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The properties of a newly developed single crystal superalloyknown as STAL-15 -is described which is suitable for use in first stage blades of industrial gas turbines (IGTs). With 15 wt.% Cr and 4.55 wt.% Al, the alloy combines good corrosion and oxidation resistance with sufficient creep and fatigue performance. Traditionally, polycrystalline alloys such as IN792 and IN738LC or the single crystal alloy PWA-1483 have been used for this application; unfortunately they display only limited resistance to environmental degradation. The new alloy does not display this weakness and is therefore highly optimised for IGT applications. The new alloy is shown to be an alumina (Al2O3) former; the mechanisms behind the Al2O3-formation process are studied and the effects arising from changes in the chemical composition have been modelled. In addition, the mechanical properties in terms of creep and fatigue resistance are demonstrated together and the alloy stability evaluated during long term (up to 10,000 hours) exposure. For such applications, the new alloy is superior to existing nickel-based single crystal superalloys designed for aeroengine applications and which are optimised for very high creep resistance. The absence of Re contributes to a lower cost of alloy stock, and enhanced castability.

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Recent work within DDIT has shown that Ni base superalloys like HAYNES230, Co base superalloys like HAYNES188, super stainless steels like HAYNES HR-120, and stainless steels like 253MA are similar from a materials modelling point of view. They are austenitic, delivered solutioned, and precipitate secondary carbides and other brittle phases in service and during cyclic tests at elevated temperature. These new phases result in a significantly reduced RT ductility, while the high temperature ductility is at most moderately reduced. Therefore, TMF cycles, which repeatedly go down to low temperatures, see an embrittled alloy whereas LCF tests at Tmax (in the TMF cycle) do not. This suggests that the classical use of LCF data at Tmax might given non-conservative life estimates. Literature studies and materials testing have confirmed that TMF data may be well below LCF data at Tmax verifying the non-conservatism of the classical methodology. Furthermore, the cyclic life tends to decrease with decreasing Tmin in TMF tests, and IP TMF is usually more detrimental than OP TMF due to creep-fatigue interaction. While standard TMF tests are closer to reality than LCF tests, we are still not certain that they capture all detrimental effects under component cycling, and are running additional, carefully planned, TMF tests on aged specimen at low Tmin values to improve the analysis. More tests, especially biaxial IP TMF tests, will eventually be needed to get a comprehensive picture. A new TMF data backed model has, however, already shown a higher precision when compared with service experience than the classical creep-fatigue methodology which is calibrated with LCF data at Tmax. Further testing and analysis will enable us to refine the TMF model and extend it to additional ACP alloys. The main input to the TMF model is the stabilised inelastic strain range, as calculated by the constitutive model described in an earlier ASME Turbo paper, GT2002-30659.