Material scientists are increasingly looking at the design of natural structural mineralized composites such as bone or nacre in search of new design concepts16. These natural materials develop unique mechanical response from very simple components. They usually exhibit complex anisotropic architectures with layered, columnar or fibrous motifs17. Furthermore, to a large extent their properties depend on the careful engineering of interfaces at the chemical and structural levels. These design concepts have also been employed in synthetic composites and, in particular, weak interfaces are often used in ceramic-based materials as a way to promote toughness through mechanisms such a crack deflection or fibre pull-out18. Here graphene opens new opportunities as its two-dimensional (2D) structure is very well adapted to interfacial engineering. Carbon has been used before to create weak interfaces, for example, in layered ceramic materials15,19,20. However, in these systems the ceramic layers are usually hundreds of microns thick and the interfaces are also in the micron range and relatively flat. Nature also uses interfacial roughness to promote friction during crack propagation and enhance fracture resistance. This strategy has been more difficult to replicate synthetically but, for example, Mirkhalaf et al.21 increased the toughness of glass by laser engraving wavy internal interfaces. However, the waviness and layer thickness were in the hundreds of micrometers range and the procedure significantly reduced the strength of the material. One of the critical features of some natural systems that has been very difficult to replicate synthetically is the presence of very thin (few nanometres) soft interfaces separating hard, mineral layers16. In general, and with few exceptions, the architectural motives of most synthetic structures are still orders of magnitude larger than in their natural counterparts22,23.
It is interesting to compare the properties of the system with ceramic laminates that use C or BN to form a thin, relatively weak interface between SiC or Si3N4 layers19,20,36. In these composites the thicknesses of the ceramic layer and the interfaces are one to two orders of magnitude larger than those of the materials described here. In addition the layers are flat and continuous with lengths of centimetres. The samples are usually tested in bending with the load applied perpendicular to the layers. Increases of the work of fracture between two to three orders of magnitude have been reported in these materials. However, one of the main causes seems to be crack deflection. Cracks form and run along the interfaces in some cases even before the maximum force is reached in the load deflection curve and they can run for distances of up to millimetres in the set-ups used in the papers. The material described here is closer to a brick and mortar structure and crack deflection is much more limited. In addition, the work on layered materials used high-performance technical ceramics where here we have used a brittle glass-ceramic to prove the concept.
Layered Ti-Al metal composite (LMC) fabricated by hot-pressing and hot-rolling process displays higher ductility than that of both components. In this paper, a combination of digital image correlation (DIC) and X-ray tomography revealed that strain delocalization and constrained crack distribution are the origin of extraordinary tensile ductility. Strain delocalization was derived from the transfer of strain partitioning between Ti and Al layer, which relieved effectively the strain localization of LMC. Furthermore, the extensive cracks of LMC were restricted in the interface due to constraint effect. Layered architecture constrained the distribution of cracks and significantly relieved the strain localization. Meanwhile, the transfer of strain partitioning and constrained crack distribution were believed to inhibit the strain localization of Ti and change the deformation mechanisms of Ti. Our finding enriches current understanding about simultaneously improving the strength and ductility by structural design.
In this work, layered Ti-Al metal composite (LMC) was prepared by hot-press and multi-pass hot-rolling process of commercially pure Ti and Al sheets. The deformation behavior of LMC was analyzed during in-situ tensile test to reveal the effect of local strain distribution on mechanical properties using DIC. In addition, the distribution of cracks in fractured LMC was studied by X-ray tomography to reveal the effect of constraint effect on crack initiation and propagation.
(a) Fractured composite; (b) 2D tomographic slice of yz plane in (a), (c) 3D distribution of cracks in LMC; the 3D rendering of abstracted Ti: (d) fractured abstracted Ti; (e) 3D distribution of cracks in abstracted Ti.
The 3D morphology of LMC after tensile fracture was shown in Fig. 6a. Fractured composite displayed a severely plastic deformation with a shear fracture. Close inspection of damage in LMC shows that there were three major types of damage (Fig. 6b): (i) locally interfacial micro-cracks between Ti and Al layers; (ii) transverse cracks, which were restrained by Ti layer without sequential propagation during deformation and finally appeared in Al layer approaching the region of fracture; (iii) delamination, which was formed after whole interface was stripped at a high strain level. Figure 6c shows that the damage of LMC developed progressively from the low εeq to high εeq. At a low εeq level, a few micro-cracks occurred within the interface due to early incompatible deformation. As the εeq increases, the cracks propagated along the interface by inheriting sequent damage elsewhere, but we found that the cracks were steadily restricted in the interface without transverse propagation. Once the interface delamination occurred, constraint effect would disappear locally. Without constraint effect, the Al layers quickly necked and failed at a higher εeq level, resulting in the ultimate fracture of LMC. 2b1af7f3a8