Tailoring an ultrafine-grained VNbMoTaW refractory high entropy alloy with ultrahigh strength

Ultrafine-grained (UFG) VNbMoTaW refractory high entropy alloy (RHEA), reinforced with carbide particles, was prepared through a novel powder metallurgy route. Initially, pre-alloyed RHEA powder, characterized by body-centered cubic (BCC) phase, ∼88 nm particle size and good dispersity, was fabricated via a wet-chemical (WC) method. The high quality of the nano powder enabled low-temperature sintering with suppressed grain growth. Subsequently, a dense UFG VNbMoTaW RHEA bulk sample was fabricated by spark plasma sintering (SPS). It provided uniform microstructure with 99 % theoretical density and ∼320 nm grain size. The residual carbon from powder facilitated the precipitation of carbide phases in the sintering process. These carbide precipitates were identified by a high-entropy characteristic, ∼180 nm particle size and a volume fraction of 8.3 %, serving as effective reinforcing agents for the RHEA matrix. Benefiting from the high configurational entropy, ultrafine grain size, and precipitate phase, the bulk RHEA exhibited excellent mechanical properties in terms of a high hardness of 10.9 GPa and a compressive strength of 4.2 GPa with an acceptable strain of 10.7 %. We expect such a powder metallurgy route should also be applicable to other systems of RHEAs.