THE ICOSAHEDRAL AND RELATED CRYSTAL APPROXIMANT PHASES IN TI-ZR-NI ALLOYS

The discovery of the icosahedral phase (i-phase) and higher-order crystal approximant phases in rapidly quenched Ti-Zr-Ni alloys is reported. Transmission electron microscopy (TEM) studies reveal a series of rational approximants, including 3/2 and 5/3 and structures intermediate between the 2/1 and the i-phase, which form in the same alloys. Si is demonstrated to influence strongly the phase formation and structural disorder. Alloys containing less than 2 at.% Si give near-resolution-limited X-ray diffraction patterns, although TEM studies still show a phase mixture of i-phase and approximants. X-ray peak widths increase with increasing Si, indicating a decreasing grain size and an increased structural disorder. Samples containing more than 4 at.% Si are amorphous. Differential thermal analysis and external heating experiments show that the i-phase and approximant phases are stable to 650-degrees-C. Unlike Ti 3d transition metal quasicrystals, no evidence of localized diffuse scattering is observed in either the i-phase or the crystal approximants.