ATOMIC STRUCTURES OF TI BASED DECAGONAL APPROXIMANT PHASES

Despite the many similarities between icosahedrally ordered phases in Al- and Ti-transition metal (TM) alloys, decagonal phases have been found only in Al-based systems. The closest Ti-TM decagonal approximant reported in the literature is the Ti-TM-Si lambda-phase. Starting with the atomic positions of the b.c.c. TiCrSi alpha-phase, we develop a stoichiometrically correct atomic model for the lambda-phase that accurately predicts the observed selected area diffraction patterns and stereographic Kickuchi band maps, and is in agreement with high-resolution micrographs. We present a fully decorated decagonal glass model with cluster matching rules determined from analysis of the atomic clusters. Diffraction simulations from this model strongly resemble measured diffraction patterns from another decagonal approximant phase, referred to as the delta-phase. High-resolution data from the delta-phase are also presented. Comparisons between the local atomic structures of Al-based decagonal phases and Ti-based decagonal approximant phases lead to a possible explanation for the apparent lack of a true decagonal phase in Ti-based alloys.