On the stability of chemical order in small ordered-alloy particles

The reduction in the chemical order of small particles of long-range ordered alloys has recently been discussed by Fukami and Tanaka (Phil. Mag. Lett. 84 33 (2004)) in the framework of the Bragg-Williams model, in apparent accordance with experiment. We re-examine the discussion in the same framework with a closer inspection of the quantitative details. While it reproduces fairly well the lowering of the order-disorder transition temperature of Cu ₃Au particles of about 10 nm in size, the model is unable to explain the extended stability of the disordered state observed for smaller particles at room temperature; the order-disorder transition temperature does not fall below half of the transition temperature of bulk materials.