The origin and the effect of the fcc phase in sintered HfNbTaTiZr

The growing interest in preparing refractory high entropy alloys via powder metallurgy reveals significant sensitivity to impurities, influencing the phase composition and microstructure and, consequently, the mechanical properties. Mechanical alloying of commercial purity powders is a cost-effective way of material preparation leading to a very fine microstructure when spark plasma sintering is used. However, the atmosphere of the milling process plays a significant role. This study shows that the phase composition of the HfNbTaTiZr alloy may be tailored by milling in ambient atmosphere, in argon or in nitrogen. Formation of a new fcc-ZrHfTi phase as a complex nitrogen and oxygen compound was elucidated and its significant hardening effect was shown. Finally, exceptional hardness was achieved by the synergic effect of nano-grained structure and high volume fraction of fcc-ZrHfTi phase.