Extraordinary toughening enhancement in nonstoichiometric vanadium carbide

•VC1-x composites have a quasi-monophasic fcc structure with coherent precipitates.•A doubling of fracture toughness is obtained in VC1-x without any second phase.•VC1-x show an unadulterated transgranular fracture feature, leading to toughening.•Coherent toughening and amorphous bridging toughening make a prodigious toughening.