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Xing-Wang Cheng and Zhao-Hui Zhanga) School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China; and National Key Laboratory of Science and Technology on Materials under Shock and Impact, Beijing, 100081, People’s Republic of China

Zheng-Yang Hu and Sheng-Lin Li School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, People’s Republic of China (Received 9 March 2016; accepted 16 May 2016)

Bulk nanocrystalline (NC) silvers were fabricated by spark plasma sintering process. The effects of sintering temperature on physical and mechanical properties of the NC silvers were investigated. The results indicate that no impurities were introduced into the bulk compacts during the preparation procedure. Both the density and the electrical conductivity of the NC Ag increase with an increase in sintering temperature. However, the micro-hardness and ultimate tensile strength (UTS) of the bulk compacts increase initially and then decrease with increasing sintering temperature. The NC Ag sintered at 500 °C exhibits the highest micro-hardness of 85.3 HV along with the best compression yield strength of 379 MPa and the highest UTS of 534 MPa. The deterioration of the mechanical properties of the NC Ag sintered at 550 °C should be attributed to the rapid grain growth.

I. INTRODUCTION

Bulk nanocrystalline (NC) materials are polycrystalline materials consisting of grains in a nanometer range from 1 to 100 nm. They were widely studied in the past few decades due to their unusual mechanical properties and outstanding physical and chemical properties.1–4 In particular, a conventional metal can get a 7-fold or even decuple increase in tensile strength when the grain size reaches the nanoscale, assumably due to the grainboundary strengthening mechanism known as the Hall–Petch effect.2 For example, tensile strengths as high as 1.5 GPa in NC-iron and 0.28 GPa in NC-aluminum have been reported.5,6 According to the mechanism of fine-grain strengthening, the plasticity of NC materials can also be increased. However, the bulk NC materials usually exhibit undesirable low ductility, especially for the NC metals fabricated by powder metallurgy method. Thus, the fabrication of the NC materials with both high strength and good ductility has become a hot research focus in recent years.7–11

Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.212 J. Mater. Res., Vol. 31, No. 15, Aug 15, 2016

Up to now, many techniques have been developed to fabricate bulk NC materials, such as severe plastic deformation,12–16 crystallization of amorphous solids,17 powder metallurgy, 18,19 etc. 20–22 In these methods, powder metallurgy is considered to be suitable to fabricate the bulk NC materials due to its simplicity and feasibility. However, this method is hard to generate NC materials with controllable, stable, and homogeneous nanostructures because the metal nanoparticles are easy to get