Mechanical Properties of Nanocrystalline Ni in Relation to its Microstructure
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Mechanical Properties of Nanocrystalline Ni in Relation to its Microstructure F. Dalla Torre1, H. Van Swygenhoven1, M. Victoria2, R. Schaeublin2, W. Wagner1 1 Paul Scherrer Institut, SWITZERLAND; 2 EPFL Lausanne, Dept. of Fusion Technology CRPP, SWITZERLAND. ABSTRACT Mechanical properties of nanocrystalline Ni made by Inert Gas Condensation and Electrodeposition are presented in relation to their microstructure. Significant plasticity is only observed at elevated temperatures for both types of nanocrystalline Ni. However, a higher temperature is needed in the Inert gas condensated material. Careful analysis of the microstructure by means of X-ray diffraction and conventional electron microscopy reveal initial differences in as-prepared samples. The change in microstructure during deformation at elevated temperatures and during heat treatment without external load is investigated and information about the deformation mechanisms is reported. INTRODUCTION Because nanocrystalline samples are usually very small, research on mechanical properties has concentrated mainly on measurements of hardness as function of grain size. Results at the smallest grain sizes remain controversial: some results even indicate a yield stress independent of the grain size, or a reverse Hall-Petch relation and others confirm an increasing yield stress with decreasing grain size [1, 2]. It has however been shown that sample imperfections and microstructure play a key factor in these controversial results [3]. Mechanical properties for larger amounts of submicrocrystalline (produced by Severe Plastic Deformation (SPD) [4]) and nanocrystalline (produced by Electrodeposition (ED) [5]) materials have been studied by means of stress-strain curves at room temperature and higher temperatures, where diffusion controlled mechanism become dominant. Superplasticity has been observed in ED Ni, having an initial mean grain size of 35nm, at temperatures above 280°C. The plasticity is accompanied by rapid grain growth [6]. Recently a tensile machine has been developed allowing tensile testing of tiny dog-bone shaped samples with a gauge length of only 300 microns [7]. This allows samples synthesised with techniques producing only very small amount of material, such as Inert Gas Condensation (IGC), to be used for tensile testing. In the present work we use a similar miniaturised tensile machine to deform nanocrystalline Ni synthesised by IGC and ED. The initial microstructure and influence of deformation and/or temperature on the microstructure are reported and compared for both materials. EXPERIMENTAL PROCEDURE Nanocrystalline Ni samples prepared by ED (supplied by Goodfellow Ltd.) and by IGC are investigated. For preparing the IGC samples, a high vacuum chamber of 2 x 10-7 mbar was filled with 3 mbar of high-purity (99.999%) helium. High-purity nickel (99.99+%) wire was evaporated from resistively heated tungsten boats, with the resulting metal clusters collected on a B2.8.1
liquid nitrogen-cooled finger. The clusters were removed from the cold finger with
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