Ageing kinetics of Ni-base superalloys

In the present work, the degradation of the gamma/gamma' microstructure of single crystal Ni-base superalloy, CMSX-8, has been studied experimentally and computationally. Two microstructure evolution processes, referred to as rafting and coarsening, have been decoupled and investigated independently. The kinetics of coarsening and rafting of gamma' precipitates have been characterised using scanning electron microscopy for specimens at temperatures ranging from 900 to 1120 degrees C and under stress levels of 25-360 MPa. A phenomenological relation describing rafting as a function of stress, temperature and time was fitted to experimental data. It was determined that coarsening of gamma' precipitates follows the LSW cube rate law and is controlled by a volume diffusion mechanism. The effective diffusivity which influences the ageing behaviour was estimated computationally using DICTRA software and the coarsening experimental results were verified by formulating and computing the effective activation energy in multicomponent systems.