Nanoscale Twins Research Articles

Microstructure and crystallographic texture of Cu-Zn alloys subjected to ECAP and flat rolling

This paper presents the results of the microstructure and crystallographic texture investigations of the Cu-Zn alloys system with different stacking fault energies (SFE) subjected to severe plastic deformation (SPD) by equal channel angular pressing (ECAP) and subsequent flat rolling. It is shown that ECAP leads to the formation of an ultrafine-grained (UFG) structure. Further flat rolling is accompanied by a decrease in the size of structural elements and the formation of nanoscale twins, which are more likely to be detected in an alloy with a lower SFE. As the deformation degree increases, the main crystallographic textures components of the investigated alloys become Brass and Goss components.

Investigation of strengthening mechanisms in an additively manufactured Haynes 230 alloy

There are abundant studies on additive manufacturing of Ni alloys, such as Inconel 718. However, the studies on the microstructure and the deformation behavior of solid solution strengthened Ni alloys, such as Haynes 230, are limited. Here we compare the microstructure and the mechanical behavior of as-printed and stress-relieved Haynes 230 Ni alloys prepared by laser powder bed fusion. Transmission electron microscopy analysis revealed M23C6 carbides decorated at the dislocation cell walls in the as-printed material transformed to well-aligned arrays of M6C nanoprecipitates separated by an array spacing of 500 nm after stress-relief (heat treatment). The nanoprecipitates in stress-relieved alloy have formed numerous aligned orientation relationships with the matrix despite their large lattice mismatch. Post-tension microscopy analyses demonstrated high-density dislocation networks in the as-printed samples, whereas high-density stacking faults and nanoscale deformation twins were observed in the stress-relieved counterparts. This study provides insights for understanding the influence of nanoprecipitates on the deformation mechanisms of additively manufactured Ni alloys.

Static recrystallized annealing treatment-induced strength-ductility trade-off in cold-rolled Co36Fe36Cr18Ni10 multi-principal alloy

Multi-component alloys are recrystallized with various heterogeneous microstructures to regulate their main properties. However, the production of conventionally structured alloys with high strength and superior ductility is exigent because ductility is typically deteriorated by strength enhancement. The microstructure features of alloys determine their mechanical properties. In this work, a new multi-component Co 36 Fe 36 Cr 18 Ni 10 alloy is developed. The alloy was prepared by arc melting and solidified in a Cu mold with a single face-centered cubic phase that provides an outstanding ductile matrix. The mechanical properties and microstructures were systematically investigated through tensile tests, electron backscatter diffraction, and transmission electron microscopy. The as-cast alloy was cold-rolled at 90% reduction and subsequently annealed at 700, 800, and 900 °C. The texture evolution formed numerous fine grains with stacking faults, dislocations, and nanoscale annealed twins in their interior, effectively improving the mechanical properties of the alloy. In single-phase alloys, the twins dominated the matrix and the efficiency of grain refinement achieved by annealing at 700 °C afforded strength improvement. The strength and ductility trade-off is attributed to a microstructure that combines incompletely recrystallized and recrystallized grain regions, forming a heterogeneous microstructure whose performance dimensions span from the submicron to the nanoscale. The microstructure of the fabricated multicomponent alloy was created by large-deformation cold rolling followed by low-temperature annealing treatment. The results show that heterogeneous designs can be effectively achieved by implementing simple low-temperature annealing processes with reduced energy consumption. This strategy provides a means for developing new generation structural materials with high strength and ductility. • The Co 36 Fe 36 Cr 18 Ni 10 alloy exhibits a beneficial combination of strength and ductility when annealed at 700°C. • Stacking faults, dislocation entanglement and nanotwins interactions are effective way to strengthen FCC structure alloys. • Low SFE promotes annealed twins formation and LAGB is the nucleation site for strain-induced martensitic phase transitions.

