Interface-driven microstructure development and ultra high strength of bulk nanostructured Cu-Nb multilayers fabricated
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Thomas Nizolek Department of Materials, University of California at Santa Barbara, Santa Barbara, California 93106
William M. Mook, Thomas A. Wynn, Rodney J. McCabe, Jason R. Mayeur, Keonwook Kang, Shijian Zheng, and Jian Wang Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Tresa M. Pollock Department of Materials, University of California at Santa Barbara, Santa Barbara, California 93106 (Received 16 November 2012; accepted 23 January 2013)
We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (;4.5 GPa) and strength (;2 GPa). I. INTRODUCTION
Recent studies have demonstrated that bulk nanostructured bimetallic composites with uniformly structured bimetal interfaces can be fabricated with severe plastic deformation (SPD).1–3 The particular SPD technique used, accumulative roll bonding (ARB), involves repeated operations of rolling, cutting, and restacking.2–5 ARB enables extremely large strains (of the order of 10) to be applied via a consistent strain path and without changing the original dimensions of the sample. When starting with two metals that are immiscible, such as copper (Cu) and niobium (Nb), the outcome is a sheet material comprised of alternating layers of two metals with individual layer thicknesses ranging from the submicrometer to the nanoscale (Fig. 1). At these fine length scales, the grains become severely elongated, with widths at least 10–20 times the thickness, and contain minimal substructure.6 The layers encompass one to two of these band-like grains. The textures of ARB copper-niobium (Cu-Nb) composites are found to deviate from those expected of severely rolled monolithic Cu or Nb.2 Most remarkably, the bimetal interfaces are oba)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.21 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
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served to stabilize to a few orientation relationships that prevail over the bulk sample. The characteristic interface that occurs most frequently involves a Cu crystal with an orientation
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