Figure 1
Design concept: the overall architecture of the hierarchical steel, as compared to that of strong and tough natural byssal threads. (a) Sandwich structures of byssal threads and (b) the hierarchical steel developed in this work. On the micro-scale, the threads are made of a matrix and granules which vary in cross-linking density, the granules containing a higher cross-link density than the matrix; this controls the difference in local mechanical properties. From the micro-scale point of view, the hierarchical steel is comprised of a hard layer and a coarse-grained layer. At the nano-scale, there are two compositions, dopa and metal (Fe3+), that comprise a single granule. At the same dimension, our hierarchical steel has a graded dual-phase structure consisting of nano-scale martensite and nano-scale austenite, which are distributed as alternating lamellar, ~5 to 6 nm thick, inside each grain of the steel.
