Decomposition behaviour of rapidly quenched Al–Nb alloys– TEM study

Two binary Al-rich Al–Nb alloys containing 4·5 and 7·7 wt-%Nb have been rapidly quenched from the liquid state using the ‘gun’ technique. Transmission electron microscopy and diffraction techniques have been extensively employed to characterize the as quenched as well as externally heat treated alloy foils. Each alloy in the as-quenched state shows only a supersaturated solid solution, indicating a substantial increase in the solid solubility limit of Nb in Al by rapid quenching. Elongated solid solution grains and defect structures have also been observed in the as-quenched foils. While no precipitation was found to occur on annealing the quenched dilute alloy (Al–4·5Nb) foils for 1 h at 673 K, precipitation along grain boundaries was observed in the concentrated alloy (Al–7·7Nb). Higher temperature annealing (≥773 K) resulted in the formation of a new metastable phase having an ordered fcc GeCa₇ type structure with a lattice parameter a ∼ 0·8 nm. This phase forms predominantly with a rodlike morphology and is arranged in a Widmanstätten pattern inside the grains, although fine precipitate particles have also been observed. On prolonged annealing at or above 773 K, the metastable phase transforms with the formation of the equilibrium tetragonal Al₃Nb phase.

MST/391