The microstructural evolution of NbAlV ternary alloys

The microstructural evolution during annealing of a series of Nb-Al-V alloys has been studied using electron microscopy. A modified isothermal section of the Nb-Al-V ternary phase diagram is presented. The B2 phase is prominent in the as-cast alloys. On annealing, needle-like A15 particles precipitate in the metastable B2 phase around the grain boundaries. These then spread throughout the grains. The transformation process is very slow when the alloys are annealed below 1273 K due to the difficulty of nucleation. Cold working before annealing can increase effectively the A15 nucleation rate and refine the microstructure.