Twin Spacing Research Articles

Pseudoelasticity in twinned α-Fe nanowires under bending

The pre-existing twin boundaries play an important role in the mechanical behaviors of metals. In the present work, we studied the bending deformation of [100]-oriented α-Fe nanowires seeded with 〈111〉/{112} twin boundaries using atomistic simulations. Bending behaviors along two different orientations were investigated. The initially flat twin boundaries become curved with a high density of 1/6 〈111〉/{112} partial dislocations under bending when the neutral plane intersects with twin plane along [01-1] direction, whereas could transform to the nonconventional {110} interfaces under bending when the neutral plane intersects with twin plane along [111] direction. The bent α-Fe nanowires could recover to the original state upon unloading, leading to a unique pseudoelasticity. The driving force for shape recovery primarily arises from the high interfacial energy, i.e. curved {112} twin boundaries or the transformed {110} interfaces. We further characterized the recoverability by the ratio η of the nanowire size (external length scale) to twin boundary spacing (internal length scale). This η-dependent recoverability matches well with the experimental data at large length scale. The present work may help understand the unique mechanical properties associated with twin boundaries in metals.

Electron localization governed plasticity in nanotwinned metals beyond the Hall-Petch type limit

While nanotwinned metals have been widely reported to show a strengthening-softening transition as the twin spacing decreases below a critical size, which is similar to the Hall-Petch limit in nanocrystalline materials, a recent study has suggested that some nanotwinned high entropy alloys (HEAs) could show continuous strengthening as long as the twin spacing decreases. Here we investigate the physical origin of the contrasting plasticity in nanotwinned face-centered-cubic (FCC) metals beyond the Hall-Petch type limit by using atomistic simulations based on density functional theory. Our results indicate that the degree of electron localization among atoms at the FCC sites and the stacking faults, which have a hexagonal-closed-packed (HCP) structure, governs the plasticity in nanotwinned metals with extremely fine twin spacing. Specifically, metals whose electrons are more localized at FCC atoms than those at stacking faults, such as conventional FCC metals, are associated with positive stacking fault energy and accordingly detwinning and strengthening-to-softening transition beyond the Hall-Petch type limit. In contrast, metals in which electrons are more localized at stacking faults than at FCC sites as found in some HEAs exhibit negative stacking fault energy and continuous strengthening by the unconventional FCC-to-HCP martensite transformation when the Hall-Petch type limit is reached. This work provides a new microscopic aspect from the electron properties to understand the macroscopic mechanical behavior of nanotwinned FCC metals which can shed some light on designing novel complex alloys.

Quantifying the effect of twin boundaries on grain boundary activities in nanotwinned copper: A molecular dynamics simulation

Twin boundaries (TBs) play an important role in work hardening of nanotwinned metals. However, the effect of TBs on grain boundaries (GBs) activities in nanotwinned metals has not given detailed analysis due to the limited spatial and temporal resolution of experimental technique and the unrealistic model with flat GBs in simulation. Therefore, the nanotwinned copper with more natural GBs was created in this study by using phase field model for molecular dynamics (MD) simulation. In addition, a novel code was developed to quantitatively analyze the GB activities including deformation and rotation in the initial deformation stage. The results show that the average flow stress in nanotwinned copper is larger than that in the twin-free model and the stress increases with decreasing twin spacing. From the statistical results of GB activities, we can find that the rotation of GBs seems to be not affected by the TBs, but the deformation of GBs will be detained by TBs which finally results in improving stress concentration on GBs. Hence, with decreasing twin spacing, the deformation mechanism shifts from partials gliding on different slip systems to partials gliding on the adjacent plane of TBs due to the stress concentration.

Dislocation plasticity and detwinning under thermal stresses in nanotwinned Ag thin films

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

Lattice buffer effect of Ti film on the epitaxial growth of Ag nanotwins on Si substrates with various orientations

Titanium thin film has been commonly used in the semiconductor industry as an adhesive layer to enhance the bonding strength between a metallic film and Si substrate. Depositing a Ti interlayer to alleviate the lattice mismatch for the epitaxial growth of nanotwinned Ag films on Si substrates with (100), (110) and (111) orientations by magnetron sputtering process provides an additional advantageous effect. The experimental results indicated that the deposited Ag films, consisting of columnar grains with a high density of nanotwins, grew normal to the Si substrates. The twin boundaries showed a strongly preferential orientation of (111), and the twin spacing ranged from 2 to 50 nm, with an average value of about 12 nm. Ag (111) nanotwins grew much better on Si (100) substrate than on Si (110) and Si (111) with Ti pre-coating, leading to the sputtered Ag nanotwinned film on Si (100) having the highest indentation hardness among the three combinations.

