Invited: Dominant role of second nearest-neighbor bonding in strength-ductility tradeoff of bcc refractory high entropy alloys
Balancing strength and ductility in body-centered cubic (bcc) refractory high entropy alloys remains a fundamental challenge due to the intrinsic trade-off between bond strength and dislocation mobility. Using density functional theory (DFT) calculations, we show that electronic structure and bond strength play a critical role in affecting both strength and ductility. Specifically, there is a noticeable decrease in the density of states at the Fermi level, N(Ef), within a short window of valence electron concentration (VEC) around 5.1 marking a crossover from metallic to covalent-type bonding in refractory bcc alloys. This change is driven by a disproportionate suppression of eg states, responsible for σ-type bonds along second nearest neighbor (2NN) directions, relative to t2g states. The resulting increase in 2NN stiffness outpaces the increase in the first nearest neighbor (1NN) stiffness and drives steep rise in Young and shear moduli, while simultaneously reducing local lattice distortion and ductility. Enrichment with higher-VEC Group VI elements (e.g., Cr, Mo, W) amplifies these effects, whereas addition of lower-VEC Group IV and V elements (e.g., Ti, Zr, Hf, V, Nb, Ta) reduces them. These results establish 2NN bonding as the dominant atomic-scale mechanism controlling the strength-ductility balance, providing a pathway for designing next-generation bcc refractory alloys.