Lattice distortion as an estimator of solid solution strengthening in high-entropy alloys

Abstract

Over the past decade, select high-entropy alloys (HEAs) have exhibited excellent structural properties, even at high temperatures, outperforming conventional alloys in some cases. Intriguingly, some reports in the literature suggest that HEA properties may be enhanced by increasing the number of elements, while another school of thought negates this notion and suggests that there is no clear dependence of mechanical properties on number of elements. We further examine this question in the context of a quinary refractory alloy system (MoTaTiWZr) and scrutinize whether number of elements in a HEA positively impact its mechanical properties. The present work showcases that certain equiatomic low- and medium- entropy alloys can exhibit superior structural properties (hardness, Young's modulus) relative to their higher-entropy counterparts composed of the same family of elements. Evidently, incorporating a higher number of constituent elements does not guarantee enhanced structural properties. Using a synergy of experimental measurements, complementary microscopic characterization and materials theory, we conclusively demonstrate that the intrinsic lattice distortion and cohesive energies are the predominant strengthening mechanisms that are reflected as high hardness and Young’s moduli of single-phase multicomponent alloys investigated in this work. Severe lattice distortion is one of the core effects of HEAs which imparts excellent room temperature structural properties and is generated by mixing multiple atom types. Likewise, a higher cohesive energy between the atoms in a lattice requires greater shear stresses to break the metallic bonds that increases the stiffness. An alloy with lower number of elements may intrinsically possess a higher cohesive energy than one with a higher number of elements within the same series, thereby outperforming the higher-entropy alloy on the structural properties.