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 Mg6Pd (beta), Mg57Pd13 (gamma), Mg56.4Pd13.5 (delta), Mg306Pd77 (epsilon), Mg78.5Pd21.5 (zeta), Mg3Pd (eta), Mg5Pd2 (theta) and Mg2Pd (iota) has been reconfirmed. The first five of these are CMAs. The beta phase melts congruently whereas the CMAs gamma, delta, epsilon and zeta form in a cascade of peritectoid and peritectic reactions in a narrow window of approximately 30 degrees C and 3 at.%. It is assumed that the stability of the Mackay icosahedron plays an important role in the phase formation of these CMAs. However, the beta 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.