Structural study of electrodeposited aluminum-manganese alloys
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INTRODUCTION
I N T E R E S T in aluminum-manganese alloys (and a number of other transition metals) has dramatically increased with the recent discovery of quasicrystal phenomena rll in many of these alloys. The essentially uncontrolled conditions present during rapid solidification have resulted in a search for alternative methods of preparing alloys containing quasicrystals such as solid-state processing t2'31 and sputtering deposition. ]4] A1-Mn alloys produced from these methods at room temperature are amorphous. Heat treatment of the amorphous phase at 270 °C to 350 °C results in the amorphous-toquasicrystalline transformation in the solid state, t3j The in situ formation of a fine-grained icosahedral phase has been observed in thin layers of alloys which were sputter deposited at 230 °C to 370 °C.[4] Aluminum alloys can also be electrodeposited from molten salt electrolytes containing A1CI3, NaC1, and the chloride salt of the solute metal at temperatures as low as 110 °C. The addition of small quantities of MnC12 to the chloroaluminate melt produces a binary aluminummanganese alloy with a near-specular surface finish, tS~ The manganese content of the electrodeposit can be rigorously controlled and is dependent upon deposition potential and the relative bulk concentrations of the electroactive species A12C17- and Mn ++ in the melt. t61 Alloy compositions ranging from 0 to 30 wt pct Mn BENJAMIN GRUSHKO, Postdoctoral Fellow, is with the Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa, Israel. GERY R. STAFFORD, Research Chemist, is with the Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted August 30, 1988.
METALLURGICAL TRANSACTIONS A
have been reported, [6.7,8] and higher manganese compositions are possible. The electrodeposition of these alloys offers many advantages over conventional solidification techniques. Since the deposition process is isothermal, one has rigorous control of the temperature at the electrolyte/electrode interface; hence, alloys of uniform composition and structure are possible. One also has an in situ measure of the deposition rate, which is proportional to the applied or measured current density. Earlier structural studies of electrodeposited A1-Mn alloys rTJ indicate that alloys of lower manganese composition consist of a supersaturated solid solution of manganese in aluminum. Alloys with greater manganese content were reported to contain an additional phase which produced a diffuse ring between the aluminum (111) and (200) reflections in electron diffraction patterns. It was proposed that this phase was a very fine-grained A16Mn intermetallic. [71 In this study, [7] well-defined reflections of this phase were observed in diffraction patterns of the heat-treated alloys. More recent work has characterized the electrochemical parameters pertinent to alloy composition and suggests that the structure of the as-deposited highmanganese alloy is that of a metallic glass, t6,81 which correspon
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