Extra-electron induced covalent strengthening and generalization of intrinsic ductile-to-brittle criterion

Gradient boosted and statistical feature selection workflow for materials property predictions

With the emergence of big data initiatives and the wealth of available chemical data, data-driven approaches are becoming a vital component of materials discovery pipelines or workflows. The screening of materials using machine-learning models, in particular, is increasingly gaining momentum to accelerate the discovery of new materials. However, the black-box treatment of machine-learning methods suffers from a lack of model interpretability, as feature relevance and interactions can be overlooked or disregarded. In addition, naive approaches to model training often lead to irrelevant features being used which necessitates the need for various regularization techniques to achieve model generalization; this incurs a high computational cost. We present a feature-selection workflow that overcomes this problem by leveraging a gradient boosting framework and statistical feature analyses to identify a subset of features, in a recursive manner, which maximizes their relevance to the target variable or classes. We subsequently obtain minimal feature redundancy through multicollinearity reduction by performing feature correlation and hierarchical cluster analyses. The features are further refined using a wrapper method, which follows a greedy search approach by evaluating all possible feature combinations against the evaluation criterion. A case study on elastic material-property prediction and a case study on the classification of materials by their metallicity are used to illustrate the use of our proposed workflow; although it is highly general, as demonstrated through our wider subsequent prediction of various material properties. Our Bayesian-optimized machine-learning models generated results, without the use of regularization techniques, which are comparable to the state-of-the-art that are reported in the scientific literature.

Elastic, optoelectronic and photocatalytic properties of semiconducting CsNbO3: first principles insights

The cubic phase of CsNbO₃ (CNO) perovskite has been hypothesized to investigate the elastic, electronic, photocatalytic, and optical properties for various technological applications using first-principles method. The pressure dependent structural stability has been confirmed from computed elastic constants. Relatively high value of elastic moduli, large hardness and toughness suggested that CNO would be applicable to design industrial machineries. The ductile to brittle transition is noticed at 20 GPa. The indirect bandgap of CNO proclaims its suitability for photovoltaic and IR photodetector applications. The total and partial density of states are calculated to show in evidence the contribution of individual atomic orbitals in the formation of bands. The pressure changes orbitals hybridization which can be substantiated by the change in the bandgap. Strong covalency of the Nb-O bond and antibonding character of Cs-O have been anticipated by the Mulliken population analysis and by the contour maps of electron charge density. The low carrier effective mass and high mobility carriers predict the good electrical conductivity of the material. The calculated values of conduction and valance band edge potential illustrate the excellent water-splitting and environmental pollutants degradation properties of CNO.

Microstructure and Wear of W-Particle-Reinforced Al Alloys Prepared by Laser Melt Injection

W-particle-reinforced Al alloys were prepared on a 7075 aluminum alloy surface via laser melt injection to improve their wear resistance, and the microstructure, microhardness, and wear resistance of the W/Al layers were studied. Scanning electron microscopy (SEM) results confirmed that a W/Al laser melting layer of about 1.5 mm thickness contained W particles, and Al₄W was formed on the surface of the Al alloys. Due to the reinforcement of the W particles and good bonding of the W and Al matrix, the melting layer showed excellent wear resistance compared to that of Al alloys.

Compression Induced Deformation Twinning Evolution in Liquid-Like Cu2Se

For practical applications of copper selenide (Cu₂Se) thermoelectric (TE) materials with liquid-like behavior, it is essential to determine the structure-property relations as a function of temperature. Here, we investigate β-Cu₂Se structure evolution during uniaxial compression over the temperature range of 400-1000 K using molecular dynamics simulations. We find that at temperatures above 800 K, Cu₂Se exhibits poor stability with breaking order that is described as a liquid-like or hybrid structure comprising a rigid Se sublattice and mobile Cu ions. A uniaxial load causes accumulated structural heterogeneity that is alleviated by diffusion-induced accommodation of local deformations. With increasing strain, the deformation mode changes into a combination of compression and shear, accompanied by restructuring in terms of twinning. Interestingly, in addition to a plastic behavior rarely found in inorganic semiconductors, we find that higher temperature promotes deformation twinning in liquid-like Cu₂Se, showing the role of thermal instability, including Cu diffusion, in structural adaptation and mechanical modulation. These findings reveal the micromechanism of hybrid structural evolution as well as performance tuning through twinning, which provides a theoretical guide toward advanced Cu₂Se TE materials design.