On the real-time atomistic deformation of nano twinned CrCoFeNi high entropy alloy

High entropy alloys (HEAs) holding several principal elements in high concentration have unprecedented combination properties. The design of strong and highly ductile HEAs has attracted extensive attention from researchers in the last decade, such as in mechanisms for inducing different types of phase and nano-sized precipitates. Since some HEAs have low stacking fault energy, nanotwins can form during the plastic deformation process or magnetron sputtering, resulting in enhanced mechanical properties due to the existence of twin boundaries. The addition of twin boundaries is implied to be a promising method in engineering HEAs. Understanding how these twin boundaries affect the mechanical properties of nanotwinned HEAs is the key to designing strong and ductile examples. In this study, we performed a large-scale molecular dynamic simulation to investigate the mechanical properties of HEAs with different twin boundary spacings at various temperatures. The results show that the strength of HEAs at all tested temperatures increases with decreasing twin boundary spacing until a lower critical value of 1.83 nm is reached, which is close to the experimental value (2 nm). The strength of the HEAs at all tested temperatures decreases as the twin boundary spacing is decreased further. The dislocation motion transitions at the critical twin boundary spacing. In the sample with a twin boundary spacing bigger than 1.83 nm, Shockley dislocations tend to intersect the twin boundaries and glide in the hardening modes; on the other hand, Shockley dislocations travel along the direction parallel to the twin boundaries in samples with a twin boundary spacing smaller than 1.83 nm, leading to detwinning and softening in the HEAs. The dislocation motion and entanglement at 1 K are respectively slower and stronger than those at 300 K; the grain boundary activity is more obvious at a higher temperature. A mechanistic theoretical model together with a Hall–Petch relationship is then proposed to consider the coupled twin boundary and temperature effect on the deformation of nanotwinned HEAs.

The wrinkling and buckling of graphene induced by nanotwinned copper matrix: A molecular dynamics study

Abstract The three-dimensional (3D) graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties, while the preparation of uniform and stable 3D graphene structures remains a challenge. In this paper, using molecular dynamics simulations, we found that the nanotwinned copper (nt-Cu) matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression. The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation, resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity. The newly formed twin boundaries (TBs) in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range. These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains. Furthermore, we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage. This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene, which may expand the application of graphene in energy storage and catalysts.

Nanotwin formation in Ni–Mo–W alloys deposited by dc magnetron sputtering

Nanotwinned metals and alloys have an attractive suite of properties, but the formation of the underlying nanostructure is generally dependent on the alloy chemistry and processing conditions. This study was undertaken to investigate the effect of deposition rate on nanotwin formation in sputter deposited Ni84Mo11W5 that possess ultrahigh strength and stability. Deposition rates of 0.5–2.3 nm/s were examined and the grain size, texture, twin spacing and hardness were characterized. Minimal variations in the underlying nanostructure and attendant hardness were observed, elucidating a wide processing window and the importance of chemistry on the formation of nanotwins in this alloy.

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.

The role of Mn on twinning behavior and tensile properties of coarse- and fine-grained Fe–Mn–C twinning-induced plasticity steels

