Determining the site occupancy of Ru in the L12 phase of a Ni-base superalloy using ALCHEMI

Small additions of Ru significantly improve the high temperature creep properties of Ni-base superalloys, but the mechanism is uncertain. In order to understand the effects of adding Ru it is important to know the atomic site(s) occupied by Ru in the superalloy structure. We used the technique ALCHEMI (Atomic site Location by CHannelling Enhanced MIcroanalysis) on a Ni-base superalloy containing Ru and encountered unexpected problems. We have explained why the ALCHEMI technique cannot accurately be used with Ni-base superalloys. We then successfully applied ALCHEMI to the simpler ternary alloy Ni-19at.%Al-3at.%Ru and found that 60% of the Ru partitions to the Al sublattice in the Ni3Al γ′ phase giving an absolute atomic site preference of 5:1. Assuming such site preference holds true for more complex alloys, this information can be used to better understand the role that this platinum group metals plays.