High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling.

International audienceCreep tests have been performed on MC2 single crystal superalloy at 950°C/200 MPa and 1150°C/80 MPa under isothermal and thermal cycling conditions with a tensile axis along the [001] direction. It is shown that the thermal cycles strongly affect the creep behaviour at 1150°C but not at 950°C. This was related to the repetitive precipitation and dissolution of γ' small rafts for the higher temperature, as revealed by quantitative characterisation of the γ/γ' microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[100]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appeared that the density of a[100] dislocations, and not their climb velocity - or diffusion rate - is the key parameter for the control of creep rate. The thermal cycles which imply the creation and subsequent dissolution of rafts, provided new dislocations which explains the acceleration of creep observed under such conditions