SOME CHARACTERISTICS OF BETA-]ZETA DEGREES AND BETA'-]ZETA DEGREES TRANSFORMATIONS IN EQUIATOMIC AGZN

The b.c.c. β phase in near equiatomic AgZn alloys undergoes an ordering reaction (to CsCl β′) at ~245°C which is insuppressible on quenching from the β field to room temperature. Upon slow cooling the β phase transforms to a trigonal phase, ζ°. Quenched β specimens (i.e. β′) also transform to ζ° during up-quenching and isothermal annealing. The β (or β′) to ζ° transformation is a crystallographic transformation in that there are onty four variants of the ζ° phase, each associated with a prior 〈111〉β′ which is parallel to the trigonal axis. Single crystals of β and β′were partially transformed at various temperatures and sectioned on low index planes to determine the shape of the growing ζ° particles. Although a single ζ° particle may be composed of more than one variant, the unique particle shape at low temperatures strongly reflects the cubic symmetry of the matrix in that growth occurs most rapidly along the 〈100〉β′directions. The double cone shaped ζ° particles formed from the β phase at high temperatures show an asymmetry in the growth rate in directions parallel and normal to the ζ° trigonal axis, and thus do not reflect the cubic symmetry of the matrix. It is proposed that the difference in particle morphology is primarily a function of the elastic and plastic properties of the cubic matrix. A growth-related substructure appearing in the ζ° particles is considered to result from the incorporation and rearrangement of dislocations. © 1969.