Comparison of Solute Partitioning between Nanocrystalline Sputtered Thin Films and Ball Milled Cu-Zr

Abstract

Ascertaining the mechanism(s) of nanocrystalline stability is a critical need in revealing how specific alloys retard grain growth. Often significant debate exists concerning such mechanisms, even in the same alloy. Here, we compared two processing methods - high-energy ball milling and thin film deposition - in the fabrication and subsequent two-step annealing (500 °C/24 hours followed by temperature ramp to 900 °C/1 min and quench) for nanocrystalline Cu-Zr. Using precession electron diffraction (PED) and atom probe tomography (APT), the grain stability and secondary phase content was quantified. The milled powder samples, Cu-1Zr (at%), revealed that the Zr solute was largely in an oxide/carbide state after milling with no significant change upon annealing. In contrast, the thin film samples, Cu-3Zr, showed nearly all elemental Zr upon deposition but significant oxidation after the vacuum anneal. The significant up-take of oxygen is contributed to the high surface area-to-volume ratio of the film coupled with columnar grains that were enriched in elemental Zr. Furthermore, upon sputter deposition, many of these boundaries were vitrified which was lost upon annealing. These glassy boundaries were not observed by PED for the powders. The consequence of when the solute reacts with contaminate species is discussed in relationship to nanocrystalline and microstructural stability. The use Zener pinning predicted grain sizes based on the quantification of the secondary phase particulates, as measured by APT, is given to better ascertain their contribution to nanocrystalline stability.