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I.

INTRODUCTION

IN previous reports, antimony (Sb) was found to enhance the strength and grain size stability of copper (Cu) when added in dilute quantities. Experimental work[1] showed that ultrafine grain (UFG) Cu (350 nm grain size) created by equal-channel angular pressing (ECAP) benefited from the addition of Sb in regard to strength and grain size stability. In UFG Cu, grain growth was found to commence at 523 K (250 C) when pure, but was retarded until 673 K (400 C) when Sb was added (up to 0.5 at. pct). Results obtained by molecular dynamics (MD) simulation[1,3] found grain size stabilization to be much more profound in nanocrystalline Cu where the grains were found to be stable during accelerated annealing at 1200 K (927 C) with a concentration of only 0.5 pct Sb. The driving force for grain growth typically exhibits an inverse proportionality to grain size, so it was intriguing to study the system experimentally. The grain size stability as found by simulation and ECAP processing was attributed to thermodynamic and kinetic contributions through segregation of the Sb. Thermodynamic analysis considering elastic and chem[1–3]

MARK A. ATWATER, Student, is with the Department of Materials Science and Engineering, North Carolina State University, 911 Partner’s Way, EB I, Room 3002 Raleigh, NC 27606, and also Assistant Professor with the Department of Applied Engineering, Safety & Technology, Millersville University, Millersville, PA 17551. Contact e-mail: maatwat2@ ncsu.edu SUHRIT MULA, Visiting Researcher, is with the Department of Materials Science and Engineering, North Carolina State University, and also Assistant Professor, with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India. RONALD O. SCATTERGOOD and CARL C. KOCH, Professors, are with the Department of Materials Science and Engineering, North Carolina State University. Manuscript submitted February 16, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

ical enthalpies can support the proposition that Sb can be an effective segregant in Cu; however, there is a caveat to the way it is calculated. Especially with regard to the elastic contribution, there can be significant variation in the predicted grain size stability depending on whether the molar volumes or atomic radii of the solvent and solute are used to determine the strain energy. Although the two methods are considered equivalent in many cases, low density, high atomic number materials such as Sb can yield drastically different results. In this work, nanocrystalline Cu-Sb alloys were studied experimentally. The alloys were generated by mechanical alloying using cryogenic, high-energy ball milling. After the alloys were annealed, X-ray diffraction and transmission electron microscopy (TEM) revealed that the grain size was significantly larger than predicted by the MD simulations, even at much lower temperatures. The contradiction between predicted results and experimental findings can be justified by the way the elastic contribution