A Quantum-Mechanical Critique of the Miedema Rules for Alloy Formation

Abstract

The chapter presents a quantum-mechanical critique of the Miedema rules for alloy formation. It is stated that although the Miedema model provides very simple and useful rules for alloy formation, the physical concepts upon which it is based find no justification in quantum mechanics. This chapter presents an analysis on the quantum-mechanical origin of the heat of formation of metallic alloys. The chapter begins by discussing the Miedema model, and the failure of the ionic model. The nature of the metallic bond in simple metals and their alloys is presented, within the nearly-free-electron approximation. It is mentioned that although there is a rearrangement of the electronic charge on the formation of an alloy, each atom remains perfectly screened within its new metallic environment, so that the ionic model is not appropriate. The nature of the metallic bond in transition metal alloys is discussed within the tightbinding approximation. The attractive contribution to the heat of formation is identified with the change in bond order which accompanies alloy formation. The chapter also presents a discussion on the significance of the Miedema parameters φ * and ρ M 1/3 . The energy change involved in the formation of an alloy is very small compared to the cohesive energy which is required to separate the atoms infinitely far apart.