Lattice Dislocations Research Articles

Influence of temperature on twinning dominated pop-ins during nanoindentation of a magnesium single crystal

The present paper examines the temperature sensitivity of tensile twinning in a magnesium single crystal during nanoindentation of the prismatic plane. High temperature indentations from 25 °C to 250 °C were employed on a well polished magnesium single crystal {10-10} plane. For a indentation curve displaying a pop-in, a single twin was seen on the sample surface using Atomic Force Microscopy (AFM) imaging. For indentations that produced no pop-in, no twinning was observed. We thus conclude the pop-in arises from a twinning event in the present case. With increasing temperature, the mean pop-in load (measured from 200 repeat indentations of each testing temperature) drops markedly. This is interpreted by the thermal activation of nucleation of lattice dislocations, which immediately trigger a twinning event. Thermal activation analysis yields activation energies that are consistent with this idea. With increasing temperature the pop-ins became deeper and the twins, after further indentation, showed more growth. It is likely that non-basal slip is activated in the stress concentrations that arise during twinning and the thermal activation of this slip accounts for the observed temperature effects. It is concluded that in interpreting the temperature sensitivity of twinning stresses, any associated lattice dislocation activity must be considered.

Исследование особенностей высокотемпературной деформации керамик из чистого карбида вольфрама с различным размером зерна

The mechanism of high-temperature creep deformation during compression tests of binderless tungsten carbide specimens with different initial particle size was studied. Tungsten carbide samples with high relative density (96.1 – 99.2 %) were obtained by high-speed spark plasma sintering (SPS) from nano-, submicron-, and micron-sized α-WC powders. The creep tests were carried out in two modes: isothermal soaking at different temperatures (1300 – 1375 °C) at a given stress, allowing to estimate the activation energy of creep, and tests by “stress jumps” at 1325 °C, allowing to estimate the value of the coefficient n in the creep equation. It is shown that the value of creep activation energy in ultrafine grained tungsten carbide with grain size ~ 0.15 µm sintered from plasma chemical nanopowders is ~ 31 kTm. This value is 1.5 – 2 times higher than the creep activation energy in fine-grained tungsten carbide samples obtained by SPS from submicron (~ 0.8 μm) and micron (~ 3 μm) industrial powders. It was found that the value of the coefficient n varies from 2.4 to 3.1, which corresponds to the case of motion of lattice dislocations in the field of uniformly located point obstacles. It has been suggested that one of the reasons for the increase in creep activation energy in tests of tungsten carbide is an increased volume fraction of W2C low carbide particles formed during high-speed sintering of plasma-chemical α-WC nanopowders with an increased concentration of adsorbed oxygen.

Porous Bismuth Nanoflowers Enriched with Lattice Dislocations for Highly Efficient Electrocatalytic Reduction of Carbon Dioxide to Formate※

Porous Bismuth Nanoflowers Enriched with Lattice Dislocations for Highly Efficient Electrocatalytic Reduction of Carbon Dioxide to Formate^※

