Nanotwinned Research Articles

Structural optimization strategies for improving the maximum strength of gradient nanotwinned metals

Many studies have shown that the excellent mechanical properties of gradient nanotwinned (NT) metals were primarily attributed to the unique micro-nano structure. In the present study, the motion resistance of mode Ⅱ and trans-twin dislocations is directly used to describe the effective stress in gradient nanotwinned (GNT) metals, combined with the modified hetero-deformation-induced (HDI) stress model, a constitutive model in connection with structural parameters has been successfully developed for the GNT metals with preferentially oriented. The proposed quantitative continuum plasticity model could investigate the role of gradient structure in tuning the strength of GNT metals. Furthermore, it has been found that the gradient element height and minimum twin thickness have significant effects on the strength of GNT metals. Therefore, the proposed model can be employed to optimize the mechanical property of GNT metals by controlling the gradient element height and minimum twin thickness.

Nano-precipitation strengthening regulated by nanotwins in CoCrNi alloy with super-high strength

The number and size of precipitates directly influence the strengthening of the high-temperature alloys. As special coherent interfaces, nano-twinned (NT) boundaries are pre-set in CoCrNi-based superalloy to regulate the distribution of nano-precipitates for further strengthening the mechanical property. High density of NTs with twin spacings of 2–200 nm is produced in CoCrNi alloy by surface mechanical rolling treatment (SMRT), and L12 (Ni3Ti)-type precipitates are induced by the low-energy NT boundaries, as well as grain boundaries, after aging at 700 °C for 4 h. The CoCrNi alloy with L12 nano-precipitates exhibits a super-high mechanical property, characterized by 2019 MPa of a yield strength (σy) and 2300 MPa of an ultimate tensile strength (σu), much higher than the CoCrNi alloy counterpart of 840 (σy) and 1100 (σu) MPa, respectively. Meanwhile, the treated CoCrNi alloy also delivers a reasonable elongation of 7.0%. The super-high strength is mainly ascribed to the high density L12 nanoprecipitates grown on the NT boundaries, while the elongation is sustained by the NTs boundaries.

Temperature- and internal structural size-dependent strength of nanotwinned face-centered cubic metals

The Nanotwin structure exhibits great potential as a strengthening medium, with its effectiveness influenced by both internal structural size and temperature. In this work, we developed a strength model that incorporates the dependence on temperature and internal structural size. This model can predict the strength of nanotwinned (NT) FCC metals with intracrystalline or intercrystalline twin boundaries featuring different internal structural sizes at varying temperatures at the inverse Hall-Petch stage. The proposed prediction model has significant importance for enhancing our understanding of the mechanical behavior exhibited by NT metals and facilitating the design of super-strong materials that can withstand extreme conditions.

Atomic insight into mechanical behavior of AuPt alloys

AuPt alloys are widely applied in electronics, aerospace, and precision equipment fields. A thorough understanding of the mechanical behavior of AuPt alloys is critical for their potential applications. Here, we leverage molecular dynamics (MD) method to for the first reveal the effects of microstructure and temperature on mechanical behavior of AuPt alloys from atomic level. The nanoindentation of monocrystalline (MC), nanotwinned (NT), and nanocrystalline (NC) AuPt alloys are performed. The results demonstrate that the mechanical properties of the MC AuPt alloy are excellent at 77 K because of dislocation strengthening while poor at high temperature due to amorphous behavior. The introduction of twinning boundary (TB) greatly improves the mechanical properties of AuPt alloy by preventing the stress spread and promoting dislocation activity. The mechanical behavior of NC AuPt alloy is strongly dependent on grain size. With increasing average grain size, the dominant plastic deformation mechanism of NC AuPt alloy changes from GB migration to dislocation motion and then to GB destruction. An appropriate grain size should be selected to inhibit GB migration or GB destruction, thereby achieving the enhancement of NC AuPt alloy through dislocation strengthening. This work provides an important theoretical basis for the subsequent design of AuPt alloys with desirable mechanical performances to extend the application prospects of noble metals.

