Secondary Twins Research Articles

Effect of Mn Addition on the Microstructures and Mechanical Properties of CoCrFeNiPd High Entropy Alloy.

CoCrFeNiPdMnx (x = 0, 0.2, 0.4, 0.6, 0.8) high entropy alloys (HEAs) were prepared and characterized. With an increase in Mn addition, the microstructures changed from dendrites (CoCrFeNiPd with a single face-centered-cubic (FCC) phase) to divorced eutectics (CoCrFeNiPdMn0.2 and CoCrFeNiPdMn0.4), to hypoeutectic microstructures (CoCrFeNiPdMn0.6), and finally to seaweed eutectic dendrites (CoCrFeNiPdMn0.8). The addition of Mn might change the interface energy anisotropy of both the FCC/liquid and MnPd-rich intermetallic compound/liquid interfaces, thus forming the seaweed eutectic dendrites. The hardness of the FCC phase was found to be highly related to the solute strengthening effect, the formation of nanotwins and the transition from CoCrFeNiPd-rich to CoCrFeNi-rich FCC phase. Hierarchical nanotwins were found in the MnPd-rich intermetallic compound and a decrease in either the spacing of primary twins or secondary twins led to an increase in hardness. The designing rules of EHEAs were discussed and the pseudo binary method was revised accordingly.

Fracture morphology and crack mechanism in pure polycrystalline magnesium under tension–compression fatigue testing

The fatigue fracture was characterized and the fracture behavior was analyzed, using scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD), the fatigue tests of two strain amplitude at room temperature were 0.5% and 1.0% respectively, and the results showed that the fatigue deformation of different strain amplitude produced two typical fatigue fracture morphology, and when the strain amplitude was 1.0%, fatigue fracture mechanism of AZ31 magnesium alloy induced by $$\{ 10\bar{1}2\}$$ twins, when the strain amplitude is 0.5%, it was induced by $$\{ 10\bar{1}2\} - \{ 10\bar{1}2\}$$ double twins. In the present study, the average thickness of primary twin is ~ 20 μm at amplitude of 0.5% and ~ 80 μm at amplitude of 1.0%. The thickness of $$\{ 10\bar{1}2\}$$ primary twins was large enough to activate $$\{ 10\bar{1}2\} - \{ 10\bar{1}2\}$$ secondary twins at a high strain amplitude, while the thickness of $$\{ 10\bar{1}2\}$$ primary twins was too narrow to activate $$\{ 10\bar{1}2\} - \{ 10\bar{1}2\}$$ secondary twins at a low strain amplitude.

Characterization and analysis on micro-hardness and microstructure evolution of brass subjected to laser shock peening

This paper is concerned with the effect of laser shock peening on micro-hardness and microstructure of brass. In order to study the plastic deformation behaviors of brass, laser shock peening with 1, 2, and 3 impacts at pulse energy of 6.5 J were performed in this experiment respectively. The micro-hardness distribution in the near-surface layer is performed by hardness tester, and the microstructure in the surface layer is determined by X-ray diffraction and transmission electron microscopy. The results indicate that higher surface micro-hardness of brass after laser shock peening impacts is induced comparing with untreated brass. Additionally, increasing the impact times of laser shock peening, the surface micro-hardness improves correspondingly. Combined with transmission electron microscopy analysis, the determinate factors for variation of surface micro-hardness owes to the twins spacing and grain refinement induced by laser shock peening. The formation and interaction of surface mechanical twins are accompanied by improving in surface strain and strain rate. The equiaxed grains with an average size of 70–100 nm are generated in the top surface layer after 3 laser shock peening impacts. Furthermore, the grain refinement mechanism in the top surface layer subjected to laser shock peening is summarized as follows: (i) parallel mechanical twins are generated due to the effect of strain deformations induced by laser shock peening; (ii) mechanical twins are subdivided into secondary twins with the formation of dense dislocation tangles at the mechanical twins boundaries; (iii) twins interaction in different directions refine the grain into nano-scale equiaxed grains.

Evolution of deformation twins with strain rate in a medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V

Microstructure evolutions of the medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V (in wt.%) with stacking-fault energy of 22 mJ m−2 during deformation at strain rate ranging of 10−2–1 s−1 were analyzed by means of X-ray diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results indicate that the twinning-induced plasticity effect is the main strengthening mechanism of the studied steel, whilst the transformation-induced plasticity effect only occurs at high strain rate. With an increase in strain rate, volume fraction of the deformation twins, in particular that of the secondary twins, increases significantly along with decreasing average size. When applied strain rate is higher than 10−1 s−1, the parallel deformation twins are turned into a crossing morphology, and the original straight twin boundaries exhibit a ladder feature, which is attributed to the interactions between regular dislocations and twin dislocations at the twin boundary. The critical strain, a key indicator of the initiation of deformation twin, decreases with increasing strain rate. In addition, the ductility and strength of medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V are mainly determined by the shape and volume fraction of deformation twins.