Microstructure, Mechanical and Tribological Properties of Advanced Layered WN/MeN (Me = Zr, Cr, Mo, Nb) Nanocomposite Coatings

Due to the increased demands for drilling and cutting tools working at extreme machining conditions, protective coatings are extensively utilized to prolong the tool life and eliminate the need for lubricants. The present work reports on the effect of a second MeN (Me = Zr, Cr, Mo, Nb) layer in WN-based nanocomposite multilayers on microstructure, phase composition, and mechanical and tribological properties. The WN/MoN multilayers have not been studied yet, and cathodic-arc physical vapor deposition (CA-PVD) has been used to fabricate studied coating systems for the first time. Moreover, first-principles calculations were performed to gain more insight into the properties of deposited multilayers. Two types of coating microstructure with different kinds of lattices were observed: (i) face-centered cubic (fcc) on fcc-W₂N (WN/CrN and WN/ZrN) and (ii) a combination of hexagonal and fcc on fcc-W₂N (WN/MoN and WN/NbN). Among the four studied systems, the WN/NbN had superior properties: the lowest specific wear rate (1.7 × 10⁻⁶ mm³/Nm) and high hardness (36 GPa) and plasticity index H/E (0.93). Low surface roughness, high elastic strain to failure, Nb₂O₅ and WO₃ tribofilms forming during sliding, ductile behavior of NbN, and nanocomposite structure contributed to high tribological performance. The results indicated the suitability of WN/NbN as a protective coating operating in challenging conditions.

Order-Tuned Deformability of Bismuth Telluride Semiconductors: An Energy-Dissipation Strategy for Large Fracture Strain

In addition to thermoelectric (TE) performance tuning through defect or strain engineering, progress in mechanical research is of increasing importance to wearable applications of bismuth telluride (Bi₂Te₃) TE semiconductors, which are limited by poor deformability. For improving dislocation-controlled deformability, we clarify an order-tuned energy-dissipation strategy that facilitates large deformation through multilayer alternating slippage and stacking fault destabilization. Given that energy dissipation and dislocation motions are governed by van der Waals sacrificial bond (SB) behavior, molecular dynamics simulation is implemented to reveal the relation between the shear deformability and lattice order changes in Bi₂Te₃ crystals. Using the disorder parameter (D) that is defined according to the configurational energy distribution, the results of strain rates and initial crack effects show how the proper design of the initial structure and external conditions can suppress strain localization that would cause structural failure from the lack of energy dissipation, resulting in large homogeneous deformation of Bi₂Te₃ nanocrystals. This study uncovers the essence of the tuning mechanism in which highly deformable Bi₂Te₃ crystals should become disordered as slowly as possible until fracture. This highlights the role of the substructure evolution of SB-defect synergy that facilitates energy dissipation and performance stability during slipping. The disorder parameter D provides a bridge between micro/local mechanics and fracture strain, hinting at the possible mechanical improvement of Bi₂Te₃ semiconductors for designing flexible TE devices through order tuning and energy dissipation.

Generalization of intrinsic ductile-to-brittle criteria by Pugh and Pettifor for materials with a cubic crystal structure

Two classical criteria, by Pugh and Pettifor, have been widely used by metallurgists to predict whether a material will be brittle or ductile. A phenomenological correlation by Pugh between metal brittleness and its shear modulus to bulk modulus ratio was established more than 60 years ago. Nearly four decades later Pettifor conducted a quantum mechanical analysis of bond hybridization in a series of intermetallics and derived a separate ductility criterion based on the difference between two single-crystal elastic constants, C₁₂–C₄₄. In this paper, we discover the link between these two criteria and show that they are identical for materials with cubic crystal structures.

