Studies on thermal stability, mechanical and electrical properties of nano crystalline Cu 99.5 Zr 0.5 alloy Debdas Roy a,b,⇑ , Mark A. Atwater a , Khaled Youssef a , John Christopher Ledford a , Ronald O. Scattergood a , Carl C. Koch a a Material Science and Engineering Department, North Carolina State University, Raleigh, NC 27606, USA b Materials and Metallurgical Engineering Department, NIFFT, Ranchi 834 003, India article info Article history: Received 13 July 2012 Received in revised form 14 October 2012 Accepted 1 November 2012 Available online 8 December 2012 Keywords: Cryomilling Nanocrystalline Thermal stabilization Shear stress Electrical resistivity abstract Cryogenic high energy ball milling was used to synthesize nanocrystalline Cu and Cu 99.5 Zr 0.5 alloys by mechanical alloying and consolidation by hot pressing at 550 °C temperature. The grain size stability of nanocrystalline Cu is improved by the Zr addition. Microstructural characterization using X-ray diffrac- tion and transmission electron microscopy provided evidence for the formation of a Cu–Zr alloy solid solution with nanocrystalline size after hot pressing. The alloy exhibited a higher hardness (3.31 GPa), and shear strength (550 MPa) than nano-crystalline pure Cu however, the electrical resistivity is increased in the alloy. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The interest in developing high strength and high conductivity copper alloys has increased recently for many applications in, for example, electrical conductor and connectors. High strength is ob- tained through microstructure refinement and this requirement has resulted in the extensive evolution of rapid solidification and mechanical alloying techniques [1,2]. Nanocrystalline (nc) materials are polycrystalline solids with either single or multi-phase microstructure with an average grain size of less than 100 nm. The grain boundary surface-to-volume ra- tio increases with decreasing grain size which results in a large percentage of atoms located in interfacial area. In coarse-grained materials this percentage is usually negligible, and it is this notice- able disparity that is primarily responsible for the major difference and often superior performance of nc materials [3]. Experimental results have shown that many elemental metals, including Sn, Pb, Al, Mg, Cu, and Pd, with nano-sized grains will undergo signif- icant coarsening even at room temperature [4–6]. The superior mechanical, electrical, and magnetic properties of bulk nanocrys- talline metals are lost when the microstructure reverts to coarse grain size. This high tendency for grain growth in nc-solids has been a substantial hindrance to the use of this class of materials in applications. To advance the commercial viability of these mate- rials, understanding their microstructural stabilization at elevated temperature is highly desirable. The proposed mechanisms for sta- bilization are kinetic (e.g. Zener pinning or solute drag [7–11]), or thermodynamic (reduction in the effective grain boundary energy [12–15]). Thermodynamic stabilization is typically accomplished by adding an oversize solute which lowers grain boundary energy upon segregation [12,16–18]. These same solutes can also form precipitates at higher temperature and can produce stabilization by Zener pinning. In the present work kinetic mechanism play ac- tive role for stabilizing grain size. 2. Experimental procedure 2.1. Materials processing Elemental powders of copper (Alfa Aesar, 99.9%) and Zr (Cerac, 99.95%, 200, +325 mesh) were added in appropriate quantities to a 440 stainless steel vial (Spex Sample Prep) with grade 25, 440 stainless steel ball bearings (Salem Specialty Ball). The ball to powder weight ratio was maintained at 10:1. All materials were loaded into the vial in an argon atmosphere (O 2 < 1 ppm) and sealed before transferring to the mill. A modified Spex 8000 mixer/mill was used to mill Cu and Cu 99.5 Zr 0.5 for 8 h at liquid nitrogen temperature (196 °C). The milled powders were annealed under 2% H 2 (bal. Ar) for 1 h at temperatures from 200 to 800 °C. Milled powders were compacted in a cylindrical (10 mm dia) tool steel die un- der an applied pressure of 2 GPa in a hot press (Fritsch Postfech-500428, Germany) at 550 °C. The pressing load on the specimen was applied for 30 min when the desired temperature was attained. The sample was then allowed to cool to room temperature and the cylindrical samples were stripped from the die for characterization. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.004 ⇑ Corresponding author at: Material Science and Engineering Department, North Carolina State University, Raleigh, NC 27606, USA. Tel.: +1 919 515 7217; fax: +1 919 515 7724. E-mail address: droy2k6@gmail.com (D. Roy). Journal of Alloys and Compounds 558 (2013) 44–49 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom