Structural-Phase Low-Stability States of BCC-Alloys with APB-Complexes in the Course of an Order-Disorder Phase Transition

Using the Monte Carlo method, the influence of the APB-complexes (a pair of shear-induced APBs along the <110>direction and a pair of thermal APBs along the <100> direction) on the low-stability states of β-brass is investigated during an order – disorder phase transition in the CuZn alloy as an example. It is shown that the formation energy of a complex of thermal APBs is higher than that of a complex of shear APBs. The contribution of APBs into disordering is essential up to the temperature of the order – disorder phase transition. The most significant fact for the long-range order in this system is the appearance of a defect in the form of an APB; the difference in the types of APBs and the plane of their occurrence do not exert a significant influence on the long-range order behavior with the temperature variations. The types of antiphase boundaries do affect the structural-energy characteristics of the system at the temperatures below that of the phase transition. It is obvious that a system with structure defects is less ordered than a defect-free system. The presence of a defect in the form of an APB favors the onset of disordering at lower temperatures: reduction of ordering in the alloy begins in the case of thermal APBs at lower temperatures compared to the case of shear APBs. In the CuZn alloy with a complex of thermal APBs along the <100> direction the first distortions of the structural order invariably appear near the Zn–Zn boundary. In the alloy with a complex of shear-induced APBs along the <100> direction the distortions of the structural order are only observed in the regions where the boundaries intersect. The presence of antiphase boundaries influences the alloy stability during heating. The CuZn alloy without distortions of structural defects is more stable than that with APBs. It is shown that the process of disordering is accompanied by the smearing and faceting of the boundaries.