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 Ti₅₃Zr₂₇Co₂₀Electron 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° C. The Laves phase transforms to the bcc solid solution phase (β -Ti) and the Ti₂Ni-type fcc structure for T > 630° C. A reversible transformation between the β -Ti and the hexagonal solid solution phase (α-Ti) is observed on temperature cycling; the Ti₂Ni-type fcc phase is stable over the entire temperature range.