Structural-Phase Low-Stability States of BCC-Intermetallic Compounds with APB Complexes

Using a mono-nickel aluminide (NiAl) as an example, the influence of APB complexes (a pair of shear APBs along the < 110 > direction and a pair of APBs along the < 100 > direction) on the low-stability pre-transitional states of BCC-intermetallic compounds is investigated by the Monte Carlo method. It is shown that in the region of the low-stability states of this compound 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 structural-phase transformation temperature. The most significant factor for the long-range ordering in the system is the appearance of a defect in the form of an APB itself, while the differences in the APB types and planes of their occurrence do not so essentially affect the long-range order behavior with the temperature variations. A system with structural defects is obviously less ordered compared to a defect-free system. The presence of a defect in the form of an APB promotes disordering of the system at lower temperatures: the degree of ordering starts to decrease in the case of thermal APBs at a lower temperature compared to the case of shear APBs. In the NiAl alloy with a complex of < 100 > APBs, the first distortions of the structural order invariably appear near the Al-Al boundary. In the alloy with a complex of shear < 110 > APBs, the distortions of the structural order are observed only in the regions where the boundaries cross. The presence of antiphase boundaries affects the alloy stability during heating. It is shown that the process of disordering is accompanied by smearing of the boundaries and their faceting.