Evaluating Deformation Behavior of a TBC-System during Thermal Gradient Mechanical Fatigue by means of High Energy X-Ray Diffraction

Abstract

Analysis of X-ray diffraction pattern is a suitable method for contact free local strain measurement of crystalline materials under thermal and mechanical load. For employing this method in TMF - testing an equipment for cyclic thermal loading of tubular specimens has been developed and implemented into a servo-hydraulic test machine at the advanced photon source (APS) at Argonne National Laboratory. The specimen’s outer surface is heated by focused infrared radiation while it’s inner surface can be cooled with pressurized air resulting in a controlled thermal gradient across the specimen’s wall. The entire test machine is mounted on a precision positioning rig, which allows for exact µm-positioning with respect to the X-ray beam. Thermal mechanical tests with and without superposed thermal gradient have been performed on superalloy specimens with a thermal barrier coating system, and simultaneously X-ray diffraction patterns were taken using a 2D detector, giving 360° lattice parameter data. Size of the beam was 50µm in width and 300µm in height, and with a step width of 30µm the sample was scanned by the high energy beam of about 65 keV. Thermal and mechanical load schemata have been varied intending to determine material properties from the respective strain response of the different layers of the coated system. Strain data were acquired in plane parallel to the specimen’s length axis and out of plane. Comprehensive results of the strain data evaluation will be presented and discussed with respect to local material properties and damage features achieved in previous TGMF-tests series on the same type of specimen.