Metales

Metals 2011, 1, 3-15; doi:10.3390/met1010003
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metals
ISSN 2075-4701 www.mdpi.com/journal/metals/ Article

Evolution of Morphology and Microstructure in Electrodeposited Nanocrystalline Al–Mg Alloy Dendrites
Sankara Sarma V. Tatiparti * and Fereshteh Ebrahimi Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611, USA; E-Mail:febra@mse.ufl.edu * Author to whom correspondence should be addressed; sankara@ufl.edu; Tel.: +1-352-846-3323; Fax: +1-352-846-3355. Received: 8 August 2011; in revised form: 23 August 2011 / Accepted: 29 August 2011 / Published: 5 September 2011

Abstract: Nanocrystalline Al–Mg dendrites were fabricated through galvanostatic electrodeposition. Initially feather-like morphology was formed exhibitingmorphological evolution to smooth globules at its tips. With eventual deposition, rough globules formed over the smooth ones. The feather-like and smooth globules possessed supersaturated face centered cubic (fcc)–Al(Mg) phase with ~7 and ~20 at.% Mg respectively. The rough globules contained hexagonal close packed (hcp)–Mg(Al) phase with ~80 at.% Mg. Microstructural examinations revealed that thefeather-like and rough globules possessed grain sizes of ~42 ± 15 and ~36 ± 12 nm respectively. The region, which exhibited morphological evolution from feather-like to smooth globules, possessed ~16 ± 7 nm grain size. The observed microstructural and compositional features were attributed to the local current density values. The formation of the Al–Mg dendrites is discussed in this paper. Keywords: Al–Mg;nanocrystalline; dendrites; morphology; microstructure

1. Introduction Several techniques have been employed in the production of nanocrystalline powders, for example solution based aggregation [1], hydrothermal method [2] and sol–gel technique [3]. Electrodeposition has been gaining importance as a promising technique for nanocrystalline powder fabrication in

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4dendritic form with several other advantages to it such as the ability to produce materials in relatively pure form and with metastable phases [4,5]. Various parameters are involved in the electrodeposition process namely, overpotential, current density, temperature, electrolyte composition, substrate, agitation, etc., some of which have substantial effect on the morphology of the deposits. Among theseparameters, the effect of potential (or current density) has been studied most extensively. For example, near equilibrium potentials (or low current densities) render faceted crystals [6] and induce epitaxial growth [7]. Under such conditions, because of the slow deposition rate, charge transfer mechanisms dominate the crystal growth and specific crystallographic facets evolve owing to theanisotropy in the interfacial energy and the growth rate. This process is usually controlled by the activated intermediate states. Employing high current densities has been shown to cause branching or dendritic growth in deposits [8]. Because of the high deposition rates, diffusion processes play significant roles in deciding the morphologies [9]. For example, in the galvanostatic deposition of Au–Agalloys, dendritic precursors were observed in the initial stages of deposition under mixed activation–diffusion controlled conditions. At higher potentials produced in the later stages of deposition, due to the onset of the spherical diffusion condition, poorly defined crystallites and globular morphologies were found [10]. In the case of the electrodeposition of Cu it has been shown that atrelatively low current densities, deposits formed with protruded tips which eventually grew in the form of carrot-like structures under activation–controlled mechanisms [11]. The development of this structure was suggested to be associated with the differences in the growth rates of the tips and the lateral sides of the growing entities. At higher deposition rates cauliflower-like structures were...