The Effect of Mg on the Fatigue-Creep Interaction Behaviour of Superalloy GH698

This investigation deals with the effect of Mg on the fatigue-creep interaction behaviour of Ni-base superalloy GH698, and the behaviour of the element Mg under creep stress.
The experimental results indicate that Mg has no influence on the pure fatigue property tested at 700℃ 1Hz tension-compression cycling. Mg increases the rupture life for pure creep test due to the prolongation of the steady state creep and the development of the tertiary creep, but has no influence on min. creep rate. Under fatigue-creep interaction the beneficial effect of Mg on fracture life still manifests itself, even though several different wave forms are used for interaction tests.All results indicate that as long as the fraction of creep fracture exists the beneficial effect of Mg on fracture life occurs.
The fractographic analysis indicates that the beneficial effect of Mg is directly related to the appearance of intergranular fracture.The inter granular fracture can be found in certain degree for all tests except pure fatigue test.
AES analysis illustrates grain boundary segregation of Mg occured after heat treatment. At the beginning of the creep the segregation of Mg redistributes under the effect of creep stress, and the Auger peak of Mg is eliminated.However, as long as the creep voids nucleate the segregation of Mg at the surface of creep voids can be found by AES analysis, and this segregation will increase with increasing the creep time.According to the AES analysis, the conclusion is that the mechanism of the beneficial effect of Mg on the propagation rate of creep void lies in decresi ng the surface diffusion coefficent and grain boundary diffusion coeffic-ent by the segregation of Mg.
Another possible effect of Mg determined by AES analysis is that Mg may react with S, and decreases the segregation of S.