BETA-PHASE DECOMPOSITION IN NB-17 AT-PERCENT AL-ALLOY

The microstructures of as-cast and heat-treated ingots and melt-spun a nd heat-treated ribbons of Nb-17 at.% Al have been studied. The result s show that Al not only enhances the A2-->B2 ordering transition of th e beta phase, but also promotes formation of the omega phase in B2. A binary o phase has been observed for the first time in the B2 matrix o f the melt-spun ribbons after prolonged heat treatment at 973 K. It is suggested that the B-2-->o phase decomposition precedes the precipita tion of the equilibrium A15 phase in the B2 matrix, since the o phase has a lower nucleation barrier than the A15 structure.