DESCRIPTION OF ORDERED BCC-B2 PHASE IN AL-PD SYSTEM

The AlPd phase with a wide homogeneity range in Al-Pd system has ordered BCC-B2 (CsCl prototype) structure. Previously the AlPd phase was described with the asymmetrical model (Al%, Pd) (Pd%, Va) in the Al-Pd system [1], Va represents a vacancy and the symbol % marks the major component in the sublattice. Extrapolation of the Al-Pd binary system to Al-Pd-Ni and also Al-Pd-Co ternary systems requires compatibility of models in corresponding binary subsystems. Therefore, comparison of the BCC-B2 model in Al-Pd system with model of the phase in Al-Ni [2] and Al-Co [3] systems was done and differences were found. To ensure the consistency with the descriptions of other binary systems with BCCB2 phase, the model for order-disorder transition was used and the (Al,Pd,Va) (Al,Pd,Va) symmetrical description for the BCC-B2 phase in Al-Pd system was created. The disordered BCC_A2 phase is not stable in the Al-Pd system, and therefore the BCC.A2 phase cannot be directly assessed. The approach presented by Dupin and Ansara [4] was used as a basis for the assessment. The phase diagram of the system calculated with symmetrical model is showed on Fig.la.