold rolling and annealing

Superior strength-ductility synergy of 316L stainless steel was achieved through medium cold rolling and subsequent annealing, which successfully fabricated heterogeneous lamella structure (HLS), reduced processing cost and improved preparation controllability. The HLS in 316L stainless steel processed by lower original rolling strain was more stable, and its microstructure was characterized with lamella recrystallized grain clusters and lamella coarse grains that were sandwiched by the mixtures of nano-sized twin bundles and ultrafine grains. The satisfactory strength-ductility synergy was mainly determined by the contents of various microstructures as well as the soft/hard interfaces in HLS. The high strength was obtained from hard ultrafine grains and nano-twin bundles. Meanwhile, the back stress induced by soft/hard interfaces also contributed to the high strength. For the favorable ductility, it was ascribed to the generation of plentiful forest dislocations in soft structures and geometrically necessary dislocations around soft/hard interfaces. It is specially noted that high yield strength (∼820 MPa) and good uniform elongation (∼10.6%) can be simultaneously obtained for the annealed 70% cold rolled 316L stainless steel. These results may markedly boost the industrial application of HLS 316L stainless steel.

Dislocation and twinning behaviors in high manganese steels in respect to hydrogen and aluminum alloying

The dislocation and twinning evolution behaviors in high manganese steels Fe-22Mn-0.6C and Fe-17Mn-1.5Al-0.6C have been investigated under tensile deformation with and without diffusive hydrogen. The notched tensile tests were interrupted once primary cracks were detected using the applied direct current potential drop measurement. In parallel, the strain distribution in the vicinity of the crack was characterized by digital image correlation using GOM optical system. The microstructure surrounding the crack was investigated by electron backscatter diffraction. Electron channeling contrast imaging was applied to reveal the evolution of dislocations, stacking faults and deformation twins with respect to the developed strain gradient and amount of hydrogen. The results show that the diffusive hydrogen at the level of 26 ppm has a conspicuous effect on initiating stacking faults, twin bundles and activating multiple deformation twinning systems in Fe-22Mn-0.6C. Eventually, the interactions between deformation twins and grain boundaries lead to grain boundary decohesion in this material. In comparison, hydrogen does not obviously affect the microstructure evolution, namely, the twinning thickness and the amount of activated twinning systems in Fe-17Mn-1.5Al-0.6C. The Al-alloyed grade reveals a postponed nucleation of deformation twins, delayed onset of the secondary twinning system and develops finer twinning lamellae in comparison to the Al-free material. These observations explain the improved resistance to hydrogen-induced cracking in Al-alloyed TWIP steels.

Hierarchical evolution and thermal stability of microstructure with deformation twins in 316 stainless steel

We report extensive nano-twin formation in 316 stainless steel (SS) and the evolution of a hierarchical microstructure through the formation of multi-scale twin bundles after uniaxial tension with uniform elongation levels of 20%, 30%, and 40%. Multiscale characterization techniques were employed to reveal the nature of these twins. The twin density increases with the increasing strain level, however, the twin width remains the same, notably reducing the mean free path of dislocations. Concurrently, significant work hardening is observed during subsequent deformation. The deformation-induced nano-twins are thermally stable up to ~800°C, shown by both interrupted and in-situ transmission electron microscopy experiments, above which the recrystallization takes place in the vicinity of the twins. Such favorable thermal stability of the twins in nano-twin strengthened 316 SS offers a promising approach for microstructurally engineering these materials for potential applications at elevated temperatures. The related strengthening mechanisms are discussed in the light of the mean free path of dislocations and the dislocation interactions with twin boundaries.

Effect of counterface materials on dynamic recrystallized structure and wear resistance of nanostructured Cu

Bulk nanostructured Cu sample with nano-scale twin bundles embedded in nano-sized grains was synthesized by using dynamic plastic deformation (DPD) technique. Dry sliding tribological properties of the DPD and the coarse grained (CG) Cu samples were investigated when sliding against different counterfaces. There is an obvious difference in the relative wear resistance of the DPD to the CG Cu as a result of the change of the counterface materials. Either for the DPD Cu or the CG Cu sample, the wear rate was much larger when sliding against Cu ball than that when sliding against WC-Co or AISI 52100 steel ball. The DPD Cu sample shows almost same wear rate compared with the CG Cu sliding against Cu or AISI 52100 steel ball, which is quite different from the enhanced wear resistance for the DPD Cu sample sliding against WC-Co ball. The worn subsurface structure for two kinds of Cu samples sliding against different counterfaces, was constituted by heavily deformed nanostructured mixing layer (NML) and ultra-fine grained dynamic recrystallization (DRX) layer. Structure characteristics of the NML and DRX layer determines wear rates of the Cu samples. When sliding against Cu or AISI 52100 steel ball, there are heavily cracked NMLs and extremely fine DRX grains in the subsurface, comparing with that sliding against WC-Co ball. This structure feature leads to quick spread of cracking and high flaking rate of the NML, which corresponds to much higher wear rate for both Cu samples.

