TITANIUM BASED QUASICRYSTALS: Titanium Based Quasicrystals and Crystal Approximants

The icosahedral phase (i-phase) occurs frequently in rapidly quenched Ti-3d transition metal (TM) binary and ternary alloys (with some silicon often required). A detailed discussion of the microstructures and diffraction features of this less well studied class of quasicrystals is presented. In contrast with previous reports, we demonstrate that a 1/1 large-unit-cell cubic phase, which is a true bcc version of the α(AlMnSi) structure, is the appropriate crystal approximant for the Ti-TM-Si i-phases, giving hints of the i-phase structure and its formation. We present the first complete structural determination of that phase. We demonstrate that a host of other high-order approximants, including the 3/2 and the 5/3, and possibly higher, Fibonacci cubic approximants, also occur in these systems. To our knowledge, this is the first report of the 5/3 approximant in any alloy. Finally, we review the features of the intense localized diffuse scattering common to most Ti-based i-phases, and discuss models for its origin.