Influence of hierarchical structure on deformation characteristics and mechanical response of nanotwinned copper

ABSTRACT The introduction of the hierarchical structure into face-centered cubic (FCC) nanotwinned (NT) metals can improve strength without losing plasticity. In present work, molecular dynamics (MD) simulations have been performed to investigate the effect of the hierarchical structure on NT copper under tensile and shear loading by comparing the differences between the hierarchical nanotwinned (HNT) and perfect nanotwinned (PNT) specimens. The results show that the hierarchical structure will alter the twin boundaries (TBs) migration mode under shear loading. The hardening process of HNT copper is dependent on the geometric structure of the grain boundary and loading direction. There is a stress gradient and additional hardening in HNT copper under tensile loading due to the hierarchical structure. This work may shed light on the role of hierarchical structure in the multiscale structure design of NT materials.

Effects of microstructure and vibration parameters on mechanical properties of nanoimprinted FeNiCrCoCu high-entropy alloys

Molecular dynamics (MD) simulations are utilized to study the mechanical behavior of FeNiCrCoCu high-entropy alloys (HEA) during nanoimprint lithography with single-crystal, polycrystal, and nano-twinned polycrystal structures. The findings of MD simulations reveal that the microstructure and vibration parameters significantly impact the loading force, elastic recovery ratio, and deformation behavior of FeNiCrCoCu HEA. The imprinting force curve revealed that the maximum loading force is in reduced order with single-crystal, nano-twinned (NT) polycrystal, and polycrystalline structures. With the polycrystalline structure, an inverse Hall-Petch relationship is observed when the grain size varies from 5.1 nm to 9.8 nm. Grain boundary (GB) plays an important role in softening material; the splitting of grains, the migration of the GBs, and grain rotation are the main deformation mechanisms of this region. For NT polycrystals, the stability of material can be enhanced due to the existence of twin boundaries (TB), and the migration of TB is explored near the imprinted region. With polycrystalline structure, the best formability is observed for specimens with a grain size of 9.8 nm, where the average elastic recovery ratio is the smallest, and the forming shape at this grain size is the best. The mold angle of 10° and 20° result in the pattern having a good symmetrical shape, suggesting a better-imprinted shape than with other angles. Moreover, the effect of high-frequency mechanical vibration is analyzed carefully in this study. The results show that the best forming ability is achieved as the vibration amplitude is 3.0 Å. As changing vibration frequencies, the frequency of 50 GHz gives the highest forming ability.

Solute synergy induced thermal stability of high-strength nanotwinned Al-Co-Zr alloys

Reducing the grain size into the nanoscale regime in metallic materials provides high mechanical strengths, however at the cost of degrading thermal stability, as grain refinement induces a high driving force for grain coarsening. In this study, we present a solute synergy strategy that stabilizes the microstructures of high strength nanotwinned (NT) Al–Co–Zr alloys. Zr solute additions promote microstructural and mechanical stability up to an annealing temperature of 400 °C. In-situ microcompression tests demonstrate concomitant high strengths and deformability in these ternary NT alloys. Density functional theory calculations provide insight into the interplay between Co and Zr solute and how they pin and stabilize incoherent twin boundaries. This work provides a strategy for enhancing both strength and thermal stability of nanocrystalline materials when combining synergistic solute pairs.

