Development of a First-Principles Computational Methodology for Predicting Long-Term Material Stability and Mechanical Performance, 18-9420

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Principal Investigators
Kwai S. Chan
Yi-Ming Pan
Yi-der Lee

Inclusive Dates:  08/01/02 - 08/01/04

Background - Nickel-base alloys such as Alloy 22 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-base alloy outer containers can undergo microstructural changes as a result of the formation of ordered Ni2(Cr, Mo) and topologically close-packed (TCP) phases. Because of slow reaction kinetics, the formation, morphological evolution, and properties of the Ni2(Cr, Mo) and TCP phases cannot be measured confidently using short-term tests over a reasonable time frame, but must be computed theoretically using first-principles computational methods.

Approach - A first-principles quantum-mechanical computational code is used to compute thermodynamic data and elastic constants for selected ordered and TCP phases in a Ni-base alloy. The thermodynamic data are used with the CALPHAD approach to compute the relevant phase diagrams. In addition, a discrete atom-dynamics approach is being developed to treat long-range ordering and the nucleation and growth kinetics of the Ni2(Cr, Mo) and TCP phases using results from first-principles computations. These methods will be integrated into a computational methodology for predicting the long-term phase stability of Ni-base alloys in a nuclear waste repository environment.

Accomplishments - First-principles quantum-mechanical computation has been performed to obtain the energy of formation for various ordered phases in the two-sublattice model of (Ni,Cr)2(Ni,Cr) and TCP phases in the Ni-Cr system, as well as the energy of formation for Ni2Mo and Ni10Mo4Cr. These results, shown in Figure 1, have been used to compute the interaction energy parameters and fill gaps in the existing thermodynamic database of Ni-base alloys. The updated thermodynamic database has been used in conjunction with the CALPHAD approach and the Thermo-Calc® software to predict the phase diagram for Ni-Cr, Figure 2. Long range ordering to form Ni2Cr (OP6 structure) at low temperatures is predicted by this approach. The formation of this ordered phase is expected to decrease the corrosion resistance and mechanical performance of Ni-base alloys.

Figure 1. First-principles computation of energy of formation (ΔEform) for (Ni,Cr)2(Ni,Cr) and TCP phases in the Ni-Cr system. Figure 2. Predicted Ni-Cr phase diagram shows the formation of Ni2Cr (OP6) at low temperatures.

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