Failure Mechanism of Pure Nickel (Ni 200/201) under Thermo-Mechanical Loading

Abstract

Cyclic loading of metallic engineering components at constant elevated or fluctuating temperature causes a complex evolution of damage which be can hardly be described in a unique and straightforward manner. Often the thermal behaviour of the base metals is to weak, so thermal barrier coatings were needed. Nickel is generally used for such thermal barrier coatings. Therefore it is necessary to study the thermo-mechanical fatigue (TMF) of this material. The lifetime of these coatings is very strong affected by the temperature loading in general, both described by nodal temperatures and their local gradient. The thermal cyclic loading takes place as thermo-mechanical and low cycle fatigue (LCF) damage regime. To classify the thermo-mechanical failure mechanism of pure nickel, OP (out of phase) and IP-TMF (in phase) test series were examined. The use of damage parameters like the unified energy approach make sense, a more detailed life time calculation for pure Nickel can be done by using the Neu-Sehitoglu model. Summary, thermomechanical loadings activate multiple damage mechanism. Surface embrittlement by oxidation is the major distinctive mechanism in addition to pure fatigue damage. Different lifetime approaches were tested and analysed to fulfil the requirements for the fatigue analysis of nickel made components.