Order--Disorder Transformation in Co sub 30 Pt sub 70 Alloy: Evidence of Wetting From the Antiphase Boundaries

A study of the order--disorder L1 sub 2 - > A1 transformation near a congruent point of the Co--Pt phase diagram is presented. This work was performed by combining in situ transmission electron microscopy observations, resistivity measurements carried out in conditions near thermodynamic equilibrium and high-resolution imaging on quenched samples. Resistivity measurements display three characteristic temperatures: 994, 1020 and 1032K. The evolution of the microstructure with temperature at different heating rates was followed in dark field images, until the order--disorder temperature was reached. It was found that at temperatures > 994K ( approx = 40K below T sub c ), the antiphase boundaries (APB) undergo a structural change; the phenomenon becomes more pronounced as the temperature increases. High-resolution images of modified APB show thin layers of disorder phase in the core of these APB. This can be described as a wetting of the antiphase boundaries by the disordered phase. Above 1030K, up to 1032K, a second mechanism is superimposed; the disordering occurs through a nucleation, growth and coarsening of disordered regions inside the ordered domains. This nucleation and growth process starts at the beginning of the two-phase region (1020K). These results are in excellent agreement with those from cluster variation method calculations performed by Kikuchi and Cahn and Finel and co-workers. Graphs, Photomicrographs. 22 ref.--AA