Fig. 1. Bouligand-type structures of bioinspired tungsten-copper composites mimicking fish scales. (a) The scales of Pagrus major and Latimeria chalumnae fish provide an effective protective role against the predation by sharks. (b, c) Micrographs and schematic illustrations of the orthogonal plywood and double-Bouligand structures in the scales of (b) Pagrus major and (c) Latimeria chalumnae fish. (d, e) 3-D microstructures of the tungsten-copper composites with bioinspired (c) orthogonal plywood and (d) double-Bouligand structures by XRT imaging. The spatial distributions of tungsten fibers within the composites are generated by filtering out the signals from the copper matrices. The micrographs of fish scales in (b) and (c) are adapted with permission from refs. [4,8].

Fig. 2. Mechanical and functional properties of bioinspired tungsten-copper composites. (a, b) Representative tensile stress-strain curves for the bioinspired tungsten-copper composites with orthogonal plywood (OP) and double-Bouligand (DB) structures at (a) room temperature and (b) 800 °C as compared to the pressure sintered composites (PS) with a uniform structure. The overall appearances of the fractured samples are shown in the insets. (c) The mass increase per unit surface area of the composites with increasing temperature measured by TGA. Representative morphologies of the surface oxides for the sintered and bioinspired composites are shown in the insets. (d, e) Comparison of the (d) hardness and (e) electrical conductivity of the bioinspired composites measured on horizontal (H) and vertical sections (V) with those of the sintered samples. Asterisks indicate statistically significant differences at a 5% level of significance according to the Student's t-test.

Fig. 3. Interpretation of the mechanical properties for the bioinspired Bouligand-type structures. (a) Schematic illustrations of the laminate composite model and coordinate system for the theoretical analysis, and the definition of the two different reorientation modes (I and II) of tungsten fibers in the composite. (b) Theoretical results on the variations in the yield and ultimate tensile strengths of the entire laminate as a function of the twisting angle between adjacent laminae for the bioinspired double-Bouligand structure (the twisting angle equals 0° for the orthogonal plywood structure). Experimental data are presented for comparison. (c-e) SEM micrographs of a fractured sample for the composite with the double-Bouligand structure after tensile testing at room temperature. (d) and (e) show the magnified views for the fracture surface and lateral profile, respectively, of the sample as indicated by the arrow and dashed box in (c).

Fig. 4. Adaptive structural reorientation behavior of the bioinspired tungsten-copper composites. (a) X-ray CT images of a fractured sample after tensile testing at room temperature showing a clear shape change and extension of the grids of tungsten meshes in the composite with the bioinspired double-Bouligand structure. (b) Variations in the reorientation angle of the tungsten fibers following the different modes (I and II) as a function of their initial orientations with respect to the loading axis. The numbers 1 and 2 denote the two different cycles of helices in the composites. (c-e) Schematic illustrations and the corresponding experimental verification based on SEM characterization concerning the micro-mechanisms for adaptive structural reorientation.