Fabrication and Mechanics of Low-stacking Fault Energy (SFE) Multicomponent Alloy films and Their Metallic Metamaterial Applications

低層錯能多主元合金薄膜的製備與力學研究及其金屬超材料應用

Student thesis: Doctoral Thesis

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Award date28 May 2021

Abstract

In recent years, three-dimensional (3D) micro-/nanolattices have appeared as a unique approach to realize superior specific strength through merits of reasonably topological optimization as well as size effect at small-scale. Nevertheless, the majority of metallic and ceramic micro-/nanolattices present an inevitable degradation of mechanical performance after cyclic loading because of local brittle failure. The key issue is the fracture mode, namely, localized brittle failure of metallic coatings usually applied to improve the strength of polymer lattices, leading to strut fracture and limiting their practical applications. Besides, multicomponent alloys (MCAs) show outstanding mechanical performances, like superior specific strength and toughness, regarded as promising structural materials. However, when reducing the grain size to the nanoscale, the insufficient plastic deformation turns into the drawback of these ultrastrong MCA films.

In this case, first, we studied the effect of Zr alloying on the phase transformation and mechanical responses of CoCrFeNiZrx MCA films. With increased Zr content, a phase transformation from crystal to amorphous is found. The intermediate crystal-glass dual-phase films show double strength than the other films. Besides, heavily nanotwins induced by pre-deformation endows a strong resistance to crack propagation, leading to unprecedented strength and crack resistance of the low-SFE CoCrNi MCA.

Next, we revealed the size effect on microstructure and mechanical responses, containing the hardness (H) and strain rate sensitivity (m) of low-SFE Al0.1CoCrFeNi MCA films. Notably, the MCA films achieved ultrahigh hardness with decreased grain size (d) or film thickness (h). Additionally, the activation volume was monotonously decreased with decreased d and h, indicating increased grain boundary activities.

Combined with the above strategies, through the coating of CoCrNiTi0.1 microalloyed MCA with extremely low SFE, we prepared ultratough MCA-coated nanolattices. In particular, nanolattices with the thinnest film can repeatedly sustain strains without strut failure by unique surface wrinkling. Besides, the MCA nanolattices presented excellent energy absorption and specific strength due to the MCA film’s ultrahigh strength, offering an approach for improved mechanical and functional applications.