Phase formation and stability in sputter deposited Be--Nb compounds

The phase relations and thermal stabilities of phases in sputter deposited Be--Nb alloys (3--14% Nb) have been studied. An amorphous phase forms during deposition at ambient temperatures (30 degree C) for compositions >5% Nb. Metastable crystalline phases form during deposition at 350--450 degree C as well as during annealing of the amorphous phase. One metastable bcc phase designated as Be12Nb' can be explained by an extremely high fault density in the Be12Nb structure and is a precursor phase to the formation of both Be12Nb and Be17Nb2. Another metastable bcc phase also forms during high temperature deposition. Transformation to the equilibrium phases occurs at temperatures >800 degree C. A stoichiometric range of about 5.5--7.8% is indicated for the Be12Nb phase and is probably due to vacancies on the Nb sublattice. No evidence of a Be5Nb phase was found and the Be17Nb2 phase is stable to room temperature