Deformation Induced Hierarchical Twinning Coupled with Omega Transformation in a Metastable \u03b2-Ti Alloy

Hierarchical twinning, at multiple length scales, was noted in a metastable body-centered cubic (bcc) β-titanium alloy on tensile deformation. Site-specific characterization within the deformation bands, carried out using EBSD and TEM, revealed {332} <113> type primary bcc twins, containing different variants of secondary and tertiary twins, as well as the formation of stress-induced martensite (α”). Within the primary {332} <113> type twin, “destruction” of the prior quenched-in athermal ω phase was observed, while a stress-induced ω phase reforms within the tertiary twins, revealing the intricate nature of coupling between deformation twinning and displacive ω transformation.

Effect of Twinning Behavior on Dynamic Recrystallization During Extrusion of AZ31 Mg Alloy

A new severe plastic deformation method, viz. integrated dual-directional extrusion and spiral deformation, was adopted to process AZ31 Mg alloy and found to be very effective for refining the grains and softening the texture. We also investigated how twinning in AZ31 alloy affected the dynamic recrystallization (DRX) during hot extrusion, revealing that multiple twins can offer increased nucleation sites and enough energy to trigger DRX. Moreover, even after dual-directional extrusion and spiral deformation, a misorientation angle of ~ 86° remained between the primary tension twin and matrix or ~ 60° between the secondary tension twin and primary twin, being induced by {10–12} tension twin variants (primary and secondary) and resulting in a dispersed misorientation angle distribution. Nucleation of DRX occurred at the tip of preexisting twins, although primary and secondary tension twins can also act as nucleation sites for DRX.

Evolution of Fracture Mode in Nano-twinned Fe-1.1C-12.5Mn Steel

The evolution of fracture mode was investigated in Fe-1.1C-12.5Mn steel with prior introduced nano-twinned structures. The thicknesses of the prior twin plates were manipulated by thermo-mechanical treatments. During the plastic deformation, thin prior twin plates (30–40 nm) were penetrated by dislocations which activated secondary twin systems, and therefore dimple features were promoted in the fractured surfaces due to dislocation accommodation within the nano-twin plates. However, dislocation pile-up on the boundaries of the thick prior twin plates (90 nm) led to trans-granular fracture and formation of cracks. Moreover, prior twin elimination enhanced inter-granular feature appearance in the fractured surfaces because of dislocation accumulation on grain boundaries.

The subsurface frictional hardening: A new approach to improve the high-speed wear performance of Ti-29Nb-14Ta-4.5Zr alloy against Ti-6Al-4V extra-low interstitial

In the present work, the subsurface microstructure evolutions of Ti-29Nb-14Ta-4.5Zr (TNTZ) during high speed-sliding wear against Ti-6Al-4V extra-low interstitial has been studied in detail. Its correlation with surface features characteristics is also discussed. Progressive substructure development and mechanical twin formation have been proposed to be the main subsurface deformation mechanisms. The microstructure of the subsurface deformed regions consists of both primary and secondary twins holding ∑3:{112}<111> and ∑11:{332}<113> orientation relationships even at the early stage of sliding. Surprisingly, examinations indicate that an ultra-fine grained metal mixed layer is formed underneath the wear track, the extent of which increases in relation to the sliding distance. The latter has been justified considering the progression of primary mechanisms under a high imposed strain rate and the resulting tangential force. Despite the previously reported results for the same couples, the tribological performance of the TNTZ experimented alloy is significantly improved. This is well discussed relying on the increased subsurface hardenability of the alloy under the high-speed wear regime, which effectively supports the superficial oxide layer and prevents the premature micro-crack formation.

Shock-induced two types of {10[formula omitted]2} sequential twinning in Titanium

Shock loading causes complicated twinning processes in Ti and Ti alloys. In this work, two types of {101¯2} sequential twinning processes are for the first time reported according to electron backscatter diffraction (EBSD) characterization of titanium sheets that are subjected to shock loading. One is described as {112¯1}⇒P→{101¯2} (TiII⇒P→TjI), i.e., {101¯2} sequential twinning is activated in the parent grain (P) along with {112¯1} twins. The other is described as {112¯2}⇒{112¯4}→{101¯2} (CiI⇒CjII→TkI), i.e., {101¯2} secondary twinning is stimulated inside {112¯4} twin by co-zone {112¯2} twin. A statistical analysis according to EBSD characterization reveals the well-defined crystallographic relations between sequential {101¯2} twin variants and the primary/incoming twins, i.e., TiII⇒P→TiIorTi+1I and CiI⇒Ci+3II → TiI or Ti+1I. We proposed and examined two sequential twinning mechanisms, (i) emissary twinning disconnections at steps along twin boundary for the first case and (ii) shear transformation into the primary twin for the second case, based on dislocation theory and the deformation accommodation ability of the sequential {101¯2} twinning to the shear deformation of the primary/incoming twin. Crystal plasticity modelling is performed to calculate the local stress field associated with the proposed twinning mechanisms. The results demonstrate that the preferred twin variant observed in experiments has the maximum resolved shear stress among the six twin variants. Our work suggests that complicated twinning processes under shock loading obey crystallographic relations according to deformation accommodation ability. The findings from this study can be implemented into meso-/macro-scale crystal plasticity models for predicting mechanical behaviors and texture evolution of polycrystalline aggregates of hexagonal materials.

Steps and [formula omitted] secondary twinning associated with [formula omitted] twin in titanium

{112¯2} twinning commonly takes place in α-titanium (α-Ti). High-resolution transmission electron microscopies (HRTEM) explored various steps along {112¯2} coherent twin boundary. Topological model of {112¯2} twin revealed twinning disconnections (TDs) that are represented by (bi, ih{112¯2}) corresponding to a step height ih{112¯2} and a shear vector bi. Atomistic simulations were conducted to study the energies and kinetics of TDs. Combining microscopies and atomistic simulations, we concluded that (b3, 3h{112¯2}) is the elementary TD and (b1, h{112¯2}) is the reassembly TD. Steps observed in HRTEM thus can be treated as a reassembly of (b3, 3h{112¯2}) TDs and (b1, h{112¯2}) TDs. In addition, Electron Backscatter Diffraction (EBSD) maps revealed {112¯2}→{112¯1} double twins in α-Ti. Using two-dimensional and three-dimensional atomistic simulations, we demonstrated the nucleation of (b1, h{112¯2}) TD and {112¯2}→{112¯1} double twin through the interaction between basal <a> dislocation and {112¯2} twin. Our results enrich the understanding of {112¯2} twinning including TDs, steps, and {112¯1} secondary twins in hexagonal metals.

Strain rate dependence on the evolution of microstructure and deformation mechanism during nanoscale deformation in low carbon-high Mn TWIP steel

We elucidate here the strain rate dependence on the deformation behavior and accompanying deformation mechanism in Fe-30Mn-0.3C TWIP steel via nanoscale deformation experiments and post-mortem microscopy of the deformed region. The nanoindentaion hardness increased with increased strain rate from 0.01 s-1 to 1 s-1, and exhibited a positive strain rate sensitivity of 0.095 with an activation volume of 18b3. At a low strain rate, dislocations dominated the deformation behavior with a high density of 2.7 × 1016 m-2. With increased strain rate, the dislocations decreased and the stacking faults and nanotwins gradually increased. However, nanotwins with secondary twins were the dominant deformation process at high strain rate of 1 s-1. The deformation behavior was significantly impacted by the interplay between strain rate, stacking fault energy and deformation mechanisms. A critical theoretical analysis suggested that the strain rate influenced the critical shear stress for twinning and dislocation slip, resulting in the change in deformation mechanism from dislocation slip to twinning.

Accumulative roll bonding (ARB)-processed high-manganese twinning induced plasticity (TWIP) steel with extraordinary strength and reasonable ductility

Accumulative roll bonding (ARB) process at room temperature was used to fabricate a nanostructured twinning-induced plasticity (TWIP) steel. The effect of the number of ARB cycles from 1 to 3 on the microstructure and mechanical properties of Fe-31Mn-3Al-3Si (wt%) TWIP steel was investigated. The results showed that the microstructure of the 1-cycle ARB-processed sample mainly contained the primary and secondary mechanical twins. However, in the microstructure of 2-cycle ARB-processed sample, new configurations of small nanotwins which they are known as the hierarchical nanotwinned (HNT) structures were observed between the secondary mechanical twins. In the microstructure of 3-cycle ARB-processed sample, new subdivided grains with a mean size of about 100 nm were produced. Additionally, the 3-cycle ARB-processed TWIP steel sample exhibited extraordinary strength (about 1.2 GPa) and reasonable ductility (about 15%). The mechanical behavior of ARB-processed TWIP steel was interpreted as the evidence for microstructural features such as the mechanical twins (especially, the HNT structures) and nano/ultrafine grains.

