A NEW APPROACH TO INTERFACE SEGREGATION - SURFACE DANGLING AND INTERATOMIC BOND EFFECTS IN BINARY-ALLOYS

First a new theory and method of calculation, describing also surface interatomic bonds with environmental atoms is outlined. This theory is termed the MTCIP-1A (modern thermodynamic calculation of interface properties-first approximation). The method permits the calculation of the X1 thermodynamic equilibrium composition (given by the mole fraction of a selected component, e.g. the solute component) of the topmost atomic sublayer of a binary solid. The value of X1 is shown to be determined by the following factors: the "mixing", the "non-bonding", the "bonding" and the "real mixture" factors. The first three are described here more deeply. The numerical calculations are shown in detail for an AuPd alloy with a bulk Au content of 25 at%, with (100) surface orientation, with a temperature T = 773.2 K and under an environmental oxygen partial pressure PO = 5 × 10-10 Pa. Further results are outlined (in appendix 6) for T = 293.2 K and PO = 5 × 10-5 Pa (much stronger oxidizing conditions). The X1Au value at% in the first case (Au segregation) while it is practically 0 at% in the second case (Pd segregation). Results on this alloy, obtained by ISS (ion scattering spectroscopy) are used for comparison. The initial Au content of the surface (after sputtering) was 29 at%. After 15 min. of equilibration in the first case 75 at% was measured (Au segregation). In the second case just 26 at% was obtained (Pd segregation compared with the initial composition). The calculated X1Au value is close to the experimental one at 773.2 K. At T = 293.2 K the agreement seems to be only qualitative. This is due to the slow approach to equilibrium (from 29 to about 0 at% of Au) by diffusion at such a low temperature and to the relatively small time of equilibration. Several other cases giving good agreement between theory and experimental results are found in other works mentioned here. © 1990.