The addition of ruthenium can control Topologically Close Packed (TCP) phases precipitation in nickel-base superalloys. Thus it becomes possible to add more solid solution strengthening elements such as molybdenum or rhenium. 4th generation nickel-base single crystal (SC) superalloys, with platinum group metals (PGMs) elements such as ruthenium, show superior creep strengths, although the change in the TCP phases crystallography or compositions have not yet been reported.
   This study was carried out based on a 3rd generation nickel-base SC superalloy, TMS-121, which contains 5 mass pct rhenium and is known to precipitate TCP phases; a series of alloys with different amounts of ruthenium addition to TMS-121, TMS-138 and TMS-138+, were cast and heat treated in a variety of time and temperature conditions. TEM microstructural observations at different aging time and temperatures showed that, R phase, which is the most harmful phase in TCP phases with large amounts of γ′envelope, disappeared by the addition of ruthenium. It was also found that 2 mass% ruthenium addition increased the solubility limit of multi-phase nickel-base superalloys by about 7-8%.