ИССЛЕДОВАНИЕ ВЛИЯНИЯ СКОРОСТИ ДЕФОРМАЦИИ НА СКЛОННОСТЬ УГЛЕРОДИСТОЙ СТАЛИ У8 К ДЕФОРМАЦИОННОМУ СТАРЕНИЮ

The effects of the strain rate and degree on the scale of the strain aging effect in carbon steel U8 with lamellar pearlite microstructure, which is a eutectoid mixture of ferrite plates and brittle plates of secondary cementite Fe3C have been investigated. The microstructure and properties of Russian steel U8 after quasistatic compression, dynamic compression (shock processing), and explosion processing have been investigated. A low-temperature annealing (150–500 °C) of the non-deformed steel U8 has been shown to result in a monotonous reduction of hardness. The quasistatic compression and shock processing of the carbon steel were found to result in the strain aging manifested as an increasing hardness at annealing of the deformed steel U8. The increasing of the deformation degree and strain rate was accompanied by the increasing of hardness of the annealed steel. The explosion treatment was shown not to result in a notable improvement of the mechanical properties of carbon steel as well as in a change of the morphology of cementite plates. The deformation aging effect in steel U8 after the explosion loading was absent. The strain aging effect in U8 steel after quasistatic compression and dynamic loading was suggested to originate from an increasing of the lattice dislocation density, the long-range internal stress fields from which lead to the dissociation of the cementite Fe3C plates and to increasing of the concentration of carbon atoms in the ferrite crystal lattice. The absence of the strain aging effect in the carbon steel after the explosion loading is due to a small degree of the explosion deformation that doesn't allow providing an essential increasing of the lattice dislocation density in ferrite.

Atomistic modeling of interface strengthening in Al-Si eutectic alloys

Al-Si cast alloys are usually composed of α-Al and Al-Si eutectic. Si flakes and Al matrix generally hold cube-on-cube orientation relationship with the primary interface (111)Al∥(111)Si. Extensive experimental studies demonstrated that Si flakes cannot significantly improve mechanical properties of Al-Si cast alloys. We hypothesize that the weak strengthening effect associated with Si flakes might be attributed to thermomechanical properties of Al-Si interfaces besides their morphologies. To characterize Al-Si interfaces with a large lattice mismatch (> 30%), we proposed the quasi-coincident site lattice (Q-CSL) as reference lattice, and demonstrated that the Q-CSL Al-Si coherent interface has three characteristic coherent structures, one stable and low energy structure and two metastable and high energy structures. The translation vectors for the same type of coherent Q-CSL structures are consistent with three displacement shift complete (DSC) vectors. The two metastable structures can be obtained by shifting the low energy structure with three partial DSC vectors. Semi-coherent interface is composed of the low energy Q-CSL patches and three sets of interface misfit dislocations with Burgers vectors same as the DSC vectors. Atomistic simulations revealed that Al-Si interface exhibits low shear resistance. Ideal shear strength of the Q-CSL coherent interface is 110 MPa and semi-coherent interface is 20 MPa. The low shear resistance is attributed to the glide of interface misfit dislocations. Al-Si interface also exhibits low formation and migration energies of point defects. Owing to low shear strength and low formation and migration energies of point defects, interface sliding or shear readily happen under mechanical loading or during dislocation-interface interactions. Lattice dislocations can cross slip onto or climb along Al-Si interfaces. These reactions decrease the number of accumulated dislocation loops around Si flakes and promote nucleation and emission of lattice dislocations from Al-Si interfaces to matrix, consequently reduce the repulsive force on approaching dislocations and weaken Si flakes strengthening effect. In situ tension and compression tests in a scanning electron microscope reveal relatively weak strengthening effect due to Si flakes, consistent with the computed dislocation interaction with interfaces and shear behavior of interfaces.

Lattice-dislocated Bi nanosheets for electrocatalytic reduction of carbon dioxide to formate over a wide potential window

Electrochemical reduction of CO2 to HCOOH (ERC-HCOOH) is one of the most feasible and economically valuable ways to achieve carbon neutrality. Unfortunately, achieving optimal activity and selectivity for ERC-HCOOH remains a challenge. Herein, ultrathin Bi nanosheets (NS) with lattice dislocations (LD-Bi) were prepared by the topological transformation of Bi2O2CO3 NS under high current conditions. LD-Bi exhibited excellent activity and selectivity as well as stability in ERC-HCOOH. Electrochemical tests and DFT calculations revealed that the excellent performance of LD-Bi was attributed to lattice dislocations, which can induce the production of more active sites on the catalyst surface and improve the electronic transfer ability. In addition, LD-Bi was beneficial to enhance the adsorption of CO2 and key reaction intermediates (OCHO*), thus improving the reaction kinetics. The result provides a unique perspective on the crucial role of lattice dislocations, which may have a significant impact on highly selective electrochemical conversion of CO2.