Electronic, phononic and superconducting properties of trigonal Li2MSi2(MIr, Rh)

We have usedab initiodensity functional theory to study electronic, mechanical, phononic, and superconducting properties of Li2MSi2(M= Ir, Rh), which has recently been produced as a new type of transition metal-based ternary compound in the trigonal structure (Horiganeet al2019New J. Phys.21093056). The calculated electronic band structure and the density of states indicate that the Li2IrSi2and Li2RhSi2compounds are in metallic character. Mechanical properties such as elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature were calculated for these compounds. The calculated results suggest that the compounds are mechanically stable and behave in a ductile manner. The phonon spectra have no imaginary frequency, which proves that these compounds are dynamically stable. Electron-phonon coupling parameters confirm that they are weak-coupling superconductors. Although the influence of spin-orbit coupling in superconductivity is not significant for these compounds, it has a very small influence on electronic structure for Li2IrSi2. The calculated critical temperature (Tcμ⋆=0.11) values of 3.29 K for Li2IrSi2and 2.82 K for Li2RhSi2agree well with experimental estimates.

Trends in elastic properties of Ti-Ta alloys from first-principles calculations

The martensitic start temperature (M_s) is a technologically fundamental characteristic of high-temperature shape memory alloys. We have recently shown [Chakrabortyet al2016Phys. Rev.B94224104] that the two key features in describing the composition dependence ofM_sare theT= 0 K phase stability and the difference in vibrational entropy which, within the Debye model, is directly linked to the elastic properties. Here, we use density functional theory together with special quasi-random structures to study the elastic properties of disordered martensite and austenite Ti-Ta alloys as a function of composition. We observe a softening in the tetragonal shear elastic constant of the austenite phase at low Ta content and anon-linearbehavior in the shear elastic constant of the martensite. A minimum of 12.5% Ta is required to stabilize the austenite phase atT= 0 K. Further, the shear elastic constants and Young's modulus of martensite exhibit a maximum for Ta concentrations close to 30%. Phenomenological, elastic-constant-based criteria suggest that the addition of Ta enhances the strength, but reduces the ductile character of the alloys. In addition, the directional elastic stiffness, calculated for both martensite and austenite, becomes more isotropic with increasing Ta content. The reported trends in elastic properties as a function of composition may serve as a guide in the design of alloys with optimized properties in this interesting class of materials.

Ultra-high piezoelectric coefficients and strain-sensitive Curie temperature in hydrogen-bonded systems

We propose a new approach to obtain ultra-high piezoelectric coefficients that can be infinitely large theoretically, where ferroelectrics with strain-sensitive Curie temperature are necessary. We show the first-principles plus Monte Carlo simulation evidence that many hydrogen-bonded ferroelectrics (e.g. organic PhMDA) can be ideal candidates, which are also flexible and lead-free. Owing to the specific features of hydrogen bonding, their proton hopping barrier will drastically increase with prolonged proton transfer distance, while their hydrogen-bonded network can be easily compressed or stretched due to softness of hydrogen bonds. Their barriers as well as the Curie temperature can be approximately doubled upon a tensile strain as low as 2%. Their Curie temperature can be tuned exactly to room temperature by fixing a strain in one direction, and in another direction, an unprecedented ultra-high piezoelectric coefficient of 2058 pC/N can be obtained. This value is even underestimated and can be greatly enhanced when applying a smaller strain. Aside from sensors, they can also be utilized for converting either mechanical or thermal energies into electrical energies due to high pyroelectric coefficients.

Ultrahigh-strain ferroelasticity in two-dimensional honeycomb monolayers: from covalent to metallic bonding

We propose a possible ferroelastic switching pathway of two-dimensional (2D) honeycomb lattice (including graphene, BN, stanene, etc.) that may swap its armchair and zigzag direction, reversing an unprecedented strain of 73.2%. Our ab initio calculations reveal that such pathway cannot work in covalent systems like graphene and BN; for monolayer with metallic bonds like stanene, stanane and InBi that have all been synthesized, however, such pathway can be feasible with a low switching barrier (<0.15 eV) and stress (<graphene upon 1% tensile strain), also with the highest energy/stress point in the elastic region. Their distinct behaviors are attributed to the different feature of covalent bonds and metallic bonds: the former is rigid with directionality, while the latter is malleable with ductility. A general trend of linear decrease in switching barrier with uprising metallicity for the same group compounds is revealed. Similar behaviors can be extended to bulk zinc-blended or wurtzite structure that can be deemed as multilayer stacking of buckled monolayer. Binary compounds like InBi monolayer are even multiferroics with both in-plane and vertical ferroelectricity as well as nontrivial topological properties.

