Publications

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Grain size and texture of Ni electrodeposited from sulfamate baths depend greatly on current density. Increasing grain size is observed with increasing current density and the deposit texture changes from (110) at current densities lower than 5 mA cm{sup -2} to (100) for higher current densities. Co-deposition of Mn modifies the deposit structure by favoring the growth of the (110) texture and decreasing the average grain size even as the current density increases. While the average Mn film content increases with increasing current density, local Mn concentrations are a more complex function of deposition parameters, as indicated by atom probe tomography measurements. In both direct-current plated and pulse plated films, large variations on a nanometer scale in local Mn concentration are observed.

The effects of Mg alloying on the temporal evolution of Al{sub 3}Sc (L1{sub 2} structure) nanoscale precipitates are investigated, focusing on the morphology and coarsening kinetics of Al{sub 3}Sc precipitates in an Al-2.2 Mg-0.12 Sc at.% alloy aged between 300 and 400 C. Approximately spheroidal precipitates are obtained after aging at 300 C and irregular morphologies are observed at 400 C. The coarsening behavior is studied using conventional and high-resolution transmission electron microscopies to obtain the temporal evolution of the precipitate radius, and atom-probe tomography is employed to measure the Sc concentration in the {alpha}-matrix. The coarsening kinetics are analyzed using a coarsening model developed by Kuehmann and Voorhees for ternary systems [Kuehmann CJ, Voorhees PW. Metall Mater Trans A 1996;27:937]. Values of the interfacial free energy and diffusion coefficient for Sc diffusion in this Al-Mg-Sc alloy at 300 C are independently calculated, and are in good agreement with the calculated value of interfacial free energy and the experimental diffusivity obtained for the Al-Sc system.

Controlling the distribution of chemical constituents within complex, structurally heterogeneous systems represents one of the fundamental challenges of alloy design. We demonstrate how the combination of recent developments in sophisticated experimental high resolution characterization techniques and ab initio theoretical methods provide the basis for a detailed level of understanding of the microscopic factors governing compositional distributions in metallic alloys. In a study of the partitioning of Mg in two-phase ternary Al-Sc-Mg alloys by atom-probe tomography, we identify a large Mg concentration enhancement at the coherent {alpha}-Al/Al{sub 3}Sc heterophase interface with a relative Gibbsian interfacial excess of Mg with respect to Al and Sc, {Lambda}{sub Mg}{sup rel}, equal to 1.9 {+-} 0.5 atom nm{sup -2}. The corresponding calculated value of {Lambda}{sub Mg}{sup rel} is -1.2 atom nm{sup -2}. Theoretical ab initio investigations establish an equilibrium driving force for Mg interfacial segregation that is primarily chemical in nature and reflects the strength of the Mg-Sc interactions in an Al-rich alloy.

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