Electrodeposited Al-Mn Alloys with Microcrystalline, Nanocrystalline, Amorphous and Nano-quasicrystalline Structures

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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 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. The binary aluminum-manganese (Al-Mn) system has stimulated much curiosity in the scientific community because of the rich variety of equilibrium and metastable phases it exhibits, which include solid solutions, at least nine intermetallic phases, icosahedral and decagonal quasicrystalline phases, as well as amorphous structures. Interestingly, a great many of these phases can be produced by a single technique, i.e., electrodeposition from acidic chloroaluminate salts [1-13]. While phases predicted by the equilibrium phase diagram have been electrodeposited in the temperature range of 250 to 425 o C [6, 8], non-equilibrium phases, such as the quasicrystalline phases, amorphous phase, and supersaturated FCC phase, have been deposited at lower temperatures from 150 to 325 o Despite the significant number of works conducted on electrodeposited Al-Mn alloys [2-14], only a few studies have provided detailed characterization of the deposited structure. In 1966, Read and Shores presented transmission electron diffraction patterns of Al-Mn alloys electrodeposited from a chloroaluminate molten salt electrolyte (AlCl 3-KCl-NaCl-MnCl 2) presumably at 200° C [10]. Their data suggested that for alloys with low Mn content below about 6 at.%, a single FCC phase was deposited, with higher Mn content promoting apparently finer grains. At somewhat higher Mn contents up to about 12 at.%, a second phase coexisted with the FCC phase, as evidenced by an additional diffuse reflection between the first and second rings in the electron diffraction patterns. The authors identified the second phase as …