Theoretical Prediction of Strengthening in Nanocrystalline Cu with Multi-Element Grain Boundary Segregation Decoration.

The composition of grain boundaries (GBs) determines their mechanical behavior, which in turn affects the mechanical properties of nanocrystalline materials. Inspired by GB segregation and the concept of high-entropy alloys (HEAs), we investigated, respectively, the mechanical responses of nanocrystalline Cu samples with and without multi-element GBs, as well as the grain size effects, aiming to explore the effects of GB composition decoration on mechanical properties. Our results show that introducing multi-element segregation GBs can significantly improve the mechanical properties of nanocrystalline Cu by effectively inhibiting GB migration and sliding. Additionally, we proposed an improved a theoretical model that can reasonably describe the strengths of the materials with multi-element or single-element segregation GBs. Notably, the introduction of multi-element segregation GBs inhibits both migration and sliding behavior, with migration being more effectively suppressed than sliding. These results present a novel approach for designing high-performance nanometallic materials and offer valuable insights into the role of GB composition decoration in enhancing mechanical properties.

Mechanism analysis of the carrier viscosity effect on shear stress of magnetorheological fluids.

Shear stress is an important index to evaluate the rheological behavior of magnetorheological fluids (MRFs), which is not only related to the properties of ferromagnetic particles, but also the viscosity of the carrier. However, the research related to the carrier viscosity is quite lacking, and the mechanism of its effect on shear stress is still unclear. In this work, the carrier viscosity effect on the microstructure of MRFs under shearing was investigated via numerical simulations, and the relationship between chain inclination and carrier viscosity was presented for the first time. It was found that the deflection angle of the chain increases with the increase of carrier viscosity. Based on the simulation results, the relationship between the shear resistance induced by the magnetic field and the deflection angle of the chain was studied. Finally, a constitutive model incorporating the mechanism of the viscosity effect on shear stress was proposed, and the calculated results agreed well with the experimental data. This work provides new insights into the effect of carrier viscosity and can help us to better understand the corresponding microscopic mechanism.

Applications of cellulose-based composites and their derivatives for microwave absorption and electromagnetic shielding.

Cellulose, a shining star of nano-dimensional self-assembled unit materials, has perfect biocompatibility, mechanical toughness, low density, and powerful modification potential characteristics, all of which make cellulose and its derivatives gained wide attention in various applications, especially for the expanding market for microwave absorption (MA) and electromagnetic interference (EMI) shielding. In this paper, the latest research progresses of cellulose and its derivatives in MA and EMI shielding, including the state-of-the-art design concepts, synthetic strategies, and electromagnetic characters, were summarized. Different types of cellulose-based electromagnetic components have been classified according to their electromagnetic mechanism of action (shielding or absorption), filler properties (dielectric, magnetic or electrical conductivity), and structural expression (film or aerogel). The benefits stemming from its applications are analyzed, providing novelties and unique perspectives for relevant research. Finally, the main obstacles and bottlenecks for further applications were analyzed, and the trend and prospects of cellulose material's future research were proposed.

Plastic deformation and strengthening mechanism of FCC/HCP nano-laminated dual-phase CoCrFeMnNi high entropy alloy.

FCC-structured CoCrFeMnNi high entropy alloy (HEA) has attracted abroad interests for years because of its excellent mechanical properties, except for strength. Recent experiments have reported a kind of nano-laminated dual-phase (NLDP) FCC/HCP structure that can strengthen the HEA. However, it is still unknown why the HEA can be strengthened by this kind of NLDP structure. Here, we employ molecular dynamics simulations to study the atomistic strengthening mechanism of the NLDP HEA. Dislocation-assisted multiple plastic deformation mechanisms in both FCC and HCP single phase HEAs are observed, and amorphization is also found in the plasticity of HCP phase, which are consistent with the previous experimental characterizations. The HCP phase possesses higher strength because of its higher stacking fault energy, higher Peierls-Nabarro stress and less active dislocation slip systems. It is also found that the introduction of HCP phase can enhance the mechanical properties, including yield stress, yield strain and plastic flow stress, of the NLDP HEAs, which also show volume fraction dependence. And the phase boundary plays crucial roles in the deformation and strengthening of the NLDP HEAs. The plastic deformation of the NLDP HEAs can be divided into two stages, i.e. stage I (plasticity only appears in FCC lamella) and stage II (plasticity in both FCC and HCP lamellas). With the increase of volume fraction, the lamella thickness of FCC matrix phase decreases, leading to continuous strengthening of yield properties and flow stress of stage I because of suppressed dislocation nucleation and confined dislocation motion in FCC matrix phase by the phase boundary. While there is no monotonous relationship between the flow stresses of stage II and the increasing volume fraction of HCP phase, which can be attributed to the competitive mechanisms between strengthening effect of phase boundary on the dislocation motion in FCC phase and softening effect of phase boundary on the dislocation motion in HCP phase. The results should be helpful for understanding the underlying physical mechanism of strengthening of HEAs with NLDP structure.