Screening of suitable cationic dopants for solar absorber material CZTS/Se: A first principles study

The earth abundant and non-toxic solar absorber material kesterite Cu2ZnSn(S/Se)4 has been studied to achieve high power conversion efficiency beyond various limitations, such as secondary phases, antisite defects, band gap adjustment and microstructure. To alleviate these hurdles, we employed screening based approach to find suitable cationic dopant that can promote the current density and the theoretical maximum upper limit of the energy conversion efficiency (P(%)) of CZTS/Se solar devices. For this task, the hybrid functional (Heyd, Scuseria and Ernzerhof, HSE06) were used to study the electronic and optical properties of cation (Al, Sb, Ga, Ba) doped CZTS/Se. Our in-depth investigation reveals that the Sb atom is suitable dopant of CZTS/CZTSe and also it has comparable bulk modulus as of pure material. The optical absorption coefficient of Sb doped CZTS/Se is considerably larger than the pure materials because of easy formation of visible range exciton due to the presence of defect state below the Fermi level, which leads to an increase in the current density and P(%). Our results demonstrate that the lower formation energy, preferable energy gap and excellent optical absorption of the Sb doped CZTS/Se make it potential component for relatively high efficient solar cells.

Insight into Physical and Thermodynamic Properties of X3Ir (X = Ti, V, Cr, Nb and Mo) Compounds Influenced by Refractory Elements: A First-Principles Calculation

The effects of refractory metals on physical and thermodynamic properties of X3Ir (X = Ti, V, Cr, Nb and Mo) compounds were investigated using local density approximation (LDA) and generalized gradient approximation (GGA) methods within the first-principles calculations based on density functional theory. The optimized lattice parameters were both in good compliance with the experimental parameters. The GGA method could achieve an improved structural optimization compared to the LDA method, and thus was utilized to predict the elastic, thermodynamic and electronic properties of X3Ir (X = Ti, V, Cr, Nb and Mo) compounds. The calculated mechanical properties (i.e., elastic constants, elastic moduli and elastic anisotropic behaviors) were rationalized and discussed in these intermetallics. For instance, the derived bulk moduli exhibited the sequence of Ti3Ir < Nb3Ir < V3Ir < Cr3Ir < Mo3Ir. This behavior was discussed in terms of the volume of unit cell and electron density. Furthermore, Debye temperatures were derived and were found to show good consistency with the experimental values, indicating the precision of our calculations. Finally, the electronic structures were analyzed to explain the ductile essences in the iridium compounds.

Tuning structure and mechanical properties of Ta-C coatings by N-alloying and vacancy population

Tailoring mechanical properties of transition metal carbides by substituting carbon with nitrogen atoms is a highly interesting approach, as thereby the bonding state changes towards a more metallic like character and thus ductility can be increased. Based on ab initio calculations we could prove experimentally, that up to a nitrogen content of about 68% on the non-metallic sublattice, Ta-C-N crystals prevail a face centered cubic structure for sputter deposited thin films. The cubic structure is partly stabilized by non-metallic as well as Ta vacancies - the latter are decisive for nitrogen rich compositions. With increasing nitrogen content, the originally super-hard fcc-TaC_0.71 thin films soften from 40 GPa to 26 GPa for TaC_0.33N_0.67, accompanied by a decrease of the indentation modulus. With increasing nitrogen on the non-metallic sublattice (hence, decreasing C) the damage tolerance of Ta-C based coatings increases, when characterized after the Pugh and Pettifor criteria. Consequently, varying the non-metallic sublattice population allows for an effective tuning and designing of intrinsic coating properties.

