Overcoming brittleness in NbMoTaW refractory high entropy alloy: A review of strength–ductility strategies and challenges

NbMoTaW and its derived refractory high-entropy alloys (RHEAs) exhibiting a single body-centered cubic (BCC) lattice structure have attracted widespread attention due to their exceptional strength at high temperatures. However, their practical applications are limited by room-temperature brittleness. While the BCC stability ensures resistance to thermal softening, its intrinsic high barrier to dislocation movements reduces the intrinsic ductility. Extensive research has been focused on mitigating this issue through microstructural and compositional modification. This review first discusses the fundamental mechanisms governing the brittleness in NbMoTaW and related BCC RHEAs, including grain boundary embrittlement, intrinsic properties and atomic environments. Then promising results from alloying and processing optimization are highlighted. Finally, the industrial feasibility and potential directions for alloy design and development are outlined.