Tailoring the Mechanical Properties of Earth-Abundant Transition Metal Borides via Bonding Optimization
2020
Zhang, Ziyan | Tehrani, Aria Mansouri | Brgoch, Jakoah
Borides containing 4d or 5d transition metals are among the most common types of high hardness materials because of the high valence electron density of the metals combined with short, covalent main group bonding. Unfortunately, even though these compounds have outstanding mechanical properties, most 4d and 5d transition metals are expensive with low natural availability. Studying their 3d counterparts such as Cr₃B₄ which has an experimentally measured Vickers hardness of 26 GPa, may provide insight on the structure–property relationship that can be used to develop earth-abundant, 3d transition metal based high hardness materials. The origin of the mechanical properties in Cr₃B₄ was therefore computationally investigated using density functional theory. The changes in bonding as a function of composition were analyzed through crystal orbital Hamilton population calculations and revealed that the bonding in Cr₃B₄ could be optimized by changing the valence electron count via vanadium substitution. The calculations also verify that as the bonding states are optimized, the mechanical properties are improved. Further influencing the bonding by introducing Ti across the hypothetical “V₃–ₓTiₓB₄” solid solution shows significant deterioration of the mechanical properties as the occupied bonding states are depleted. These results prove the relationship between bonding and intrinsic mechanical properties and indicate that bonding optimization through elemental substitution is an effective approach to tailor the properties of structural materials.
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