Studies on thermal stability, mechanical and electrical properties of nano crystalline Cu99.5Zr0.5 alloy

https://doi.org/10.1016/j.jallcom.2012.11.004Get rights and content

Abstract

Cryogenic high energy ball milling was used to synthesize nanocrystalline Cu and Cu99.5Zr0.5 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 diffraction 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.

Highlights

► 0.5 at.% of Zr addition is an excellent candidate for stabilizing nanocrystalline Cu. ► The Cu99.5Zr0.5 alloy shows better mechanical properties than nanocrystalline Cu after hot-pressing at 550 °C. ► However, electrical resistivity is higher than nanocrystalline Cu.

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 obtained 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 ratio 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 noticeable 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 significant coarsening even at room temperature [4], [5], [6]. The superior mechanical, electrical, and magnetic properties of bulk nanocrystalline 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 materials, understanding their microstructural stabilization at elevated temperature is highly desirable. The proposed mechanisms for stabilization are kinetic (e.g. Zener pinning or solute drag [7], [8], [9], [10], [11]), or thermodynamic (reduction in the effective grain boundary energy [12], [13], [14], [15]). Thermodynamic stabilization is typically accomplished by adding an oversize solute which lowers grain boundary energy upon segregation [12], [16], [17], [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 active role for stabilizing grain size.

Section snippets

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 (O2 < 1 ppm) and sealed before transferring to the mill. A modified Spex 8000 mixer/mill was used to mill Cu and Cu99.5 Zr0.5 for 8 h at

Results and discussion

The XRD patterns for each annealing temperature for Cu are shown in Fig. 1. It can be seen that up to 400 °C the pattern shows negligible change. The noticeable peak sharpening occurs above 400 °C. The XRD patterns for the Cu99.5Zr0.5 alloy after each annealing temperature are shown in Fig. 2. It can be seen that even up to 800 °C the pattern shows relatively small change. The most notable sharpening take places for the high angle peaks (i.e. 311 and 222) at 800 °C. The X-ray scans do not show

Conclusion

0.5 at.% Zr is an excellent candidate for stabilizing nanocrystalline Cu when mechanically alloyed at cryogenic temperature. Cu99.5Zr0.5 can be maintained at a nanoscale grain size at >0.9Tm. This was confirmed with data collected by microhardness, X-ray diffraction. High temperature hot pressing is a necessary for fully dense bulk sample. Lower (0.5) at.% of Zr addition in pure Cu increase hardness, shear strength due to synergy effect of grain size and Zener pining at lower temperature and

Acknowledgements

The work was sponsored by the Indo-US Science & Technology Forum, Govt of India, under the grant of Indo-US Fellowship No – IUSSTF Felloship/2011/8 dated 17/3/2011 and US office of Naval Research under Grant Number N00014-10-1-0168.

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