Tensile deformation behavior and deformation twinning of an equimolar CoCrFeMnNi high-entropy alloy

The tensile deformation and strain hardening behaviors of an equimolar CoCrFeMnNi high-entropy alloy (HEA) were investigated and compared with low and medium entropy equiatomic alloys (LEA and MEA). The HEA had a lower yield strength than the MEA because the addition of Mn weakens solid solution hardening in the HEA. However, deformation twinning induced the multiple stage strain hardening behavior of the HEA and enhanced strength and elongation. Using tensile-interrupted electron backscatter diffraction analysis, geometrically necessary dislocations were observed as plume-shaped features in grain interior, and a considerable texture was characterized, which is typical of face centered cubic metals. Moreover, the relationship between favorably oriented grains and twinning in the HEA bore a clear resemblance to the same tendency in TWIP steels. The thickness of the twin bundles was less than 100nm. A high density of stacking defects was found in the nanotwins. Nano twinning and stacking faults were found to contribute to the remarkable mechanical properties. Deformation induced twinning not only demonstrated the dynamic Hall-Petch effect but also changed dislocation cell substructures into microband structures.

A hybrid refining mechanism of microstructure of 304 stainless steel subjected to ECAP at 500 °C

The mechanism of microstructure evolution of 304 stainless steel (SS) during equal-channel angular pressing (ECAP) at 500°C was investigated systematically by optical microscope (OM), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results show that the refining mechanism of microstructure of 304 SS during ECAP behaviors as a hybrid model consisting of dislocation-subdivision mechanism and twin fragmentation mechanism, resulting in three kinds of nanostructure domains: (1) the equiaxial nano-grains with curving grain boundaries (GBs) having high angles of misorientation resulted from the cross-slip and movement of dislocations; (2) the equiaxial nano-grains with smooth and distinct boundaries resulted from the intersection between primary deforming twin bundles; (3) the secondary nano-twins formed in the primary deforming twins. The average nano-grain size ultimately obtained is in the range of 80nm to 120nm and the nano-twins have a width of 20–40nm and a length of 100 to a few hundred nm. A low content about 10% (V/V) of deformation-induced martensite (DIM) is obtained in the first pass and remains almost constant during further passes of ECAP. The refining mechanism of DIM is also a hybrid model, including the refinement of banded DIM by the intersection with slipping bands and the newly induced fine martensite in submicron and nanometer grained austenite matrix.

On the more persistently-enhanced strain hardening in carbon-increased Fe–Mn–C twinning-induced plasticity steel

We show more persistently-enhanced strain hardening with improved tensile strength and maintained high plasticity in Fe–18Mn–1.0C than in Fe–22Mn–0.6C steel. This is primarily attributed to the gradual release of more densely-spaced thinner twins and twin bundles favored by the increased dynamic interactions between carbon and dislocations in the carbon-increased steel.

Influence of large cold strain on the microstructural evolution for a magnesium alloy subjected to multi-pass cold drawing

Abstract Cold-drawn AZ31 Mg alloy wires with a wide range of strains up to ~1.2 were prepared by multi-pass cold drawing to investigate the influence of cold strain on microstructural evolution. With the drawing, plastic deformation was initially dominated by twinning, particularly {10–10} twinning, but gradually becoming by slips especially after twinning was exhausted at large strains>1. More importantly, pronounced refined crystallites with diameter ~100 nm were achieved through profuse intersections across twin bundles starting from strains larger than ~0.6, and contributed to the weakening of {0002} fiber texture. This refinement could be interpreted as twin fragmentation related to dynamic recovery; and to some extent it implied a possibility of fabrication of ultrafine-grained structure by cold severe plastic deformation for magnesium alloys.

Multi-Scale Correlative Microscopy Investigation of Both Structure and Chemistry of Deformation Twin Bundles in Fe–Mn–C Steel

A multi-scale investigation of twin bundles in Fe-22Mn-0.6C (wt%) twinning-induced plasticity steel after tensile deformation has been carried out by truly correlative means; using electron channelling contrast imaging combined with electron backscatter diffraction, high-resolution secondary ion mass spectrometry, scanning transmission electron microscopy, and atom probe tomography on the exact same region of interest in the sample. It was revealed that there was no significant segregation of Mn or C to the twin boundary interfaces.