Bulk and dislocation core diffusion in Al-Mo solid solutions obtained through rapid-solidification processing

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Abstract
Samples of Al-2.2at.% Mo and Al-0.7at.% Mo have been prepared as solid solutions by melt-spinning, and have then been annealed at temperatures between 643 and 923 K. Analytical transmission electron microscopy has been used to determine the factors that control the rate of return to equilibrium in these supersaturated solid solutions. It has been found that in the Al-2.2at% Mo alloy the diffusion of Mo, to form the equilibrium phase Al12 Mo, is accompanied by the formation of dislocation networks, which separate regions that contain 2.2 at% Mo from regions of virtually pure Al, where the Mo has been removed from solution. The dislocations accommodate the change in lattice parameter between these two regions and provide diffusion paths to grain boundaries, where Al12 Mo is precipitated. In the case of Al-0.7 at.% Mo it has been found that diffusion occurs either along grain boundaries, during grain growth, or through the bulk of the grains, but that no dislocations are generated during annealing. Measurements of the width of the denuded zones at stationary grain boundaries in this dilute alloy have been used to determine the kinetics of diffusion of Mo in Al and it has been found that these may be described by the equation, D = D0 exp(- Ed/kT) with values of D0 = 0.28 (+0.13, −0.09) cm2 s−1 and Ed = 2.3 ± 0.3 eV. The significance of these results is discussed in terms of the published diffusion data for transition metals in Al.