Interface-driven mechanisms in cubic/noncubic nanolaminates at different scales
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Introduction Many next-generation engineering systems will rely on high-performance metals with strength and toughness several times those in use today. One popular class of interfacedominant materials are nanolayered composite thin films. These materials have a two-dimensional structure and are comprised of a layered stack of two different metals, wherein the individual layer thickness h has nanoscale dimensions (usually h < 100 nm), and the density of interfaces is unusually high.1–4 Studies on nanolayered films report exceptional structural properties compared to those of their constituents or volume average values of their constituents, such as strengths that are more than 5–10 times higher, hardness values that are several orders of magnitude higher, and greater microstructural stability in harsh environments, such as irradiation, impact, or elevated temperatures.5–11 The combination of high strength with other desirable structural properties can be attributed to the physical dominance of the biphase interfaces in the material. Spacing between layers has nanoscale dimensions (2–50 nm) and is spanned most often by a single crystal. The density of biphase interfaces in a typical nanolaminate is unusually high, significantly affecting the mechanical behavior, or in some cases, the intrinsic properties of the adjoining phases. Consequently, the nanolaminate concept has gained widespread attention,
with the number of composite material systems being studied growing rapidly, such as Cu/Nb, Cu/Ni, Cu/Ag, Cu/Cr, Cu/Mo, Al/Nb, Fe/W, and Cu/Ta.1,4,12–14 While in principle nanolaminates can be made with any two-phase, bimetallic system, the constituent metals often have a cubic crystal structure, such as face-centered cubic (fcc) or body-centered cubic (bcc), and considerably fewer studies have focused on nanolaminates combining noncubic/cubic phases. Many important structural metals used today have a noncubic crystal structure. Some examples of well-known, low-symmetry noncubic materials are hexagonal close-packed (hcp) metals, such as Mg and Ti and their alloys, and orthorhombic materials, such as uranium. The hcp class of materials alone are technologically relevant, bearing desirable intrinsic properties, such as low specific density, fatigue resistance, biocompatibility, corrosion resistance, and radiation resistance of Zr, just to name a few. At present, for even the coarse-grained traditional form, there is an increasing demand to use these materials more often and more broadly in structural applications within the automotive, aircraft, aerospace, biomedical, and nuclear industries.15–21 While clearly possessing important structural attributes, one concern with noncubic materials is that their deformation behavior is anisotropic. This arises not only from the structural complexity, from the electronic scale to the scale of the crystal,
I.J. Beyerlein, Department of Mechanical Engineering, Materials Department, University of California, Santa Barbara, USA; [email protected] J. Wang, Department of Mechanical and
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