Surface and bulk order-disorder transitions

Abstract

Interrelation of surface an bulk order-disorder transitions in binary alloys A X B 1− X is discussed within the framework of the Bragg-Williams approximation, using the fcc (100) and bcc (110) surfaces as an example. In an fcc alloy with a (100) surface, layers parallel to the surface undergo alternately second-order transitions. On the other hand, the bulk fcc alloy reveals a first-order transition from the disordered phase to a mixture of the disordered and the AB 3 − (or A 3B −) type ordered phases except for X = 0.5. The origin of the bulk first-order transition is discussed by taking into account order-disorder transitions in the surface region. For comparison, the bulk second-order transition in a bcc alloy is also discussed.