A theoretical investigation of mixing thermodynamics, age-hardening potential, and electronic structure of ternary M(1)1-x M(2)xB2 alloys with AlB2 type structure

Ceramic transition metal diboride superlattices with improved ductility and fracture toughness screened by ab initio calculations

Inherent brittleness, which easily leads to crack formation and propagation during use, is a serious problem for protective ceramic thin-film applications. Superlattice architectures, with alternating nm-thick layers of typically softer/stiffer materials, have been proven powerful method to improve the mechanical performance of, e.g., cubic transition metal nitride ceramics. Using high-throughput first-principles calculations, we propose that superlattice structures hold promise also for enhancing mechanical properties and fracture resistance of transition metal diborides with two competing hexagonal phases, [Formula: see text] and [Formula: see text]. We study 264 possible combinations of [Formula: see text], [Formula: see text] or [Formula: see text] MB[Formula: see text] (where M [Formula: see text] Al or group 3-6 transition metal) diboride superlattices. Based on energetic stability considerations, together with restrictions for lattice and shear modulus mismatch ([Formula: see text], [Formula: see text] GPa), we select 33 superlattice systems for further investigations. The identified systems are analysed in terms of mechanical stability and elastic constants, [Formula: see text], where the latter provide indication of in-plane vs. out-of-plane strength ([Formula: see text], [Formula: see text]) and ductility ([Formula: see text], [Formula: see text]). The superlattice ability to resist brittle cleavage along interfaces is estimated by Griffith's formula for fracture toughness. The [Formula: see text]-type TiB[Formula: see text]/MB[Formula: see text] (M [Formula: see text] Mo, W), HfB[Formula: see text]/WB[Formula: see text], VB[Formula: see text]/MB[Formula: see text] (M [Formula: see text] Cr, Mo), NbB[Formula: see text]/MB[Formula: see text] (M [Formula: see text] Mo, W), and [Formula: see text]-type AlB[Formula: see text]/MB[Formula: see text] (M [Formula: see text] Nb, Ta, Mo, W), are suggested as the most promising candidates providing atomic-scale basis for enhanced toughness and resistance to crack growth.

First-principles demonstration of band filling-induced significant improvement in thermodynamic stability and mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions

Mixtures of different metal diborides in the form of solid solutions are promising materials for hard-coating applications. Herein, we study the mixing thermodynamics and the mechanical properties of AlB[Formula: see text]-structured Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions using the first-principles method, based on the density functional theory, and the cluster-expansion formalism. Our thermodynamic investigation reveals that the two diborides readily mix with one another to form a continuous series of stable solid solutions in the pseudo-binary TaB[Formula: see text] [Formula: see text]ScB[Formula: see text] system even at absolute zero. Interestingly, the elastic moduli as well as the hardness of the solid solutions show significant positive deviations from the linear Vegard's rule evaluated between those of ScB[Formula: see text] and TaB[Formula: see text]. In case of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text], the degrees of deviation from such linear trends can be as large as 25, 20, and 40% for the shear modulus, the Young's modulus, and the hardness, respectively. The improvement in the stability as well as the mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text] solid solutions relative to their constituent compounds is found to be related to the effect of electronic band filling, induced upon mixing TaB[Formula: see text] with ScB[Formula: see text]. These findings not only demonstrate the prominent role of band filling in enhancing the stability and the mechanical properties of Sc[Formula: see text]Ta[Formula: see text]B[Formula: see text], but also it can potentially open up a possibility for designing stable/metastable metal diboride-based solid solutions with superior and widely tunable mechanical properties for hard-coating applications.

Mechanical properties and multifunctionality of AlB2-type transition metal diborides. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022;35:074002. [PMID: 36541538 DOI: 10.1088/1361-648x/aca85f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Transition metal diborides (TMdBs,P6/mmm, AlB₂-type) have attracted much attention for decades, due to TMdBs can be conductors, superconductors, magnetism materials, and catalysts. The layered structure caused by the borophene subunit is the source of functions and also makes TMdBs a potential bank of Mbene. However, TMdBs also exhibit high hardness which is not supposed to have in the layered structure. The high hardness of TMdBs arises from covalent bonds of boron-boron (B-B) and strongp-dorbit hybridization of B and TM. While strong B-TM bonds will eliminate the layered structure which may damage the functional properties. Understanding the basic mechanism of hardness and function is significant to achieve optimal TMdBs. This work summarizes the basic properties of TMdBs including hardness, superconductor, and catalytic properties. It can be found that Young's modulus (E) and Shear modulus (G) are beneficial for the hardness of TMdBs and the Poisson's ratio is the opposite. Increasing the atomic radius of TM brings an improvement in the hardness of TMdBs before it reaches the highest value of 1.47 Å, beyond which hardness decreases. Besides, TMdBs also have excellent activity comparable with some noble metals for hydrogen evolution reaction, which is closely related to the d-band center. More importantly, higher valence electron concentrations were found to be adverse to the hardness and superconductivity of TMdBs and greatly affect their catalytic properties. This review is of guiding significance for further exploring the relationship between structures and properties of TMdBs.

