Processing, microstructure and mechanical properties of Ni1.5CoFeCu0.8Al0.2V0.5 high entropy alloy matrix composites reinforced by in-situ synthesized vanadium carbides

Three in-situ synthesized vanadium carbides (in the volume fraction of 5%, 10% and 15%, respectively) reinforced Ni1·5CoFeCu0·8Al0·2V0.5 high entropy alloy (HEA) matrix composites were prepared by adding V and C in equal molar ratio using mechanical alloying and spark plasma sintering techniques. The formation and evolution of vanadium carbide reinforcements as well as their influences on the microstructure and mechanical properties of HEA matrix composite were investigated intensively. The results show that after milling of 70 h part of V and C from raw materials reacted incompletely through the mechanism of solid-state diffusion at the interface, resulting in the generation of V2C and [VC] in the composite powders. Subsequently, owing to the enhanced atomic diffusion during sintering V2C in the intermediate state was further carbonized into [VC] and partial [VC] was transformed to ordered V8C7 in the consolidated composites. The addition of 5 vol% and 10 vol% vanadium carbides resulted in the significant refinement of grains in the HEA matrix due to the grain boundary pinning effect induced by in-situ generated fine reinforcements. However, grains of the composite containing 15 vol% vanadium carbides are coarser than those of the base HEA due to the serious agglomeration of in-situ synthesized reinforcements. With the increase of vanadium carbides content, mechanical properties of the HEA matrix composite do not change monotonously. The composite reinforced by 10 vol% vanadium carbides performs best among the three and its yield strengths at room temperature, 500 °C, 600 °C and 700 °C are about 1.2, 1.4, 2 and 2.5 times, respectively, than those of the base HEA. The improvement in mechanical properties of composite are attributed to grain boundary strengthening and dispersion strengthening caused by in-situ synthesized vanadium carbide nanoparticles.