The role of excess vacancies in two-stage ordering in ordered alloys

Abstract

The ordering behaviour in some quenched alloys having an L1₂- or B2-type ordered structure and the effect of excess vacancies on the ordering kinetics are investigated. Differential thermal analysis in thermally disordered alloys reveals that the ordering takes place in two stages irrespective of alloy systems; the stage I ordering at lower temperatures is associated with the migration of excess vacancies and the stage II ordering at higher temperatures is by the migration of equilibrium vacancies. The kinetics of stage I ordering depends on the quenching temperature, the sample shape and the grain size because these experimental factors control the concentration of excess vacancies introduced by quenching. A relationship between these factors and the vacancy concentration is proposed in a form of a flow chart. The ordering in Cu-25 at.% Au and Cu-15 at.% Pt is strongly dependent on such vacancy behaviour as the formation of divacancies and vacancy clustering. Direct observation of antiphase domains (APDs) by electron microscopy shows that the stage I ordering in Cu-25 at.% Au is homogeneous in which APDs appear uniformly throughout the specimen, but in other alloys the stage I ordering is heterogeneous, yielding a stripe-like APD morphology running perpendicular to the grain boundary. The criterion for the occurrence of heterogeneous ordering in particular alloy systems at stage I is then suggested.