Thermal stability of nanocrystalline nickel with yttrium additions

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M. Atwater Department of Applied Engineering, Safety & Technology, Millersville University, Millersville, Pennsylvania 17551

J. Semones, R.O. Scattergood, and C.C. Koch Department of Materials Science and Engineering, NC State University, Raleigh, North Carolina, 27695-7907 (Received 24 October 2012; accepted 4 January 2013)

Nickel-yttrium nanocrystalline alloys with an as-milled grain size of approximately 6.5 nm were synthesized using high-energy cryogenic mechanical alloying. The microstructural changes due to annealing were characterized using x-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy. Experiments demonstrated that increasing yttrium content led to stabilization of the nanocrystalline grain size at elevated homologous annealing temperatures. Additionally, it was found that inadvertent contamination with nitrogen during the milling process caused the formation of yttrium nitride (YN) precipitates, which, in turn, resulted in an additional nonlinear hardening effect beyond the expected hardening due to grain-size reduction. Results reveal that kinetic pinning by YN particles is effective in retaining a nanostructure to relatively high temperatures. I. INTRODUCTION

Nanocrystalline (nc) metals having grain sizes less than 100 nm have been found to have unique physical and mechanical properties which, in some cases, are far superior to their coarser-grained counterparts. Due to their small grain size, there exists a large driving force for microstructural coarsening through the removal of the excess grain boundary area. Many face-centered cubic (FCC) metals have been reported to exhibit grain growth temperatures at low temperatures.1–5 Pure nanocrystalline nickel (nc Ni), for instance, has been shown to undergo grain growth at temperatures as low as 200 °C.6 At 420 °C, Hibbard et al.7 reported the rapid coarsening of nc Ni from 20 nm to 60 lm in just 5 days time, and there are numerous reports indicating this low temperature susceptibility to thermally induced coarsening. The collective temperature range for the grain growth onset in nc nickel is 200–400 °C.6–11 This temperature range is too low to exploit the high strength and high-temperature oxidation and corrosion resistance nc Ni is expected to provide. Despite this drawback, it is possible to design a nc microstructure that is more resistant to thermally induced grain growth. In general, the alloying of Ni with a second element has proven useful in reducing the growth rate and/or increasing the grain growth temperature over that of pure, nc Ni. Examples showing a low-to-moderate increase include, nickel-iron (Ni-Fe),11–13 nickel-phosphorus (Ni-P),14–16 nickel-cobalt (Ni-Co),17 and a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.9 J. Mater. Res., 2013

nickel-tungsten (Ni-W).18 Explanations have attributed the increased stability to the Zener pinning of grain boundaries by precipitated secondary phases and solute segrega