Sankara Sarma V Tatiparti

Abstract: Nanocrystalline supersaturated dendritic Al-Mg powders were electrodeposited using potentiostatic and galvanostatic techniques under equal-charge conditions. In potentiostatic deposition morphology depended on applied potential: featherlike at lower and globular at higher potentials. Galvanostatic deposits yielded both morphologies at any current density. Morphological evolution was observed in galvanostatic deposits from featherlike to globular. Independent of deposition technique face centered cubic (fcc) Al(+Mg) phase with ~7 atom% Mg (featherlike) with/without ~20 atom% Mg (smooth globular) composition formed at lower applied/realized potentials (or deposition rates). Higher applied/realized potentials showed hexagonal close packed (hcp) Mg(+Al) phase with ~80 atom% Mg (rough globules) over smooth globules. Potentiostatic and galvanostatic deposits were compared for their morphologies, phases and compositions.

Abstract: Nanocrystalline, supersaturated and metastable Al�Mg dendrites with stacks of smooth and rough globules were produced through electrodeposition. Rough globules always formed over the smooth ones. The smooth globules possessed face centered cubic (fcc)-Al(Mg) phase with ~ 20 at.% Mg. The rough globules contained ~ 80 at.% Mg and corresponded to hexagonal close packed (hcp)-Mg(Al) phase. Across the smooth-rough boundary a drastic jump in the composition from ~ 20 to ~ 80 at.% Mg was observed in nanometer scale. The metastable equilibrium between the supersaturated fcc and hcp phases was explained using the estimated free energy change-composition (�G�x) curves.

Abstract: Nanocrystalline Al-Mg dendrites with smooth (face centered cubic (fcc)-Al(Mg)) and rough (hexagonal close packed (hcp)-Mg(Al)) globular morphology were electrodeposited. The longitudinal sections of the rough globules exhibited bright-dark banded structure possessing Mg-poor and Mg-rich compositions respectively. The banded structure was explained by considering the fluctuations in potential(E)-time curve. More negative potentials result in Mg-rich dark bands. Mg-poor bright bands appear at less negative potentials. A mechanism was proposed for the banded structure formation in Al-Mg dendrites.

Abstract: Porous nanocrystalline supersaturated face centered cubic (fcc)-Al(Mg) dendrites with globular morphology were produced via electrodeposition. The cross-section of the globules revealed compact�disperse�compact structure along the growth direction. Initially compact globules formed due to high potential (or current density) which decreased eventually resulting in disperse-entity growth. Overlapping spherical diffusion zone formation over the disperse-entities was attributed as a reason for the compact growth at later stages. The internal structure of the globules was explained by the global potential (E)�time curve and the estimated local current densities ahead of deposit front. A growth mechanism for globular morphology was proposed using the results presented.

Abstract: Nanocrystalline Al�Mg dendrites were fabricated through galvanostatic electrodeposition. Initially feather�like morphology was formed exhibiting morphological evolution to smooth globules at its tips. With eventual deposition, rough globules formed over the smooth ones. The featherlike 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 the featherlike and rough globules possessed grain sizes of ~42 and ~36 nm respectively. The region, which exhibited morphological evolution from featherlike to smooth globules, possessed ~16 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.

Abstract: Al�Mg electrodeposition yielded face centered cubic (fcc)-Al(Mg) nanocrystalline featherlike dendrites consisting a stem and several arms exhibiting strong morphological anisotropy and microtexture. Various morphological features and preferred orientations lead to a crystallographic model suggesting that the stem and arms contain nanograins with their high energy {011} and {001} planes perpendicular to the growth directions. The dendrites grow with both low ({111}) and high ({001}) energy planes of nanograins as growth planes. The shape of these dendrites was explained using preferred orientations of nanograins within.

Abstract: Supersaturated dendritic Al�Mg alloy powders in globular form were produced using galvanostatic electrodeposition technique on polycrystalline Cu and Mg-substrates. The deposit produced on Cu-substrate possessed face centered cubic (fcc)-Al(Mg) phase with composition of ~ 18 at.% Mg. However, when Mg-substrate was used, initially hexagonal close packed (hcp)-Mg(Al) phase with ~ 77 at.% Mg was formed over which fcc-Al(Mg) phase with ~ 36 at.% Mg was nucleated. The results of the present study indicate that substrate crystal structure and estimated substrate-deposit lattice mismatch can influence the depositing phase and its composition but not the morphology of these powders.

Abstract: Al�Mg alloy powders were produced in the form of nanocrystalline dendrites using the galvanostatic electrodeposition technique. Two distinct morphologies namely, featherlike and globular, were observed under the employed deposition conditions. The featherlike morphology consisted of only the face-centered cubic (fcc)-Al(Mg) phase. However, the globular morphology was composed of the hexagonal close-packed (hcp)-Mg(Al) phase as well as the fcc-Al(Mg) phase. Compositional and TEM studies revealed that both phases were supersaturated and exhibited a nanocrystalline microstructure. Short-time potentiostatic experiments revealed that the formation of these morphologies is dependent on the applied overpotential such that at high overpotentials, only the globular morphology develops, whereas at low overpotentials, only the featherlike morphology is created. These observations are discussed in terms of the deposition rate.

