The behavior and roles of antiphase boundaries in order-disorder transformation processes

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The objective of the research is to clarify the behavior and the effects of antiphase boundaries on the order-disorder transformation process. The kinetic equation was formulated by developing a Ginzburg-Landau type expansion of the free energy in order to carry out simulation of the changes in antiphase boundary structures. From the simulations, the following results were drawn. (1) At T>T_c ( the critical temperature of ordering ), disordering undergoes by expansion of a disorder region which has formed on antiphase boundaries. (2) At T<T_c, antiphase boundaries can have unevenness on the surface where a pair of hairpins are formed. The transformation from L1_<2-s> ( long period antiphase superstructure ) to L1_2 proceeds by receding of the hairpins. (3) At a low temperature region, the surface energy of antiphase boundaries is relatively high so that antiphase boundaries tend to become flat. Therefore, periodic antiphase boundaries cannot move and then the L1_<2-s> becomes metastable.Transmission electron microscopy (TEM) and X-ray diffraction experiments were performed to examine the effects of antiphase boundaries on the order-disorder transformation. In the specimen of Cu_3Pt with a small variant size of L1_<2-s>, periodic antiphase boundaries are reduced by the hairpin mechanism to transform L1_<2-s> to L1_2. In the specimen with a large variant size, on the other hand, the L1_<2-s> phase remains metastable. High resolution TEM observation showed that in an alloy of Fe-Si the thickness of "wetting" on antiphase boundaries increases with annealing time. The results of the simulations and experiments confirmed that antiphase boundaries give significantly important effects on the process of order-disorder transformation in alloys.