Solute Stabilization for HCP-FCC Transitions: Co-Mo
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I.
INTRODUCTION
A. Applications of the Brewer-Engel Correlation to the Co-Mo System
THE Brewer-Engel theory of transition metal bonding has been extensively developed and applied to the prediction of phase diagrams. The reader is referred elsewhere to several comprehensive discussions of the Brewer-Engel theory. ~'2 The results are summarized below. This paper deals with objections that have been raised to the application of the theory and presents new experimental data to check the theory. Bonding in transition metals arises from the contributions of electrons in two different principal quantum shells. The d-electrons are in inner orbitals which are relatively localized and they participate primarily in bonding between nearest neighbors. The s and p orbitals of the outer electronic shell are well extended and are responsible for fixing the long range order and crystal structure of the metal. The recognition by Engel of this distinction between inner and outer shell electrons allows the Hume-Rothery rules 3'4 for structures of nontransition metallic phases to be extended to transition metals. The Brewer-Engel correlation is applicable to pure transition metals as well as to transition metals alloyed with either other transition or nontransition metals. It also successfully predicts the role played by nonmetallic solutes. The theory correlates structure with the average number of outer s and p electrons. The Hume-Rothery rules relate electronic configurations with up to 1.5 s, p electrons per atom to the body-centered cubic (bcc) structure; 1.7 to 2.1 s, p electrons per atom would yield the hexagonal close packed (hcp) structure, and more than 2.5 s, p electrons would yield the face centered cubic (fcc) structure. The binding of neighboring atoms due to inner shell electrons depends upon the extent of overlap of the atomic orbitals. The degree of localization increases and the bonding strength decreases in the order 5d, 4d, 3d, 5f, 4f. Consideration of these electronic influences upon bonding together with size and internal pressure contributions can provide predictions of thermodynamic properties and the L. BREWER is Principal Investigator, Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Professor, Department of Chemistry, University of California, Berkeley, CA 94720. D.G. DAVIS, formerly Graduate Student Research Assistant, Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, is now Research Chemist, Shell Development Company, Houston, TX 77001. Manuscript submitted April 26, 1983. METALLURGICALTRANSACTIONSA
resulting phase diagrams of transition metal systems as was done for binary Mo diagrams? There have been a number of tests of the Brewer-Engel correlation, and the success rate of the predictions 26 that arise from the theory is 90 to 95 pct. As an example, the crystal structures stable at high pressure are predicted by considering which modification of an element has the maximum d-bonding. The
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