THE EFFECTS OF COHERENCY ON PHASE EQUILIBRIA. J. B. Cohen, Department of Materials Science and Engineering, Robert R., McCormick School of Engineering and Applied Science, Northwestern, University, Evanston, IL 60208

Recent theories have predicted large effects on both phase composition and volume fraction if a precipitating phase is coherent, but there has been no satisfactory experimental verification. Furthermore, these effects vary with overall composition of an alloy. For example, in the presence of coherency stresses, the compositions of the phases in a two-phase region are not constant, as they are for incoherent equilibria. Such coherent phase equilibria were examined experimentally in a Ni-24 at.% V alloy, using an in-situ high-temperature X-ray attachment mounted on a diffractometer. Peak positions of the a phase shifted to higher 2q during the transition from coherency to incoherency, indicating that the vanadium content in the coherent a phase is higher than that of the incoherent phase. There was no significant peak shift observed for the 400 peak of the b phase. These peak shifts are expected from these recent theories on the role of coherency stresses on phase equilibria. (In order to calculate the coherent phase diagrams from these theories for comparison with experiment, the elastic constants and misfit versus temperature were also measured.) Other experiments will also be described. This research was funded by the National Science Foundation (Grant DMR 8417961) and was performed with D. K.Na and P. W. Voorhees.