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[Feature article]Optimizing HEA film thickness to improve the toughness of its composite microlattice metamaterials

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Release Date: 2021-01-26 Visited: 

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1. Introduction 

Recently, the incorporation of high-entropy alloys (HEAs) in composite microlattice structures have yielded superior mechanical performance and desirable functional properties compared to conventional metallic lattices. However, the modulus mismatch and poor adhesion between the soft polymer core and stiff metallic film coating often results in delamination of the film and brittle strut fracture at relatively low strain levels (typically below 10%). In this work, by utilizing metal size effect, Surjadi et al. from City University of Hong Kong reported that “Optimizing film thickness to delay strut fracture in high-entropy alloy composite microlattices” on International Journal of Extreme Manufacturing (IJEM). In this work, the authors demonstrated that by optimizing the HEA film thickness in a CoCrNiFe-coated microlattice, they can completely suppress delamination and significantly delay the onset of strut fracture, consequently increasing the specific strength by up to 50%. This work presents an efficient strategy to improve the properties of metal-composite mechanical metamaterials for structural applications.

2. Background

Emerging mechanical metamaterials including metallic microlattice metamaterials have attracted increasing interest in the recent years owing to its ability to exhibit excellent mechanical properties such as high stiffness and strength at low densities. Among the various strategies implemented, metal/alloy composite microlattices present a straightforward but effective method to elevate the stiffness and strength at even lower weight. Furthermore, high-entropy entropy alloys (HEAs), consisting of four or five primary constituent metal elements, have recently been implemented in composite micro- and nanolattices to exhibit superior specific strength and tunable properties compared to conventional metals/alloys. However, these HEA microlattices still experience brittle strut fracture at low compressive strains (~ 7%) despite its high strength. This phenomenon has also been observed in most of the composite microlattice reported thus far and is primarily caused by the insufficient strength of the core or delamination of the film.

3. Recent Advances

HEA composite lattice design and microfabrication process

Firstly, the octet lattice was designed and optimized using a CAD software (SolidWorks) and the polymer microlattices were fabricated via PµSL (BMF Technologies). The octet geometry was chosen due to its stretching-dominated behavior, allowing it to withstand larger loads than a bending-dominated lattice at similar relative densities. The octet lattice has a unit cell size of 2 mm and a strut diameter of 0.2 mm. Subsequently, a thin layer of CoCrNiFe HEA film were then deposited onto polymer scaffold via magnetron sputtering at room temperature.

Figure 1. (a) Representative unit cell of the HEA-coated octet lattice fabricated. (b, c) Schematic illustration of the PµSL printing technology (b) to fabricate the polymer lattices and DC magnetron sputtering (c) used to deposit a thin HEA coating on the polymer lattice to obtain the composite lattices.

in situ uniaxial compression of polymer and composite microlattices

From Fig. 2a, it could be seen that all the microlattices show elastic strut buckling at low compressive strains. However, the microlattice with higher coating thickness (i.e. 400 nm) exhibit localized strut fracture prior to the pristine polymer (i.e. 0 nm) and composite microlattice with lower coating thickness (i.e. 100 nm). On the other hand, the microlattice with thinner coating only began to display strut fracture at a relatively much higher compressive strain (~ 17%) compared to that of the composite microlattice with thicker coating (~ 8.5%) and is similar to pure polymer microlattice (~ 18%).

Figure 2. (a) Deformation behavior of the microlattices up to the strain at which strut fracture starts to occur. (b) Representative stress-strain curves of the polymer and composite microlattices coated with 100 nm and 400 nm of HEA film.

Post-compression SEM images of the composite microlattices with different film thicknesses

The improved strength and ductility of the composite microlattice with thinner coating is mainly attributed to the brittle-to-ductile transition of the HEA film caused by the size reduction. The microlattice with 100 nm coating retains its smooth surface morphology without any apparent cracks even after deformation. Conversely, the microlattice with 400 nm coating exhibit multiple cracks on the surface of its struts after compression. The cross-sectional morphology of the fractured strut also indicates some delamination and brittle fracture surface, implying that it undergoes little to no plastic deformation. This explains the largely delayed onset of fracture in the composite microlattices with thinner film (100 nm), which results in higher peak stress at lower densities.  

Figure 3. (a, b) The post-compression SEM image of an octet unit cell, (c, d) Outer fracture surface morphology, and (e, f) cross-sectional morphology for the lattices coated with 100 nm and 400 nm of CoCrNiFe HEA, respectively.

4. Perspectives

In this article, the authors employed Projection Micro-stereolithography (PµSL) and magnetron sputtering to fabricate CoCrNiFe-coated composite microlattice. PµSL offers a combination of high resolution and relatively large build area, enabling the fabrication of centimeter-scale samples with micro-scale resolution. Furthermore, by optimizing the film thickness, they have introduced that it is possible to completely suppress delamination and drastically delay the onset of strut fracture. This strategy may apply for other advanced metallic/alloy composite microlattice design for robust mechanical and functional applications.

5. About the Authors

Yang Lu is currently Associate Professor in the Department of Mechanical Engineering at City University of Hong Kong (CityU). He directs the Nanomanufacturing Laboratory at CityU Shenzhen Research Institute, which focuses on nanomechanics and nanomanufacturing. He has hosted a number of research projects supported by the National Natural Science Foundation of China, the Research Grants Council of Hong Kong, and the Shenzhen Science and Technology Innovation Committee. As the first or corresponding author, he has published more than 70 articles in academic journals such as Science, Nature Nanotechnology, Nature Communication, Science Advances, etc. He has won Excellent Young Scientists Fund (Hong Kong and Macau); Early Career Award 2013/14, University Grants Committee (UGC) of Hong Kong; 2019 Outstanding Supervisor Award, City University of Hong Kong; The President’s Awards 2017, City University of Hong Kong. He is also the Associate Editor of Materials Today and the Young Editorial Board member of Science China: Technological Sciences.

Homepage: http://www.cityu.edu.hk/mne/yanglu/

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