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

Microstructural investigations on rapidly solidified Zr3Al based alloys (bi nary Zr3Al and ternary Zr3Al-3Nb and Zr3Al-10Nb) have revealed some unusual phase transformation sequences. The Zr5Al3 phase (D8(8) structure) has bee n found to occur in both the rapidly solidified ternary alloys unlike in th e rapidly solidified stoichiometric Zr3Al alloy in which the Zr2Al phase (B 8(2) structure) has been found to be present. The evolution of the D8(8) ph ase, which could be regarded as one of the ordered derivatives of the omega phase, could be described in terms of a superimposition of replacive and d isplacive ordering waves in the beta phase. The orientation relationship be tween the beta and the D8(8) phases has been established. The microstructur al changes occurring in the rapidly solidified Zr3Al-Nb alloys during aging have been examined. It has been found that on aging the D8(8) phase gets t ransformed into the B8(2) phase which, on continued aging, transforms to ot her metastable and equilibrium phases, depending upon the aging temperature . The observed sequence of phase transformations involving different struct urally related phases has been along the direction of progressively close p acked structures. The symmetry changes associated with the sequence of omeg a related transformations have been summarized in the form of a symmetry tr ee. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.