Einfluss des Versetzungsgleitverhaltens und der Vorgeschichteabhängigkeit auf das Ermüdungsverhalten von Nickelbasis-Superlegierungen und Nickel im VHCF-Bereich

In this work a contribution to the understanding of the cyclic deformation behaviour of precipitation-hardened nickel-based superalloys (Nimonic 80A and Nimonic 75) and pure Nickel was developed for the VHCF regime. Modern technical alloys often undergo a mecha-nical pre-treatment during the production process to optimize their mechanical properties. Therefore, the aim was a comprehensive characterization of the influence of a defined predeformation up to 8% total strain (pre-strain history dependence) and the precipitation condition (peak-aged, overaged and precipitation-free) on the fatigue behaviour in the VHCF range (at room temperature (RT) and isothermal loading temperature up to 800°C). On the basis of microstructural analyses by means of light-, scanning and transmission electron microscopy new insights regarding damage mechanisms at very complex loading conditions (with a very localized and inhomogeneously distributed plastic deformation) could be generated for the VHCF regime. Due to the choice of materials tested, the dislocation slip character could be identified as crucial factor for the fatigue behaviour in the VHCF range. In addition to the planar gliding nickel-based superalloys, pure nickel (wavy slip behaviour) was investigated as reference material. The VHCF experiments were performed with three ultra-modern testing machines allowing test frequencies of around 130 Hz, 760 Hz and 20 kHz. Hence, within a reasonable test time 107 to 1010 cycles could be achieved.
Surprisingly, the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime for temperatures up to 600°C as compared to the peak-aged condition. Accordingly, the VHCF-behaviour does neither correspond to the static strength nor to the fatigue behaviour in the conventional range. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease in cyclic lifetime for all precipitation conditions. Irrespective of test conditions, crack initiation occurs at or close to the surface irrespective of the surface condition. An increasingly negative effect of the prior load history on the cyclic life was observed at RT with increasing hardening and can be attributed to surface roughening in combination with higher notch sensitivities. Nickel shows a contrary behaviour due to a more homogeneous wavy slip behaviour in the VHCF range. According to the VHCF life time, the negative pre-strain dependence history decreases with increasing test temperature (up to 600 °C) for all precipitation conditions. This can be partially explained by the beginning of recovery. However, additional effects not related to the strain-hardened microstructure play a role, such as the formation of oxide layers and the decrease in notch sensitivity. For the overaged condition, 4% predeformation led to the highest VHCF fatigue resistance at 600 °C, due to the relatively stable recovered dislocation arrangement. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease in cyclic lifetime for all precipitation conditions with or without a mechanical prehistory.
The characterization of the cyclic strength together with the analysis of the influence of the microstructure (such as the corresponding dislocation slip behaviour and the relevant dislocation/particle interaction mechanisms, the micro crack formation in the emerging oxide layer etc.) resulted in the development of a schematic description of the deformation behaviour. The findings underline the complexity of damage behaviour due to the competing failure modes. Until now the investigations show, that for a fail-safe life time prediction model for nickel-based superalloys the observed phenomena in the VHCF regime still constitute an insufficient basis. However, the results allow preliminary recommendations as to the influence of production and loading conditions related factors on the VHCF behaviour of the alloys investigated.