Gradient Structure Design of High- and Medium- Entropy Alloy via Novel Surface Modification Techniques

The high- and medium-entropy alloys (HEAs and MEAs) are a new class of structural materials, which generally consists of three or more elements and exist either in equiatomic or non-equiatomic compositions. These materials have been studied extensively in the last decades due to their excellent mechanical properties even at extremely low temperatures. Nevertheless, the development and understanding of its structural ability are still unqualified for industrial applications. One of the main concerns is strengthening the alloy efficiently and economically up to industrial standards. Recent studies showed that a heterogeneous structure could improve the mechanical properties of these alloys to the extent of avoiding a strength-ductility trade-off. This study explores the methods of fabricating gradient structure in HEAs and MEAs through novel surface treatments, namely ultrasonic nanocrystal surface modification (UNSM) and aerosol deposition (AD). These processes induced a severe plastic deformation on the surfaces, ultimately producing a heterogeneous structure within the alloys. The deformation was performed in a fully-recrystallized microstructure, which then produces a dislocated and refined grain and undeformed grain. The microstructure features a significant hardness and strength improvement nearly twice compared to the alloys without surface treatment. The study provides guidelines for utilizing the novel design of the alloy and the concept of the heterogeneous structure to develop the HEAs and MEAs for further structural application.