Hard and tough electrodeposited aluminum-manganese alloys with tailored nanostructures

Abstract

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 combination of high hardness, toughness and lightweight is thus achieved in our electrodeposited Al-Mn alloys.