ELECTRODEPOSITION MECHANISM OF ALUMINUM-MANGANESE ALLOYS WITH AMORPHOUS AND QUASI-CRYSTAL PHASES

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The electrodeposition of Al-Mn alloys from the molten chlorides salts was carried out to investigate the codeposition mechanism.The effect of a current density on the alloy composition and on the cathode current efficiency was investigated. The result shows characteristic feature that the Mn content in the deposits is kept almost constant at relatively lower current densities. At higher current densities, the Mn content in the deposits shifts to reach another constant values which are depended on the bath composition and determined by the limiting aurrent densities of Al and Mn. Then, the polarization curves were measured for Al and Al-Mn deposition. Both depositions of Al and Mn from the alloy plating baths occur at the same potential and no potential region where Mn or Al deposited individually is observed. The partial polarization curves for Al deposition show that the potential of Al deposition shifts to less noble direction by 70mV due to the codeposition with Mn.The morphology and the structure of the Al-Mn alloys were investigated using scanning electron microscope, X-ray diffraction.The alloys became lustrous with an increase in Mn content. X-ray diffraction patterns of these alloys showed that the alloys of lower Mn content consisted of a-Al crystalline phase. With an increase in Mn content, the intensities of the diffraction peaks from the crystalline phase decreased while a broad peak appeared. The electron diffraction pattern of ca.7mass%Mn alloy showed alpha-Al polycrystalline phase. With an increasing Mn content in deposit.the alloy phases changed from alpha-Al to amorphous phase. These phases showed lamellar structure which was parallel to the substrate. In electron diffraction images of Al-Mn alloy of ca.30mass%Mn, four broad rings were observed and they were reflected from microcrystalline intermetallic compound Al_6Mn with the grain size of 3 to 5 nm.