Compositional Disorder, Magnetism, and Their Interplay in Metallic Alloys
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COMPOSITIONAL DISORDER, MAGNETISM, AND THEIR INTERPLAY IN METALLIC ALLOYS 4 3 2 D. D. Johnson', J. B. Staunton , F. J. Pinski , B. L. Gyorffy , and G. M. Stocks I Sandia National Laboratories, Livermore, CA 94551-0969 2 University of Warwick, Coventry, U. K. CV4 7AL 3 University of Cincinnati, Cincinnati, OH 45220 4 University of Bristol, Bristol, U. K. BS8 iTL 5 Oak Ridge National Laboratory, Oak Ridge,TN 37831
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ABSTRACT Chemical disorder leads to a variety of intriguing phenomena in alloys which have yet to be fully understood, particularly those phenomena occuring when chemical and magnetic effects interplay with one another. For example, magnetic order gives rise to chemical ordering in alloys, as in Ni-rich NiFe alloys. Two examples of the interplay of chemical disorder and magnetism will be discussed. Our recently developed ab-initio Landau (mean-field) theory for calculating the chemical-chemical, magneto-chemical, and magnetic-magnetic correlation functions in substitutional random alloys is used to describe electronic/magnetic mechanisms (e.g. in FeV) which give rise to the chemical short-range order as determined by neutron, X-ray, or electron diffuse scattering intensities. New developments within this approach that account for charge rearrangement effects will be mentioned. These calculations are performed within the multiplescattering framework, developed by Korringa, Kohn, and Rostoker (KKR), combined with the coherent potential approximation (CPA) to describe the disorder. This approach allows a firstprinciples description of the electronic structure of the high-temperature, chemically disordered state and its instability to ordering at low temperatures. This method provides not only a direct comparison of diffuse scattering data with theory but a means to understand more fully the underlying mechanisms which drive chemical and/or magnetic ordering. INTRODUCTION Several fundamental questions in metallurgy remain unanswered from a basic point of view. Why do some metallic alloys mix (while others do not)? What happens to such solutions as they are cooled from the melt? Such phenomena have only been described empirically, and a more fundamental picture is lacking. At the root of alloy theory [1], and of crucial importance to its subsequent exploitation in the design of new alloys, are the electronic mechanisms responsible for such desirable properties as stability of phases, strength, resistance to swelling, etc. Such mechanisms are driven by underlying electronic forces and are most evident than in an alloy's chemical ordering tendencies. In metallic alloys it is necessary to solve the many-electron problem as accurately and realistically as possible and to establish a theory for the appropriate atomic correlation functions in terms of this solution. Such correlation functions then reveal the electronic mechanisms that determine the compositional, or magnetic, ordering in the alloy. In many instances, correlations at temperatures well above a chemical or magnetic ordering transition are indicative
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