Enhanced Hardness in Transition-Metal Monocarbides via Optimal Occupancy of Bonding Orbitals

Insights into the anomalous hardness of the tantalum carbides from dislocation mobility

The tantalum carbides, TaCx, have been repeatedly shown to harden dramatically with some loss of carbon content, then soften with further decarburization. First observed in 1963, this anomalous hardness behavior has been reproduced for decades without satisfactory explanation. Prior attempts to characterize this phenomenon using elastic stiffnesses have failed to reproduce the anomalous hardness behavior. In this work, we demonstrate a change in slip system preference from {111}B1 to {110}B1 in TaCx as x decreases, while no such transition is observed in TiCx. We find this to be the primary mechanism of the anomalous hardness, arising from reduced energetic favorability of dissociation of dislocations on {111}B1 into Shockley partials at lower carbon contents. We also present experimental hardness measurements for bulk and thin-film TaCx at different carbon contents. An anomalous hardness peak is observed in the bulk samples, but not in the thin films, due to loss of dislocation plasticity in the nanocrystalline films.

Discovery of Metastable W3P Single Crystals with High Hardness and Superconductivity

Hard and superconducting materials play significant roles in their respective application areas and are also crucial research fields in condensed matter physics. Materials with the key properties of both hard and superconducting properties could lead to technology development, but it is also full of challenges. Herein, we report the synthesis of high-quality metastable W3P single crystals with superconductivity and excellent mechanical properties. The synergistic effect of temperature and pressure was effective in suppressing further decomposition of metastable W3P as-synthesized by our synthesis technique (high-pressure and high-temperature method). The transport and magnetic measurements indicate that W3P is a typical type-II BCS superconductor, displaying a superconducting transition temperature of 5.9 K and an impressive critical magnetic field of 4.35 T. Theory calculations reveal a metallic property in W3P, and the phonon modes of the vibration of W atoms are important for electron-phonon interaction. Meanwhile, W3P shows excellent mechanical properties with a high fracture toughness of 8 MPa m1/2 and an impressive asymptotic hardness of 22 GPa, which is currently reported as being the hardest among transition metal phosphides. It opens up a new class of advanced materials that combine excellent mechanical properties with superconductivity.

Coexistence of Superhardness and Metal-Like Electrical Conductivity in High-Entropy Dodecaboride Composite with Atomic-Scale Interlocks

High electrical conductivity and super high hardness are two sought-after material properties, but both are contradictory because the effective suppression of dislocation movement generally increases the scattering of conducting electrons. Here we synthesized a high-entropy dodecaboride composite (HEDC) with a large number of atomic-scale interlocking layers. It shows a Vickers hardness of 51.2 ± 3.6 GPa under an applied load of 0.49 N and an electrical resistivity of 44.5 μΩ·cm at room temperature. Such HEDC achieves superhardness by inheriting the high intrinsic hardness of its constituent phases and restricting the dislocation motion to further enhance the extrinsic hardness through forming numerous atom-scale interlocks between different slip systems. Moreover, the HEDC maintains the excellent electrical conductivity of the constituent borides, and the competition between two correlating structures produces the special kind of coherent boundary that minimizes the scattering of conducting electrons and does not largely deteriorate the electrical conductivity.