Introduction Biomimetic structures can effectively improve the mechanical properties of materials, but are difficult to construct in metallic systems. The structure-property relationships of biomimetic metal composites also remain unclear. This paper fabricates Mg-Ti composites b

Introduction

Bionic structures can effectively improve the mechanical properties of materials, but are difficult to build in metal systems. The structure-property relationships of biomimetic metal composites also remain unclear. This paper fabricates Mg-Ti composite materials by pressureless infiltration of pure magnesium melt into the Ti-6Al-4V scaffold of 3D printed . The composition of the composite material is continuous, interpenetrating in 3-D space and exhibiting specific spatial arrangements with biomimetic brick and mortar, buligan and cross-laminated structures. These structures promote efficient stress transfer, delocalize damage, and resist cracking, resulting in higher strength and ductility than composites with discrete reinforcements. Furthermore, they activate a series of extrinsic toughening mechanisms, including crack deflection/twisting and uncracked ligament bridging, which enable the crack tip to shield the applied stress and result in a “Γ”-shaped rising resistance to fracture R-curve.

Strength and fracture toughness are two important mechanical properties of structural materials, but they often show a mutually exclusive relationship. By drawing lessons from natural biomaterials, bio-inspired structural design has become an effective way to improve the performance of artificial materials; in particular, it provides new opportunities to achieve synergistic enhancement of strength and fracture toughness. Specifically, the simple additive manufacturing of polymers makes them ideal prototypes for exploring structure-property relationships in biological and biomimetic materials. However, the structural applications of these materials are often limited by their low strength and poor heat resistance. There are also questions about the applicability of structure-property relationships derived from polymeric materials to other material systems such as metals and alloys, given their significantly different deformation mechanisms. Building biomimetic structures in metallic materials is more challenging than in polymers. This is mainly due to the difficulty in controlling the microstructure of metallic materials during traditional manufacturing processes, which often involve high temperatures and pressures. To date, most biomimetic metal materials have been limited to those with nacreous layered structures fabricated through directional non-isometric reinforcements. However, exceptions have been made recently, where fish scale-like structures were created in Cu-W and Mg-Ti systems by infiltrating the metal melt into the fibrous structure of another metal with a higher melting point.

Metal additive manufacturing technology, represented by three-dimensional (3-D) printing with selective laser or electron beam melting, provides an effective method for processing metal materials in a "bottom-up" manner similar to nature. They are particularly effective in producing porous metal scaffolds with complex pre-designed structures, thus opening new opportunities to implement biomimetic designs . Various biomimetic structures have been used to optimize the mechanical properties of additively manufactured metal stents. However, such building structures in dense metal materials may be hampered by several limitations. First, additive manufacturing techniques for metals are largely limited to a single material system (or a single component, even if it is composed of multiple components or components); when two or more components are involved, the process can become very complicated. In contrast, most biological materials in nature contain at least two components with significantly different stiffnesses. Relatively rigid and compliant components are often bicontinuous and topologically interconnected in 3D space, forming specific interpenetrating phase architectures. Similar structures have been built in polymer composites through 3-D printing and have proven effective in improving properties including stiffness, strength, fracture toughness and energy dissipation capabilities, but are less effective in 3-D printing metal materials. Second, although direct comparison of mechanical properties between different types of biomimetic structures is crucial for their selection in materials design, this has not yet been achieved experimentally for metallic systems. Third, the structure-property relationship is the basis for structural optimization to improve performance, but this remains largely unclear for biomimetic metal materials with different structures, except for those that mimic nacre. In particular, the specific strengthening and toughening mechanisms associated with these structures in metallic systems remain unexplored.

In order to solve the above problems, Professor Zhang Zhefeng's team from the Institute of Metal Research, Chinese Academy of Sciences, and Professor Robert O. Ritchie of the University of California, Berkeley, and others, used a two-step method to create a set of Mg-Ti interpenetrating composite materials containing different bionic structures. , specifically including: (i) 3-D printing of open porous Ti-6Al-4V scaffolds with biomimetic structures, and (ii) pressureless infiltration of the scaffolds with pure magnesium melt. Ti-6Al-4V alloy and Mg were chosen as ingredients because of their high specific strength, low density and good biocompatibility . Furthermore, they exhibit large differences in stiffness, which are qualitatively similar to biological materials. Three types of biomimetic structures were designed: (i) solid structures that mimic nacre, (ii) twisted plywood or so-called Bouligand structures that mimic arthropod exoskeletons, and (iii) crosses that mimic conches or bivalve shells. Layered structure.

