New phase space of hardness materials and synergic enhancement of hardness and toughness in superconducting Ti2Co and Ti4Co2X (X = B, C, N, O)

TL;DR

This study discovers new alloy materials, Ti2Co and Ti4Co2X, with enhanced hardness and toughness, expanding the phase space of alloy materials and revealing mechanisms behind their mechanical properties.

Contribution

The paper introduces novel alloy semiconducting materials with significantly improved hardness and toughness, surpassing traditional alloy limits and expanding the known phase space.

Findings

Ti2Co exhibits high hardness (6.7 GPa) but low toughness (1.51 MPa·m^0.5).

Oxygen incorporation in Ti4Co2X improves both hardness and toughness.

Enhanced properties are linked to specific bonding and electron concentration changes.

Abstract

Compared to traditional superhard materials with high electron density 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 Hvexp = 6.7 GPa and low fracture toughness of KICexp =1.51 MPa m0.5. Unexpectedly, the former value is nearly triple of the Hvcal = 2.66 GPa predicted by density functional theory (DFT) calculations and the latter value is about one or two orders of magnitude smaller than that of ordinary titanium alloy materials (KICexp = 30-120 MPa m0.5).These specifications place Ti2Co far from the phase space of the known alloy materials, but close to medium hardness materials such as MgO or TiO2. Upon incorporation of oxygen into structural void positions, both values were simultaneously improved for Ti4Co2O to = 9.7 GPa and 2.19 MPa m0.5, 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 Hvexp and Hvcal of Ti2Co suggests underlying mechanism concerning the absence of the O(16d) or Ti2-O bonds in the O-(Ti2)6 octahedron.Our discovery expands the phase space of alloy materials and illuminates the path of exploring superconducting materials with excellent mechanical performances.