Einfluss einer zyklischen Belastung auf die Versprödungskinetik von Legierungen am Beispiel der 475°C-Versprödung von Duplexstahl und der dynamischen Versprödung einer Nickelbasislegierung

The objective of this study was to investigate the dependence of high temperature embrittlement mechanisms on high temperature fatigue and vice versa. As model embrittlement mechanisms the 475°C Embrittlement of ferritic austenitic duplex stainless steel and the Dynamic Embrittlement of nickel-based superalloys were selected.

The 475°C Embrittlement is a thermally activated decomposition of the ferritic phase which hardens the material. In contrast to this a cyclic plastic deformation weakens the steel by a deformation-induced dissolution of the decomposition. Fatigue tests with different frequencies, loading amplitudes at room temperature and at 475°C with Duplex Stainless Steel in different states of embrittlement show that the ongoing 475°C Embrittlement and the deformation-induced dissolution are competing mechanisms. It depends on the frequency, the loading amplitude and the temperature which mechanism is dominant. Applying the model of the yield stress distribution function to the hysteresis branches of the fatigue tests allows an analysis of the fatigue behaviour of each phase individually. This analysis shows that the global fatigue behaviour for the test conditions applied in this study is mainly controlled by the ferritic phase.

According to the existing understanding of Dynamic Embrittlement it is an oxygen grain boundary diffusion arising by tensile stress at elevated temperatures with the result of a fast intercrystalline crack propagation. In reference tests under vacuum conditions without oxygen grain boundary diffusion, a slow transcrystalline fracture appears. To analyse the Dynamic Embrittlement, the crack propagation was tested at 650°C with different
frequencies and superimposed hold times in the fatigue cycle at maximum stress. The results shows that the existing model of Dynamic Embrittlement needs to be adapted to the effects of cyclic plastic deformation. In hold times, the oxygen grain boundary diffusion in front of the crack tip builds a damage zone which cracks because of the cyclic plastic deformation between two hold times. In a model presented in this study the quantities of inter- (damage by Dynamic Embrittlement) and transcrystalline (damage by fatigue) fracture can be related to their ratio on the fracture surfaces. Therefore, the influence of high temperature fatigue on high temperature embrittlement depends on the embrittlement mechanism itself.