Site occupancies, point defect concentrations, and solid solution hardening in B2 (Ni,Fe)Al

Abstract

A novel approach was used to characterize the site occupancies and point defect concentrations in the B2-ordered (Ni,Fe)Al single phase field over a wide range of composition and temperature. This approach combined atom location by channeling enhanced microanalysis (ALCHEMI) determinations of atomic site occupancies and vacancy concentration measurements. The triple defect structure was observed across the entire phase field. Qualitative thermodynamic predictions such as that Fe anti-site are more stable than Ni anti-sites were confirmed. However, in some portions of the phase field it was found that Ni anti-site concentrations were higher than expected from simplistic thermodynamic predictions of site preference. Such predictions are clearly inadequate if a quantitative determination of site occupancies is desired. The importance of this type of characterization in interpreting solid solution hardening (SSH) in ordered compounds was illustrated by considering three portions of the (Ni,Fe)Al phase field. In all three cases the solute atoms did not directly harden, but affected the hardness by altering the concentrations of other point defects.