Nanometer-size polycrystallization in bcc → hcp+C15 structural change of a Ti-30 mol % Cr alloy

The crystallographic features of a bcc --> hcp + C15 structural change in a Ti-30 mol%Cr alloy have been examined by transmission electron microscopy. When the alloy was annealed at 873 K below the eutectoid temperature, the structural change occurred in the following five steps: bcc --> bcc + Zone I --> bcc + Zone II --> bcc + Zone II + hcp --> bcc + hcp + C15 --> hcp + C15. The interesting features of the change are that the diffuse omega state is present in the metastable bee matrix, and that in the fourth step the C 15 grains with a size of about 2 nm appear in the bee matrix around the hcp region. As a result of these extremely small C 15 grains, the final microstructure, which was obtained from the 5-h-and 100-h-annealed samples, consists of the hcp and C15 grains with an average size of about 100 nm. On the basis of these results, the physical origin of the nm-size polycrystallization in the appearance of the C15 phase is discussed in relation to both the presence of diffuse omega state in the metastable bcc matrix and the formation of the complex coordinated polyhedra in the C15 structure.