In situ TEM observations of the order-disorder transition at interfaces in Cu₃Au

Abstract

The order-disorder phase transformation at anti-phase boundaries and coherent twin boundaries in Cu₃Au was investigated by in situ heating experiments in the electron microscope. With increasing temperature, a wetting of anti-phase boundaries and coherent twin boundaries with the disordered phase was observed, followed by a heterogeneous nucleation of disorder at these interfaces, associated with a narrow two-phase region below the critical temperature T _c. The wetting transition has a continuous (higher-order) character, analogous to the phase transition occurring at free surfaces. The degree of order or the width of the wetted area is dependent on temperature and on the type of interface, e.g. areas of a coherent twin boundary with different structures. In the two-phase region, a slow increase of a disordered layer (S = 0) adjacent to interfaces occurs at constant temperature until the equilibrium fraction of disordered material has been reached (first-order transition). The extension of the disordered layer is very dependent on the type of interface. If the temperature is not stable enough or if the temperature increase is too fast, heterogeneous nucleation of disorder also occurs within the ordered domains, presumably at the free surface of the thin foil. The observations on APBs are compatible with the results obtained previously on quenched Cu₃Au or with the results from in situ heating experiments on Ll₂-ordered Co₃Pt₇. However in our paper various interfaces have been investigated, with the emphasis on coherent twin boundaries.