Cyclic deformation, oxidation damage and fatigue crack growth in nickel-based superalloys

Cyclic deformation, oxidation damage and fatigue crack growth in nickel-based superalloys have been studied. Strain-controlled low cycle fatigue tests showed strong anisotropy and strain-rate dependency for a directionally solidified alloy. Essentially, the material showed different stiffness and fatigue life when the loading direction was changed from parallel to normal to the solidification direction. Imposition of dwells at peak strain level resulted in stress relaxation. Stabilise devolution of stress amplitude up to failure indicated limited cyclic softening/hardening. Crystal plasticity modelling was carried out to simulate cyclic deformation of the material, using finite element models developed from EBSD analysis of failed specimens. The model showed good predictive capability for the observed low cycle fatigue behaviour. To explain the reduced fatigue life in specimens loaded normal to the solidification direction, a study of stress distribution was conducted using the finite element crystal plasticity model. It was discovered that grain misorientations arising from a cluster of smaller grains caused severe localised stress concentrations, leading to earlier crack initiation and shortened fatigue life. Oxidation damage of single-crystal and directionally-solidified alloys was studied by testing thin-disc specimens under isothermal conditions. [Continues.]