A crystal plasticity model for metal matrix composites considering thermal mismatch stress induced dislocations and twins

Originated at heterogeneous interfaces with distinct coefficient of thermal expansion (CTE), thermal mismatch stress is one of the critical influential factors to mechanical properties of metal matrix composites (MMCs). This stress is normally accommodated plastically by various defects, for example, high-density dislocations and twins in Al near heterogeneous interfaces in SiC/Al composites. Basic knowledge on the influence of defect characteristics is important but difficult to extrapolate from experimental results. However, existed theoretical models more focus on the influence of dislocation density, but less focus on defects variety, volume and distribution. In this paper, we propose a physics-based crystal plasticity model that has the capability of dealing with thermal mismatch stress induced dislocations and twins (denoted as TMDT model). The proposed TMDT model that is implemented in the Visco-Plastic Self-Consistent (VPSC) method considers defect heterogeneous distribution (gradient range), defect type (dislocations vs. twins) and defect volume fraction (twin spacing vs. twin volume). We demonstrate the validity and the capability of the VPSC-TMDT model in SiC/Al composites with thermal mismatch induced dislocations or twins. Furthermore, this model predicts the ultra-high strength of Graphene/Copper composites with high-density nanoscale twins, which is in turn the future aim for such nanocomposites.

Toughening and Crack Healing Mechanisms in Nanotwinned Diamond Composites with Various Polytypes.

As an emerging ceramic material, recently synthesized nanotwinned diamond composites with various polytypes embedded in nanoscale twins exhibit unprecedented fracture toughness without sacrificing hardness. However, the toughening and crack healing mechanisms at the atomic scale and the associated crack propagation process of nanotwinned diamond composites remain mysterious. Here, we perform large-scale atomistic simulations of crack propagation in nanotwinned diamond composites to explore the underlying toughening and crack healing mechanisms in nanotwinned diamond composites. Our simulation results show that nanotwinned diamond composites have a higher fracture energy than single-crystalline and nanotwinned diamonds, which originates from multiple toughening mechanisms, including twin boundary and phase boundary impeding crack propagation, crack deflection and zigzag paths in nanotwins and sinuous paths in polytypes, and the formation of disordered atom clusters. More remarkably, our simulations reproduce more detailed crack propagation processes at the atomic scale, which is inaccessible by experiments. Moreover, our simulations reveal that crack healing occurs due to the rebonding of atoms on fracture surfaces during unloading and that the extent of crack healing is associated with whether the crack surfaces are clean. Our current study provides mechanistic insights into a fundamental understanding of toughening and crack healing mechanisms in nanotwinned diamond composites.

Mechanical properties and thermal stability of gradient structured Zr via cyclic skin-pass cryorolling

Gradient-structured Zr sheets were fabricated by cyclic skin-pass cryorolling and cyclic skin-pass rolling. The mechanical properties and microstructural evolution of the gradient-structured Zr after rolling and annealing were investigated. The results indicated that the cyclic skin-pass cryorolled samples exhibited higher tensile strengths (σb = 557 MPa) and slightly lower fracture elongations (εu = 17.1%) than the cyclic skin-pass rolled samples due to high dislocation storage. After annealing at 150 °C–450 °C for 1 h, high-resolution transmission electron microscopy results showed that the ultrafine-grained layer of the cyclic skin-pass cryorolled samples exhibited better thermal stability, which was attributed to the formation of nanoscale twins and incomplete recovery of dislocations.

An Study of Nanoscale Mechanical Twins in GH4169 Alloy by Laser Shot Peening Processing

In order to study the mechanism of the fatigue strengthening using laser shot peening in GH4169 alloy, micro-structural and nanoscale mechanical twins (MT) at different depth below the top surface subjected to laser shot peening processing (LSP) were investigated by means of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations. In terms of the experimental observations and analyses, the formation of refined grains and nanoscale MT mechanism at the near surface of GH4169 alloy as a function of LSP treament can be summarized as follows: (i) two direction low density of MTs divide the initial coarse grains into submicron rhombic blocks; (ii) high density of MTs aligned in two directions subdivide the submicron rhombic blocks into nanoscale rhombic MT blocks; (iii) the third direction MT further refine the nanoscale rhombic MT blocks into nanoscale triangular MT blocks; (iv) some of subdivided blocks evolve into refined grains. An ultra-high strain rate induced by ultra-short laser pulse plays a key role in the formation of refined grains and nanoscale MT during plastic deformation of GH4169 alloy subjected to LSP treatment.