Novel Class of Rhenium Borides Based on Hexagonal Boron Networks Interconnected by Short B2 Dumbbells

Transition metal borides are known due to their attractive mechanical, electronic, refractive, and other properties. A new class of rhenium borides was identified by synchrotron single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells between 26 and 75 GPa. Recoverable to ambient conditions, compounds rhenium triboride (ReB₃) and rhenium tetraboride (ReB₄) consist of close-packed single layers of rhenium atoms alternating with boron networks built from puckered hexagonal layers, which link short bonded (∼1.7 Å) axially oriented B₂ dumbbells. The short and incompressible Re-B and B-B bonds oriented along the hexagonal c-axis contribute to low axial compressibility comparable with the linear compressibility of diamond. Sub-millimeter samples of ReB₃ and ReB₄ were synthesized in a large-volume press at pressures as low as 33 GPa and used for material characterization. Crystals of both compounds are metallic and hard (Vickers hardness, H _V = 34(3) GPa). Geometrical, crystal-chemical, and theoretical analysis considerations suggest that potential ReB _x compounds with x > 4 can be based on the same principle of structural organization as in ReB₃ and ReB₄ and possess similar mechanical and electronic properties.

Synthesis and Characterisation of Nanocomposite Mo-Fe-B Thin Films Deposited by Magnetron Sputtering

Several ternary phases are known in the Mo-Fe-B system. Previous ab initio calculations have predicted that they should exhibit a tempting mix of mechanical and magnetic properties. In this study, we have deposited Mo-Fe-B films with a Fe-content varying from 0–37 at.% using non-reactive DC (direct current) magnetron sputtering. The phase composition, microstructure, and mechanical properties were investigated using X-ray diffraction, scanning transmission electron microscopy, and nanoindentation measurements. Films deposited at 300 °C and with >7 at.% Fe are nanocomposites consisting of two amorphous phases: a metal-rich phase and a metal-deficient phase. Hardness and elastic modulus were reduced with increasing Fe-content from ~29 to ~19 GPa and ~526 to ~353 GPa, respectively. These values result in H³/E² ratios of 0.089–0.052 GPa, thereby indicating brittle behaviour of the films. Also, no indication of crystalline ternary phases was observed at temperatures up to 600 °C, suggesting that higher temperatures are required for such films to form.

Ab initio inspired design of ternary boride thin films

The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB₂, VB₂, etc.) tend to be chemically more stable in the AlB₂ structure type, whereas late transition metal diborides (WB₂, ReB₂, etc.) are preferably stabilized in the W₂B_5−x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB₂ or W₂B_5−x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB₂, TcB₂, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.

New design for highly durable infrared-reflective coatings

The fundamental challenge in designing durable infrared-reflective coatings is achieving the ideal combination of both high reflectivity and durability. Satisfying these competing demands is traditionally achieved by deposition of durable layers on highly reflective metals. We overturn the traditional logic of 'first reflectivity and then durability' and propose an alternative of 'first durability and then reflectivity': First, a transition-metal compound is selected as a durable base; then its reflectivity is improved by incorporating silver/gold to form an alloy or by overcoating a multilayer stack. Two validation experiments prove that the new strategy works extremely well: the coatings thus obtained have infrared reflectivities close to that of aluminum, and their hardness and acid and salt corrosion resistances are 27-50, 400-1 500 and 7 500-25 000 times that of aluminum. The traditional mirror coating (e.g., Al/SiO2 films) is more suitable for moderate environments, while our mirror coating that was obtained by the new strategy (e.g., an Ag-doped hafnium nitride film) is more suitable for harsh environments, such as ones with dust, windblown sand, moisture, acid rain or salt fog. This work opens up new opportunities for highly durable infrared-reflective coatings and rejuvenates the study of transition metal compounds in a completely new area of optics.

Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics

The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)(1-x)(B13C2)(x), as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)(1-x)(B13C2)(x) is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young's and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)(1-x)(B13C2)(x) system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)(1-x)(B13C2)(x). The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

Theoretical study of phase stability and elastic properties of T 0.75Y0.75B14 (T = Sc, Ti, V, Y, Zr, Nb, Si)

In this study the phase stability, elastic properties, and plastic behaviour of icosahedral transition metal borides T 0.75Y0.75B14 (T = Sc, Ti, V, Y, Zr, Nb, Si) have been investigated using density functional theory. Phase stability critically depends on the charge transferred by T and Y to the B icosahedra. For the metal sublattice occupancy investigated here, the minimum energy of formation is identified at an effective B icosahedra charge of - 1.8 ± 0.1. This charge corridor encompasses the highest phase stability among all the reported icosahedral transition metal boride systems so far. This data provides guidance for future experimental efforts: from a wear-resistance point of view, Sc0.75Y0.75B14, Ti0.75Y0.75B14, and Zr0.75Y0.75B14 exhibit a rather unique and attractive combination of large Young's modulus values ranging from 488 to 514 GPa with the highest phase stability for icosahedral transition metals borides reported so far.

Influence of boron vacancies on phase stability, bonding and structure of MB₂ (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) with AlB₂ type structure

Transition metal diborides in hexagonal AlB2 type structure typically form stable MB2 phases for group IV elements (M  =  Ti, Zr, Hf). For group V (M  =  V, Nb, Ta) and group VI (M  =  Cr, Mo, W) the stability is reduced and an alternative hexagonal rhombohedral MB2 structure becomes more stable. In this work we investigate the effect of vacancies on the B-site in hexagonal MB2 and its influence on the phase stability and the structure for TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, CrB2, MoB2, and WB2 using first-principles calculations. Selected phases are also analyzed with respect to electronic and bonding properties. We identify trends showing that MB2 with M from group V and IV are stabilized when introducing B-vacancies, consistent with a decrease in the number of states at the Fermi level and by strengthening of the B-M interaction. The stabilization upon vacancy formation also increases when going from M in period 4 to period 6. For TiB2, ZrB2, and HfB2, introduction of B-vacancies have a destabilizing effect due to occupation of B-B antibonding orbitals close to the Fermi level and an increase in states at the Fermi level.