Synergetic enhancement of hardness and toughness in new superconductors Ti₂Co and Ti₄Co₂O

Compared to traditional superhard materials with high electron density and short and strong covalent bonds, alloy materials mainly composed of metallic bonding structures typically have great toughness and lower hardness. Breaking through the limits of alloy materials is a preface and long-term topic, which is of great significance and value for improving the comprehensive mechanical properties of alloy materials. Here, we report on the discovery of a cubic alloy semiconducting material Ti₂Co with large Vickers of hardness H_v^exp ~ 6.7 GPa and low fracture toughness of K_IC^exp ~ 1.51 MPa·m^1/2. Unexpectedly, the H_v^exp ~ 6.7 GPa is nearly triple of the H_v^cal ~ 2.66 GPa predicted by density functional theory (DFT) calculations and the KICexp ~ 1.51 MPa·m^1/2 is about one or two orders of magnitude smaller than that of ordinary titanium alloy materials (K_IC^exp ~ 30-120 MPa·m^1/2). These specifications place Ti₂Co far from the phase space of the known alloy materials. Upon incorporation of oxygen into structural void positions, both values were simultaneously improved for Ti₄Co₂O to ~ 9.7 GPa and ~ 2.19 MPa·m^1/2, respectively. Further DFT calculations on the electron localization function of Ti₄Co₂X (X = B, C, N, O) vs. the interstitial elements indicate that these simultaneous improvements originate from the coexistence of Ti-Co metallic bonds, the emergence of newly oriented Ti-X covalent bonds, and the increase of electron concentration. Moreover, the large difference between H_v^exp and H_v^cal of Ti₂Co suggests underlying mechanism concerning the absence of the O(16d) or Ti₂-O bonds in the O-(Ti₂)₆ octahedron. This discovery proposes a new pathway to simultaneously improve the comprehensive mechanical performances and illuminates the path of exploring superconducting materials with excellent mechanical performances.