Effect of Pressure on Order and Stability in Alloys: The Case of Al-Ge
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EFFECT OF PRESSURE ON ORDER AND STABILITY IN ALLOYS: THE CASE OF Al-Ge P. E. A. TURCHI* and M. SLUITER*; and G. M. STOCKS" *Lawrence Livermore National Laboratory, Condensed Matter Division (L-268), Livermore, CA 94550 "*Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge TN 37831-6114
ABSTRACT A paxameter-free approach to phase stability in substitutional alloys is applied to the influence of pressure on order-disorder phenomena in Al-Ge. The methodology is based upon an application of the Generalized Perturbation Method to the Korringa-Kohn-Rostoker scattering formulation of the Coherent Potential Approximation. For fcc-based AI-Ge alloys, it is shown that the tendency towards phase separation at normal pressure originates from a structural difference between the pure species. By applying pressure, the structural energy difference is reduced, and a significant increase in the tendency towards order, especially for Al-rich alloys, is theoretically observed, leading, as an example, to the possible observation of a DO 22 ordered state around 25 at. pct. Ge. The electronic origin of the ordering tendencies induced by pressure is discussed and the theoretical predictions are related to experimental facts.
INTRODUCTION AND METHODOLOGY The synthesis of new phases under pressure and the study of their structure-properties relationships yield valuable information on the general laws of phase formation in multi-component alloys, and provide an important insight in the conditions of occurrence of particular phases in systems with typical phase diagrams [1]. The choice of binary alloys with elements from group IIB through VB as model systems for the study of the influence of pressure on phase transformations is motivated for the following reasons. First, their phase diagrams, at normal pressure, are quite simple since they exhibit either a limited solubility-eutectic type or a single intermediate phase with, in general, no magnetic order. Second, most B group elements display a tendency toward polymorphism under pressure. And finally, the low melting points and the absence of new intermediate phases formed under pressure make their experimental study easy. With this respect, Al-Ge alloys have drawn particular attention [1, 2, 3, 4, 5]. This system exhibits, at atmospheric pressure, simple eutectic phase diagrams with limited mutual solubility of the components in the solid state [6]. The terminal Al-rich solid solution is less than 2.8 at. pct. Ge, at normal pressure. Such low solubility represents an equilibrium state of Al relative to the semiconductor modifications of Ge, and is characteristic of metal-semiconductor equilibria. Meanwhile, based on atomic size factors and electronegativity differences, favorable mutual solubility is expected. In fact, a substantial increase in solubility of Ge in Al has been observed under high pressure [3, 4, 5] as well as by rapid quenching from the liquid state [2]. An increase in the applied pressure extends the range of solubility of Ge in the fcc lattice of Al fr
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