Influence and comparison of contaminate partitioning on nanocrystalline stability in sputter-deposited and ball-milled Cu–Zr alloys

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 compare two processing methods—high-energy ball milling and thin film deposition—in the fabrication and subsequent two-step annealing (500 °C/24 h followed by a temperature ramp to 900 °C whereupon the sample was held for 1 min and quenched) 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 sample 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 sample showed nearly all elemental Zr upon deposition but significant oxidation after the vacuum anneal. The significant uptake 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 in the as-deposited state. Furthermore, upon sputter deposition, many of these boundaries were vitrified which was lost upon annealing. These glassy boundaries were not observed by PED of the powders. The consequence of when the solute reacts with contaminate species is discussed in relation to nanocrystalline and microstructural stability. The use of Zener pinning predicted grain sizes, based on the quantification of the secondary phase particulates measured by APT, are given to better ascertain their contribution to nanocrystalline stability.