Probing lattice defects in crystalline battery cathode using hard X-ray nanoprobe with data-driven modeling

Lattice defects, e.g., dislocations and grain boundaries, critically impact the properties of crystalline battery cathode materials. A longstanding challenge is to probe the meso‑scale heterogeneity and evolution of lattice defects with sensitivity to atomic-scale details. Herein, we tackle this issue with a unique combination of X-ray nanoprobe diffractive imaging and advanced machine learning techniques. The domains with different lattice defect configuration within a single-crystalline LiCoO2 cathode particle are faithfully revealed using our approach. We further visualize the rearrangement of grain boundaries and local crystallinity upon mild thermal annealing. These results pave a direct way to the understanding of crystalline battery materials’ response under external stimuli with high fidelity, which provides valuable empirical guidance to defect-engineering strategies for improving the cathode materials against aggressive battery operation.

Differential Geometry Approach to Continuous Model of Micro-Structural Defects in Finite Elasto-Plasticity

This paper concerns finite elasto-plasticity of crystalline materials with micro-structural defects. We revisit the basic concepts: plastic distortion and decomposition of the plastic connection. The body is endowed with a structure of differential manifold. The plastic distortion is an incompatible diffeomorphism. The metric induced by the plastic distortion on the intermediate configuration (considered to be a differential manifold) is a key point in the theory, in defining the defects related to point defects, or extra-matter. The so-called plastic connection is metric, with plastic metric tensor expressed in terms of the plastic distortion and its adjoint. We prove an appropriate decomposition of the plastic connection, without any supposition concerning the non-metricity of plastic connection. All types of the lattice defects, dislocations, disclinations, and point defects are described in terms of the densities related to the elements that characterize the decomposition theorem for plastic connection. As a novelty, the measure of the interplay of the possible lattice defects is introduced via the Cartan torsion tensor. To justify the given definitions, the proposed measures of defects are compared to their counterparts corresponding to a classical framework of continuum mechanics. Thus, their physical meanings can be emphasized at once.

Octacalcium phosphate crystals including a higher density dislocation improve its materials osteogenecity

Herein, we show that the enhanced osteogenecity of octacalcium phosphate (OCP) biomaterial, recently identified as an important element in hybrid organic–inorganic nanocomposites involved in the initial hydroxyapatite crystal expansion in mammal bones, results from an enhanced chemical property, stemming from the presence of lattice strain and dislocations. Two types of OCPs were synthesized by wet-chemical processing in the presence (c-OCP) and absence (w-OCP) of gelatin, respectively, and subjected to structural, chemical, and biological analyses. High-resolution transmission electron microscopy (HRTEM) and fast Fourier transform (FFT) analyses revealed that c-OCP crystals contained approximately six times higher edge dislocations with Burgers vectors perpendicular to a-axis than that in the case of w-OCP. The dissolution of c-OCP crystal in tris-HCl buffer occurred toward the long axis of the crystal, most likely, toward the lattice strain along the c-axis direction, while w-OCP crystal dissolved toward the a-axis direction. The study suggested that the increment of internal energy by the higher dislocation density contributed promoting c-OCP dissolution and hydrolysis through decreasing the activation energy. c-OCP crystal displayed enhanced in vitro mesenchymal stem 2D cell and 3D spheroid differentiation, in vivo bone formation, and apatite crystallographic orientation in critical-sized rat calvarial defect model as compared to w-OCP crystal, at the same time, converting to apatite structure earlier than w-OCP. The present study demonstrates that dislocation-related dissolution along with enhanced conversion of OCP is a determinant in bone induction, which may be relevant to normal bone development utilizing OCP biomaterials.

