CRACK INITIATION IN A COATED AND AN UNCOATED NICKEL-BASE SUPERALLOY UNDER TMF CONDITIONS

A thermomechanical fatigue (TMF) cycle, intended to represent aeroengi ne blade working conditions, was selected for carrying out tests on un coated and aluminide coated SRR99 samples until test piece failure. Op tical images of the surface of test pieces were collected during testi ng to monitor surface crack initiation and accompanying transformation s. Using these images, surface changes were quantified as a function o f time. Post test each sample,rns taken through an incremental polishi ng procedure to allow damage in the tested material to be studied as a function of depth, using optical microscope-based quantitative metall ography and scanning electron microscopy (SEM). The relationship betwe en 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 obse rved life reductions.