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:  11/01/03 – 12/15/05

Background - Nickel-based 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-based alloy outer containers can undergo microstructural changes caused by 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-based 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-based 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, Figure 1. 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-based alloys. The elastic constants and theoretical strengths of the OP6 phases (Ni2Cr and Ni2Mo) have been computed using the first-principles computational approach, Figure 2, and used to estimate their mechanical properties.

Figure 1. Predicted Ni-Cr phase diagram shows the formation of Ni2Cr (OP6) at low temperatures. Figure 2. Theoretical tensile stress-strain curves of Ni2Cr and Ni2Mo computed via the first-principles computational method.

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