Boron-induced microstructure evolution and related wear behavior in NbMoTaWBx high-entropy alloys and composites at room and elevated temperatures

Refractory high entropy alloys (RHEAs) such as NbMoTaW exhibit potential for wear-resistant applications due to their high hardness and thermal stability at elevated temperatures. The addition of boron (B) may lead to a transformation from RHEAs into refractory high entropy composites (RHECs) due to the formation of hard boride phases. This transition is anticipated to significantly increase the hardness and wear resistance over a large temperature range. In this study, we investigate the impact of boride reinforcements on the microstructure and wear performance of the NbMoTaWB_x (x = 0–2) at room and elevated temperatures up to 600 °C. The results show the following: (i) The microstructure evolves from a dendritic microstructure to a complex microstructure with eutectic phase, dispersed body-centered cubic (BCC) particles and boride phases, showing the transition from RHEAs to RHECs; (ii) At room temperature, the wear rate decreases with increasing boron content, with abrasive wear as the primary mode and oxidation wear as minor contribution; (iii) At 600 °C, the wear rate increases with increasing boron content, and the dominant wear mode is oxidation-abrasion.