Bond Strength - A Key Cause of Local Lattice Distortion in BCC High Entropy Alloys

Chemical randomness produces a range of bond lengths manifested in the form of local lattice distortion (LLD), a key concept in high-entropy alloys (HEAs) linked to solid-solution-like strengthening. LLD has been attributed mainly to atomic-radii mismatch, but recent observations of charge transfer make atomic radii dynamic and dependent on local bonding. We show that bonding strength plays a comparable role in LLD. Density functional theory calculations reveal that stronger bonding, quantified by valence electron configuration (VEC) and elastic moduli, reduces LLD. Alloys with weak bonding display a wide spread of bond lengths where the first-nearest neighbor distance can even exceed the second, yielding high distortion. Moreover, LLD correlates with electronic occupancy at the Fermi level, where higher occupancy results in larger distortion thereby connecting LLD to pseudo-gap in the bimodal density of states of BCC HEAs. We conclude that both atomic size mismatch and bond strength jointly govern LLD.