The precipitation reactions occurring in concentrated Ta-Hf and Nb-Hf alloys over the temperature range 600° to 1400°C were investigated by X-ray diffraction and transmission microscopy. In Ta-Hf alloys containing 30 to 62 at. pct Hf and Nb-Hf alloys containing 25 to 50 at. pct Hf a stable bcc solid solution exists at elevated temperature. When quenched from the single phase region and aged at lower temperatures both alloys precipitate α phase, an hcp hafnium-rich phase. Transmission microscopy showed that in the Nb-Hf system the precipitation reaction begins with the formation of coherent zones. Further aging causes coarsening into discs, still at least partially coherent. When coherency is lost the α morphology changes to rod-type. The effect of small amounts of oxygen added to these alloys during aging is an increase in the unit cell size of the α phase. Oxygen concentrations up to ~0.5 at. pct do not cause observable changes in the phase relations. These results cannot be reconciled with the miscibility gap-monotectoid type phase diagrams reported by some earlier investigators and indicate that the proper phase diagram for both systems is the β-isomorphous type.
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