Investigations the structural, optical and photovoltaic properties of La doped TiO2 photoanode based dye sensitized solar cells

Films of TiO 2 , La-doped TiO 2 (La–TiO 2 ), La–TiO 2 /TiO 2 and TiO 2 /La–TiO 2 are prepared by sol-gel (dip coating) route. The structural properties like phase, grain size, dislocation line density, inter-planner distance, and lattice parameters are calculated by X-ray diffractometer (XRD). All films have anatase phase which is good for solar cells application. Film of La-doped TiO 2 /TiO 2 has high grain size. The optical properties like transmittance, absorbance, refractive index, extinction coefficient, dielectric constants and band gap are measured by UV–Vis spectroscopy. The film of La-doped TiO 2 /TiO 2 has low bandgap and high transmittance in visible range making it a good candidate for dye sensitized solar cells (DSSCs). DSSCs from these electrodes are prepared by using electrolyte (iodide/tri-iodide) and dye (N719). 1% La–TiO 2 /TiO 2 based cell has highest open-circuit voltage (0.74 V), Fill-Factor (0.73), short-circuit current density (6.57 mA/cm 2 ), and efficiency (3.53%) as compared to other cells. Low value of bandgap energy, weak recombination rate, improvement in charge transfer rate is mainly responsible to achieve highest efficiency rate (3.53%). • Bi-layers films of TiO 2 and La-TiO 2 are deposited by sol-gel technique. • 1% La-TiO 2 /TiO 2 film has large grain size, small E g and high transmittance in the visible region. • 1% La-TiO 2 /TiO 2 film has maximum efficiency (3.53%).

The role of a shear planar mesodefect in the nucleation of a crack at a grain junction due to athermal grain boundary sliding

Ductile fracture of polycrystalline metals is usually preceded by a long deformation stage, during which grains of polycrystal are gradually divided into mutually misoriented regions (fragments) separated by strain-induced grain boundaries. The size of these regions usually does not exceed 0.2 – 0.4 mm. At such small sizes of fragments, classical models of crack nucleation based on the concept of lattice dislocation pile-ups retarded by grain boundaries become incorrect. In recent years, models have been developed to describe the nucleation of cracks under the action of elastic stress fields of rotational and shear-type mesodefects formed at the grain junctions and grain boundaries due to inhomogeneous plastic deformation in the ensemble of polycrystal grains. In this paper we consider the possibility of the nucleation of microcracks at the grain junction due to athermal sliding along the grain boundary, containing a planar mesodefect of the shear-type induced by strain. It is assumed that a planar mesodefect, represented in the initial state by uniformly distributed glissile components of orientational misfit dislocations, loses its stability when the external stress exceeds a certain threshold value. As a result of the strain-induced grain boundary sliding and plastic shear retarding, a stress concentrator arises near the triple junction of grains, creating conditions for the appearance of a Zener crack. The dependences of the critical external stress for the nucleation of microcracks on the length of the mesodefect, its strength and the threshold stress of athermal sliding are obtained. It is shown that the presence of a mesodefect at the grain boundary can lead to a significant decrease of the crack nucleation stress in comparison with the case of pure grain boundary sliding. It is concluded that the proposed model can be considered as one of the possible mechanisms for the nucleation of microcracks in materials with a fragmented structure.

Electroshock treatment dependent microstructural evolution and mechanical properties of near-β titanium alloy manufactured by directed energy deposition

Effects of electroshock treatment (EST) on the microstructural evolution and mechanical properties of near-β titanium alloy (Ti-55531) formed by directed energy deposition (DED) was studied in this work. With the increase in EST time, the average hardness of specimen decreased from 426 HV to 316 HV, and the fracture strain increased significantly, which was attributed to the uniform dispersion of α phase along grain boundaries and inside the β grains. After EST, the texture intensity decreased in terms of the orientation distribution function (ODF), which was ascribed to the redistribution of α phase. Moreover, more atomic vacancies and lattice distortion were formed near the α/β interfaces, which were verified by transmission electron microscopy (TEM) observation and ascribed to the migration of atoms near the interface under EST. External loadings facilitated the dislocation motion and lattice distortions near the interfaces, which resulted in the reduction in hardness and the improvement in ductility. The above results indicated that EST can quickly alter the microstructure and mechanical properties of DED titanium alloys as a simple and energy-saving method.

