Development of a First-Principles Computational Methodology for Predicting Long-Term Material Stability and Mechanical Performance, 18-R9420

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

Inclusive Dates:  10/01/03 – 12/15/05

Background - Ni-base alloys such as Alloy 22 (Ni-21Cr-16Mo-4Fe-3W in wt pct) 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 due to 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 has been used to compute thermodynamic data, elastic constants, and theoretical strengths 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. Using first-principles computational results as input, a discrete atom-dynamics approach has been utilized to treat long-range ordering and the growth kinetics of the Ni2(Cr,Mo) phases. These methods have been integrated into a methodology for predicting the long-term phase stability and mechanical properties of Ni-base alloys in a potential 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 have been used in conjunction with the CALPHAD approach and the Thermo-Calc® software to predict the phase diagram for Ni-Cr. Long range ordering to form Ni2Cr (oI6 phase) at low temperatures is predicted by this approach, Figure 1. The first-principles thermodynamics data were applied to compute the phase fractions of Alloy 22 as a function of temperature. The model correctly predicted the presence of the Ni2(Cr,Mo), the µ and P type TCP phases, and γ´ in a γ matrix, Figure 2. The oI6 and µ phases were not predicted without the first-principles thermodynamics data. An accurate prediction of ordered phase formation allows a better assessment of the corrosion resistance and mechanical performance of Alloy 22 under long-term storage in a nuclear waste repository environment.

Figure 1. Predicted and measured ordering boundaries for Ni-Cr. Ni2Cr was predicted to be a line compound with a narrow phase field (dotted line). Figure 2. Plot of phase fractions predicted for Alloy 22 using Thermo-Calc® and a Ni database updated with first-principles thermodynamic data.

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