Mechanical Properties in Metal-Organic Frameworks: Emerging Opportunities and Challenges for Device Functionality and Technological Applications

Some of the most remarkable recent developments in metal-organic framework (MOF) performance properties can only be rationalized by the mechanical properties endowed by their hybrid inorganic-organic nanoporous structures. While these characteristics create intriguing application prospects, the same attributes also present challenges that will need to be overcome to enable the integration of MOFs with technologies where these promising traits can be exploited. In this review, emerging opportunities and challenges are identified for MOF-enabled device functionality and technological applications that arise from their fascinating mechanical properties. This is discussed not only in the context of their more well-studied gas storage and separation applications, but also for instances where MOFs serve as components of functional nanodevices. Recent advances in understanding MOF mechanical structure-property relationships due to attributes such as defects and interpenetration are highlighted, and open questions related to state-of-the-art computational approaches for quantifying their mechanical properties are critically discussed.

Elastic properties of bulk and low-dimensional materials using Van der Waals density functional

In this work, we present a high-throughput first-principles study of elastic properties of bulk and monolayer materials mainly using the vdW-DF-optB88 functional. We discuss the trends on the elastic response with respect to changes in dimensionality. We identify a relation between exfoliation energy and elastic constants for layered materials that can help to guide the search for vdW bonding in materials. We also predicted a few novel materials with auxetic behavior. The uncertainty in structural and elastic properties due to the inclusion of vdW interactions is discussed. We investigated 11,067 bulk and 257 monolayer materials. Lastly, we found that the trends in elastic constants for bulk and their monolayer counterparts can be very different. All the computational results are made publicly available at easy-to-use websites: https://www.ctcms.nist.gov/~knc6/JVASP.html and https://jarvis.nist.gov/. Our dataset can be used to identify stiff and flexible materials for industrial applications.

Vacancy-induced brittle to ductile transition of W-M co-doped Al3Ti (M=Si, Ge, Sn and Pb)

We investigated the effect of vacancy formation on brittle (D0₂₂) to ductile (L1₂-like) transition in Al₃Ti using DFT calculations. The well-known pseudogap on the density of states of Al₃Ti migrates towards its Fermi level from far above, via a W − M co-doping strategy, where M is Si, Ge, Sn or Pb respectively. In particular, by a W − M co-doping the underline electronic structure of the pseudogap approaches an octahedral (L1₂: t_2g, e_g) from the tetragonal (D0₂₂: e_g, b_2g, a_1g, b_1g) crystal field. Our calculations demonstrated that (1) a W-doping is responsible for the close up of the energy gap between a_1g and b_1g so that they tend to merge into an e_g symmetry, and (2) all M-doping lead to a narrower gap between e_g and b_2g (moving towards a t_2g symmetry). Thus, a brittle to ductile transition in Al₃Ti is possible by adopting this W − M co-doping strategy. We further recommend the use of W-Pb co-doped Al₃Ti to replace the less anodic Al electrode in Al-battery, due to its improved ductility and high Al diffusivity. Finally this study opens a new field in physics to tailor mechanical properties by manipulating electron energy level(s) towards higher symmetry via vacancy optimization.

Elastic and mechanical softening in boron-doped diamond

Alternative approaches to evaluating the hardness and elastic properties of materials exhibiting physical properties comparable to pure diamond have recently become necessary. The classic linear relationship between shear modulus (G) and Vickers hardness (H_V), along with more recent non-linear formulations based on Pugh’s modulus extending into the superhard region (H_V > 40 GPa) have guided synthesis and identification of novel superabrasives. These schemes rely on accurately quantifying H_V of diamond-like materials approaching or potentially exceeding the hardness of the diamond indenter, leading to debate about methodology and the very definition of hardness. Elasticity measurements on such materials are equally challenging. Here we used a high-precision, GHz-ultrasonic interferometer in conjunction with a newly developed optical contact micrometer and 3D optical microscopy of indentations to evaluate elasticity-hardness relations in the ultrahard range (H_V > 80 GPa) by examining single-crystal boron-doped diamond (BDD) with boron contents ranging from 50–3000 ppm. We observe a drastic elastic-mechanical softening in highly doped BDD relative to the trends observed for superhard materials, providing insight into elasticity-hardness relations for ultrahard materials.

