Low Cycle Fatigue and thermomechanical fatigue behaviour of coated and uncoated IN738 superalloy

Results of isothermal Low Cycle Fatigue (LCF) tests made at 850 degrees C on uncoated IN738LC samples are compared with similar tests carried out on specimens with two types of coating, namely a platinum aluminide coating (RT-22) and a plasma sprayed MCrAlY coating (A995). When compared on the basis of identical values of applied strain range, it is found that the presence of the MCrAlY coating did not significantly lower the cyclic life with respect to that obtained for the un-coated material. The Thermo-Mechanical Fatigue - Linear Out of Phase (TMF-LOP) cycling mode was also applied, in tests on the coated and uncoated specimens of IN738. Interestingly, the TMF-LOP lives for the coated samples were considerably inferior than the TMF-LOP lives of the uncoated specimens. A modest diminution of life for the RT22 coated specimens was also confirmed. The crucial feature, leading to the observed dramatic life shortening of the coated samples of RT22 in TMF-LOP tests with respect to the isothermal ones was attributed to be the reduction of the elastic-plastic ductility of the coating, occurring in the cold part of the thermomechanical cycle when a large tensile stress is occurring with a low value of the temperature.