Electro-mechanical coupling in FCC metal rhodium from first-principles simulations

Electroplasticity in metals has been controversial since it is extremely hard to decouple the electron-induced heating in experiment. To illustrate the origin of this phenomenon, we applied density-functional theory (DFT) calculations to examine the response of injecting electrons and holes to face-centered-cubic metal, Rh, under finite shear deformation. We firstly examine the single crystal and find that single-crystal Rh can become more ductile/stronger with the electron/hole injection due to the less/more charge distribution between the sheared bonds cross the slip plane under deformation. Then the nanoscale twins are introduced to illustrate microstructure effect, and the simulation results indicated that nanoscale twins weaken the Rh, leading to a softening effect. With injected electron/hole, the nanotwinned structure displays the same more ductile/stronger effects as the single-crystal Rh. The origin of these effects arises from the weakened/enhanced bonding along twin boundaries. This study provides some insights on the origin of electroplasticity in metals.

Temperature-Induced Phase Transition Characteristics of [001]-Oriented 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 (PZN-7%PT) Single Crystal by Using Piezoresponse Force Microscopy.

The evolution of the domain structures of [001]-oriented relaxor ferroelectric 0.93PbZn1/3Nb2/3O3-0.07PbTiO3 (PZN-7%PT) single crystals as a function of temperature was investigated in situ by using piezoresponse force microscopy (PFM). It was found that the local domain structure of PZN-7%PT single crystals at room temperature is rhombohedral with nanoscale twins. Temperature-dependent domain structures showed that the phase transition process is a collective process and that the sample underwent a sequence of rhombohedral (R) → monoclinic (Mc) → tetragonal (T) → cubic (C) phase transformations when the temperature increased from 25 °C to 170 °C. The results provide direct observation of the phase transition evolution of PZN-7%PT single crystals as a function of temperature, which is of great significance to fully understand the relationships between the domain structure and phase structure of PZN-7%PT single crystals.

Nanoscale twinning in Fe–Mn–Al–Ni martensite: a backscatter Kikuchi diffraction study

Iron-based Fe-Mn-Al-Ni shape-memory alloys are of rather low materials cost and show remarkable pseudoelastic properties. To further understand the martensitic transformation giving rise to the pseudoelastic properties, different Fe-Mn-Al-Ni alloys have been heat treated at 1473 K and quenched in ice water. The martensite, which is formed from a body-centred cubic austenite, is commonly described as face-centered cubic (f.c.c.), even though there are also more complex, polytypical descriptions of martensite. The presently studied backscatter Kikuchi diffraction (BKD) patterns have been evaluated, showing a structure more complex than simple f.c.c. This structure can be described by nanoscale twins, diffracting simultaneously in the exciting volume. The twinned structure shows a tetragonal distortion, not uncommon for martensite in spite of the lack of interstitial elements. These features are evaluated by comparing the measured BKD patterns with dynamically simulated ones.

Effect of defect interactions with interstitial Ag in the lattice of BixSb2−xTe3 alloys and their thermoelectric properties

In this study, the effect of Ag defects on the lattice and thermoelectric properties of Ag-doped BixSb2−xTe3 (BST) alloys are systematically investigated. Using density-functional calculations, we reveal that Ag impurity defects occupying interstitial sites in the BST affect the formation of intrinsic defects in the host materials. Ag interstitial defects, which are responsible for increasing the lattice parameter of the c axis, lower the formation energies of BiTe and SbTe acceptor defects more effectively in the Bi-rich BST than in the Bi-poor BST. In addition, these Ag interstitials induce the formation of AgBi and AgSb in BixSb2−xTe3. Therefore, Ag-induced defect interactions can be used to finely optimize the defects and doping density by varying the Bi content. Furthermore, the phonon thermal conductivity is reduced with the formation of nanoscale twin structures with various stacking faults. Finally, a high figure of merit of 1.4 at 423 K is achieved for p-type BST alloys.

