Crack Initiation in a Coated and an Uncoated Nickel-Base Superalloy Under TMF Conditions

Abstract

A thermomechanical fatigue (TMF) cycle, intended to represent aeroengine blade working conditions, was selected for carrying out tests on uncoated and aluminide coated SRR99 samples until test piece failure. Optical images of the surface of test pieces were collected during testing to monitor surface crack initiation and accompanying transformations. Using these images, surface changes were quantified as a function of time. Post test each sample was taken through an incremental polishing procedure to allow damage in the tested material to be studied as a function of depth, using optical microscope-based quantitative metallography and scanning electron microscopy (SEM). The relationship between the observed surface changes and the damage built-up subsurface was examined. Differences in damage density on the surface and subsurface planes between coated and uncoated samples could accommodate the observed life reductions.