Thermo-mechanical fatigue of a wrought nickel based alloy

Abstract

The nickel-based alloy Haynes 230 was investigated under low-cycle fatigue (LCF) and thermo-mechanical fatigue loading at different temperatures and with different phase shift. Whereas the yield strength is low, causing high plastic strains in every cycle, appreciable cyclic hardening up to double the yield strength was observed. The crack propagation rates were evaluated at different temperatures, with different mean stress and a standard waveform. A dwell signal was also tested. The crack propagation was modelled assuming a plasticity controlled fatigue part, which was correlated to the crack tip opening displacement by modified cyclic J-integral. At higher temperatures, an additional thermally activated oxide embrittlement increased the crack growth rate. Both mechanisms were considered separately in the model as they work independently. This model enabled us to describe the experimental growth rate, but cannot account for the threshold effects. As the cyclic life was found to be controlled by crack propagation starting from carbides or oxide spikes, life was predicted based on crack propagation. The results compare well for LCF within a scatterband of factor 2 at the lower cycles to failure. For higher cycle numbers, the measured lives were higher, which was assumed to be caused by the lack of a crack growth threshold in the model.