Evolution of ordered ω phases in (Zr3Al)-Nb alloys

Microstructural investigations on rapidly solidified Zr3Al based alloys (binary Zr3Al and ternary Zr3Al-3Nb and Zr3Al-10Nb) have revealed some unusual phase transformation sequences. The Zr5Al3 phase (D88 structure) has been found to occur in both the rapidly solidified ternary alloys unlike in the rapidly solidified stoichiometric Zr3Al alloy in which the ZrAl phase (B82 structure) has been found to be present. The evolution of the D88 phase, which could be regarded as one of the ordered derivatives of the ω phase, could be described in terms of a superimposition of replacive and displacive ordering waves in the β phase. The orientation relationship between the β and the D88 phases has been established. The microstructural changes occurring in the rapidly solidified Zr3Al-Nb alloys during aging have been examined. It has been found that on aging the D88 phase gets transformed into the B82 phase which, on continued aging, transforms to other metastable and equilibrium phases, depending upon the aging temperature. The observed sequence of phase transformations involving different structurally related phases has been along the direction of progressively close packed structures. The symmetry changes associated with the sequence of ω related transformations have been summarized in the form of a symmetry tree