To improve the high temperature strength, solid solution strengthening elements with amounts well above the solubility limit imposed by the solvus, have been added to high-strength Ni-base superalloys. Under long-term exposure at elevated temperatures, the formation of Tertiary phases causes a serious degradation in terms of creep strength. The Tertiary phases, also known as Topologically Close Packed (TCP) phases, and can be classified into μ, P, R, and σ types. Recently, it has been found that Platinum Group Metals (PGMs) elements are effective in controlling precipitation of TCP phases, although the exact mechanism of this phenomenon is not well understood.
This study was carried out based on a 3rd generation Ni-base superalloy, TMS-121, which contains 5 mass pct rhenium and is known to precipitate TCP phases; a series of alloys with different amount of ruthenium additions to TMS-121 were cast and heat treated in a variety of time and temperature conditions. Based on the microstructural examinations, Time Temperature Transformation (TTT) diagrams were constructed by heating the alloys at different aging time and temperatures. These diagrams indicated the critical amount of TCP phases that were known to deteriorate the high temperature strength. It was concluded that Ru addition is effective in stabilizing the microstructure by increasing the solubility of solid solution strengthening elements in Ni-base superalloys at high temperatures. It was also found that delaying the precipitation of TCP phases at all temperature ranges.