Preparation, characterization, and phase evolution of nanocrystalline Cu/Ti doped MoNbTa-based multi-principal element alloys

As multi-principal element alloys (MPEAs) continue to draw attention from researchers, there is a growing demand for MPEAs with a combination of body-centered cubic (BCC) and face-centered cubic (FCC) phases because of their intriguing properties. In the present study, we prepared MPEAs with BCC and FCC phases using composition optimization, mechanical alloying (MA), and powder sintering. Two alloys Mo25Nb25Ta25Cu25 and Mo28Nb28Ta28Cu8Ti8 (in at.%) were prepared from elemental powders. The phase evolution and composition were investigated from the powder processing stage to the annealing stage, using X-ray diffractometry and electron microscopy. As the milling times increase, the crystallite sizes were significantly refined. For the as-sintered Mo25Nb25Ta25Cu25, two different BCC structures were observed along with an FCC phase enriched in Cu. In contrast, the sintered Mo28Nb28Ta28Cu8Ti8 also exhibited the same dual-phase BCC structures but contained Ti-rich microprecipitates. The sintered Mo28Nb28Ta28Cu8Ti8 had a better balance of mechanical strength (1302 MPa) and plasticity (20%) compared to Mo25Nb25Ta25Cu25 alloy. Gibbs free energy calculations showed that the dual-phase BCC + FCC structure arose because of the miscibility gap between Cu and the refractory elements.