Phase transformations in Nb-Al-Ti alloys

Phase relationships as well as morphological and crystallographic features in Nb-rich Nb-Al and Nb-Al-Ti alloys have been investigated. The phase boundaries involving the bcc and Nb3Al (Al5 structure) were experimentally determined and several isothermal sections of the Nb-rich corner of the Nb-Al-Ti phase diagram established. The present findings show that (a) the solubility of Al in Nb is considerably less than that 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 + Nb3Al phase field has been systematically studied in these alloys. A wide variety of morphological features were found to be associated with the Nb3Al precipitates that formed in the bcc/B2 matrix during isothermal heat treatments. The lengthening kinetics of the plate-shaped Nb3Al precipitates were also studied.