The ordering tie-line method for sublattice occupancy in intermetallic compounds

Abstract

A new method for representing sublattice occupancy in intermetallic compounds has been developed. This new approach is based on the concept that the atom configuration in an ordered alloy with two sublattices can be described by an ordering tie-line (OTL). The OTL is similar to a tie-line in a ternary phase diagram and is defined as the tie-line connecting the 'compositions' of the individual sublattices when these are plotted on a compositional diagram. There are two properties of an OTL, namely the slope of the line and its extent, that is the location of the compositional end points. One method of determining the slope involves application of the ALCHEMI technique, whereas the determination of the end points requires some additional information (often intuition!). Thus, the results of ALCHEMI experiments, which are essentially characteristic X-ray counts, are reduced to 'apparent compositions'; these compositions lie on the OTL which is determined simply as the best fit of a line through the measured apparent compositions. The advantage of OTL analysis is to provide a physical way of assessing the ordering scheme of complex multicomponent ordered alloys, especially those whose may have no obvious stoichiometry. A secondary but significant advantage involves the simplification of the analysis and interpretation of experimental data obtained in ALCHEMI experiments, that is without using complicated formulations normally required in these procedures. To illustrate this new method, it has been applied to the analysis of data from several ALCHEMI experiments reported in the literature. It is demonstrated that OTL analysis yields the same results as those deduced by conventional analyses, but without involving complex formulations, and in a more informative way as far as physical significance and compositional accuracy are concerned.