EVOLUTION OF ORDERED to PHASES IN (Zr_3Al)-Nb ALLOYS

摘要

Microstructural investigations on rapidly solidified Zr_3Al based alloys (binary Zr_3Al and ternary Zr_3Al-3Nb and Zr_3Al-10Nb) have revealed some unusual phase transformation sequences. The Zr_5Al_3 phase (D8_8 structure) has been found to occur in both the rapidly solidified ternary alloys unlike in the rapidly solidified stoichiometric Zr_3Al alloy in which the Zr_2Al phase (B8_2 structure) has been found to be present. The evolution of the D8_8 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 fi phase. The orientation relationship between the β and the D8_8 phases has been established. The microstructural changes occurring in the rapidly solidified Zr_3Al-Nb alloys during aging have been exam- ined. It has been found that on aging the D8_8 phase gets transformed into the B8_2 phase which, on contin- ued 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.