Mechanical, electrical properties and microstructures of combinatorial Ni-Mo-W alloy films

Nickel-molybdenum-tungsten (Ni-Mo-W) alloys have been suggested for applications of advanced micro-electro-mechanical systems (MEMS) in extreme environments due to their high strength, good electrical conductivity, and excellent thermal stability. In this work, we have carried out combinatorial experiments to reveal composition-dependent phase formation, physical properties, and thermal stability of Ni-Mo-W alloy films. Combinatorial synthesis through a magnetron sputtering process fabricated thin films with a composition range of Ni 56–92 Mo 6–38 W 0.5–7.5 . Structural analysis for the as-deposited combinatorial film revealed that a nanotwinned and strongly textured nano-columnar fcc structure was formed in the Ni-rich compositions, while solute content greater than 30% resulted in the formation of metallic glasses. The specimen exhibited very high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ ∙ cm), and both the properties were found to increase with solute concentration. Annealing of the combinatorial film at 600 ℃ for 1 h revealed that nanotwinned structure is thermally stable for high Ni concentrations. Metallic glasses with high solute concentration also exhibited high thermal stability and were not crystallized after annealing. Alloys with intermediate solute content experienced the transformation from the amorphous to fcc nanocrystalline phase with nanotwins. The annealed combinatorial film maintained high hardness and moderate electrical resistivity after annealing. • Composition-dependent phase formation and physical properties of Ni 56–92 Mo 6–38 W 0.5–7.5 alloy films were investigated. • A nanotwinned (NT) and nano-columnar fcc structure was formed in the Ni-rich compositions. • The solute content greater than 30% resulted in the formation of metallic glasses. • The specimen exhibited high hardness (8.8–12.5 GPa) and moderate electrical resistivity (85–135 μΩ · cm). • The NT structure with high Ni content and metallic glasses with high solute concentration were thermally stable.

High strength and ductility in a heterostructured nanotwinned Ni film

We report the synthesis and mechanical behavior of two nanotwinned (NT) Ni films with different distributions of twin widths along the film growth direction. The NT Ni films, deposited on Si (111) substrates with an Ag buffer layer, are composed of two crystallographic variants with (111) out-of-plane orientation, which results in the formation of twin boundaries at their interfaces. The uniaxial stress-strain response of the freestanding NT Ni films was measured using MEMS tensile testing stages. While both films exhibited high strength, the film with a larger mean and broader distribution of twin widths exhibited significant strain hardening as well, which led to uniform elongation of ∼10%. These results suggest that by controlling the twin width distribution it is possible to realize a synergistic combination of strength and ductility in NT metal films.

Nanotwinned medium entropy alloy CoCrFeNi thin films with ultra-high hardness: Modifying residual stress without scarifying hardness through tuning substrate bias

High entropy alloy (HEA) and highly nanotwinned (NT) material with improved ductility and high strength demonstrate promising potential compared to nanocrystalline materials and traditional metallic alloys. In this study, CoCrFeNi-based medium entropy alloy (MEA) thin films with nanotwinned structure (NT) was fabricated by sputtering technology and their corresponding properties and microstructure were investigated. The NT-MEA thin films show simple face centered cubic (FCC) structure with nearly 100% (111) texture and twin thickness in 1.8–2.8 nm. The NT-MEA thin films exhibit both low electrical resistivity (100 ± 2 μΩ-cm) and high hardness (8.0–9.15 GPa) with low roughness (Rms < 1.3 nm). By controlling the substrate bias, the residual stress of the CoCrFeNi thin film can be manipulated from compressive (−0.89 GPa) to tensile (+0.6 GPa) without scarifying the hardness.

Tuning the strength-ductility synergy of nanograined Cu through nanotwin volume fraction

Nanograined (NG) metals with nanotwinned (NT) regions can overcome the inferior ductility of NG metals and achieve high strength and modest ductility. Based on the strain gradient plasticity and Johnson–Cook failure criterion, we simulate the dependences of their strength and ductility on volume fraction, twin spacing, as well as shape and distribution of NT regions in NG Cu. It is found that these factors have significant impact on the overall ductility. In particular, the overall ductility abnormally decreases with the increase in the volume fraction of NT regions, which is directly related to the failure modes of this material system. Interface debonding can explain the above abnormal decrease in overall ductility. In addition, with the increase in twin spacing, fracture of NT regions can cause different reversals of overall ductility. We also found that, when the NT regions are of the oblique square type, the overall ductility is significantly lower than when they are of the circular and square types. In most cases, the array arrangement of NT regions is superior to the staggered arrangement for the improvement of the overall ductility. It is believed that these reported results can contribute to a deeper understanding of this novel material system.

