Thermo-mechanical Out-of-Phase Fatigue Life of Overlay Coated IN-738LC Gas Turbine Material

Abstract

Thermomechanical fatigue (TMF) is a unique type of fatigue process in which a component is simultaneously subjected to fluctuating loads and temperature. Isothermal life prediction techniques are often not applicable to TMF conditions since mechanical properties are temperature dependent with different damage mechanisms. There are two major cycles in TMF: the in-phase (IP) cycle where the maximum strain peak coincides with the maximum temperature and the out-of-phase (OP) cycle where the maximum strain and the lowest temperature coincide. Experimental and analytical methods are developed to address the effect of thermomechanical strain cycling on coated nickel base superalloy IN-738LC material which is a γ' (Ni 3 Al) strengthened material used primarily for land based gas turbine blades. The coating system was a NiCoCrAlY overlay type. Tubular specimens in the two conditions, coated and uncoated, were primarily tested in out-of-phase (OP) TMF loading with a temperature range of 482-871°C. Using a viscoplastic concept which accounts for strain/temperature cycling response of substrate and coatings in terms of hysteresis loops which characterize the evolution of stress/strain/cycle up to mid-life cycle, a life prediction model was developed incorporating the effect of creep (strain hold-period), environment, and temperature. Test results show the OP TMF type cycle is the most damaging cycle for the coated IN-738LC material when compared to both in-phase and isothermal cycles. All experiments were strain-controlled with a triangular waveform and a strain-ratio A = e a m p /e m e a n = ∞.