Effect of superimposed high cycle fatigue loadings on the out-of-phase thermal-mechanical fatigue behaviour of CoCr22Ni22W14

The thermal-mechanical fatigue (TMF) behaviour of CoCr22Ni22W14 was investigated under total strain controlled out-of-phase (OP) experiments without and with superimposed high cycle fatigue (HCF) loadings. The minimum temperature Tmin was 200 °C and the maximum temperature Tmax was varied between 750 and 1200 °C. The mechanical strain amplitude εa,tme during pure TMF tests was kept equal to the thermal strain amplitude εtth and the superimposed HCF amplitude εa,tHCF was varied between 0.05 and 0.2%. In both loading conditions cyclic hardening is observed, which is the less pronounced the higher Tmax is. Only at Tmax=1200 °C, cyclic softening appears after cyclic hardening in the first cycles as a result of creep damage accumulation. With increasing superimposed HCF amplitudes, the cyclic deformation behaviour is obviously more and more determined by the superimposed HCF loadings. Due to the dynamic relaxation processes at higher temperatures, tensile mean stresses develop during all TMF tests performed. Under TMF-OP conditions a significant lifetime reduction is observed as a result of superimposed HCF loadings. This lifetime reduction increases with growing HCF amplitudes and may approach 90% of the fatigue lifetime obtained from pure OP experiments. For each Tmax the dependence between the total strain amplitude (εa,t=εa,tme+εa,tHCF) and the number of cycles to failure can be described as a potential function (εa,t=A×NB−b) with an exponent b which decreases with increasing Tmax and which depends on the material properties at different temperatures.