In this work, these three biomimetic structures were constructed by 3-D printing Ti-6Al-4V scaffolds, followed by pressureless infiltration of Mg to form Mg-Ti composites. The process takes advantage of the large melting point difference between the two components without violent chemical reactions, as well as the excellent wettability of the magnesium melt with the Ti-6Al-4V alloy. We aimed to evaluate and compare the damage tolerance, specifically strength, toughness and impact resistance of these composites, reveal their structure-property relationships, and elucidate the toughening mechanisms associated with their specific biomimetic architecture. On this basis, we attempt to provide guidance for architectural selection and design of bionic metal materials. Additionally, we believe our composites may also have potential for structural and biomedical applications. Relevant research results were published in the internationally renowned journal Nature Communications under the title "On the damage tolerance of 3-D printed Mg-Ti interpenetrating-phase compoSites with bioinspired architectures".

Paper link: https://www.nature.com/articles/s41467-022-30873-9

Three types of Ti-6Al-4V scaffolds were prepared by pressureless infiltration of pure magnesium melt into 3-D printed Ti-6Al-4V scaffolds. Type Mg-Ti composite material, which has bionic brick structure, Bouligand and cross-layered structure. Both the Mg and Ti-6Al-4V components are continuous, interpenetrate in 3D space, and exhibit a specific spatial arrangement in the biomimetic composite material that is qualitatively consistent with the structure in its natural prototype. Adjacent reinforcement layers are connected to each other via welded joints formed during the 3-D printing process. Precipitates appear at the grain boundaries of Mg and at the interface between components. The tensile properties of

biomimetic composites are all better than composites reinforced with discrete Ti-6Al-4V particles, but are closely related to their specific structures. The Young's modulus and strength of composites can be combined with their architectural characteristics by modifying the classical laminate theory, specifically linked to the orientation of their components. This may provide a theoretical basis for the architectural selection and design of biomimetic composite materials.

Figure 1: Biomimetic structure of 3-D printed Ti-6Al-4V scaffold and its infiltrated Mg-Ti composite.

Figure 2: Fine microstructure, phase composition and element distribution of biomimetic Mg-Ti composite materials. a-c: Micrographs of Mg-Ti composite material, b: Ti-6Al-4V reinforcement material, c: Micrographs of Mg matrix and interface region between Mg and Ti-6Al-4V phases. d: Micro-area XRD patterns of magnesium matrix within grains (top) and grain boundaries (bottom). e: Area distribution of the elements Mg, Al, Ti, Si and V in the area corresponding to the image in c obtained by EDS measurement.

Figure 3: Uniaxial tensile properties and damage characteristics of biomimetic Mg-Ti composites with three different structures. a: Tensile engineering stress-strain curve and b overall fracture morphology of the composite, tensile test. The loading configuration of is illustrated in the inset of a. Data for coarse-grained pure magnesium are also shown in (a) for comparison. c–e: SEM images and X-ray computed tomography (CT) volume rendering of a tensile sample unloaded before fracture of (c) solid, (d) Bouligand and (e) cross-laminated structures.The CT images are processed by filtering out the signal of Mg and highlighting the crack area in yellow. The inset in e magnifies the slip band within Mg and the deflection and branching of cracks in the cross-laminated structured composite.

Figure 4: Lamination theory analysis of Young’s modulus and strength of three bionic structures.

bionic composite material shows a "Γ" shaped R curve, and its J integral shows an upward trend, but with the crack expansion rate, it shows a downward trend. The biomimetic structure induces a series of extrinsic toughening mechanisms, including crack deflection/distortion and uncracked ligament bridging to protect the crack tip from applied stress. These mechanisms are further facilitated by microcracks preceding the crack tip. Only the cross-laminated structure showed significant anisotropy in impact toughness when loaded in different directions.

Figure 5: Fracture and impact toughness of biomimetic Mg-Ti composites with different structures.

Figure 6: Cracking morphology and toughening mechanism of biomimetic Mg-Ti composite materials. SEM images and CT volumetric images of quasi-static fracture toughness samples of (a) solid, (b) Bouligand and (c) cross-laminated structured composites. CT images are processed by filtering out component signals and highlighting crack areas. The white arrow in (b) indicates the microcrack before the crack tip. The inset in (c) magnifies the zigzag cracking path and the resulting frictional sliding between the crack faces, as indicated by the arrows.

Figure 7: Comparison of mechanical properties of bionic Mg-Ti composites with different structures.

Among the three biomimetic structures, the cross-laminated structure is the most effective in reinforcing the material, delocalizing damage and resisting crack propagation. This structure gives the composite an optimal combination of mechanical properties, including strength, elongation at break, work of fracture, and fracture and impact toughness. This is mainly attributed to its hierarchical nature, where changes in compositional orientation and interfaces are active at different length scales.

Bionically inspired magnesium-titanium composites may have potential for structural and biomedical applications. The significant strengthening and toughening efficiency of biomimetic structures can further be used to develop new biomimetic metal materials. Current lines of theoretical analysis and treatment can provide the means to design and construct architectures in a more precise and efficient manner.

Source: Frontier Materials