Effects of Anisotropy and In-Plane Grain Boundary in Cu/Pd Multilayered Films with Cube-on-Cube and Twinned Interface.

In crystalline materials, grain boundary and anisotropy of crystal structure affect their mechanical properties. The effects of interfacial structure on the mechanical properties may be diverse when the multilayer film is loaded along different directions. In this work, we performed a series of molecular dynamics simulations of the tension of in-plane single and polycrystalline Cu/Pd multilayered films with cube-on-cube (COC) and twinned interfaces to explore the effects of the interfacial structure, loading direction and in-plane grain boundaries on their mechanical properties. The interfacial misfit dislocation lines become bent after relaxation, and the high temperature of 300 K was found as a necessary condition. When stretched along 〈110〉 direction, the strengthening effect of the COC interface is more noticeable; however, when stretched along 〈112〉 direction, the twin interface's strengthening effect is more visible, showing the anisotropic effect of interfacial structure on mechanical properties. However, in the in-plane honeycomb polycrystalline sample, the twin interface showed a pronounced strengthening effect, and no jogged dislocations were observed.

Plastic Deformation and Hardening Mechanisms of a Nano-twinned Cubic Boron Nitride Ceramic.

A very interesting experimental finding shows that a nano-twinned cubic boron nitride (NT-cBN) ceramic has size-dependent hardness. In order to reveal the hardening mechanism of NT-cBN, the plastic deformation mechanism of single-crystalline cubic boron nitride (SC-cBN) under nano-indentation is first studied, and then that of NT-cBN is further investigated using atomistic simulations with a parameter-modified Tersoff potential. It is found that the plastic deformation of SC-cBN under nano-indentation is mainly attributed to serial dislocation behaviors, such as the formation of dislocation embryos, shear loops, and prismatic loops. In comparison, for NT-cBN, the plastic behavior is much more complex, which is influenced by a dislocation blockage, absorption, dissociation, and re-nucleation due to the interaction between dislocations and twin boundaries (TBs). From the plastic deformation mechanism of NT-cBN, it is found that the size-dependent hardening behavior of NT-cBN is a competitive result between the hardening sources, including slip transfer, dislocation accumulation, and suppression of dislocation nucleation, and the softening sources, including TBs being destroyed, parallel slips of dislocations, and the formation of new sites for dislocation nucleation. The smaller the distance between the adjacent TBs, the more dominant the role of hardening sources is, resulting in the high size-dependent hardness of NT-cBN. The results in this paper should be helpful for the optimized design of high strength and toughness of nano-structured cBN ceramics.

A quick measuring method for yield temperatures of materials under constant prestress at ultra-high temperatures.

A new method for the rapid measurement of the yield temperatures of the material subjected to constant prestress at ultra-high temperatures was proposed, combining electric heating with non-contact image measurement technology. In this method, the yield temperature could be determined with a small number of thermal cycles in which the peak temperature increases with an increase in the number of thermal cycles, and in each cycle, the material is heated to a prescribed peak temperature and then cooled. There are two pairs of tiny sharp protrusions in the central part of the specimen. During the test, the surface optical image is recorded and analyzed with code Halcon to determine the temperature and elongation of the specimen. The time of each test is less than 20 min, so the influence of over-heating on the properties and microstructure of the material can be avoided. It is worth noting that the developed method can effectively reduce the test time and cost without breaking the specimen, and should have great potential in testing for the mechanical properties of materials at ultra-high temperatures.

Repeated Thermal Shock Behavior of ZrB2-SiC-Graphite Composite under Pre-Stress in Air and Ar Atmospheres.

The thermo-chemo-mechanical coupling on the thermal shock resistance of 20 vol%-ZrB2-15 vol%-SiC-graphite composite is investigated with the use of a self-developed material testing system. In each test, a specimen under prescribed constant tensile pre-stress (σ0 = 0, 10, 20 and 30 MPa) was subjected to 60 cycles of thermal shock. In each cycle, the specimen was heated from room temperature to 2000 °C within 5 s in an air atmosphere or an Ar atmosphere. The residual flexural strength of each specimen was tested, and the fracture morphology was characterized by using scanning electron microscopy (SEM). There were three different regions in the fracture surface of a specimen tested in the air, while no such difference could be observed in the fracture surfaces of the specimens that were tested in Ar. The residual flexural strength of the composite that was tested in Ar generally decreases with the increase of σ0. However, in the range of 0 ≤ σ0 ≤ 10 MPa, the residual flexural strength of the composite that was tested in the air ascended with the increase of σ0 due to the healing effect of oxidation, but it descended thereafter with a further increase of σ0, as the effect pre-stress that became prominent.

