The chemical driving force for rafting in superalloys

Abstract

Until recently, all theories of the driving force for rafting have considered the compositions of the two phases to be fixed, although accepting that the rate of rafting might be controlled by diffusion. When plastic flow occurs, the difference in elastic constants becomes negligible. A high energy density builds up in the transverse {gamma} sheets, and rafting occurs by outward motion of the transverse interfaces, reducing the volume which has a high energy density. The analysis considers only the change in enthalpy between two states, one in which the two phases have the compositions which are in equilibrium in the absence of external stress, the external stress has been applied, but no diffusion has occurred, and one in which the two phases have the homogeneous compositions which are in equilibrium under the applied stress. The authors do not attempt to treat the intermediate configuration in which some diffusion has occurred, but the compositions of the phases are inhomogeneous.