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Site Selection and Pseudo-Clustering Behaviors of Alloying Elements in Aluminum-Lean gamma-TiAl Intermetallics

Site selection and pseudo-clustering behaviors of the various M alloying elements in Al-lean Ti50Al50-X M (X) (X = 1, 2, 3, 4, and 5 at. pct) intermetallics have been investigated by means of the ordering energy-dependent and long-range-order forced fast Monte Carlo simulation method. The ordering energies have been calculated via pseudopotential approximation in the electronic theory of alloys up to the third coordination sphere (CS) taking the anisotropic nature of tetragonal L1(0)-type structure of gamma-TiAl into account. It was shown that the site occupation characteristics of the M alloying element atoms in gamma-TiAl intermetallics are governed by the relative magnitude of partial ordering energies between Ti-M and Al-M atomic pairs. However, the sign of partial ordering energies of these atomic pairs at the first CS becomes important in determining the clustering behavior and controls the dissolution modes of alloying element atoms in the gamma-TiAl matrix. The pseudo-clustering behavior of alloying elements reveals three dissolution modes, namely, random dissolution (mode I), planar clustering in two dimensions (mode II), and three-dimensional (3-D) clustering (mode III) of the M occupant atoms.