The twinning behavior, strain hardening and tensile properties of coarse- and fine-grained Fe-13/18/22Mn-1.0C twinning-induced plasticity (TWIP) steels were investigated by microstructural observations and mechanical tests. These steels have stacking fault energy ranging from ~27 to ~37 mJ/m2 with increasing Mn content, and thus deformation twinning is the dominant mechanism. Tensile test results show that, with increasing Mn content, the yield and tensile strengths decrease slightly for both coarse- and fine-grained steels while the ductility increases significantly, especially for coarse-grained steels. Microstructural observations indicate that, at lower strains, an increase in Mn content suppresses the formation of twins, resulting in a lower twinning rate; however, at intermediate and higher strains, even higher twinning rate is observed in higher-Mn steels. In addition, increasing Mn content enables deformation twins to form in a wider range of strain. Such a varying twinning behavior with Mn content is even more apparent for coarse-grained steels. Furthermore, the sizes of twins vary with Mn content and also with grain size: both the thickness and spacing of twins or twin bundlers increase with increasing Mn content but decrease with refining grain sizes. Moreover, increasing Mn content decreases strain-hardening rate, but allows persistent strain hardening to higher strains. These strain-hardening behaviors and tensile properties observed in the twinning-dominated Fe–Mn–C steels with different Mn contents are correlated well their microstructural evolutions.

Deformation mechanisms of the subgranular cellular structures in selective laser melted 316L stainless steel

Selective Laser Melting (SLM) of certain alloys such as Stainless Steel yields a unique microstructure consisting of complex subgranular cell structures produced by ultra-fast cooling during solidification. Owing to the presence of these cell structures, SLM-fabricated 316L Stainless Steel displays a superior yield strength and hardness. Despite the significance of the distinctive cellular structures, their deformation characteristics and the mechanisms responsible for superior mechanical properties are not well-understood yet. In this paper, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The atomic configurations in the computational models of the cell structures are inspired by the SEM images of the microstructure of 316L Stainless Steel. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Ni concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. However, the flow stress is found to be the highest when both the cell boundary and cell interior are austenitic with higher Ni concentration in the cell interior. Moreover, governed by the initial dislocation density, the subgranular cell size has an inverse relationship with mechanical strength. Finally, the nano-twinned cells exhibit higher strength in comparison with twin-free cells as a result of the dislocation pinning at the twin boundaries. The investigation on the effect of twin spacing reveals a Hall-Petch type phenomenon in which smaller twin spacing strengthens the nano-twinned subgranular cells further.

Characterization of heterogeneous–nano structure in austenitic stainless steel: crystal orientations and hardness distribution

Hardness distribution in the heterogeneous–nano (HN) structure developed in a heavily cold-rolled SUS316LN austenitic stainless steel was systematically investigated. The HN structure consisted of twin domains surrounded by shear bands, which were further embedded in the conventional low-angle lamellae. The twining planes of {111} in the twin domains were nearly parallel to the rolling direction (RD). The average twin spacing was 20–40 nm. The longitudinal direction of the lamellae was also nearly parallel to the RD, and the average interboundary spacing was about 100 nm. Rather shortened ultra-fine grains with an average size of 100 nm were well developed within the shear bands. Microstructural observations using the transmission Kikuchi diffraction technique revealed that both of the disorientation and the value of the kernel average misorientation in the twin domains were relatively low compared to those in the shear bands and lamellar grains, and the misorientation angle of twin boundaries within the twin domain still remained 60°. On the other hand, the misorientation angles among grains within the shear bands were considerably high. The intense strain localization within the shear bands during cold rolling would result in the instant formation of ultra-fine grains surrounded by high-angle boundaries. These results suggested that dislocation density within twin domains was lower than that of the shear bands and lamellar grains. Although the twin domains possess a smaller interboundary spacing than that in the shear bands and lamellar grains, nanoindentation tests revealed that hardnesses were almost identical among the component nanostructures. The nearly identical hardness among the component nanostructures would be ascribed to a lower amount of dislocation strengthening in the twin domains than that in the shear bands and lamellae.

Tensile Properties of -Oriented Nanotwinned Cu with Different Columnar Grain Structures.

We performed tensile tests on highly <111>-oriented nanotwinned copper (nt-Cu) foils with different columnar grain structures. For a systematic study, we altered the microstructure of the foils by tuning the electroplating electrolyte and annealing temperatures under a nitrogen atmosphere. The results show that the yield strength ranges from 300 to 700 MPa, and elongation spans from 5% to 25%. Knowing the measured twin spacing and average grain size, and combining the confined layer slip with the Hall–Petch equation, we calculated the theoretical yield strength of the nt-Cu with different microstructures, and the theoretic values match the experiment results. Owing to the unique crystal orientation properties of <111>-oriented columnar grains, dislocations induced by slip are very limited. The Schmid factor of grains along the tensile axis direction is highly identical, so the plastic deformation is much more suitably explained by the Schmid factor model. Thus, we replace the Taylor factor with the Schmid factor in the slip model of nt-Cu.

