The role of diffusion in determining the controlling creep mechanisms in Al-Zn solid-solutions: Part I

This is the first of a two part paper in which the role of diffusion in determining the rate controlling creep mechanisms in binary solid solutions is investigated. Part I deals with the appropriate diffusion coefficients for climb and glide controlled creep, while Part II involves a comparison of the climb and glide creep equations with experiment. The first part begins with a review of a number of diffusion coefficients that have been proposed for creep in binary solid solutions. We show that the diffusion coefficient for climb is while that for glide is . Two other diffusion coefficients that have been used in the literature are shown to be incorrect. To experimentally verify the theoretical diffusion coefficients, the creep behavior of Al-Zn alpha solid solutions was studied over a composition range of 0 to 60 at. pct Zn. Values of and the thermodynamic factor (1 +d lnγ _i/d lnX i) are taken from the literature and used to calculate D_cland D_glas a function of composition at 360 and 410°. It is found that the thermodynamic factor strongly affects the diffusion coefficientD _gl for glide-controlled creep. When these curves forD _cl andD _gl are compared with experimental creep data for the Al-Zn system,D _gl is found to explain the decrease in creep rate observed at the miscibility gap composition. Formerly Graduate Research Assistant