Deformation mechanism of bimodal nanotwinned Cu-Ag alloy with grain boundary affect zone segregation

The deformation mechanism of bimodal nanotwinned (NT) Cu-Ag alloys with grain boundary affect zone (GBAZ) segregation has been investigated through molecular dynamics (MD) simulations. The bimodal NT Cu-Ag alloys model is established by embedding a coarse NT grain into the ultrafine grained matrix. The solute Ag atoms in Cu-Ag alloy polycrystalline matrix are segregated in the GBAZ (generated by equidistance scaling method). The coarse NT grain inclusion phase can impart high strength resulting from the twin boundaries (TB) strengthening regime. Through the uniaxial tensile simulations, the strength of bimodal NT samples is found to be related with the three factors, including coarse NT grain size, TB inclination angles as well as TB spacings (TBSs). The dislocation activities and corresponding deformation mechanisms are explored in detail.

A novel shock-induced multistage phase transformation and underlying mechanism in textured Nano-Twinned Cu

Nano-Twinned (NT) Cu under shock loading parallel to twin planes was investigated via nonequilibrium molecular dynamics (NEMD) simulations, with particular attention to the shock response and intrinsic deformation mechanism. During the shock compression process, a novel multi-stage phase transformation (MPT) scenario, i.e., from FCC phase to BCC phase and then to HCP phase, happens due to shock-enhanced twin boundary (TB) sliding. Both increasing the compressive strain caused by shock and decreasing the twin lamellae in the NT Cu can promote the MPT, with which the yield strength retains but the spallation strength increases. The critical shock pressure triggering the MPT depends exponentially on lamellae twin thickness (T). These new results have great potentials for improving the impact resistance of metals by tailoring their internal nanostructures.

Microstructure and composition dependence of mechanical characteristics of nanoimprinted AlCoCrFeNi high-entropy alloys

Molecular dynamics is applied to explore the deformation mechanism and crystal structure development of the AlCoCrFeNi high-entropy alloys under nanoimprinting. The influences of crystal structure, alloy composition, grain size, and twin boundary distance on the mechanical properties are carefully analyzed. The imprinting load indicates that the highest loading force is in ascending order with polycrystalline, nano-twinned (NT) polycrystalline, and monocrystalline. The change in alloy composition suggests that the imprinting force increases as the Al content in the alloy increases. The reverse Hall–Petch relation found for the polycrystalline structure, while the Hall–Petch and reverse Hall–Petch relations are discovered in the NT-polycrystalline, which is due to the interactions between the dislocations and grain/twin boundaries (GBs/TBs). The deformation behavior shows that shear strain and local stress are concentrated not only around the punch but also on GBs and adjacent to GBs. The slide and twist of the GBs play a major in controlling the deformation mechanism of polycrystalline structure. The twin boundary migrations are detected during the nanoimprinting of the NT-polycrystalline. Furthermore, the elastic recovery of material is insensitive to changes in alloy composition and grain size, and the formability of the pattern is higher with a decrease in TB distance.

Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism

Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs.Graphic abstract

Molecular dynamics study on the fracture mechanism in bimodal nanotwinned Cu with a composite structure

Utilizing molecular dynamics simulations, the authors investigate the notion of bimodal nanotwinned (NT) microstructure in strength-ductility trade-off. Using a coarse circular grain with twins embedded in a sea of nano-grains (without twins) as a surrogate, the paper explores three scenarios pertaining to the strength and failure strain mapping: (i) the effect of twin boundary (TB) inclination; (ii) the effect of coarse-grain size; (ii) the effect of twin boundaries spacings (TBSs). The central result is that there exists an optimum grain size at which the strength and failure strain are simultaneously improved. Meanwhile, the fracture mechanisms of bimodal NT microstructure are demonstrated by analyzing dislocation-TB interactions, which involve TB migration, deformation twinning, void generation, etc.

