Abstract
The degradation process and lifetime of the directionally solidified Ni-base superalloy Alloy 247 DS was investigated under out-of-phase thermo-mechanical fatigue (OP-TMF) loading in relation to the properties and microstructure of oxidation protection coatings. The influence of two coating systems applied by low pressure plasma spraying (i) MCrAlY coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl-topcoat was studied.The MCrAlY coating did not cause significant changes in failure behaviour and TMF life of the superalloy, whereas duplex coating reduced TMF life significantly because of accelerated fatigue crack initiation and propagation into the substrate material. Due to much lower fracture strains of the duplex coating than of MCrAlY coating the crack formation in the duplex coatings was already observed at very early stages of lifetime. Furthermore, crack propagation was found to be significantly affected by crack branching at the interface between coating and substrate alloy.
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