Acoustic emission analysis of out-of-Phase thermo-mechanical fatigue of coated Ni-base superalloys

Abstract

Abstract The acoustic emission (AE) activity was monitored during the thermo-mechanical fatigue (TMF) testing of Ni-base superalloys (CM186), in both single-crystalline and directionally solidified form, coated by a modified aluminide. The TMF tests were carried out with 180° phase difference between mechanical strain and temperature (out of phase) at two different mechanical strain ranges (Δem=0.8 and 1.0%, R=-∞) and with temperature cycled between 350° and 950°C. For reference and AE source identification, TMF with AE monitoring was also performed for the uncoated bulk materials. In-situ monitoring of crack initiation and propagation on the surface of the specimens by means of a fully automated video imaging system was also implemented. Both AE monitoring and video imaging were synchronised with the TMF cycle. The AE data were analyzed statistically with respect to various parameters (such as amplitude, number of counts, duration, rise time, total energy, etc., and the TMF parameters of their occurrence, stress, temperature, cycle number). Within the eleven-dimensional parameter space, lower-dimensional descriptor subspaces were identified by correlation hierarchy that characterize different groups of AE events. From the analysis of the AE signals various types of information were obtained, such as: (1) the effects of the coating on the AE activity, (2) early surface vs. bulk damage and the subsequent growth of fatigue cracks, respectively.