A Statistical Learning Framework for Materials Science: Application to Elastic Moduli of k-nary Inorganic Polycrystalline Compounds

Materials scientists increasingly employ machine or statistical learning (SL) techniques to accelerate materials discovery and design. Such pursuits benefit from pooling training data across, and thus being able to generalize predictions over, k-nary compounds of diverse chemistries and structures. This work presents a SL framework that addresses challenges in materials science applications, where datasets are diverse but of modest size, and extreme values are often of interest. Our advances include the application of power or Hölder means to construct descriptors that generalize over chemistry and crystal structure, and the incorporation of multivariate local regression within a gradient boosting framework. The approach is demonstrated by developing SL models to predict bulk and shear moduli (K and G, respectively) for polycrystalline inorganic compounds, using 1,940 compounds from a growing database of calculated elastic moduli for metals, semiconductors and insulators. The usefulness of the models is illustrated by screening for superhard materials.

Discovery of elusive structures of multifunctional transition-metal borides

A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.

Ductile-to-brittle transition and materials’ resistance to amorphization by irradiation damage

By summarizing over seven hundred elastic constants of materials with various crystal structures, we have found that ductile-to-brittle transition can be reflected by the change of G/B, an important indicator of bonding type.

Predictive analytics for crystalline materials: bulk modulus

The machine learning-based generalized model developed for forecasting bulk moduli of various types of stoichiometric and non-stoichiometric crystalline materials. The web application (ThermoEl) deploying the developed predictive model is available for public use.

Charting the complete elastic properties of inorganic crystalline compounds

The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

Ab initio study of AlxMoNbTiV high-entropy alloys

The Al(x)MoNbTiV (x = 0-1.5) high-entropy alloys (HEAs) adopt a single solid-solution phase, having the body centered cubic (bcc) crystal structure. Here we employ the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to investigate the equilibrium volume, elastic constants, and polycrystalline elastic moduli of Al(x)MoNbTiV HEAs. A comparison between the ab initio and experimental equilibrium volumes demonstrates the validity and accuracy of the present approach. Our results indicate that Al addition decreases the thermodynamic stability of the bcc structure with respect to face-centered cubic and hexagonal close packed lattices. For the elastically isotropic Al(0.4)MoNbTiV HEAs, the valence electron concentration (VEC) is about 4.82, which is slightly different from VEC ∼ 4.72 obtained for the isotropic Gum metals and refractory--HEAs.

Mechanical properties and structural features of novel Fe-based bulk metallic glasses with unprecedented plasticity

Fe-based bulk metallic glasses (BMGs) have attracted great attention due to their unique magnetic and mechanical properties, but few applications have been materialized because of their brittleness at room temperature. Here we report a new Fe(50)Ni(30)P(13)C(7) BMG which exhibits unprecedented compressive plasticity (>20%) at room temperature without final fracture. The mechanism of unprecedented plasticity for this new Fe-based BMG was also investigated. It was discovered that the ductile Fe(50)Ni(30)P(13)C(7) BMG is composed of unique clusters mainly linked by less directional metal-metal bonds which are inclined to accommodate shear strain and absorbed energy in the front of crack tip. This conclusion was further verified by the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy experiments of Fe(80-x)Ni(x)P(13)C(7) (x = 0, 10, 20, 30) and Fe(72-x)Ni(x)B(20)Si(4)Nb(4) (x = 0, 7.2, 14.4, 21.6, 28.8) glassy systems. The results also indicate a strong correlation between the p-d hybridization and plasticity, verifying that the transition from brittle to ductile induced by Ni addition is due to the change of bonding characteristics in atomic configurations. Thus, we can design the plasticity of Fe-based BMGs and open up a new possible pathway for manufacturing BMGs with high strength and plasticity.

Polytypism in superhard transition-metal triborides

The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB₃ and MoB₃ with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB₃ manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities.

Variable-composition structural optimization and experimental verification of MnB3and MnB4

Variable-composition evolutionary algorithm calculations combined with first-principles calculations have uncovered four viable group compounds, Mn₂B, MnB, MnB₄and previously never reported MnB₃, in the well-known Mn–B binary system.