Thermo-mechanical fatigue investigations on Nickel base superalloys and creep of NiAl thin films

Abstract

In service, many components and structures of aero jet engines, are exposed to a complex superposition of varying mechanically and thermally induced forces. This cyclic loading is most pronounced during the start-up and the shut-down sequence of the engine, usually combined with high temperature transients, and is responsible for a serious reduction in lifetime, compared to isothermal operating conditions. A detailed knowledge of this interaction between varying temperatures and loads is of considerable importance for precise lifetime calculations. In order to characterise and scientifically describe the material behaviour under thermo-mechanical fatigue (TMF) exposure, laboratory experiments are performed under strictly defined conditions. The main challenge for experimental investigations is the precise temperature control required to simulate the fast thermal transients under operating conditions. In any component, the lifetime behaviour is dominated by three different damage mechanisms: fatigue, oxidation and creep. In order to protect the component surface against oxidising atmosphere in gas turbines the substrate material is coated with different protective coating systems. This work compares the thermal mechanical fatigue behaviour of two different substrate materials, Nimonic 90 as the “classic” matrix and PM1000, an oxide dispersoide strengthened powder metallurgical superalloy under TMF loadings. Additionally, the creep behaviour and the lifetime analysis of a b−nickel aluminide diffusion coating system is described.