A review on advanced manufacturing strategies for MoNbTaW refractory high-entropy alloys: Processing, structure–property relationships, and environmental performance

Abstract MoNbTaW refractory high-entropy alloys (RHEAs) have attracted considerable interest for extreme environment applications due to their exceptional high-temperature strength, thermal stability, and simple body-centered cubic (BCC) solid-solution structure. This review provides a comprehensive analysis of their processing routes, microstructural evolution, mechanical properties, strengthening mechanisms, and environmental behavior. Fabrication methods, including arc melting, spark plasma sintering, and additive manufacturing, are comparatively evaluated with respect to phase stability, compositional uniformity, and defect evolution. Microstructural transitions from dendritic segregation in cast alloys to refined and dense structures in powder-processed and additively manufactured systems are critically examined. The mechanical response is discussed in relation to temperature, composition, and processing, highlighting the roles of solid solution strengthening, grain boundary effects, and secondary phase reinforcement. Despite their high strength, limited room-temperature ductility remains a key challenge, necessitating compositional tuning and microstructural engineering. High-temperature oxidation and wear behavior are also assessed, revealing limitations associated with non-protective oxide scale formation and spallation. Surface engineering strategies, including coatings and thin films, are identified as effective approaches to enhance environmental resistance. This review provides a framework for advancing MoNbTaW RHEAs for high-temperature applications.