Mechanical properties and strengthening mechanism of the nano-sized m-ZrO2 ceramic particle reinforced NbMoTaW refractory high-entropy alloy

Body-centered cubic (BCC) refractory high-entropy alloys (RHEAs) possess the typical advantage of high strength. However, RHEAs often exhibit apparent brittleness at room temperature, which remains great challenges for their engineering applications. In this study, NbMoTaW with 0 and 1 wt% m-ZrO2 nano-sized ceramic particles RHEAs have been successfully fabricated by ball milling and spark plasma sintering (SPS). The effect of m-ZrO2 ceramic particles on the microstructures and mechanical properties of NbMoTaW RHEAs was investigated. Results show that the matrix of alloys was characterized by single-phase BCC. Microstructural analysis shows that the grains in the single-phase BCC alloy have been refined by 39.0% through introduction of oxide nano-ceramic particles. The m-ZrO2 ceramic addition could effectively enhance both the hardness and strength of the NbMoTaW RHEA, accompanied by a slight decrease in plasticity. Compared with the vacuum arc-melting (VAM) method, the sintered NbMoTaW-1ZrO2 RHEA shows a significant increase in comprehensive compressive properties, especially the ductility. The yield strength, ultimate compressive strength and fracture strain of sintered NbMoTaW-1ZrO2 RHEA are 1775 MPa, 2118 MPa and 12.4%, respectively, surprisingly increased by 67.8%, 74.9% and 376.9%. A quantitative yield strength model correlating m-ZrO2 ceramic particle size, volume fraction and NbMoTaW-1ZrO2 grain size was built. The addition of fine m-ZrO2 ceramic resulted in Orowan strengthening and fine grains, thereby increasing yield strength of alloy. The superior combination of mechanical properties of the NbMoTaW-1ZrO2 RHEA could promote it to be a promising alloy in engineering applications.