High-throughput screening for superhard carbon and boron nitride allotropes with superior stiffness and strength

In search of intrinsically superhard materials with superior stiffness and strength, we performed a comprehensive high-throughput hunting on hundreds of carbon and BN allotropes based on energetic and mechanical criteria. Our results suggest that at ambient pressure, an approximate linear relationship exists between the ideal strengths and elastic moduli in two allotrope regions with high elastic moduli, while no carbon (BN) allotrope can possess both superior stiffness and strength than diamond (c-BN). With further consideration of pressure induced stiffening and strengthening, it is interestingly found that the strength enhancement shows distinct characteristic trend, resulting in some intriguing ultrastiffening and strengthening phenomena. In particular, a superdense carbon allotrope termed as tI12-C was unexpectedly discovered to possess superior stiffness and strength than diamond under high pressure. Electronic structure analysis indicates that an increasing charge accumulation appearing in tI12-C under pressure is responsible for its ultra-stiffening and strengthening phenomena, differing from the appearance of abnormal charge depletions and the accompanied metallization in diamond under applied strain. These findings provide a fundamental basis for screening the novel superhard carbon and BN allotropes based on mechanical criteria, and highlight the importance to understand the effect of strain tunable electronic structure on mechanical response of materials.Web of Science13716415

Designing ultrastrong 5d transition metal diborides with excellent stability for harsh service environments

Much effort was devoted towards the rational design of ultrastrong transition metal borides (TMBs) with remarkable mechanical properties and excellent stabilities, owing to promising applications in machining, drilling tools and protective coatings for the aerospace industry. Although an enormous number of investigations have been performed on these TMBs under normal conditions, studies on the stability and mechanical strength in harsh high-pressure environments, which are critical for safe service behavior and a realistic understanding of stabilities and strengthening mechanisms, are yet nearly absent. In this work, taking 5d TMB2 (TM = Hf, Ta, W, Re, Os, Ir and Pt) as an illustration, we performed comprehensive high-throughput first-principles screening for thermodynamically stable and metastable structures under various pressures. Four experimentally observed structures are found to be thermodynamically feasible for most 5d TMB2 (TM = Hf, Ta, W, Re, Os and Ir) at 0 and 100 GPa. By exploiting orbital-decomposed electronic structures, we reveal that the pressure-induced stabilization and phase transitions of 5d TMB2 can be rationalized by the splitting of bonding and antibonding states around the Fermi level. Further investigations on the pressure-induced strengthening indicate that 5d TMB2 in the hP6[194] structure exhibit a profound strengthening effect under high pressure, which can be rationalized by the proposed strengthening factor eta, but eta fails in the oP6[59] structure due to the changed instability modes at different pressures. These findings suggest the necessity to explore the plasticity parameters for a realistic understanding of pressure-induced strengthening in TMBs, providing a strong argument for rules based on bond parameters at equilibrium in designing strong solids.Web of Science2129161071609

Stacking stability and sliding mechanism in weakly bonded 2D transition metal carbides by van der Waals force

The stability of the stacked two-dimensional (2D) transition metal carbides and their interlayered friction in different configurations are comparatively studied by means of density functional theory (DFT). At equilibrium, a larger interlayer distance corresponds to a smaller binding energy, suggesting an easier sliding between them. The oxygen-functionalized M2CO2 possesses much lower sliding resistance than the bare ones due to the strong metallic interactions between the stacked M2C layers. Compared to the parallel stacking order of M2CO2-I, the mirror stacked M2CO2-II possesses better lubricant properties. At strained states, normal compression substantially enhances the sliding barrier owing to more charges transferring from the M to O atom. Furthermore, the in-plane biaxial strain may effectively hinder the interlayer sliding, while the uniaxial strain fundamentally modifies the preferred sliding pathway due to anisotropic expansion of surface electronic state. These results highlight that the functionalized MXenes with strain-controllable frictional properties are promising lubricating materials because of their low sliding energy barrier and excellent mechanical properties.Web of Science788559195591

Atomistic insight into the dislocation nucleation at crystalline/crystalline and crystalline/amorphous interfaces without full symmetry

