Extraordinary toughening enhancement in nonstoichiometric vanadium carbide

Abstract

• VC 1- x composites have a quasi-monophasic fcc structure with coherent precipitates. • A doubling of fracture toughness is obtained in VC 1- x without any second phase. • VC 1- x show an unadulterated transgranular fracture feature, leading to toughening. • Coherent toughening and amorphous bridging toughening make a prodigious toughening. Improving fracture toughness, which has gone through decades, is a long-standing topic and is particularly important for safety-critical applications. In refractory transition metal carbides (RTMCs), remarkable toughening is usually achieved by adding metallic binders, however, resulting in a drastic deterioration of hardness and thermal stability. Here, we report a novel self-toughening strategy for synthesizing high-toughness RTMCs. Using mechanical alloying (MA) and spark plasma sintering (SPS), we synthesized nonstoichiometric VC 1- x (0.5 ≤ 1- x ≤ 0.6) with a quasi-monophasic microstructure containing a carbon-rich matrix and carbon-poor precipitates. Significantly, the VC 0.5 sintered at 1400°C shows a good trade-off of high hardness of 20.5±0.5 GPa and fracture toughness of 7.1±0.2 MPa m 1/2 . The fracture toughness of VC 0.5 increases by more than 100% accompanied by 7% hardness loss, compared with that of stoichiometric VC. The microstructure characterization and fracture behavior analysis demonstrate that the extraordinary toughening enhancement is attributed to a self-toughening strategy combined with coherency toughening and amorphous bridging toughening, which may offer an efficient pathway for developing high-performance structural ceramics.