Icosahedral-phase formation and stability in Ti-Zr-Co alloys

We present the first report of icosahedral phase (i-phase) formation in rapidly quenched alloys of Ti53Zr27Co20. Electron diffraction patterns of i(TiZrCo) contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti-3d transition metal-Si-O and Ti-Zr-Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti-Zr-Ni i-phases, showing none of these features. The quasilattice constant for i(TiZrCo), a(q) = 0.51 nm, is close to that of i(TiZrNi). The i-phase in rapidly quenched Ti-Zr-Co alloys is deeply metastable and transforms exothermically to the hexagonal Laves phase about 500 degrees C. The Laves phase transforms to the bcc solid solution phase (beta-Ti) and the Ti2Ni-type fee structure for T > 630 degrees C. A reversible transformation between the beta-Ti and the hexagonal solid solution phase (alpha-Ti) is observed on temperature cycling; the Ti2Ni-type fee phase is stable over the entire temperature range.