High hardness and fatigue resistance of CoCrFeMnNi high entropy alloy films with ultrahigh-density nanotwins

Development of film materials has been limited by the hardness-fatigue resistance trade-off. The purpose of the present study was to obtain films with a combination of both high hardness and strong fatigue resistance. To achieve this, CoCrFeMnNi high entropy alloy films (HEAFs) were fabricated with three different structures: amorphous, high-density nanotwinned crystal structure with twin spacings of 2.2–5.6 nm, and ultrahigh-density nanotwinned columnar grains with twin spacings of 1.2–2.5 nm. Nanoindentation with dynamic mechanical analysis was used to measure the hardness and perform the fatigue tests. While higher twin densities could dissipate more energy by detwinning during fatigue loading to enhance the fatigue resistance, twin spacings larger than and small than 2 nm could, respectively, result in hardening and softening. Our results showed a high hardness of ~9 GPa and fair fatigue resistance (~104 cycles) for both amorphous and high-density nanotwinned crystalline layers. For the ultrahigh-density nanotwinned columnar grain structure, a high hardness of ~8.5 GPa and an excellent fatigue resistance (~106 cycles) were obtained. The outstanding fatigue resistance and high hardness were attributed to the synergistic effect of strain hardening and detwinning of ultrahigh-density nanotwins. The results not only enable CoCrFeMnNi HEAFs with a predominant combination of hardness and fatigue resistance, but also shed light on a new perspective for overcoming the conflict between hardness and fatigue resistance in film materials for microelectromechanical applications.

Digital Twin Reference Model Development to Prevent Operators’ Risk in Process Plants

In the literature, many applications of Digital Twin methodologies in the manufacturing, construction and oil and gas sectors have been proposed, but there is still no reference model specifically developed for risk control and prevention. In this context, this work develops a Digital Twin reference model in order to define conceptual guidelines to support the implementation of Digital Twin for risk prediction and prevention. The reference model proposed in this paper is made up of four main layers (Process industry physical space, Communication system, Digital Twin and User space), while the implementation steps of the reference model have been divided into five phases (Development of the risk assessment plan, Development of the communication and control system, Development of Digital Twin tools, Tools integration in a Digital Twin perspective and models and Platform validation). During the design and implementation phases of a Digital Twin, different criticalities must be taken into consideration concerning the need for deterministic transactions, a large number of pervasive devices, and standardization issues. Practical implications of the proposed reference model regard the possibility to detect, identify and develop corrective actions that can affect the safety of operators, the reduction of maintenance and operating costs, and more general improvements of the company business by intervening both in strictly technological and organizational terms.

Size-dependent formation and thermal stability of high-order twins in hierarchical nanotwinned metals

Introducing hierarchical twins into nanotwinned (NT) materials is regarded as an effective way to further improve their mechanical properties. It can be imagined that, with the increase of the order of hierarchical twins, it is insufficient to solely take single twin spacing into consideration. For example, the effect of the spacings of primary and secondary twins should be considered together for tertiary twinning. By virtue of theoretical modelling and atomistic simulations, we investigate the influence of low-order twin spacings on high-order twinning. The optimization strategy of high-order twin density and spacings with respect to low-order twin spacings are proposed. It is demonstrated that there exists a trade-off between high-order twin density and twin spacing which can be tuned by the low-order twin spacings. In addition, the atomistic deformation mechanisms related to low-order twin spacings are discussed. Different size-dependent propagation behaviors of partial dislocations are unveiled, relying on the combination of low-order twin spacings. At last, the great thermal stability of high-order twins is also verified, which is attributed to a strong pinning effect of partial dislocations onto low-order twins, leading to a special stress partitioning phenomenon. Our findings may provide a theoretical benchmark for the fabrication of high-order hierarchical nanotwinned (HNT) structures and thus, assisting the design of high-performance mechanical materials.

Dislocation Plasticity and Detwinning Under Thermal Stresses in Nanotwinned Ag Thin Films

Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic signature in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic - ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the signature thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars.

