Coarsening of γ′ in Ni–Al alloys aged under uniaxial compression: II. Diffusion under stress and retardation of coarsening kinetics

An elasticity-based model is developed for predicting how the coefficient of diffusion, D, changes under an applied compressive stress. Stress affects the activation energy for motion, QM, of an atom, as well as its jump distance, l. Since a cubic crystal is slightly distorted tetragonally under uniaxial stress, QM and l, hence D, both become different in directions normal and parallel to the stress axis. QM is calculated using a dynamical model of diffusion due to Flynn [Point defects and diffusion. London: Oxford University Press; 1972]. Using the elastic constants for Ni–Al alloys, and taking into account the effect of the hydrostatic component of the applied stress on D, the model predicts that D decreases by about 6% under an applied stress of 150 MPa. This reduction does not account entirely for the retardation of coarsening kinetics observed experimentally, but given the simplicity of the model the result is satisfactory.