Advanced Materials & Processes

FEB 2015

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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C opper-niobium nanolaminates, composed of alternating layers of Cu and Nb, display high strengths (>1 GPa) and intriguing properties due to their highly oriented nanocrystalline mi- crostructure. Compared to either pure Cu or Nb, these composite materials have superior hardness, flow strength, and radiation damage resistance [1-2] . While nanolaminates were previously only available in thin-film form (< 40 µm thick), recent advances in the severe plastic de- formation process of accumulative roll bonding (ARB) have enabled production of bulk nanolaminates (Fig. 1). Four-mm thick Cu-Nb nanolaminates containing more than 200,000 individual layers (lay- er thickness of <20 nm) are now routinely synthesized for research purposes. The ability to produce bulk metal- lic nanolaminates has greatly expanded the potential structural applications for these materials, motivating investi- gations into formability, joining tech- niques, and deformation behavior. This article presents an overview of both the ARB process and the materials' defor- mation behavior. SYNTHESIS METHOD The ARB process uses an iterative sequence of cleaning, stacking, cold roll bonding, and cutting to create and re- fine a lamellar microstructure. Starting materials consist of coarse-grained sin- gle-phase sheets of Cu and Nb which are degreased, wirebrushed, and alternately stacked. Materials are bonded together using a single-pass 50% rolling reduc- tion. Next, the roll bonded sheet is cut in half, cleaned, and re-stacked in prepa- ration for the next cycle. Iterating these steps increases the number of layers ex- ponentially while decreasing layer thick- ness (Fig. 2). Sheet thicknesses remain nearly constant during process- ing, and can be increased by cutting and bonding three or more pieces together. This processing technique allows extreme strains to be imparted to the material while maintaining the desired sheet thickness. Strains exceed those typically encountered during conventional rolling. For exam- ple, synthesis of 15 nm Cu-Nb nanolaminates requires a roll- ing strain of 11.8. Imagine a U.S. nickel coin (nearly the same thicknesses as the sheets used for ARB). If it were convention- ally rolled to a strain of 11.8, the length of the rolled strip would exceed 2 km. While ARB has been ap- plied to several diferent bimet- al systems [3-5] , the Cu-Nb sys- tem ofers low solid solubility as well as similar flow stresses for the two phases. These char- acteristics result in excellent microstructural stability during ARB processing, allowing pro- duction of nanolaminates with continuous layers and individu- al layer thicknesses as small as 10 nm [6] . DEfORMATION BEHAvIOR The large dimensions of ARB Cu-Nb nanolaminates have facilitated a wide variety of bulk mechanical tests (i.e., ten- sile tests, miniature Charpy impact tests, and fatigue tests) that are dificult, if not impossible, to perform on traditional thin film nanolaminates. In many cases, these tests confirm the extraordinary proper- ties of metallic nanolaminates. For ex- ample, tensile specimens of ARB Cu-Nb nanolaminates with a 30 nm layer thick- ness have a strain-to-failure of approx- imately 8% and a flow strength of 1200 MPa (a 5× increase in strength compared to pure Cu or Nb) [7] . However, bulk me- chanical testing also reveals an unusual deformation mode during compression. During layer-parallel compression of nanolaminate specimens with layer thicknesses below 100 nm, a pronounced inhomogeneous shape change occurs A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y 2 0 1 5 1 9 Fig. 1 — A 4-mm-thick piece of Cu-Nb nanolaminate material synthesized using the ARB process held by Thomas Nizolek. For comparison, a thin-film Cu-Nb multilayer synthesized using physical vapor deposition is shown in the bottom right. Fig. 2 — Optical micrographs show cross-sections of Cu-Nb laminates at various stages during the ARB process.

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