A case study of complex metallic alloy phases: Structure and disorder phenomena of Mg-Pd compounds

In order to achieve a deeper insight into the relation between the homogeneity range, crystal structure and disorder phenomena of complex metallic alloy phases (CMAs) we have redetermined the Mg-Pd phase diagram in the range 60-100 at.% Mg. The existence of the intermediate phases Mg 6 Pd (ß), Mg 57 Pd 13 (γ), Mg 56.4 Pd 13.5 (δ), Mg 306 Pd 77 (ε), Mg 78.5 Pd 21.5 (ζ), Mg 3 Pd (ν), Mg 5 Pd 2 (θ) and Mg 2 Pd (τ) has been reconfirmed. The first five of these are CMAs. The ß phase melts congruently whereas the CMAs d, and form in a cascade of peritectoid and peritectic reactions in a narrow window of approximately 30°C and 3at.%. It is assumed that the stability of the Mackay icosahedron plays an important role in the phase formation of these CMAs. However, the ß phase reveals an intricate pattern of disorder at the atomic positions of the Mackay icosahedron. Therefore, the concept of the stability of the Mackay cluster should be used only as a rule of thumb.