Determining the site occupancy of Ru in the L1 2 phase of a Ni-base superalloy using ALCHEMI A.P. Ofori a, * , C.J. Rossouw b , C.J. Humphreys a a Department of Materials and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK b CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South, Vic. 3169, Australia Received 17 March 2004; received in revised form 4 September 2004; accepted 13 September 2004 Available online 14 October 2004 Abstract 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 super- alloy 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 Ni 3 Al c 0 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. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: ALCHEMI; Nickel alloys; Nickel aluminides; Ruthenium 1. Introduction Nickel-base superalloys are used extensively for tur- bine blades in gas turbines of aeroengines and power plants. There are two primary phases present in such an alloy, the matrix c phase (based on nickel, fcc) which surrounds the precipitate c 0 phase (based on Ni 3 Al, L1 2 ). Typically these materials contain in excess of twelve elements, their complex composition resulting from extensive alloy development over many years. This paper focuses on the effect of the platinum group metal Ru on the structure of the latest generation of turbine blade single crystal Ni-base superalloys. In particular it reports the determination of the sublattice site distri- bution of Ru in the c 0 phase. Ru partitions preferentially to the c phase in a c/c 0 superalloy [1] with approximately 2 at.% going to c for every 1 at.% that goes to c 0 . The addition of Ru is of particular interest due to some re- cent studies suggesting that this results in a significant improvement in high temperature mechanical properties of third generation single crystal alloys [2,3]. The phase diagram of the basic c–c 0 system for Ni-based superalloys is shown in Fig. 1. The multitude of alloying additions modify important mechanical properties such as elastic constants, and physical proper- ties such as lattice parameter, but the underlying two phase c–c 0 microstructure needs to be preserved. A fur- ther limitation for advanced superalloys used in high temperature turbine blades is that the c 0 volume fraction has to be kept between approximately 65–75%. This is required in order to optimise the high temperature (900–1100 °C) creep behaviour which, at present, deter- mines the life of the alloy. 1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.09.007 * Corresponding author. Tel.: +44 7815 174459; fax: +44 1223 334437. E-mail address: Tony.Ofori@cantab.net (A.P. Ofori). Acta Materialia 53 (2005) 97–110 www.actamat-journals.com