Site occupancy in the Re-W sigma phase

Abstract

The site occupancy in the Re-W sigma phase is studied using neutron-diffraction experiments and using a first-principles statistical thermodynamics approach. We show that the five inequivalent lattice sites of the sigma phase display a specific site-occupation sequence with increasing W composition which is explained by a theoretical analysis.

Modeling Topologically Close-Packed Phases in Superalloys: Valence-Dependent Bond-Order Potentials Based on Ab-Initio Calculations

“…(We show only the convex hull for each structure type as obtained from our set of DFT calculations.) Our results are in very good agreement with previous DFT calculations of the Re-W σ phase [15]. We find that all the binary tcp phases that we considered have a positive heat of formation and are therefore unstable against phase separation to Re (hcp) and W (bcc), regardless of their chemical composition.…”

“…The structural relaxations (of unit cell and internal degrees of freedom) were converged to a maximum force of less than 1 meV/Å. The results presented in the following correspond to the structural stability at a temperature of 0 K. At finite temperatures the occupancy of the individual sites may vary strongly, as was shown for the Re-W σ phase [15] and the Ni-Nb µ phase [16] previously.…”

“…Ref. [15]). The heat of formation for the various structures as obtained from our DFT calculations are compiled in Fig.…”

Non-Stoichiometry and Calphad Modeling of Frank-Kasper Phases

“…A very good agreement of the Generalized Gradient Approximation (GGA) DFT-BWA results [36] with experimental site occupancies was also obtain for another FK phase: the χ phase in Mo-Re ( Figure 5, right), with also a relative agreement obtained between the computed and experimental phase diagrams of Mo-Re and W-Re systems. [29], and obtained by BWA at 300 K (right) [32]. Experimental points from [29].…”

“…The first significant work done on the σ phase has been performed on the two systems Cr-Fe [28] and Re-W [29]. The energies associated to the change of local atomic configurations have been parameterized by a set of Effective Cluster Interactions (ECI).…”