An orthorhombic D0₂₂-like precursor to Al₈Mo₃ in the Al–Mo–Ti system

A peculiar checkerboard microstructure has been observed in as-cast Mo-rich Al–Mo–Ti alloys with slightly less than 75 at.% Al. It contains a Mo-rich tetragonal D0₂₂-Al₃Ti phase as well as an orthorhombically distorted variant thereof, with a somewhat higher Mo content, referred to as D0₂₂’. Both phases are fcc-based superstructures originating mainly from ordering of transition metal (TM) vs. Al. The measured lattice parameters of the D0₂₂’ phase were evaluated using strain tensors quantifying the strain with respect to some average fcc substructure. Thereby, it was shown that the D0₂₂’ structure possesses some common features with the binary Al₈Mo₃ and Al₃Mo fcc-based superstructures known from the Al–Mo system. It is made likely that D0₂₂’ as well as Al₈Mo₃ and Al₃Mo (with Mo being a group-vi transition metal) are some continuation of the series of structures observed for Al₃TM with TM from group iii via iv to v with increasing average number of electrons per atom. This view is supported by first-principles calculations which indicate elastic instability of a hypothetical D0₂₂-Al₃Mo structure towards an orthorhombically distorted structure. The checkerboard microstructure likely forms directly after solidification from a homogeneous D0₂₂ structure by decomposition into Mo-richer (D0₂₂’) and -poorer regions. The phase interface orientations can be reconciled with the deformation necessary to transform D0₂₂ into D0₂₂’. Further annealing at elevated temperatures leads to decomposition of the microstructures, resulting in formation of Al₈Mo₃ and Mo-depleted D0₂₂, indicating that the D0₂₂’ phase is metastable.