In situ tension-tension strain path changes of cold-rolled Mg AZ31B

The mechanical behavior of cold-rolled Mg AZ31B is studied during in-plane multiaxial loading and tension-tension strain path changes performed on cruciform samples using in situ neutron diffraction and EBSD. The results are compared with uniaxial tension loading of dogbone-shaped samples measured with in situ neutron diffraction and acoustic emission. The activity of slip and twinning mechanisms and the active twin variants are discussed for the different strain paths. It is shown that initial strains of 4–5% cause a strengthened yield stress during reload for strain path change angles of 90 and 135°. The strengthening is primarily due to dislocation accumulation during the initial load impeding dislocation motion during the reload. The twinning observed during the prestrain activates complex multivariant secondary twinning which may also contribute to the strengthening in the reload.

Orthogonal shear process in Ni-Mn-Sn single crystal

A coordinate change of conjugation twin boundaries in Ni-Mn-Sn single crystals is studied with scanning electron microscopy. During mechanical training, the so-called orthogonal shear process initiates variant reorientation leading to the replacement of the main with conjugation type I twin boundaries. The experimental data demonstrate that highly organized redistribution of adjacent variants, in the neighboring areas, is then critical for the removal of conjugation boundaries by coordinated secondary twinning. In addition, a more complex variant reorientation mechanism is uncovered and it is found to allow for transitioning between non-conjugate twin systems. The proposed mechanism appears consistent with the orientation relationship between austenite and martensite, which is controlled by a shear deformation and martensite crystallography leading to an equal distribution of martensitic variants. This contribution complements the existing theory of martensite crystallography providing direct evidence for the general nature of the orthogonal shear process as a first step in variant reorganization during mechanical training of martensite.

Formation of 60° [formula omitted] boundaries between [formula omitted] twin variants in deformation of a magnesium alloy

Interrupted tensile testing along the normal direction of a rolled AZ31 magnesium plate was conducted at various strain levels from 6% to 15.5%. Prior to deformation the specimens were annealed for grains to coarsen such that multiple twin variants in one grain can be activated and interaction between variants can be better resolved. The grain structure and texture evolution were then examined via electron backscatter diffraction. The results reveal that multiple primary {101̅2}P twin variants inside individual grains grow and impinge, forming profuse boundaries of 60°〈011̅0〉 orientation relationship. These boundaries are close to {112̅2}〈112̅3̅〉 twin relationship, however, they are not formed by twin nucleation and are only a product of interaction between the primary twins. The morphology of these special boundaries are highly irregular. Atomistic simulations were performed to understand such interaction, and the results show that such 60° 〈011̅0〉 boundaries have limited mobility. Thus, a primary twin variant is able to grow at the expense of other primary twin variants. Our experimental results also show that {101̅2} twinning is active throughout the deformation until the specimens are fractured. As the strain increases, secondary {101̅2}S twins are activated inside the primary twins. As the strain further increases to near fracture, {101̅1}S secondary twins are activated inside the primary twins, followed by activation of tertiary {101̅2}T twins inside the {101̅1}S secondary twins, a process known as {101̅1}−{101̅2} double twinning. The formation of irregular 60°〈011̅0〉 boundaries and the sequential twinning at large strains shed new light on the twinning mechanism in Mg and other hexagonal close-packed metals.

The fatigue crack growth in hierarchically nano-twinned materials

The dislocation emission-based model is established to reveal the fatigue crack growth in polycrystalline metals with hierarchically nano-twinned structures (HTS). The analysis illustrates that the presence of HTS can effectively prevent fatigue crack propagation along the boundaries of primary twins during plastic deformation. For the same primary twin spacing λ1, the fatigue fracture toughness is enhanced first with the decreasing secondary twin spacing λ2, reaching the maximum at the critical λ2, and then reduced as λ2 becomes even smaller. It is found that the smaller the spacing λ1, the smaller the critical spacing λ2. Moreover, there also exists optimal twin spacing in primary twin lamellae. In addition, the proposed theoretical model suggests that the fatigue crack growth rate reduces with decreases of secondary twin spacing λ2 when spacing λ2 is above the critical value, as observed in molecular dynamics simulations. The present results provide insights to optimize the microstructures for achieving high plastic deformation levels in nano-twinned metals with HTS.

