Long-range order and ordering kinetics in CoPt3

The Co-Pt system is well suited for studies of order-disorder by field-ion microscopy. Because of this, and the fact that detailed information on ordering is available for only one Ll2 alloy (Cu3Au), an extensive X-ray study was made of CoPt3. Long-range order falls to lower values (S ≈ 0.64) in this alloy than in Cu3Au. There is a two-phase field nearT c (685°C) extending for about 20°C. The ordered phase has a smaller lattice parameter than the disordered phase close toT c, but at room temperature the reverse is true. Ordering (when the domain size is large) follows Rothstein’s kinetic theory, with an activation energy of 74.0(9) kcal per g-atom. Domain growth is similar to grain growth, with a time exponent of about 0.45 and an average activation energy of about 62 kcal per g-atom. The antiphase domain structure is isotropic, in contrast to Cu3Au. There is little difference in atomic volume of cobalt and platinum in the ordered state as compared to the pure elements.