Direct Measurement of Diffusion Flux in Alloys Via Flux Separation

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Abstract
Despite the importance of atomic diffusion in controlling high temperature deformation, it remains difficult to generally determine diffusion flux through alloys. We present thermomechanical nanomolding, where a nanomold is filled by the alloy’s diffusion flux under a stress gradient, to determine the flux diffusing through an alloy’s microstructure. This flux is collected in the nanomold forming nanorods. Length and composition analyses of the formed nanorods allow us to determine rate and composition of the flux, and further allow estimation of the constituents’ diffusivities in this flux. We verify this technique on metals and simple alloys, and then reveal diffusive flux in general alloys. Notably, for alloys that can access a eutectic composition in their alloy system, the flux’s composition is that of the eutectic, which can be very different from the alloy’s nominal composition. Moreover, the flux’s overall diffusivity is greatly enhanced. This, so far unknown, eutectic mechanism is present in the majority of multicomponent alloys. These insights into diffusion flux in alloys can be used to guide the development of alloys with particularly low diffusivity or particularly high diffusivity. More generally, the presented method provides a novel toolbox to reveal the rich underlying diffusion-controlled mechanisms of deformation of alloys in general.

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