The selection of precipitate habit planes in Cr-32 Wt Pct Ni

Abstract

Causes are investigated for the changes in precipitate crystal structure (fcc to 9R) and in morphology (degenerate plate to plate), which are observed to take place in Cr-Ni alloys as the reaction temperature decreases. Transmission electron microscopy (TEM) study is performed to determine the matrix/precipitate orientation relationship, habit plane, and growth ledge spacing. O-lattice modeling is used to show that it is likely that the metastable 9R phase forms as a transition phase at lower reaction temperatures because lattice matching at the bcc/9R habit 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 diffusional mechanism is explained by the small lattice invariant deformation required to produce an invariant plane in Cr-Ni. Under this circumstance, the PTMC habit plane nearly coincides with the best-matching interface that presumably appears and is predicted by O-lattice theory.