COMPARISON OF CYCLIC DEFORMATION BEHAVIOUR OF NICKEL-BASED SUPERALLOYS IN THE LOW AND VERY HIGH CYCLE FATIGUE RANGE AND INFLUENCE OF CONTROL MODE

摘要

In the VHCF regime the diversity of failure origin broadens with the complexity of the material's microstructure as well as its susceptibility to transformation processes due to the loading condition. The characteristic VHCF behaviour after a monotonic predeformation, was studied on polycrystalline nickel-based superalloys (Nimonic 75 and Nimonic 80A) in the precipitation-free, peak-aged and overaged condition and on pure nickel. On the basis of cyclic deformation curves the difference in strain response to load-controlled fatigue tests at room temperature for amplitudes in the low cycle fatigue (LCF) and very high cycle fatigue (VHCF) range is discussed. As was expected for the LCF regime, elastic and plastic strain contributes to the transient behaviour observed for all precipitation conditions. Surprisingly, cyclic deformation curves at designated VHCF amplitudes at R = -1 at RT reveal a negative total mean strain and a slightly open stress-strain hysteresis loop as a result of prestraining. Under load control a monotonic predeformation and subsequent cycling at room temperature (RT) lead to an increased lifetime for the precipitation-free condition whereas for the peak-aged and overaged condition fatigue life is not affected in the LCF regime. In contrast, cyclic strength in the VHCF regime decreases after prestraining with increasing particle hardening.