Genesis of visible gold in pyrite in the Zhaoxian gold deposit, Jiaodong gold province, China: Constraints from EBSD micro-structural and LA-ICP-MS elemental analyses

Pyrite (FeS2) is one of the most common hosts for gold (Au) in different types of gold deposits, including visible and invisible gold. For visible gold in pyrite, it remains unclear whether it resulted from remobilization of invisible gold in the gold-hosting pyrite or from external input. This paper reports a case study from the Zhaoxian gold deposit, which is the deep extension of the well-known Jiaojia gold deposit, in the Jiaodong gold province. The texture and composition of Au-bearing pyrite from the Zhaoxian deposit were studied with petrographic, Electron Back-Scattered Diffraction (EBSD) mapping, and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) methods. Four types of pyrite (Py-1 to Py-4), corresponding to four generations of veins, i.e., the pre-ore V1, syn-ore V2 and V3, and post-ore V4, were distinguished. Py-2 is volumetrically the most abundant among the four, and is the main host of visible gold. LA-ICP-MS analyses of pyrite indicate that gold contents in most analysis points are below the detection limit, with measured Au contents ranging from 0.02 to 0.17 ppm (median 0.09 ppm) in Py-1, and 0.03 to 2.45 ppm (median 0.07 ppm) in Py-2. EBSD analyses indicate that pyrite (Py-2) display brittle deformation features including microstructures, lattice dislocation, and grain boundary displacement. Visible gold in Py-2 occurs in micro-fractures, cavities, and grain boundaries. The generally low contents of Au in pyrite, including Py-2, and the brittle deformation features, suggest that the visible gold grains did not result from remobilization of invisible gold from pyrite. Rather, the majority of gold was introduced by fluids after the formation of the gold-hosting pyrite. These fluids were derived from the same sources as those precipitating the gold-hosting pyrite, but were more evolved and enriched in Au. Circulation of these auriferous ore-forming fluids in microfractures of pyrite and interstitial space resulted in precipitation of visible gold.

Temperature-dependent universal dislocation structures and transition of plasticity enhancing mechanisms of the Fe40Mn40Co10Cr10 high entropy alloy

The FCC Fe40Mn40Co10Cr10 (at.%) high entropy alloy exhibits deformation twinning for room temperature deformation and deformation-induced HCP transformation for subzero deformation. Since the systematic investigation of temperature-dependent dislocation structures is not available, we present an in-depth characterization of the defects involved in deformation at room temperature (298 K) and subzero temperature (223 K) of the cold-rolled and annealed Fe40Mn40Co10Cr10 alloy. The material deformed at 223 K shows a higher strain hardening rate than the material deformed at room temperature while both materials show large ductility, 48% for the 223 K deformation and 55% for the 298 K deformation. The main deformation mechanisms of the investigated HEA include the development of inhomogeneous dislocation structures and interaction between dislocations and deformation twin/mechanically induced HCP martensite. The stacking fault energy measured using TEM weak-beam dark-field imaging of dissociated dislocations is 20±9 mJ/m2 at 298 K. The Fe40Mn40Co10Cr10 alloy exhibiting a positive temperature dependence of SFE leads to a decrease of SFE as deformation temperature decreases from 298 K to 223 K. The decrease of SFE results in the transition from deformation twinning to deformation-induced HCP transformation. Further, at higher strains at 223 K, kink banding of HCP and reverse transformation from HCP to FCC were observed, which could account for strain accommodation and stress relaxation, and the large ductility. The 298 K deformation leads to various types of dislocation structures: Extended dislocations, Taylor lattice of perfect dislocations, dislocation loops, highly dense dislocation walls, cell blocks, and cell structures. The observed dislocation structures at 298 K and 223 K are similar suggesting the minor effect of SFE on dislocation structures.