Anisotropic strengthening of nanotwin bundles in heterogeneous nanostructured Cu: Effect of deformation compatibility

Anisotropic tensile behaviors of a heterogeneous nanostructured Cu composed of isotropic nanograin matrix and anisotropic nanotwin bundles were investigated under loading directions parallel, normal, and 45° inclined to the twin boundaries (TBs). Distinct from the anisotropic strengthening effect in the homogeneous nanotwinned (NT) counterpart, the heterostructure exhibits the highest strength under parallel tension, and moderate strength under normal tension. High-resolution digital image correlation analysis indicated an orientation-dependent deformation compatibility between the nanotwin bundles and nanograin matrix, i.e., compatible deformation in the parallel orientation; but significant incompatibility in orientations both normal and 45° inclined to TBs. The results reveal that the strengthening effect of nanotwins in heterogeneous nanostructure is not only dependent on the strength of themselves, but also influenced by the deformation compatibility with surrounding components. The orientation-dependent deformation compatibility was attributed to the interactions between isotropic shear bands in nanograin matrix and anisotropic deformation in nanotwin bundles.

Influences of Nano-structured Thermal Stability on the Intergranular Corrosion of High-Carbon Austenitic Heat-Resistant Steel

The influences of nano-structured thermal stability on the intergranular corrosion (IGC) of Super304H steel were investigated by electrochemical tests and surface analysis in this study. It was found that IGC in nano-twinned (NT) Super304H SS during the aging process was governed by the formation of nano-scale M23C6 precipitates, which generated Cr-depletion zones, and fast healing of the Cr-depletion zones due to the rapid infusion of Cr atoms from the matrix. Conversely, the nano-grains (NG) with poor thermal stability could accelerate the nucleation of sigma phase at recrystallizing interfaces after short-time aging at 650 °C, thereby deteriorating the IGC resistance. The thermal stability of the NT structure was superior to that of the NG structure, which preserved the IGC resistance of Super304H SS during the aging process. Therefore, it was critical to keep the SP strain below the saturation value to achieve high thermal stability, good IGC resistance, and suppression of recrystallisation-induced precipitation.

In-situ studies on the mechanical properties of He ion irradiated nanotwinned Ag

Recent studies show that twin boundaries are effective defect sinks in eliminating radiation-induced defects. However, the influence of radiation-induced defects on the mechanical behavior of nanotwinned (NT) materials is less well understood. In this study, we investigate the mechanical properties of He ion irradiated NT Ag. In-situ micropillar compression tests show that deformation induces prominent detwinning in as-deposited NT Ag, whereas He ion irradiation alleviates detwinning during deformation of NT Ag. The radiation hardening mechanism and the influence of He bubbles on the deformation behavior of NT Ag are discussed.

An insight into Mg alloying effects on Cu thin films: microstructural evolution and mechanical behavior

How to design ultra-strong, light-weight Cu alloys is a long-term pursuit in materials community, which is technically superior and cost-effective for their promising energy-saving applications. In this work, we prepared Cu-Mg alloyed thin films to study light element Mg alloying effects on the microstructure, hardness and strain rate sensitivity (SRS) of nanocrystalline Cu thin films. In the studied Mg concentration-range spanning from 0 at.% to 16.8 at.%, both the grain size and the twin spacing decrease monotonously with increasing Mg composition while Cu-2.8 at.% Mg sample has the highest twin fraction of ∼75%. A combined strengthening model was employed to quantify the Mg concentration-dependent hardness of nanotwinned (NT) Cu-Mg thin films, in which the grain/twin boundary facilitates strengthening while the solute Mg atoms induce softening. Both the constant rate of loading tests and the nanoindentation creep tests uncover that compared with pure Cu samples, the NT Cu-Mg thin films manifest much lower SRS, particularly in the creep tests, owing to the activation of dynamic strain aging effects.