Theoretical prediction for mechanical properties in nanotwinned and hierarchically nanotwinned metals with different twin orders

The mechanism-based plasticity model on the mechanical properties in nanotwinned and hierarchically nanotwinned metals is further developed, in which the analytical formulae of flow stress and failure strain with respect to various factors are provided. The strength, ductility and fracture toughness in metals with different twin orders are analyzed systemically based on the theoretical model, and the effects of grain size and twin spacing are emphasized. It shows that the effect of twin spacing is significant, and high strength combined with good ductility can be achieved by optimizing the combination of twin spacing in different layers of twin lamellae. Regarding to metals with different twin orders, the variation of mechanical properties with the twin spacing is different, and relatively good mechanical performance can also be achieved in materials with low twin order by adjusting the twin spacing, and it could even be better than that in metals with higher twin orders. Besides, grain size also contributes to the mechanical properties, there may be a critical grain size at which the ductility in the hierarchically nanotwinned metal reaches the peak value, and the relationship between the mechanical property and the twin spacing would change in the case of extremely small grain size. Related analysis is of great importance for the material design in which the optimization of mechanical properties is achieved and the manufacture difficulty can be reduced.

Digital twin-enabled Graduation Intelligent Manufacturing System for fixed-position assembly islands

Abstract The layout of fixed-position assembly islands is widely used in the heavy equipment industry, where the product remains at one assembly island for its entire assembly period, while required workers, equipment, and materials are moved to the island according to the assembly plan. Such layout is not only suitable for producing bulky or fragile products, but also offers considerable flexibility and competitive operational efficiency for products with medium variety and volumes. However, due to inherent complexity of the product, sophisticated assembly operations heavily rely on skilled operators, and the complexity and uncertainty are high and amplified by such massive manual interventions as well as the unique routing patterns of the fixed-position assembly process. Aiming at reducing the complexity and uncertainty, this paper introduces a digital twin-enabled Graduation Intelligent Manufacturing System (DT-GiMS) for fixed-position assembly islands. Inspired by the success of graduation ceremony, an assembly system-Graduation Manufacturing System (GMS) is proposed for fixed-position assembly islands, in which job tickets, setup tickets, operation tickets, and logistics tickets are designed to organize the production activities. Following the concept of digital twin, unified digital representations with appropriate sets of information are created at object level, product level, and system level, respectively. Through Internet of Things (IoT), smart gateway, Web 3D and industrial wearable technologies, vital information including identity, status, geometric model, and production process can be captured and mapped in physical space, and converged and synchronized with their digital representations in twin (cloud) space on a real-time basis. The overall framework of DT-GiMS is presented with physical layer, digital layer, and service layer. Real-time convergence and synchronization among them ensure that right resources are allocated and utilized to the right activities at the right time with enhanced visibility. Considering customer demand and production capacity constraints, real-time ticket pool management mechanisms are proposed to manage production activities in a near-optimal way under DT-GiMS. With the support of cloud-based services provided in service layer in DT-GiMS, managers could easily make production decisions, and onsite operators could efficiently complete their daily tasks with nearly error-free operations with enhanced visibility. A demonstrative case is carried out to verify the effectiveness of the proposed concept and approach.

The structural rearrangement with secondary reinforcement in graphene/nanotwinned copper nanocomposites: A molecular dynamics study

The incorporation of graphene into nanotwinned copper is a promising approach to obtain a material with superior mechanical properties, while related research is rarely reported. In this study, we design several graphene/nanotwinned copper nanocomposites and investigate their compressive behaviors by molecular dynamics simulation. An unusual structural rearrangement is observed under compression at an atomic level. More specifically, graphene provides a supporting skeleton for the lattice rotation of nanotwinned copper under certain conditions, resulting in the annihilation of dislocations with the recovery of elasticity. Such a rearrangement process improves the strength and durability of the nanocomposites because of the intensive support provided by periodic graphene wrinkles and new twin boundaries. The interaction between graphene and nanotwinned copper matrix can be enhanced by decreasing twin spacing and introducing multilayer graphene. The symmetrical lattice orientation in the matrix, parallel graphene-twin boundaries, and a suitable compression direction collectively contribute to a perfect structural rearrangement.