THE ORDERING TIE-LINE METHOD FOR SUBLATTICE OCCUPANCY IN INTERMETALLIC COMPOUNDS

A new method for representing sublattice occupancy in intermetallic co mpounds has been developed. This new approach is based on the concept that the atom configuration in an ordered alloy with two sublattices c an 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 co nnecting the 'compositions' of the individual sublattices when these a re 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 determinati on of the end points requires some additional information (often intui tion!). Thus, the results of ALCHEMI experiments, which are essentiall y characteristic X-ray counts, are reduced to 'apparent compositions'; these compositions lie on the OTL which is determined simply as the b est fit of a line through the measured apparent compositions. The adva ntage of OTL analysis is to provide a physical way of assessing the or dering scheme of complex multicomponent ordered alloys, especially tho se whose may have no obvious stoichiometry. A secondary but significan t advantage involves the simplification of the analysis and interpreta tion of experimental data obtained in ALCHEMI experiments, that is wit hout using complicated formulations normally required in these procedu res. To illustrate this new method, it has been applied to the analysi s 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 formu lations, and in a more informative way as far as physical significance and compositional accuracy are concerned.