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

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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.