Growth of nanotwinned Cu nanowires in modified anodic aluminum oxide templates

Modified anodic aluminum oxide (AAO) templates are developed to deposit copper nanowires (CuNWs) with high-density nanoscale twin boundaries. By controlling anodization potential during oxide barrier thinning process a porous AAO array is directly formed on an Al foil without tedious contact electrode deposition and substrate transfer processes. The nanotwinned CuNWs electrodeposited in the modified AAO template reveal shorter nanocrystalline transition region than that in the conventional AAO template. The influence of nanocrystalline and nanotwin structure on electrical resistivity of Cu nanowires is also evaluated.

Mechanical properties of FeMnCoCr high entropy alloy alloyed with C/Si at low temperatures

Microstructural evolution and mechanical properties of FeMnCoCr high entropy alloy containing C and Si at various temperatures were investigated. The experimental results demonstrate that the microstructure of the HEAs alloy is a single-phase face-centered cubic solid solution, and the C and Si are completely dissolved in the matrix. The stacking fault energy of the alloy is about 33.96 mJ/m2. Therefore, twins will be easy to form during the deformation process. Benefit from the synergistic effect of solid solution strengthening and twinning induced plasticity, the alloy shows a trade-off between high strength and good plasticity. Ultimate tensile strength of 757 MPa and considerable fracture ductility of 60.5% is obtained at room temperature. As the deformation temperature decreased to 227 K, the strength and plasticity are increased to 907 MPa and 69.6%, respectively. Under the low temperature, primary mechanical twins are suppressed, and more geometrically necessary dislocations and secondary nanoscale twins could be observed. The HEAs show a high work-hardening rate, which is related to dislocation slip being hindered by primary and secondary nano-twins.

Zr50Cu25Ni7.5Co17.5 high-temperature shape memory alloy with excellent thermal stability and large recovery strain, and the associated microstructural deformation mechanism

In this study, we produced a novel Zr50Cu25Ni7.5Co17.5 high-temperature shape memory alloy with the aim of achieving high thermal stability and a good shape memory effect. Differential scanning calorimetry and the compression cycle were utilized to determine the phase transformation temperature and shape memory effect of the alloy, and transmission electron microscopy (TEM) was employed to investigate the microstructure and clarify the corresponding deformation mechanism. We found that the Zr50Cu25Ni7.5Co17.5 alloy exhibited excellent thermal stability and achieved the best maximum recovery strain of 6.87% (8% pre-strain) observed to date, making it a promising high temperature shape memory alloy. The TEM results indicated that, during the primary stage of the compression cycle, a small number of the (001) compound twins were de-twinning and numerous contraction twins formed. As the stress increased, increasing numbers of nanoscale (021) and (111) type-I deformation twins formed, and the shift displacement of the type-B planar defect expanded from 3d ~ (001) to 8d ~ (001). Furthermore, the newly found (111) type-I deformation twin and the extension of the type-B planar defect that contributes to the large recovery strain in the Zr50Cu25Ni7.5Co17.5 alloy were observed.

Hierarchically structured diamond composite with exceptional toughness.

The well known trade-off between hardness and toughness (resistance to fracture) makes simultaneous improvement of both properties challenging, especially in diamond. The hardness of diamond can be increased through nanostructuring strategies1,2, among which the formation of high-density nanoscale twins - crystalline regions related by symmetry - also toughens diamond2. In materials other than diamond, there are several other promising approaches to enhancing toughness in addition to nanotwinning3, such as bio-inspired laminated composite toughening4-7, transformation toughening8 and dual-phase toughening9, but there has been little research into such approaches in diamond. Here we report the structural characterization of a diamond composite hierarchically assembled with coherently interfaced diamond polytypes (different stacking sequences), interwoven nanotwins and interlocked nanograins. The architecture of the composite enhances toughness more than nanotwinning alone, without sacrificing hardness. Single-edge notched beam tests yield a toughness up to five times that of synthetic diamond10, even greater than that of magnesium alloys. When fracture occurs, a crack propagates through diamond nanotwins of the 3C (cubic) polytype along {111} planes, via a zigzag path. As the crack encounters regions of non-3C polytypes, its propagation is diffused into sinuous fractures, with local transformation into 3C diamond near the fracture surfaces. Both processes dissipate strain energy, thereby enhancing toughness. This work could prove useful in making superhard materials and engineering ceramics. By using structural architecture with synergetic effects of hardening and toughening, the trade-off between hardness and toughness may eventually be surmounted.

