Occurrence of short-range-ordered incommensurate states in γ-TiAl

Abstract

This paper examines a lesser-known form of quasiperiodicity in alloys, namely, quasiperiodicity in superlattice structures. Unlike quasicrystals, a quasiperiodic superlattice (QPSL) structure has an underlying periodic lattice and it is the arrangement of the two atomic species on this lattice that makes the superlattice quasiperiodic. Central to the discussion will be a QPSL structure, comprising three different building blocks (tiles), whose approximants are the Al₅Ti₃ and Al₁₁Ti₇ structures in Al-rich γ-TiAl. The three types of tiles represent unimolecular clusters of three different superlattice structures derived from γ-TiAl. Two different types of antiphase boundaries (APBs) occur in Al₅Ti₃. Their energies have been changed continuously in a Monte Carlo simulation to study transformations, either to the QPSL or to known periodic superlattice structures in Al–Ti. Some of the intermediate stages of transformation are short-range ordered states, which exhibit intensity maxima at incommensurate positions in the reciprocal space and different geometrical patterns of diffuse intensity. Very good agreement has been found in the simulation results, the results of a 4D to 2D projection scheme and TEM observations.