Inapparent Strengthening Effect of Twin Interface in Cu/Pd Multilayered Films with a Large Lattice Mismatch.

It has been found that there are two kinds of interfaces in a Cu/Pd multilayered film, namely, cube-on-cube and twin. However, the effects of the interfacial structure and modulation period on the mechanical properties of a Cu/Pd multilayered film remain unclear. In this work, molecular dynamics simulations of Cu/Pd multilayered film with different interfaces and modulation periods under in-plane tension are performed to investigate the effects of the interfacial structure and modulation period. The interface misfit dislocation net exhibits a periodic triangular distribution, while the residual internal stress can be released through the bending of dislocation lines. With the increase of the modulation period, the maximum stress shows an upward trend, while the flow stress declines. It was found that the maximum stress and flow stress of the sample with a cube-on-cube interface is higher than that of the sample with a twin interface, which is different from the traditional cognition. This unusual phenomenon is mainly attributed to the discontinuity and unevenness of the twin boundaries caused by the extremely severe lattice mismatch.

Super Ductility of Nanoglass Aluminium Nitride.

Ceramics have been widely used in many fields because of their distinctive properties, however, brittle fracture usually limits their application. To solve this problem, nanoglass ceramics were developed. In this article, we numerically investigated the mechanical properties of nanoglass aluminium nitride (ng-AlN) with different glassy grain sizes under tension using molecular dynamics simulations. It was found that ng-AlN exhibits super ductility and tends to deform uniformly without the formation of voids as the glassy grain size decreases to about 1 nm, which was attributed to a large number of uniformly distributed shear transformation zones (STZs). We further investigated the effects of temperature and strain rate on ng-AlNd = 1 nm, which showed that temperature insignificantly influences the elastic modulus, while the dependence of the ultimate strength on temperature follows the T2/3 scaling law. Meanwhile, the ultimate strength of ng-AlNd = 1 nm is positively correlated with the strain rate, following a power function relationship.

Anisotropic Phase Transformation in B2 Crystalline CuZr Alloy.

B2 phase copper-zirconium (CuZr) particles are often used as an enhancement agent to improve the toughness of metallic glass; however, the orientation dependence of its phase transformation behaviors under loading remains unclear. In this work, molecular dynamics simulation of uniaxial tension and compression of B2 phase CuZr along different crystallographic orientation are performed to investigate the orientation-related mechanical response and phase transformation mechanisms. It was found that the mechanical behavior of CuZr exhibits obvious tension/compression asymmetry, but their failure mode is mainly local amorphization. Three different phase transformation behaviors, B2→FCC, B2→BCT, and B2→HCP, were observed in tension and compression along [001], and tension along [110], respectively. The transformations are realized by lattice rotation (~ 5°), uniform deformation and separation between Cu and Zr atomic layers, respectively. Before failure by local amorphization, phase transformation region can be recovered after unloading, showing the superelasticity.

Detwinning Mechanism for Nanotwinned Cubic Boron Nitride with Unprecedented Strength: A First-Principles Study.

Synthesized nanotwinned cubic boron nitride (nt-cBN) and nanotwinned diamond (nt-diamond) exhibit extremely high hardness and excellent stability, in which nanotwinned structure plays a crucial role. Here we reveal by first-principles calculations a strengthening mechanism of detwinning, which is induced by partial slip on a glide-set plane. We found that continuous partial slip in the nanotwinned structure under large shear strain can effectively delay the structural graphitization and promote the phase transition from twin structure to cubic structure, which helps to increase the maximum strain range and peak stress. Moreover, ab initio molecular dynamics simulation reveals a stabilization mechanism for nanotwin. These results can help us to understand the unprecedented strength and stability arising from the twin boundaries.

Higher Strength and Ductility than Diamond: Nanotwinned Diamond/Cubic Boron Nitride Multilayer.

Recently, the nanotwinned structure has attracted considerable attention because of unprecedented improvement in its mechanical properties, thermal stability, and other properties. Here, we introduce the nanotwinned structure between two superhard materials [diamond and cubic boron nitride (cBN)] and obtain a nanotwinned diamond/cBN multilayered material with ultrahigh strength and unprecedented ductility. Under continuous shear deformation, the stress and total energy in the material develop in a zigzag way because of atomic reconfiguration. Further research shows that atomic reconfiguration occurs preferentially in the cBN region, followed by that in the diamond region by partial slip, and finally occurs at the interface through alternate "exchange" of the positions of C and B atoms. This multilevel stress release model can account for the significant increase in the strain range and peak stress of nanotwinned materials. These results could provide available information for the design of superhard materials with multilevel resistance to plastic deformation.

Molecular Dynamics Simulation on B3-GaN Thin Films under Nanoindentation.

