Solving Strength–Toughness Dilemma in Superhard Transition-Metal Diborides via a Distinct Chemically Tuned Solid Solution Approach

Abstract

Solid solution strengthening enhances hardness of metals by introducing solute atoms to create local distortions in base crystal lattice, which impedes dislocation motion and plastic deformation, leading to increased strength but reduced ductility and toughness. In sharp contrast, superhard materials comprising covalent bonds exhibit high strength but low toughness via a distinct mechanism dictated by brittle bond deformation, showcasing another prominent scenario of classic strength–toughness tradeoff dilemma. Solving this less explored and understood problem presents a formidable challenge that requires a viable strategy of tuning main load-bearing bonds in these strong but brittle materials to achieve concurrent enhancement of the peak stress and related strain range. Here, we demonstrate a chemically tuned solid solution approach that simultaneously enhances hardness and toughness of superhard transition-metal diboride Ta 1− x Zr x B 2 . This striking phenomenon is achieved by introducing solute atom Zr that has lower electronegativity than solvent atom Ta to reduce the charge depletion on the main load-bearing B–B bonds during indentation, leading to prolonged deformation that gives rise to notably higher strain range and the corresponding peak stress. This finding highlights the crucial role of properly matched contrasting relative electronegativity of solute and solvent atoms in creating concurrent strengthening and toughening and opens a promising avenue for rational design of enhanced mechanical properties in a large class of transition-metal borides. This strategy of concurrent strength–toughness optimization via solute-atom-induced chemical tuning of the main load-bearing bonding charge is expected to work in broader classes of materials, such as nitrides and carbides.

Remarkable Wear Resistance in a Complex Concentrated Alloy with Nanohierarchical Architecture and Composition Undulation

“…The hardness of NiX and Ni 3 X gradually decreases from 6.1 GPa and 4.3 GPa at RT to 3.7 GPa and 2.5 GPa at 800 °C, respectively. The high thermal hardness of NiX arises from the solid solution strengthening and the resultant heterostructures [ 38 , 39 ]. The trend in wear rate of the Ni 3 X with temperature is similar to that of the conventional alloys.…”

Balance between Activity and Stability of Single Metal and Intermetallic Compounds for Electrocatalytic Hydrogen Evolution Reaction

“…As shown in Figure a, the −COHP curve of the Re–Re bond for ZrRe 2 is in the antibonding state at E F , which is different from that of the Re metal (Figure b), while the antibonding character of the Ru–Ru bond around E F for ZrRu 2 is even more profound. As mentioned earlier, this may be attributed to the low electronegativity of Zr (χ = 1.33), which transfers electron density to Ru (χ = 2.20) and Re (χ = 1.90) to occupy a more antibonding state when these elements combine as compounds. − As to the case of ZrRe 1.75 Ru 0.25 , replacing a minor portion of Re with Ru slightly populates the antibonding state of M–M bonds at E F (M = Ru/Re; purple line in Figure c) while it will not change the electronic structure dramatically (similar to ZrRe 2 ). Overall, the antibonding population of M–M bonds at E F follows the order of ZrRu 2 > ZrRe 1.75 Ru 0.25 > ZrRe 2 .…”

“…This way, it may display high HER activity while maintaining stability during long-term operation. In order to further boost HER efficiency, Zr metal with low electronegativity (χ = 1.33, Pauling scale) and high corrosion resistance to acid or alkaline electrolytes was also incorporated in the system. − Zr belongs to the early transition metals, and bonding between Zr and Ru/Re atoms will be in a bonding state that can strengthen phase stability. In addition, based on our previous work, alloying Zr and Re in the system can demonstrate similar HER activity in both acidic and alkaline electrolytes because the OH poison effect and the change of Δ G H with neighbored OH are not profound .…”

First-principles demonstration of band filling-induced significant improvement in thermodynamic stability and mechanical properties of Sc$$_{1-x}$$Ta$$_{x}$$B$$_{2}$$ solid solutions

“…Double-metal diborides with chemical formula of M M B , where M and M are two distinct metallic elements (Mg, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ru, Os, and Re, for example) and 0 x 1, have increasingly gained interest in the recent years as promising hard-coating materials for cutting tools, thanks to their high thermal and chemical stabilities as well as good mechanical properties 1 – 13 . It has lately been shown from the theoretical and experimental aspects that, by adding the second metallic element M to the diboride compounds M B , the stabilities and properties of the resulting double-metal diborides M M B can be significantly improved and thus become superior to those of the constituent diboride compounds, either M B or M B 3 – 8 , 10 – 13 . For instance, alloying TaB with ZrB to form thin films of Ta Zr B results in improvement in hardness and toughness of the films, relative to those of TaB and ZrB films 7 , 10 , 13 .…”

“…It has lately been shown from the theoretical and experimental aspects that, by adding the second metallic element M to the diboride compounds M B , the stabilities and properties of the resulting double-metal diborides M M B can be significantly improved and thus become superior to those of the constituent diboride compounds, either M B or M B 3 – 8 , 10 – 13 . For instance, alloying TaB with ZrB to form thin films of Ta Zr B results in improvement in hardness and toughness of the films, relative to those of TaB and ZrB films 7 , 10 , 13 . Another example is that introducing Cr (Al) atoms into ZrB (TiB ) films increases wear, oxidation, and corrosion resistances of the films, and the mechanical properties of the resulting off-stoichiometric films of Zr Cr B (Ti Al B ) can be improved or tuned via controlling the films’ composition 6 , 8 , 11 .…”