Simple thermodynamical treatment for the site preference in L1 2 intermetallic compounds Al 3Ti-X

Abstract

Ternary substitutional elements are often added to binary intermetallic compounds in order to modify their mechanical properties. Al[sub 3]Ti, which has a tetragonal D0[sub 22] structure with a low symmetry resulting in its brittleness, is transformed to a closely related cubic L1[sub 2] structure with higher symmetry by the addition of various ternary elements, X. Among these ternary L1[sub 2] compounds, Al[sub 3]Ti-X, those with X = Cr, Mn and Ag have revealed a considerable improvement of ductility without the losing superior characteristics of Al[sub 3]Ti. For understanding the role of the ternary elements, X, on the property changes, it is important to know the site preference of X as well as of the host elements, Al and Ti, in the lattice. Recently, Kogachi et al. and the present authors have investigated the question of the site preferences in the Al[sub 3]Ti-X L1[sub 2] compounds by means of x-ray diffractometry. According to their results, ternary elements, X, with X = Cr, Mn, Fe, Ni, Cu, Pd and Ag prefer the Al-site. Simultaneously, some Al and Ti atoms occupy their respective wrong sites. The number of these observed anti-structure defects (ASD) depended on the systems, and its change was well expressedmore » in terms of the number of d-shell electrons of X as shown in Fig. 1. The present work is concerned with such an interesting system dependence of the ASD fraction in the L1[sub 2] compounds Al[sub 3]Ti-X, and for interpreting this behavior, a simple thermodynamical treatment is made, based on the Bragg-Williams approximation.« less