Multiscale Mechanics Study of Metallic Mechanical Metamaterials and Their Multifunctional Applications

Abstract

Metallic components serve as the cornerstone of modern industry. As a new paradigm of materials, metallic mechanical metamaterials can integrate the concept of architecture and intrinsic properties of materials, enabling the creation of distinct mechanical performance and advanced functionalities beyond conventional bulk structural materials. Recent advances in additive manufacturing or 3D printing technologies allows us to create the three-dimensional architected materials with unprecedented spatial freedom. Besides the advantages in the creation of component-scale topology structure, different metallic additive manufacturing technologies can also endow unique material-level design characteristics with microstructural complexity, such as grain morphologies, molten pools, and growth nanotwins. Therefore, cross-scale coordination mechanisms understanding for mechanical metamaterials from the nano/microscale to the macroscale exhibits the critical role to obtain customized mechanical performance and multifunctionality. In this thesis, based on the cross-scale investigation, we aim to explore the mechanical response of mechanical metamaterials in time scale and length scale beyond their quasi-static mechanical properties, reveal the material-structure coordination mechanism, colonized some unexplored multifunctional application space.

We first conducted a systematical review about the research progress of mechanical metamaterials including the materials systems, design strategies, and fabrication methodologies. Furthermore, we provided an overview of the current capabilities of mechanical metamaterials beyond their quasi-static mechanical properties, including the dynamic mechanical behavior at large strain rate and emerging multifunctional engineering applications.

We then fabricated a series of lightweight and high-strength medium entropy alloy microlattices via a self-developed micro-selective laser melting technology. Based on in situ projectile impact experiments with a large strain rate range from quasi-static to 10⁵ s^-1, we demonstrate ultrahigh energy dissipation across seven orders of magnitude in strain rate in micro-architected medium-entropy alloys, surpassing existing metals/alloys. Multiscale characterization revealed architecture- and material-induced dynamic toughening mechanisms. Architecture-induced amplification of the effective dynamic stress in the metamaterial can activate hierarchical microstructure deformation mechanisms encompassing distorted dislocation cells, highly activated multiple deformation twins, hexagonal nano-lamellas, nano-sized amorphous bands, and restricted void nucleation, facilitating a hierarchical energy dissipation at varying stress levels, thereby contributing to the effective reinforcement of the material under extreme dynamic loading conditions.

To meet the requirement for the decreased package size available and high cooling efficiency, we then developed high-precision and high-performance pure copper and copper alloy mechanical metamaterials with high surface areas and excellent thermal properties via selective laser melting technology. Firstly, in term of copper alloy development, through increasing the Cr to Nb atomic ratio based on commercial CuCrNb alloys, we introduce the dual nanoprecipitations strategy enables us to successfully fabricate high-precision CuCrNb microlattices with the exceptional printability, high mechanical strength, and homogeneous deformability until densification strain. Then, we report a facile and cost-effective oxide dispersion strategy to additively manufacture a high printing resolution and unprecedented mechanical properties pure copper microlattices. Ultrafine and well-dispersed Cu₂O nanoparticles into the copper powder can effectively improve the laser absorptivity and molten pool viscosity, contributing to the achievement of low surface roughness and high-precision capability. Our work demonstrate that copper/copper alloys mechanical metamaterials have great potential for future high-precision thermal dissipation applications.

Subsequently, based on the architectural concept of metamaterials, we developed a metamaterials catalyst with controllable strain engineering. Through elaborately employing screw dislocation driven growth model during the synthesis of metamaterial catalyst, we can in-situ constructed edge-rich surface nanoterrace with layer thickness below 10 nm, directly extending the hierarchy range from the architecture scale to the active sites scale without extra post-treatment. Abundant Gaussian curvatures variety on metamaterial catalyst making them ideal supports for inducing local strain. We observed that the twist behavior of nanoterrace was significantly determined by the curvature factor of the architecture, thus elaborately introducing into the curvature-related lattice strain engineering. As a typical application demonstration of strained metamaterial catalysts, nitrate-to-ammonia conversion experiment reveals a superior electrocatalysis activity and long-term stability with the highest FE_NH3 of 95.4%, NH₃ yield rate of ~20.58 mg h⁻¹ cm⁻² at 0.1 M NO₃^– concentration, and stably running of more than 500 h. This strategy offers a paradigm to develop next-generation catalyst with digitally controllable lattice strain configurations.

In summary, this thesis systematically studied the material-structure-performance relationship of mechanical metamaterials at time and length scales to overcome various challenges in conventional bulk structural materials region. We also explored some multifunctional characteristics of mechanical metamaterials in interdisciplinary fields beyond mechanics. The strategies proposed provide a pathway to develop next-generation high-performance mechanical metamaterials for functionalized application in the future.

Date of Award	2 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shijun ZHAO (Supervisor) & Yang Lu (External Co-Supervisor)