The competition between surface segregation and compositional ordering in alloys: theory and experimental observations of segregation versus temperature peaked curves

The tendency to form ordered structures in metallic alloys can be manifested in prominent deviations from the equilibrium surface segregation behavior characteristic to random solid solutions. Depending on the relative strength of atomic interactions, the occurrence of long-range order (LRO) or short-range order (SRO) can suppress the segregation tendency of an alloy constituent. Since this suppressing effect is weakened with increasing temperature, segregation levels can increase concurrently with entropy-driven compositional disordering. In ordered alloys, such endothermic segregation process should be followed by the well-known monotonously decreasing McLean–Langmuir temperature dependence starting near the order–disorder transition temperature. The resultant peaked segregation versus temperature curve, expected for systems with relatively strong ordering tendencies, has been relatively little explored theoretically or experimentally. Some basic theoretical considerations regarding segregation/ordering competition as well as pertinent experimental results are briefly discussed, including surface segregation affected by LRO, especially in Pt3Sn(111) alloy and SRO effects (Al in Ni–9%Al solid solution).