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Nanostructured Al–Mn alloys are proposed as high-strength low-density materials, which can be electroformed (i.e., produced electrolytically and removed from the substrate) from ionic liquid. A variety of current waveforms, including direct current (DC) and pulsed current (PC), are used to electrodeposit nanostructured Al–Mn alloys, with some PC methods producing significant improvements in film ductility. Transmission electron microscopy observations point to a number of structural advantages induced by PC that apparently ductilize the Al–Mn alloys: (i) grain refinement to the nanocrystalline range without the introduction of a competing amorphous phase, (ii) unimodal nanocrystalline grain size distribution, and (iii) more homogeneous structure. The significant increase in apparent ductility in the PC alloys is also apparently related to stress- or deformation-induced grain growth, which leads to alloys with unique combinations of specific hardness and film ductility., United States. Army Research Office (Contract W911NF-07-D-0004) (Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies)

A full diffusion kinetic Monte Carlo algorithm is used to model nanocrystalline film deposition, and study the mechanisms of grain nucleation and microstructure formation in such films. The major finding of this work is that new grain nucleation occurs predominantly on surface peaks. Consequently, development of a nanocrystalline structure is promoted by a growth surface with nanoscale roughness, on which new grains can nucleate and grow separately from one another. The grain minor dimension (in the plane of the film) is primarily dictated by surface peak spacing, which in turn is reduced at low temperatures and high deposition rates. The grain major dimension (in the growth direction) is related to the probability of nucleating new grains on top of pre-existing ones, with finer grains being formed at low temperatures and low grain boundary energies. Because vacancies kinetically pin grain boundaries, high vacancy content, which is obtained at high deposition rate, also favors nanograins. Consistent with empirical observations common in the experimental literature, it is found that as grains shrink, they transition from elongated to equiaxed., National Science Foundation (U.S.) (Grant DMI-0620304), United States. Army Research Office (Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies)

Al–Mn alloys with Mn content ranging from 0 to 15.8 at.% are prepared by electrodeposition from an ionic liquid at room temperature, and exhibit a remarkably broad range of structures. The alloys are characterized through a combination of techniques, including X-ray diffraction, electron microscopy and calorimetry. For alloys with Mn content up to 7.5 at.%, increasing Mn additions lead to a decrease in grain size of single-phase microcrystalline face-centered cubic (fcc) Al(Mn). Between 8.2 and 12.3 at.% Mn, an amorphous phase appears, accompanied by a dramatic reduction in the size of the coexisting fcc crystallites to the ∼2–50 nm level. At higher Mn contents, the structure nominally appears entirely amorphous, but is shown to contain order in the form of pre-existing nuclei of the icosahedral quasicrystalline phase. Additionally, nanoindentation tests reveal that the nanostructured and amorphous specimens have very high hardnesses that exhibit complex trends with Mn content., Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010., Includes bibliographical references (p. 131-142)., Tailoring the nanostructure of electrodeposited Al-Mn films to achieve high hardness and toughness is the overarching goal of this thesis. Binary Al-Mn alloys are electrodeposited using a conventional current waveform in a chloroaluminate electrolyte at ambient temperature. It is found that alloys with low Mn contents comprise micrometer-sized FCC grains. At intermediate Mn contents, the FCC grain size decreases abruptly to the nanometer regime upon the appearance of a secondary amorphous phase. In these dual-phase alloys, the phases are distributed in a characteristic domain-network structure. At high Mn contents, an amorphous phase that contains pre-existing nanoquasicrystalline nuclei dominates. Leveraging the effects of surface kinetics at the electrode on the alloy microstructure, a reverse-pulse current waveform is designed to tailor the grain size and phase distribution of the electrodeposits; single phase FCC alloys with nanocrystalline grains, as well as dual-phase alloys with homogeneous phase distribution are synthesized. Solute distributions in these alloys are investigated using atom probe tomography. Implanted Ga ions are used as chemical markers for the amorphous phase; this method permits more robust phase identification and measurement of their compositions. Whereas uniform Mn distributions are observed in the single phase alloys, Mn is found to weakly partition into amorphous phase of the dual-phase alloy by ~2 at.%. Micro-indentation of the reverse-pulsed alloys and the guided bend tests reveal high hardness and toughness that are comparable to steels. High hardness is attributed to a combination of solid-solution strengthening effects and structural refinement; high toughness of the nanostructured alloys arises from the activation of both grain boundary- and dislocation-mediated deformation mechanisms; malleability of the amorphous alloys stems from the simultaneous operation of multiple shear bands during deformation. An unprecedented combinatio, by Shiyun Ruan., Ph.D.