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 ω state is present in the metastable bcc matrix, and that in the fourth step the C15 grains with a size of about 2 nm appear in the bcc matrix around the hcp region. As a result of these extremely small C15 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 ω state in the metastable bcc matrix and the formation of the complex coordinated polyhedra in the C15 structure.