The B3-GaN thin film was investigated by performing large-scale molecular dynamics (MD) simulation of nanoindentation. Its plastic behavior and the corresponding mechanism were studied. Based on the analysis on indentation curve, dislocation density, and orientation dependence, it was found that the indentation depths of inceptive plasticity on (001), (110), and (111) planes were consistent with the Schmid law. The microstructure evolutions during the nanoindentation under different conditions were focused, and two formation mechanisms of prismatic loop were proposed. The "lasso"-like mechanism was similar to that in the previous research, where a shear loop can translate into a prismatic loop by cross-slip; and the extended "lasso"-like mechanism was not found to be reported. Our simulation showed that the two screw components of a shear loop will glide on another loop until they encounter each other and eventually produce a prismatic dislocation loop.

Molecular dynamics study of strengthening mechanism of nanolaminated graphene/Cu composites under compression.

Molecular dynamics simulations of nanolaminated graphene/Cu (NGCu) and pure Cu under compression are conducted to investigate the underlying strengthening mechanism of graphene and the effect of lamella thickness. It is found that the stress-strain curves of NGCu undergo 3 regimes i.e. the elastic regime I, plastic strengthening regime II and plastic flow regime III. Incorporating graphene monolayer is proved to simultaneously contribute to the strength and ductility of the composites and the lamella thickness has a great effect on the mechanical properties of NGCu composites. Different strengthening mechanisms play main role in different regimes, the transition of mechanisms is found to be related to the deformation behavior. Graphene affected zone is developed and integrated with rule of mixtures and confined layer slip model to describe the elastic properties of NGCu and the strengthening effect of the incorporated graphene.

Investigation of Interaction between Dislocation Loop and Coherent Twin Boundary in BCC Ta Film during Nanoindentation.

In this work, the interaction between dislocation loop (DL) and coherent twin boundary (CTB) in a body-centered cubic (BCC) tantalum (Ta) film during nanoindentation was investigated with molecular dynamics (MD) simulation. The formation and propagation of <111> full DLs in the nanotwinned (nt) Ta film during the indentation was observed, and it was found that CTB can strongly affect the stress distribution in the Ta film, and thus change the motion and type of dislocations. There are three kinds of mechanisms for the interaction between DL and CTB in a twinned BCC Ta film: (i) dislocation absorption, (ii) dislocation desorption, and (iii) direct slip transmission. The nucleation of twin boundary dislocations and the formation of the steps in CTB were also observed during the indentation. The mechanisms presented in this work can provide atomic images for understanding the plastic deformation of BCC metals with mirror-symmetry grain boundary structures, and provide available information for the evaluation and design of high-performance nt BCC metallic thin film coatings.

In-plane anisotropy and twin boundary effects in vanadium nitride under nanoindentation.

Twin boundaries (TBs) have been observed in and introduced into nonmetallic materials in recent years, which brought new concepts for the design of new structural materials. However, the roles of TB on the mechanical properties and strengthening/softening of transition metal nitrides remain unclear. To investigate the TB effects and the in-plane anisotropy, nanoindentations on VN (111) films with and without TB were simulated with molecular dynamics, in which a cylindrical indenter was used, and its longitudinal axis were assigned along <112> and <110>, respectively. We found that the effect of the indenter orientation is insignificant in the elastic stage, but significant in the following inelastic deformation. Different deformation mechanisms can be found for inelastic deformation, such as twinning and dislocation glide. The migration of TB can be observed, which may release the internal stress, resulting in softening; while the dislocation locking and pileup at TB can enhance the strength. We also found that the strengthening/softening induced by TB depends on the deformation mechanisms induced by indenter directions.

Molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter.

We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness. We found that both twinning partial slip (TPS) and partial slip parallel with twin boundary (PSPTB) can reduce hardness and therefore should not be ignored when evaluating mechanical properties at nanoscale. There is a critical range of twin thickness λ (~25 Å < λ < ~31 Å), in which hardness of the multilayer films is maximized. At a smaller λ, TPSs appear due to the reaction between partial dislocations and twin boundary accounts for the softening-dominated mechanism. We also found that the combination of the lowered strengthening due to confined layer slips and the softening due to TPSs and PSPTBs results in lower hardness at a larger λ.

Tomographic reconstruction of damage images in hollow cylinders using Lamb waves.

Lamb wave tomography (LWT) is a potential and efficient technique for non-destructive tomographic reconstruction of damage images in structural components or materials. A two-stage inverse algorithm proposed by the authors for quickly reconstructing the damage images was applied to hollow cylinders. An aluminum hollow cylinder with an internal surface pit and a Carbon Fiber Reinforced Plastic (CFRP) laminated hollow cylinder with an artificial internal surface damage were used to validate the proposed method. The results show that the present method is capable of successfully reconstructing the images of the above damages in a larger inspection area with much less experimental data compared to some conventional ultrasonic tomography techniques.