Morphology and crystallography of Ti2Pd phase precipitated in B2 parent and B19 martensite matrices in Ti-rich Ti–Pd alloys

Morphology and crystallography of Ti2Pd phase of C11b structure precipitated in B2 parent and B19 martensite matrices in Ti-rich Ti–Pd shape memory alloy have been investigated by transmission electron microscopy observations. Three Ti2Pd variants were recognized in the specimen aged above Af temperature, i.e., in the B2 matrix. On the other hand, four Ti2Pd variants were confirmed in the specimen aged below As temperature, i.e., in the B19 matrix. The orientation relationship between the Ti2Pd precipitate and each matrix is consistent with the number of Ti2Pd variants. The Ti2Pd precipitates in the B2 and the B19 matrices have a disk and a polygonal plate shapes, respectively. The c-axis of Ti2Pd precipitate in the B2 matrix is perpendicular to the disk surface. On the other hand, that in the B19 matrix is parallel to the plate surface. These differences are well explained by the magnitude of coherent strain.