Heterogeneity and inelasticity of deformation in a notched martensitic NiTi shape memory alloy specimen

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. At the micro-scale, in-situ measurements indicate that the martensite microstructure continuously evolves during tensile loading. However, the nano-scale twin laminate structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for martensite single-variant-scale models.

Continuous strengthening in nanotwinned high-entropy alloys enabled by martensite transformation

Strengthening by nanotwins in metals has been widely reported to have a limit, i.e., softening occurs when the twin thickness is reduced below a critical value, similar to the Hall-Petch breakdown in nanocrystalline materials. Here we report based on atomistic simulations that such a limit can be eliminated in high-entropy alloys (HEAs) by tuning their composition and the corresponding stacking fault energies. By using nanotwinned ${(\mathrm{CoCrFeMn})}_{1\text{\ensuremath{-}}x}\mathrm{N}{\mathrm{i}}_{x}$ HEA as a model system, our results indicate that continuous strengthening can be enabled in this HEA at Ni concentrations below 44% by martensite phase transformation even when the twin thickness approaches the theoretical minimum (i.e., three-atomic-layers thick). In contrast, at Ni concentration above 44%, a strengthening-softening transition appears in the HEA due to detwinning when the twin thickness decreases, which is similar to that in conventional fcc metals with nanoscale twins. A simple parameter based on the ratio between the energy barriers for detwinning and martensite transformation has been proposed to explain these findings, which should shed some light on a generalized deformation map in nanotwinned metals.

A new strategy for designing immiscible medium-entropy alloys with excellent tensile properties

Here, a new strategy for designing heterogeneous medium-entropy alloys with light-weight and excellent mechanical properties is proposed. Alx(CuFeMn)100-x (x = 0, 7.5, and 15 at%) alloys were developed by utilizing the immiscible nature of Cu–Fe alloys. The microstructures of the alloys show phase separation into Cu-rich and Fe-rich regions, and the addition of Al transforms the crystal structure from dual face-centered cubic to face-centered cubic and body-centered cubic. Phase separation of the microstructure into two domains enables further dissolution of Al into the matrix. The alloys exhibit high strength because of solid solution strengthening and hetero-deformation induced strengthening caused by heterogeneous microstructures. The presence of nano-scale twins and essential partially recrystallized microstructures also enhances the strength of the alloys. This new type of medium-entropy alloys is expected to expand the design window in physical metallurgy.

Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing

High-entropy alloys (HEAs) are a new class of materials with a potential engineering application, but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge. Here, we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing (FSP). After FSP, the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated, and the cast coarse two-phase structure (several micrometers) was changed into an ultrafine-grained single-phase structure (150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins. This unique microstructure was mainly attributed to the severe plastic deformation during FSP, and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs. Furthermore, FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa, respectively, which were > 1.5 times higher than those of the base material. The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins. This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.

Simultaneously enhanced strength and ductility in a metastable β-Ti alloy by stress-induced hierarchical twin structure

A novel metastable β-Ti alloy Ti-4Mo-3Cr-1Fe with high strength and high ductility was developed through controlling the alloyʼs stability and deformation mechanisms. The microstructure consists of randomly oriented β-grains containing an even distribution of athermal ω precipitates. Under tensile loading, the alloy exhibits unprecedented and comprehensive mechanical properties including a high yield strength of 870 MPa, excellent total elongation of 41% and an ultrahigh strain hardening rate of 2.5 GPa. Based on investigations of deformation microstructures, the superior mechanical properties are attributed to stress-induced formation of a complex nano-scale hierarchical twin structure which is promoted by reversion of ω precipitates.