Misfit dislocations at bimetal interfaces play a decisive role in determining various deformation behaviors by carrying the shear sliding, serving as a barrier for dislocation transmission and a source of dislocation nucleation. However, when the interface does not possess the distinct feature of misfit dislocations, the nucleation mechanism of lattice dislocations at the interfaces cannot be simply quantified by previously developed atomistic mechanisms based on characteristic misfit dislocations. Using crystalline/crystalline interfaces with a large lattice mismatch and crystalline/amorphous interfaces without local symmetry as prototypes, we show for the first time that the dislocation nucleation at such interfaces is attributable to the localized strain heterogeneities by modifying the volumetric and shear strain components at the atomic level to mechanically respond to different loadings. Using atomic strain tensor analysis, we found that in-plane localized shearing plays a critical role in the emission of lattice dislocations from interfaces, while the corresponding normal components of the volumetric strain tensor will dominate the character of the nucleated lattice dislocation by modifying the atomic excess volume at the interface to overcome the barrier to dislocation nucleation. Further exploration of various crystalline/amorphous interfaces by varying the chemical composition of the amorphous side indicates that chemical heterogeneity may substantially change the strain heterogeneity by forming a different clustered structure at the interface, resulting in the preferred choice of nucleation sites at the boundary regions that can be defined as nano shear traces (NSTs). These results provide a foundation to investigate the effects of strain and chemical heterogeneities in order to provide a realistic explanation of interface mediated deformation mechanisms and an efficient solution to tune interface dominated plasticity.Web of Science16226725

A generalized solid strengthening rule for biocompatible Zn-based alloys, a comparison with Mg-based alloys

Solid solution strengthening has been widely used in designing various high-performance biocompatible Mg-based alloys, but its transferability to other biocompatible metals such as Zn-based alloys is questionable or nearly absent. In the present study, an ab initio informed Peierls-Nabarro model and Leyson et al.'s strengthening model are used for a systematic investigation on solute strengthening in Zn-based alloys, which is compared with the widely studied Mg-based alloys. Although an inverse relationship was revealed between volume misfit epsilon (b) and chemical misfit epsilon (SFE) for both Zn-based and Mg-based alloys, most solutes would however result in positive epsilon (b) and negative epsilon (SFE) for Zn-based alloys, differing from Mg-based alloys. With epsilon (b) and epsilon (SFE) as two key descriptors, a generalized scaling diagram is finally drawn for a fast evaluation of solid solution strengthening in Zn-based alloys, indicating that the alkaline-earth and rare earth elements are better strengtheners for Zn-based alloys, which provides a general rule in designing novel biocompatible materials.Web of Science2140226382262

First-principles design of strong solids: Approaches and applications

In the design of strong solids, especially hard and superhard materials, this review article attempts to critically cover an extended field of first-principles derived mechanical properties by considering both intrinsic (i.e., crystal structures, bonding nature and strength) and extrinsic (i.e., nanostructures and interface characteristics) parameters. For the intrinsic parameters, firstly, the bonding topology and nature, elastic property and ductility-brittleness criterion provide critical physics on the understanding of the mechanical response of a crystal. Secondly, the ideal strength model, the generalized stacking fault energy model, and ab initio informed Peierls-Nabarro model uniquely quantify the fracture and plastic resistance of a crystal. Taking the extrinsic parameters into further consideration, the recent progress of first-principles investigations on the mechanical behavior of nanostructured solids and heterogeneous interfaces is selectively reviewed, targeted as the origin and/or carrier of the fracture or plastic deformation. These extrinsic parameters include the work of adhesion, the critical stresses for interfacial cleavage and glide and so on. Finally, by classifying the strong solids into intrinsically and extrinsically hard/superhard materials, two different rules are proposed: (1) three-dimensional short covalent bond networks with sufficiently high ideal strength and Peierls resistance and (2) nanosized crystallites/layers glued by strongly bonded thin interfaces.Web of Science82649

Plastic flow between nanometer-spaced planar defects in nanostructured diamond and boron nitride

