First-Principles Computation of Diffusion Coefficients for Ni-Based Alloys, 18-R9568

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Principal Investigators
Kwai S. Chan
Yi-Ming Pan
Yi-Der Lee
Wuwei Liang

Inclusive Dates:  10/03/05 – 03/27/07

Background - Ni-based alloys are being considered for use as the outer container of the waste package for the disposal of high-level nuclear waste. During fabrication processes and long term storage, Ni-based alloy outer containers can undergo sluggish microstructural changes that reduce corrosion resistance due to the formation of several topologically close-packed (TCP) phases. To assess the long-term performance of Ni-alloy containers, an accurate prediction of these microstructural changes is required, necessitating the need for knowledge of the diffusion coefficients of solute elements in Ni as a function of composition and temperature.

Approach - A first-principles computational methodology has been developed to calculate the mobility of transition metals in binary Ni solid solutions and vice versa. A local density-based full-potential linearized augmented plane wave (FLAPW) code, WIEN2K, was utilized to compute the energy of vacancy formation and the energy barrier for solute migration using an n-atom supercell. These results were then utilized to compute the activation energy for diffusion, the correlation factor, and the mobility of the solute atom in Ni alloys over a wide composition range. The first-principles solute mobility results are compared against empirically derived mobility databases to assess the current approach for predicting the composition dependence of solute mobility in Ni alloys.

Accomplishments - First-principles computational procedures have been developed and utilized to compute the energy of vacancy formation, migration energy, and activation energy for diffusion of Mo, Cr, Fe, W, and Ni in Ni-Mo-Cr-Fe-W alloys such as Alloy 22 (Ni-21.2Cr-15.5Mo-4Fe-3W, in weight percent, UNS N06022) to establish a mobility database for these alloys. The theoretical mobility database is compared against experimental mobility database from the literature to assess the accuracy and validity of the first-principles computational approach. The comparison indicates that the mobility of Mo, Cr, Fe, W, and Ni atoms in Ni-Mo-Cr-Fe-W alloys is predicted by the first-principles computational methodology over a wide range of temperatures and alloy compositions.

Figure 1. Supercell used to compute the migration energy of Cr in Ni-Mo-Cr alloys. Mo: blue; Ni: maroon; Cr: white; red atom: Cr moving toward a vacancy. Figure 2. Computed and experimental values of the diffusion coefficient of Cr in a Ni-Cr alloy as a function of reciprocal temperature.

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