Impact of Cr substitution on the structural, morphological, dielectric, and electrical properties of MgCo[formula omitted] nanoparticles synthesized via solid-state reaction route

This paper highlights the preparation and characterization of MgCo2O4 and Cr-doped MgCo 2O4 nanoparticles grown using a solid-state reaction route. The crystallinity and crystallographic phase structure have been investigated by the X-ray diffraction (XRD), which evidences the cubic crystal structure of MgCo2O4 with space group Fd3m. The crystallites of pristine MgCo2O4 have an average size of 18.88 nm, as determined by Scherrer’s formula. It exhibits an increment of crystallite size from 18.88 nm to 21.60 nm with Cr ions substitution in the MgCo2O4 matrix. Halder–Wagner (H–W) technique reveals the parallel results of crystallite size followed by the inversed behavior of dislocation density and lattice strain. The study of FESEM illustrates the aggregation of spherical nanostructures with well-dispersed grains. UV–Vis spectroscopy depicts an enhancement in absorption accompanied by blue shift and increase in Eg from 4.22 eV to 4.85 eV after Cr addition. Dielectric measurements have revealed that the dielectric constant is noticed to reduce with the introduction of Cr into the MgCo2O4 crystal framework. The electrical resistivity reduces after Cr impurity is incorporated into MgCo2O4 and has been found to decrease with increasing temperature. Impedance analysis has been carried out on MgCo2O4 and Cr-doped MgCo2O4, and it is observed that Cr doping improves the impedance.

Laser powder bed fusion of 17–4 PH stainless steel: A comparative study on the effect of heat treatment on the microstructure evolution and mechanical properties

17–4 PH (precipitation hardening) stainless steel is commonly used for the fabrication of complicated molds with conformal cooling channels using laser powder bed fusion process ( L -PBF). However, their microstructure in the as-printed condition varies notably with the chemical composition of the feedstock powder, resulting in different age-hardening behavior. In the present investigation, 17–4 PH stainless steel components were fabricated by L -PBF from two different feedstock powders, and subsequently subjected to different combinations of post-process heat treatments. It was observed that the microstructure in as-printed conditions could be almost fully martensitic or ferritic, depending on the ratio of Cr eq /Ni eq of the feedstock powder. Aging treatment at 480 °C improved the yield and ultimate tensile strengths of the as-printed components. However, specimens with martensitic structures exhibited accelerated age-hardening response compared with the ferritic specimens due to the higher lattice distortion and dislocation accumulation, resulting in the “dislocation pipe diffusion mechanism”. It was also found that the martensitic structures were highly susceptible to the formation of reverted austenite during direct aging treatment, where 19.5% of austenite phase appeared in the microstructure after 15 h of direct aging. Higher fractions of reverted austenite activates the transformation induced plasticity and improves the ductility of heat treated specimens. The results of the present study can be used to tailor the microstructure of the L -PBF printed 17–4 PH stainless steel by post-process heat treatments to achieve a good combination of mechanical properties.

On incipient plasticity in the vicinity of grain boundaries in aluminum bicrystals: Experimental and simulation nanoindentation study

The local mechanical behavior near <100> symmetric tilt grain boundaries in aluminum bicrystals were studied by the nanoindentation technique as well as by computer simulations. Experimentally, grain boundaries with misorientation angles 8.7°, 13.8° and 18.8° were examined. Two well pronounced pop-in events were observed on the load – penetration depth curves measured during indentation in the close vicinity of the studied boundaries. The load of the pop-ins, observed close to the boundaries, practically did not differ from that obtained in the grain interior. This was interpreted as evidence that the boundaries with misorientations in the examined angular range do not represent specific sites for sources of lattice dislocations. The load, at which the second pop-ins took place, substantially increased with increasing misorientation angle of the examined boundaries. Quasistatic molecular dynamics simulations were performed to identify the details of the interaction between grain boundary and dislocations generated during indentation. For this purpose, the bicrystals with similar geometry and misorientation angles, as investigated experimentally, were computed. The simulation results showed that the direct transmission of incoming dislocations across the grain boundary was the primary mechanism for the plastic flow transfer past the boundary. The analysis of the results of both experiments and simulations provided evidence that the capability of grain boundaries to act as a barrier for the motion of incoming dislocations depends crucially on grain boundary structure.

