Phase formation in electrodeposited and thermally annealed Al-Mn alloys

Abstract

Aluminum-manganese alloys with compositions ranging from 0 to 50 wt pct Mn were electrodeposited onto copper substrates from a chloroaluminate molten salt electrolyte containing MnCl₂ at temperatures of 150 °C to 325 °C. The structures of these electrodeposits were then compared to those observed when metastable electrodeposits were thermally annealed at 200 °C to 610 °C. The alloys were characterized by scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy, and X-ray diffraction. At deposition temperatures of 150 °C to 250 °C, no stable structure other than the strongly supersaturated and highly dislocated Al-face-centered cubic (fcc) solid solution is observed. An amorphous phase and body-centered cubic (bcc) Al₈Mn₅ are observed at higher manganese compositions. In the temperature range of 250 °C to 325 °C, some of the phases predicted by the equilibrium phase diagram, such as Al₆Mn and Al₁₁Mn₄, are electrodeposited. The direct deposition of the icosahedral and decagonal phases has been demonstrated at 325 °C. Thermal annealing of the amorphous phase at temperatures higher than 225 °C results in its transformation to the icosahedral phase with a grain size much smaller than that obtained in the electrodeposited icosahedral phase. Additional annealing at higher temperatures does not result in any detectable coarsening of the icosahedral phase; instead, crystals of Al₆Mn or Al₁₁Mn₄ grow into the regions once occupied by the icosahedral phase. The crystalline Al₆Mn phase which forms as the result of thermal annealing shows a structural deviation from the equilibrium phase. As-deposited alloys comprised of 2-to 3-nm-thick amorphous regions separated by fcc-Al grains failed to crystallize after 30 minutes annealing at 500 °C.