Nature-Inspired Hierarchical Steels

Abstract

Materials can be made strong, but as such they are often brittle and prone to fracture when under stress. Inspired by the exceptionally strong and ductile structure of byssal threads found in certain mussels, we have designed and manufactured a multi-hierarchical steel, based on an inexpensive austenitic stainless steel, which defeats this "conflict" by possessing both superior strength and ductility. These excellent mechanical properties are realized by structurally introducing sandwich structures at both the macro- and nano-scales, the latter via an isometric, alternating, dual-phase crystal phases comprising nano-band austenite and nano-lamellar martensite, without change in chemical composition. Our experiments (transmission and scanning electron microscopy, electron back-scattered diffraction, nano-indentation and tensile tests) and micromechanics simulation results reveal a synergy of mechanisms underlying such exceptional properties. This synergy is key to the development of vastly superior mechanical properties, and may provide a unique strategy for the future development of new super strong and tough (damage-tolerant), lightweight and inexpensive structural materials.

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.

Figure 2

The overall structure of the hierarchical steel as compared to that of other strong and tough steels. (a) Schematic illustrations of the microstructure of a dual-phase steel showing the martensite phase (yellow lumps) randomly embedded in the ferrite matrix. (b) TRIP steel showing the martensite (yellow lumps) and austenite (blue plates) phases embedded in the ferrite matrix; note that there are no gradient structures in either the dual-phase or TRIP steels. (c) classical gradient steel displaying a traditional gradient in grain size. (d) our hierarchical steel showing a sandwich structure of nano-scale dual-phase (martensite and austenite) in the surface region (extending some 300 µm from the surface) with a coarser-grained austenitic region in the core of the steel.

Figure 3

Multi-scale structural characterization of the hierarchical steel. (a) Schematic of the gradient in grain size distribution of our hierarchical steel with depth from one topmost surface (outlined by dashed line) to the lower surface of the steel (~1000 µm in total). (b) The EBSD phase map and (c) SEM micrograph across the cross-section. (d) Nano-sized martensite and austenite phase distribution in one typical grain. Grain sizes are about 15 to 20 nm at the topmost surface, outlined by dashed line in (b),(c) and (e). A typical dark-field TEM image taken at ~100 nm deep underneath the surface of our steel where bright area/bands are ε-hcp martensite. (f) The SAED patterns corresponding to (e) clearly show the combination of diffraction spots from both austenite and ε-hcp martensite. (g) Schematic illustration of the SAED patterns of (e) The relatively weak spots (yellow spots) are from the ε-hcp martensite, with the axis of $[2\overline{1}\overline{1}0]$ hcp, and the relatively strong spots (blue spots) are from the austenite, with the axis of [011] fcc. The austenite has the following orientation relationship with the ε-hcp martensite: [011] fcc and $[2\overline{1}\overline{1}0]$ hcp, and $(\overline{1}\overline{1}1)\text{fcc}//(0002)\text{hcp}$. (h) shows a high-resolution image of the dash box selection in (e) which illustrates the alternative arrangement of the two nano-scale phases: martensite and austenite (both with thicknesses of ~5 to 7 nm). The nano-lamellar martensite produced by strain-induced phase transformation from austenite matrix. (i) shows an IFFT image corresponding to the selection in (h).

Figure 4

Comparison of mechanical properties of the hierarchical steel with those of other steels. (a) Hardness distribution along the cross-section of an as-annealed steel, as compared to our hierarchical steel. Results indicate that the hierarchical steel (red scattering) attains larger improvements in hardness as compared to as-annealed steels. (b) Uniaxial tensile tests show engineering stress–strain curves of the hierarchical steel (in red triangle scattering), as compared with that of as-annealed steel; inset graphs show the corresponding true stress-strain curves. Representative tensile properties of steels are shown in (c) and (d). The data are for dual-phase steels, TRIP-steels, classical gradient steels and our hierarchical steel (this work). Red triangle data points represent the tensile properties of the hierarchical steel. A comparison of the respective yield (c) and ultimate tensile strengths (d) of dual-phase steels, TRIP-steels, classical gradient steels and hierarchical steels, indicates that the hierarchical steels have a far superior combination of strength and ductility in comparison to other steels.

Figure 5

Microstructure after tensile test (a) TEM image of the tensile sample near the fracture zone showing high density of dislocations and (b) magnified zone of the white rectangle in (a), showing the numerous dislocation pile-ups.