PHASE-TRANSFORMATIONS IN NB-AL-TI ALLOYS

Phase relationships as well as morphological and crystallographic feat ures in Nb-rich Nb-Al and Nb-Al-Ti alloys have been investigated. The phase boundaries involving the bcc and Nb3Al (Al5 structure) were expe rimentally determined and several isothermal sections of the Nb-rich c orner of the Nb-Al-Ti phase diagram established. The present findings show that (a) the solubility of Al in Nb is considerably less than tha t reported previously, (b) the high-temperature bcc phase undergoes an ordering transformation to the B2 structure, and (c) the omega phase also forms in these alloys. The sequence of decomposition of the high- temperature bcc phase during isothermal decomposition in the bcc + Nb3 Al phase field has been systematically studied in these alloys. A wide variety of morphological features were found to be associated with th e Nb3Al precipitates that formed in the bcc/B2 matrix during isotherma l heat treatments. The lengthening kinetics of the plate-shaped Nb3Al precipitates were also studied.