Modeling and Analysis of Yielding and Strain Hardening in Metastable High‐Entropy Alloys

Metastable dual‐phase high‐entropy alloys (HEAs) have become an attractive paradigm as structural materials due to their outstanding mechanical properties compared with single‐phase HEAs, which originate from the multiple strengthening mechanisms. Herein, a theoretical model is established by integrating the effects of lattice distortion, dislocation, grain boundary, and phase transformation, to study the mechanical responses of metastable HEAs during uniaxial tensile deformation. The results show the contribution of various mechanisms to the strength, and strain hardening can be determined quantitatively. In particular, the face‐centered cubic (FCC) to body‐centered cubic (BCC) phase transformation would dominate the flow stress with plastic strain to exceed 6%, due to the strong phase interface strengthening and the formation of hard BCC phase. This work provides a microstructure‐based constitutive model for predicting the flow stress and strain hardening properties of metastable HEAs.

Study on wear behavior of FeNiCrCoCu high entropy alloy coating on Cu substrate based on molecular dynamics

The high entropy alloy (HEA) is a potential material for coating because of its high hardness and wear resistance. In order to study the wear performance and protection ability of FeNiCrCoCu HEA coating on Cu substrate, the friction behavior of abrasive particle and material wear and damage were studied by molecular dynamics (MD). The cutting of depth and velocity of abrasive particle were used as variables for simulation. By analyzing the mechanical data of results, it was found that the friction coefficient of abrasive particle decreased gradually with the increase of the cutting of velocity. Dislocation extraction analysis (DXA) and Common neighbor analysis (CNA) were used to analyze the lattice structure and dislocations. By analyzing the above data, the wear and damage of the material was obtained, and the inhibition behavior of Hirth dislocation lock on dislocation nucleation was found. Cu substrate damage results showed that HEA coatings could effectively improve the wear resistance of the material surface. In the case of small cutting of depth, the high stress region in the material was concentrated below the abrasive particle, and the material subsurface damage would be intensified. This research can provide a theoretical basis for the application of HEA coatings.

Mechanistic insights into interface-facilitated dislocation nucleation and phase transformation at semicoherent bimetal interfaces

The nucleation of lattice dislocations and interface sliding at bimetal interfaces are two fundamental mechanisms of plasticity that are responsible for the mechanical responses of nanostructured materials; however, the interface-facilitated phase transformation is rarely considered owing to its relatively high energy barrier for activation. Taking the bimetal hcp/bcc interfaces with Pitch-Schrader and Burgers orientation relationships (ORs) as an illustration, we show that both non-basal dislocation nucleation and hcp-to-bcc phase transformation can be activated at the interface under external loading when the basal slip systems are effectively suppressed. The non-basal dislocation nucleation is shown to be closely related to the dynamic evolution of misfit dislocation patterns at the semicoherent interface, in which the 1/6[022¯3¯] pyramidal dislocation is not strictly parallel to the (011¯1) stacking fault plane owing to the corrugated feature. In contrast to non-basal dislocation nucleation, phase transformation requires specific crystallographic ORs of the constituent metals under certain loading conditions, which corresponds to the process of alternate shuffle and shear deformation that involves atomistic migration. To further reveal the competition between non-basal dislocation nucleation and phase transformation, a series of twisted interface models were constructed to systematically investigate the optimal condition of the interface geometry for phase transformation. The phase transformation occurred only when the dislocation nucleation was further hindered at some specific twist angles, suggesting a strong dependence of phase transformation on the interface structure. These findings provide a foundation to the atomistic mechanism of various interface-mediated deformation and a solution to tune interface-facilitated plasticity via interface engineering.