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Damage-tolerant architected materials inspired by crystal microstructure, Pham, Liu, Todd, Lertthanasarn, Nature, 2019

Novelty/impact/significance:

Unlike most architected materials with periodic identical units and thus immediate collapse upon yielding, novel crystal-inspired structures by utilizing the toughening/strengthening mechanisms of crystalline metals (e.g., grain boundaries, precipitates and phases) are developed to possess superior robustness and damage-tolerance.

The success use of the hardening principles from metallurgy to architected materials enlarges and enhances the structural design for desired properties.

Scientific question:

How to avoid the catastrophic, post-yielding collapse of architected materials by structural design?

Key of how:

The catastrophic drop of load-bearing ability of architected materials lies in their periodic, self-similar structures, similar as the metallic single crystals with single dislocation slip mode and localized strain and decreased stress. The latter issue has been addressed in metallurgy by structural toughening mechanisms, e.g., grains boundaries, different phases/lattices to control/suppress dislocation behaviors.

By applying/mimicking these metallurgical principles of microscale crystalline metals on macroscale lattice structures (such as grain-boundary v.s. misorientation between lattice domains), the designed architected materials are strengthened and damage-tolerance enhanced.

Major points:

1. Crystalline materials featuring ordered arrangement of atoms (its crystal lattice being defined by the unit cell) consist of single crystals, which have unit cells with the same type and orientation, and polycrystals that have many domains with different lattice orientations (each domain is called a crystal grain).

2. During plastic deformation, single crystals have single dislocation slip model leading to strain localization and thus stress decrease for further deformation, while polycrystals have grain boundaries to impede the rapid dislocation slip and retain stress level, as described by the Hall-Petch relationship. Other mechanisms controlling dislocation slip include precipitates and phases (due to changing the lattice parameters). Manipulating the grain size and distribution, precipitates and phases can control the strength and toughness of crystalline metals.

3. Architected materials with high designability and special properties like negative Poisson’s ratio, designed by computer software and fabricated by 3D printing, are mostly focused on lattice materials with a single orientation, which collapse upon yielding because shear bands occur and cause deformation localization on specific planes and stress drop. This is similar to the dislocation slip in single crystals.

4. Given the similarity between single crystals and single-lattice structured architected materials (nodes v.s. atoms, struts v.s. atomic bonds), it is proposed that by mimicking the microstructure of crystal metals (grain boundary, precipitates and phases) on a macroscale(distinct from related reports), architected materials can be structurally optimized for high damage-tolerance combining the metallurgical hardening mechanisms and architected materials.

Grain boundary hardening v.s. Polycrystal-inspired architected material

5. The misorientation between two adjoining macro-lattice domains (each is termed a meta-grain) creates a boundary. Symmetric slip in a twinned bi-crystal and then multi-meta-grain lattices (for Hall-Petch relationship) are studied. 3D printed architected materials, each with two meta-grains bonded by a twin boundary (all 40x40x40mm^3 global volume) were tested in compression, and corresponding FEM predicts well the location and direction of the shear bands.

Both show symmetric shear bands parallel to the planes with maximum shear stress in each meta-grain, while certain differences (different slip planes) exist.

Using twin boundary to increase the number of meta-grains while maintaining the volume (reducing the meta-grain size), the shear bands are controlled by the boundary and the overall yield strength and flow stress increase, confirming the size effect (boundary strengthening) and the effect similar to the Hall-Petch relationship in architected materials.

PS: what if non-twin boundary but other normal boundary is used?

6. The simplicity and accurate control to vary the meta-grain size while keeping the same boundary type, which is impossible for practical metallurgy, provides an effective way to study the grain size effect in metallurgy.

7. Despite a slight lower yield strength, the polycrystal-like structures show effective crack stopping at the boundaries (due to the change in lattice orientation) and large increase in energy absorption (due to the boundaries restricting deformation and retaining strength after yielding) than the single oriented lattice (194 v.s. 1309 kJ/m^3).

8. The method is applicable to a variety of base materials, forming desired polycrystal-like structures by a brittle polymer, an elasto-plastic polymers and an austenitic stainless steel, and showing the same general behavior in forming shear bands, despite the differences in specific constitutive stress-strain relationships for different materials. The latter only influences the strengthening degree or speed, which affects the material-based terms but not the Hall-Petch relationship.

This also implies that a metal based polycrystal-like structure possesses multi-strengthening from both the meta-grain boundary and the metal crystallographic microstructure (work-hardening).

Precipitation hardening

9. Precipitates are created by introducing embedded lattice domains (meta-precipitates) with different lattice type, spacing and orientation into the matrix lattice, and the strut diameter represents coherent bonding between the precipitates and the matrix.

The fcc matrix containing face-centered tetragonal meta-precipitates show higher strength than the single fcc lattice, as the harder meta-precipitates restrict and control the shear-band propagation (shear bands stop at and bow around the interfaces between the meta-precipitates and the matrix), the governing effect similar to the Orowan hardening effect.

Multi-phase hardening

10. Multiple phases are imitated by assigning different lattice types to different macro-lattice domains, e.g., a structure consisting of two phases with fcc in the top and bottom layers and bcc in the middle layer. The multi-phase architected material show the strength mainly from the hard fcc phase while plastic deformation by the soft bcc domain, indicating another way to modulate the shear bands behavior of architected materials.

11. The ability of phase transformation in the crystal-inspired structures is demonstrated by the fabricated Kresling lattice altering its nodal arrangement under compression, and the pseudo-superelasticity by the Kresling lattice in loading-unloading behavior. This demonstrates that the phase transformation can be mimicked via the reversible alternation of node arrangement, thus for high energy absorption.

Multiscale hierarchical crystal structures

12. The created crystal-like architected materials are multiscale and hierarchical, consisting of nano-to-microscale crystal structure (the base material) and the meso-to-macroscale crystal-like lattice structures. Both can be varied readily across multiple length scales for tailored properties, and the interplay between the material structure and the macro lattice structure can provide synergistic strengthening effects.

 

Borrowing the known strengthening and toughening mechanisms from metallurgy are promisingly new means to tailor/enhance the properties/performances of architected materials.

The parallel between the mesoscale crystal-like architected materials (boundary between misorienting lattice domains, meta-precipitates, different macro-lattice domains) and metallic alloys (twin grain boundary, precipitates, multi-phases) in structure, mechanical behavior, and mechanisms demonstrated comprehensively in this work opens up new opportunities for designing and engineering advanced architected materials, a versatile approach with a large design and optimization space.

The ease and accuracy to control certain lattice structural features (such as grain size, boundary type) at meso/macroscale for studying relevant structure-property relationships also represents an alternative, effective way for studying complex phenomena in metallurgy.

 Very interesting and influential work.

Here is the link of the fulltext: https://www.nature.com/articles/s41586-018-0850-3

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