Setting material benchmarks at large-strain limits via ultimate strengths

Structural stability and durability are two foundational attributes underpinning all material functionalities, yet traditional approach only sets stability criteria in terms of elastic parameters derived at small strains, then extends their use to probing strength and durability at large strains via empirical relations due to a lack of accurate material benchmarks at strong deformation. Such extrapolations, however, may cause major quantitative or even qualitative deviations in assessing material behaviors at large strains when the elastic parameters fail to capture distinct underlying physics under strong deformations. Here, we introduce ultimate strengths, defined by peak stresses on diverse deformation paths from first-principles calculations, to set accurate and robust benchmarks for assessing materials at large-strain limits. We take transition-metal diborides as an exemplary class of materials to showcase strong directional anisotropy and load dependence of stress responses at large strains, in sharp contrast to the behaviors predicted by elastic parameters. We elucidate the impact and mechanism of load-constrained deformation, bond charge distribution, and electronic band structure on mechanical responses at elastic or dynamic stability limits. This work fulfills a longstanding need in materials science to set robust and accurate benchmarks that are tailored as explicit descriptors for key material characteristics under strong deformations.