Jeffrey David Althoff

Vibrational thermodynamics: coupling of chemical order and size effects

The effects of chemical order on the vibrational entropy have been studied using first-principles and semi-empirical potential methods. Pseudopotential calculations on the Pd_3V system show that the vibrational entropy decreases by 0.07k_B upon disordering in the high-temperature limit. The decrease in entropy contradicts what would be expected from simple bonding arguments, but can be explained by the influence of size effects on the vibrations. In addition, the embedded-atom method is used to study the effects of local environments on the entropic contributions of individual Ni and Al atoms in Ni_3Al. It is found that increasing numbers of Al nearest neighbours decreases the vibrational entropy of an atom when relaxations are not included. When the system is relaxed, this effect disappears, and the local entropy is approximately uniform with increasing number of Al neighbours. These results are explained in terms of the large size mismatch between Ni and Al. In addition, a local cluster expansion is used to show how the relaxations increase the importance of long-range and multisite interactions.

Local environment effects in the vibrational properties of disordered alloys: An embedded-atom method study of Ni3Al and Cu3Au

Local environment effects are important for providing a framework for understanding the changes in vibrational properties that result from disordering. In the present work the effects of local environments on thermodynamic quantities are examined using the embedded-atom method (EAM) for Ni3Al and Cu3Au. Projections of the density of states onto different local environments are performed, and a local cluster expansion is calculated. It is found that the contribution to the entropy from a given atom is primarily determined by the atom and its first few neighbor shells. Relaxations are seen to qualitatively change the dependence of the entropy on local environment, changing the sign of the dominant interactions. Also, relaxations are found to extend the range of point and pair interactions and to increase the importance of multisite interactions. These results suggest that a special quasi-random structure (SQS), a small supercell constructed to approximate the local environments of the disordered phase, might be able to reproduce the disordered phase vibrational thermodynamics. It is found that an eight-atom SQS can accurately represent the vibrational thermodynamic properties of the disordered phase, implying that it could be a powerful tool for firstprinciples vibrational studies.

Embedded atom method calculations of vibrational thermodynamic properties of ordered and disordered Ni3Al

Abstract Recent work had suggested that vibrational effects can play a significant role in determining alloy phase equilibria. In order to better understand these effects, we investigate the vibrational properties of disordered and ordered Ni3Al using the embedded atom method and calculate vibrational thermodynamic quantities within the quasi-harmonic approximation. The vibrational entropy is found to be strongly dependent on volume. For fully relaxed structures the dependence on lattice decoration of the vibrational entropy is compared to that suggested by recent experimental results.

Approximations for vibrational thermodynamics of disordered alloys: Effective supercells and the quasiharmonic method

Recent work has suggested that vibrational effects can play a significant role in determining alloy phase equilibria. In order to better understand these effects and the methods used in their calculation, the authors investigate the vibrational properties of disordered Ni{sub 3}Al using the Embedded-Atom Method. They examine the effectiveness of the Special Quasirandom Structure (SQS) approximation, and find that an SQS-8 can accurately represent the vibrational thermodynamics of the disordered state. By the use of Monte Carlo (MC) techniques, they also find that the quasiharmonic approximation becomes less accurate as they approach the melting temperature, but that the accuracy may be extended to higher temperatures by resorting to the MC equation of state giving the specific volume as a function of temperature.

First-principles calculations of Heusler phase precursors in the atomic short-range order of disordered bcc ternary alloys

The authors apply the recently developed first-principles theory of atomic short-range order in disordered multicomponent alloys to CuAuZn{sub 2} and AgAuZn{sub 2}, two alloys which display partially-ordered B2 phases as well as fully-ordered Heusler structures. The calculated Warren-Cowley pair correlation functions for both alloys peak at the special points and , with the peak at dominant. This is indicative of a tendency to B2 order at high temperatures, and a subsequent tendency to Heusler-type order at lower temperatures. An analysis in terms of effective interactions shows that the Au-Zn interaction drives the ordering tendency, while the Ag-Au and Cu-Au interactions contribute more to the ordering tendency--this effect is larger in CuAuZn{sub 2} and should lead to a slightly higher transition temperature for the B2 to Heusler transition in CuAuZn{sub 2} as compared to AgAuZn{sub 2}.