TRANSFORMATION CHARACTERISTICS OF ZIRCONIUM--NIOBIUM AND OTHER ALLOYS.

Work in this laboratory and elsewhere has developed the• concept that the omega reaction in Ti and Zr alloys results from an instability in the parent BCC phase and represents a unique diffusionless transformation that takes place as a second order reaction.During the past year studies have been initiated to explore the generality of this concept and the relationship be- tween this class of diffusionless transformation and classical martensitic reactions.The systems chosen for these studies have involved continuation of work on Zr -Nb alloys and initia- tion of studies on the Hf-Nb system and the intermetallic com- pound Ti-Ni.Transmission microscopy and other studies have revealed that an athermal omega reaction occurs in a Hf-32% Nb alloy.The diffuse streaking and other characteristics of this alloy are completely analogous to the corresponding behavior of a Zr -25% Nb alloy.The intermetallic compound Ti-Ni exhibits a classical mar- tensitic reaction at temperatures near room temperature and a transition phase at temperatures above MS which exhibits the diffuse streaking, reversibility and negative temperature coeffi- cient of resistivity characteristic of omega in Ti and Zr alloys.The alloys chosen for this study appear to be appropriate for advancing understanding of the instability responsible for these transitions and for exploring the relationship between these higher order reactions and classical martensitic transformations.