Ultrahigh Ductility in the BCC Nb45Ta25Ti15Hf15 Refractory Multiprincipal Element Alloy: Energetic and Electronic Origins.

Recent experimental discovery of the ultrahigh ductility of the body-centered cubic (BCC) Nb45Ta25Ti15Hf15 refractory multiprincipal element alloy (RMPEA) exhibits a clear distinction from the MoNbTaW RMPEA with low ductility. To unravel the physical origins of this remarkable ductility, this study employs first-principles calculations to investigate the factors governing intrinsic ductility of the RMPEAs. The energy calculation results indicate a stronger tendency for double cross-slip, dislocation multiplication, and twinning deformation of the Nb45Ta25Ti15Hf15 RMPEA. Integrated with fracture criteria, it is further confirmed that multiple dislocations and twins can emit from the crack tip in this alloy. Analysis of the electron structure reveals that the compositional substitution induces the charge delocalization, bonding metallization and a higher and smoother density of states curve near the Fermi level of the Nb45Ta25Ti15Hf15 RMPEA. Furthermore, increasing Nb content generally raises the calculated fault and surface energies, while favorable plasticity tendencies persist across the investigated BCC configurations. Nb addition can increase the energetic barrier to fault formation and dislocation nucleation without suppressing the intrinsic propensity for crack-tip plasticity over a broad composition range. These findings can provide energetic and electronic insights into the intrinsic ductility of the BCC RMPEAs containing group IV elements.