THE SELECTION OF PRECIPITATE HABIT PLANES IN CR-32 WT PCT NI

Causes are investigated for the changes in precipitate crystal structu re (fcc to 9R) and in morphology (degenerate plate to plate), which ar e observed to take place in Cr-Ni alloys as the reaction temperature d ecreases. Transmission electron microscopy (TEM) study is performed to determine the matrix/precipitate orientation relationship, habit plan e, and growth ledge spacing. O-lattice modeling is used to show that i t is likely that the metastable 9R phase forms as a transition phase a t lower reaction temperatures because lattice matching at the bcc/9R h abit plane is better than the matching at the bcc/fcc habit plane. The ability of the phenomenological theory of martensite crystallography (PTMC) to predict the habit plane of 9R plates precipitated by a diffu sional mechanism is explained by the small lattice invariant deformati on required to produce an invariant plane in Cr-Ni. Under this circums tance, the PTMC habit plane nearly coincides with the best-matching in terface that presumably appears and is predicted by O-lattice theory.