Residual Stress and Microstructure of CoNiCrAlY Coated Superalloys upon Thermo-Mechanical Fatigue.

it has been observed that under certain thermo-mechanical fatigue (TMF) conditions overlay coatings have a detrimental influence on the life of superalloys when compared with the uncoated material. The different thermal expansion coefficients of the coating and substrate induce residual stresses at the interface region between them. These stresses clearly influence the TMF behaviour of the material and are the main concern in this communication. The thermo-mechanical fatigue cycles, experienced at various parts of a turbine blade in service, have been reproduced in the laboratory. Residual stress measurements were performed on several coated and uncoated samples. The stresses were determined using Sin2 psi method, under a biaxial approximation, for both gamma and beta-phases throughout the TMS tests. SEM and TEM were used to monitor changes in the microstructure of both the coating and substrate material. Throughout the test an oxide layer (Al2O3) was formed and grew at the surface producing a depletion of the Al rich phase beta-NiAl in the matrix. The main mechanisms of damage were observed to be creep at high temperature and cracking at low temperature.

SEQUEIRA Antonio;  MORETTO Pietro;  BRESSERS Johan; 

1998-11-16

JRC17065

https://publications.jrc.ec.europa.eu/repository/handle/JRC17065,