The fundamental mechanisms of strengthening/hardening and toughening that may be modified by various nanometer-spaced planar defects in the ultrahard nanostructured diamond and boron nitride (BN), e.g., nanotwins, stacking faults, and coherent heterophase interfaces, are still far from understood. In the present work, by means of first-principles approaches to derive ideal strength and Peierls stress, we performed a comprehensive investigation on the effect of the nanometer-spaced planar defects on the strength and plasticity of nanostructured diamond and BN under both uniform and localized deformations. A profound strengthening under uniform strain is revealed to be closely dependent on the spacing of planar defects, yet differing from the disappearing dependence under localized strain. It is further shown that the breakage and reconstruction of covalent bonds occurs only for very small spacing of planar defects under uniform deformations, being inconsistent with the average spacing found in the experimentally prepared nanotwinned diamond and BN, thus casting a doubt on the feasibility of the previously proposed strengthening mechanism. Under localized deformations, only the planar defects of twin in c-diamond or c-BN and coherent heterophase interface in c-/h-diamond or c-/w-BN are found to increase the barrier for the parallel slip of both 1/2(110) shuffle-set full dislocation and 1/6(112) glide-set partial dislocation, resulting in the strengthening of nanostructured diamond and BN, which agrees to the experimental observation. These findings not only yield a physical insight in strengthening/toughening nanostructured diamond and BN, but highlight the importance to understand the synergetic effect of length scale and interface between planar defects in designing superhard nanostructured materials.Web of Science1011art. no. 01410

Cooperative roles of stacking fault energies on dislocation nucleation at bimetal interface through tunable potentials

By tuning consistent potentials for bimetal system, the cooperative roles of stacking fault energies (SFEs) such as intrinsic SFE (ISFE) and unstable SFE (USFE) on the dislocation nucleation are comprehensively explored. It reveals that the dislocation nucleation at semi-coherent interface is determined by the misfit dislocations, but the critical yield stress depends strongly on the values of ISFE and USFE that are function of external loadings. To account for such relationship, a modified analytical model is proposed based on the nucleation and propagation of double kinks at the interface, showing good agreement with the simulation results. These results not only resolve the previous inconsistency in the role of stacking faults on plastic deformation but shed light on evaluating the atomistic mechanism according to the synergistic effect between stacking faults and loading schemes.Web of Science193art. no. 11041

Phonon-mediated stabilization and softening of 2D transition metal carbides: case studies of Ti2CO2 and Mo2CO2

Two-dimensional transition metal carbides (MXenes) exhibit excellent thermodynamic stability, mechanical strength and flexibility, which make them promising candidates in flexible devices and reinforcements in nanocomposites. However, the dynamic stability may intrinsically determine the preferred adsorption sites of functional groups in MXenes and lead to premature failure under finite strain before approaching the elastic limits. It is found interestingly that different adsorption sites of the functional groups correspond to the different phonon stabilities and adsorption energies of MXenes, which can be attributed to different hybridization characteristics between the metal-d and O-p(z) states and delocalized electron behaviors around the metal atoms. Although both Ti2CO2 and Mo2CO2 possess high ideal strengths and superior flexibility, the premature phonon instabilities appear unexpectedly in distinct manners before approaching their elastic limits. An in-depth exploration of the soft modes and deformed electronic structures reveals that a continuously decreasing gap-opening at the point in Ti2CO2 increases after in-plane phonon instability due to the pseudo Jahn-Teller effect, differing from the out-of-plane phonon instability and semiconductor-metal transition under biaxial tension observed in MoS2. Although Mo2CO2 shows similar failure modes to graphene under uniaxial/biaxial tensions, the band crossings around the Fermi level are found to be responsible for its metallic character and elastic/phonon instabilities by modifying the elastic energy or electronic band energy, different from the gap opening appearing in graphene. Our results shed light onto the profound effect of the phonon instability on the preferable structure and strengths of MXenes, providing theoretical guidance on designing flexible MXene devices, raising a great challenge to the conventional strengthening theory by simply counting bonds.Web of Science2021146181460

Mechanistic understanding of the size effect on shock facilitated dislocation nucleation at semicoherent interfaces

Interface-facilitated dislocation nucleation dominates deformation behaviors of metallic nanolaminates under shock loadings. Recent works demonstrated a strong size effect of dislocation nucleation for bimetal nanolaminates. Herein, we demonstrate that such effect is attributed to stress effect of misfit dislocations originating from neighboring interfaces, which is consistent with analytical solutions to the coupled stress fields of misfit dislocations. When changing the relative position of neighboring layers, the size effect is modified because of the variational stress. These findings provide a rational evaluation on the observed size effect of dislocation nucleation, and a foundation in designing strong shock-resistant materials by interface engineering.Web of Science17846245