A case study of complex metallic alloy phases: structure and disorder phenomena of Mg–Pd compounds

Abstract

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 (β), Mg₅₇Pd₁₃ (γ), Mg_56.4Pd_13.5 (δ), Mg₃₀₆Pd₇₇ (ε), Mg_78.5Pd_21.5 (ζ), Mg₃Pd (η), Mg₅Pd₂ (θ) and Mg₂Pd (ι) has been reconfirmed. The first five of these are CMAs. The β phase melts congruently whereas the CMAs γ, δ, ε and ζ form in a cascade of peritectoid and peritectic reactions in a narrow window of approximately 30°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 β 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.

Acknowledgements

The authors wish to acknowledge support by Dr G. Auffermann, A. Völzke, U. Schmidt (chemical analyses), Dr M. Boström (synthesis), S. Müller (thermal analysis), Dr R. Ramlau, K. Schulze (scanning electron microscopy and energy-dispersive X-ray spectroscopy), Dr R. Cardoso and S. Hückmann (X-ray powder diffraction). We are also deeply indebted to Dr A. Leithe-Jasper, Professor R. Kniep and Professor Y. Grin whose detailed comments have helped us substantially.