Towards novel in-situ composites: MAB nanolaminate-reinforced FCI quasicrystal in multi-phase aluminum alloys

Highlights: • Fe₂AlB₂ and icosahedral phases might form in a single reaction. • Doping Al, Cu or Fe with B allows shaping grains of certain phases. • Alloys with needle-like AlB₂ are less hard than with tabular grains. • Less slender Fe₂AlB₂ grains accompanies the increase in overall hardness. • Atomic positions of B in Fe₂AlB₂ phase can be occupied by Cu. -- Abstract: A range of new in situ composites with prospective application for low-density ballistic shielding was investigated. The beneficial mechanical properties will come from the co-existence of two phases: impact-resistant face-centered icosahedral (FCI) and Fe₂AlB₂. The aim of the current study was the investigation of phase equilibria in Al-Cu-Fe-B system. Each of the elements in Al-Cu-Fe base alloy was doped with boron and the alloys were produced by means of suction casting. The composition impact on microstructure, accompanying phases identification and hardness were discussed in detail. The compositional FCI phase stability area was found to be smaller than predicted from atomic size constraint imposed by Hume-Rothery criteria for ternary system. Primary phases were Fe₂AlB₂ or Fe₄Al₁₃ in B-doped alloys. It was found that atomic positions of B in Fe₂AlB₂ phase can be occupied by Cu, what opens new route of tailoring its properties. B:Fe and B:Cu substiution in the 4–10 at% range promotes the growth of AlB₂ phase from small tabular precipitates to long needles, what is correlated with the reduction of overall hardness. B:Al substitution leads to progressive increase in thickness of Fe₂AlB₂ lamellae that accompanies the hardness improvement. It was found that dominant factor contributing to overall alloy hardness was the shape of reinforcing Ψ and ζ grains rather than volume fraction of constituent phases.