ABSTRACTVarious kinds of phase transformations, viz., spinodal decomposition, omega transformation, precipitation reactions and martensitic transformation can be induced in ternary (Zr3Al) -Nb alloys in conditions far removed from equilibrium. Transformation sequences in alloys containing 3% niobium are described and rationalized in terms of some basic tendencies such as phase separation and chemical ordering in the β (bcc) phase and displacive omega and β to α (hcp) transformations. Microstructures of rapidly solidified alloy showed a distribution of cuboidal (D88 phase) particles in the β matrix. The periodic arrangement of these particles along the <100>β directions was indicative of a spinodal transformation which preceded their formation. The β → D88 transformation could be accomplished by the superimposition of three processes, namely, chemical ordering, lattice collapse akin to ω transformation and vacancy ordering. During isothermal aging the D88 phase transformed into the B82 phase. The observed lattice correspondence and transformation morphology suggested that the D88 to B82 structural change involved the replacement of structural vacancies in the former by zirconium atoms without any reconstitution of the lattice. The evolution of the equilibrium Zr3Al (L12 structure) phase during prolonged aging were also studied.
Omega transformation.