Abstract: Al�Mg alloys were deposited using a base-electrolyte with the composition Na[AlEt4] + 2Na[Et3Al�H�AlEt3] + 2.5AlEt3 + 6toluene (where Et = C2H5). Mg was introduced into this electrolyte by employing a pure Mg anode. It was found that initially the amount of Mg in the electrolyte increased with the deposition time but eventually a steady state was reached such that the amount of Mg dissolved at the anode became equal to that deposited at the cathode. Compositional and phase analyses indicated that this state is achieved at a critical Mg/Al ratio that resulted in the formation of the hcp Mg-rich phase. By devising various component electrolytes we have attempted to understand the roles of different compounds in the base-electrolyte and have proposed a scheme for the Al�Mg alloy deposition.

Abstract: Aluminum�magnesium alloys can potentially be used as hydrogen storage materials. In order to enhance the kinetics of hydrogenation, it is desirable to have agglomerates of fine powders with very small grain size. Electrodeposition is a viable method to produce pure powders with an ultrafine structure. In this paper we report the effect of magnesium content, electrolyte temperature, and current density on the characteristics of Al�Mg deposits produced galvanostatically using an organometallic-based electrolyte. The magnesium content of the deposits improved with increasing Mg concentration in the electrolyte, temperature, and current density. The results indicate that the morphology of the deposits is significantly affected by the magnesium content. Depending on the composition, the deposits consisted of face-centered cubic-Al(+Mg) and hexagonal close-packed-Mg(+Al) phases.

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Abstract: Aluminum-magnesium alloy powders can potentially be used as hydrogen storage materials. In order to enhance the kinetics of hydrogenation it is desirable to have agglomerates of fine powders with very small grain size. In this study, nanocrystalline Al-Mg alloys in the form of powders were successfully fabricated by the electrodeposition technique using an organometallic based electrolyte. Mg was introduced into the electrolyte by a process called "pre-electrodeposition". The mechanism for Mg accumulation can be explained considering the electrode reactions as well as the chemical changes in the electrolyte. Using a copper cathode, the effects of the electrolyte composition and current density on composition of the deposit, its constituent phases and morphology were investigated. The magnesium content of the deposits improved with increasing Mg concentration in the electrolyte, temperature and current density. Depending on the composition, the deposits consisted of FCC-Al( Mg) and HCP-Mg(Al) phases and no intermetallic phase was found except for long deposition times. Generally, the deposits formed initially on the copper substrate with three dendritic morphologies namely, rod-like, feather-like and small globular, which eventually evolved into the large globular morphology. This observation is attributed to the establishment of spherical diffusion conditions at the sharp dendrite tips. Potentiostatic studies suggested that the appearance of different morphologies is associated with differing rates of deposition. While the initial dendrites consisted of the FCC Al-rich phase, the large globular morphology manifested as both FCC Al-rich and HCP Mg-rich phases, with the latter always forming over the former. The observation of formation of only the FCC phase implies that the nucleation barrier for the HCP phase on the copper substrate is quite high. The investigation of the effect of substrate, namely, Cu, graphite and Mg, revealed that the HCP phase can directly nucleate on an oxide-free Mg surface. This finding can be explained in terms of surface/interfacial energies. Detailed TEM analysis revealed that the observed morphologies consist of randomly distributed nanocrystalline grains except for the feather-like dendrites, which exhibited a strong crystallographic texture.

Abstract: Equal Channel Angular Pressing of Al-4% Li is carried out for two passes using route BC. The grain size reduced from annealed condition (8.1 �m) to two pass condition (4.11�m). The grains are separated by high angle grain boundaries (HAGB). The room temperature strength increased from annealed condition (148 MPa) to the two pass condition (270.83 MPa). The ductility at room temperature decreased from annealed condition (17.96%) to first pass condition (6.4%) and increased to 8.0% in second pass condition. The reason for this anamoly is the decreased microstain in the two pass condition. During the elevated temperature tests in the range of 400 �C �500 �C and in the strain rate range of 1X10-4 s-1-1X10-3 s-1 in the ECAP condition the maximum elongation (140.17%) is obtained at temperature of 400 �C and strain rate of 5X10-4 s-1 corresponding to strain rate sensitivity of 0.33. The reason for the drastic decrease in the ductility at 500 �C is attributed to some cracks present in the materials and rapid grain growth. In the as received condition the maximum elongation (159.48%) is obtained at temperature of 450 �C and strain rate of 1X10-3 s-1 corresponding to strain rate sensitivity of 0.2. The activation energy estimates suggest that self diffusion process is active at the testing conditions. Dimple rupture is suggested as the mode of fracture.