Secondary twinning in zinc

ABSTRACT secondary twinning inside a primary twin has been observed on the (0001) fracture surface of zinc after quenching of a single crystal grown by the Bridgman method. This type of double twinning was previously observed in magnesium with a quite different c/a axial ratio. A geometrical analysis is presented that can explain why the observed secondary twins are activated.

Interpretation of significant decrease in cryogenic-temperature Charpy impact toughness in a high manganese steel

The grain boundary segregation and precipitation and associated deformed microstructure under cryogenic-temperature and dynamic-loading conditions were systematically investigated by means of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and electron back-scattered diffraction (EBSD). The studied steels exhibit equiaxed grains with a similar grain size of 12.0–13.3 µm. The twinning and dislocation slip were observed as the main deformation modes under cryogenic-temperature and dynamic-loading conditions. However, the secondary twinning system is suppressed and plastic deformation by dislocation slip is small in the steel annealed at 800 °C for 3 h, leading to a significant decrease in Charpy impact toughness at − 196 °C. The HAADF-STEM results show that there are numerous (Cr,Mn)23C6-type carbides along grain boundaries and the Cr or C peaks have been detected at grain boundaries without (Cr,Mn)23C6-type carbides in the steel annealed at 800 °C for 3 h, whereas these carbides and Cr or C peaks were not observed in the steel annealed at 300 °C for 3 h. However, the Mn, P, S, Si and Cu peaks were not observed at grain boundaries in both steels by means of STEM-EDX. The critical shear twinning stress is high in the steel annealed at 800 °C for 3 h due to heavier grain boundary segregation. Meanwhile, the local stress concentration at grain boundaries may be relaxed because the crack may easily nucleate at grain boundaries whose cohesion is weakened by heavier grain boundary segregation or (Cr,Mn)23C6-type carbides/matrix interfaces whose bonding strength is relatively small. Therefore, the secondary twinning system is relatively hardly activated and plastic deformation by dislocation slip is relatively small for the steel annealed at 800 °C for 3 h.

Shock-induced {11[formula omitted]1}→{11[formula omitted]2} double twinning in titanium

Six twinning modes have been reported in α-titanium, including three extension twinning modes {101¯2}, {112¯1} and {112¯3} and three compression twinning modes {112¯2}, {112¯4} and {101¯1}. {101¯2} and {112¯2} twins are frequently observed without strong dependence on strain rate, while {112¯1} and {112¯4} twins are observed at high strain rate. These twinning modes and their interactions such as double twinning play significant roles in determining mechanical properties and texture evolution of α-titanium. In this work, we study double twinning associated with {112¯1} primary twin. In order to activate {112¯1} twinning, a split Hopkinson pressure bar (SHPB) device was adopted to conduct high strain rate (∼2600 s−1) compression of high purity titanium along the extrusion direction. We observed 453 {112¯1}→{112¯2} double twins but zero {112¯1}→{112¯4} double twins in six grains. Crystallographic analysis enables the classification of {112¯1}→{112¯2} double twins into Group I (29.5° <1¯100>), Group II (55°<5 5 10¯ 3>), Group III (80.6°<11¯00>) and Group IV(86.8°<5¯ 15 10¯ 3>) according to the misorientation angle and axis pair. Groups I and II dominate in the proportion of experimentally detected double twins while Groups III and IV take a small proportion. We account for these phenomena according to apparent Schmid factor, modified deformation gradient accommodation, and twin nucleation via dislocation dissociation. The results demonstrate that the preferred secondary twinning mode and corresponding variant would, to the greatest extent, relax plastic deformation associated with the primary twinning.

The crack nucleation in hierarchically nanotwinned metals

The nanocrystalline (NC) metals with hierarchical twin lamellae exhibit excellent mechanical properties. However, recent efforts are mainly focused on the effects of primary twin boundaries (TB) and grain boundaries (GB) on crack nucleation. Few investigations have been performed on secondary twin boundary (STB) effect on crack nucleation in hierarchically nanotwinned metal. A relevant model is established to describe the crack nucleation criteria quantitatively. Actually, the crack advance mainly depends on the energy of crack nucleation and the energy of pile-up dislocations. Furthermore, predictions about the site of crack nucleation are made for different hierarchically nanotwinned metals. This work will help us better understand the deformation mechanism and the failure behavior in NC materials with hierarchically nanostructures.