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A first-principles quantum-mechanical computational code has been used to compute the energy of formation for selected ordered and topologically close-packed (TCP) phases in a Ni-base alloy. The thermodynamic data are incorporated into an existing database for Ni alloys and used in conjunction with the CALPHAD approach to compute the binary Ni-Cr phase diagram. In addition, a thermodynamic model is used to treat long-range ordering and the formation of the Ni^sub 2^Cr. The phase field for ordering is compared against that predicted by the Thermo-Calc to elucidate possible implications on the long-term phase stability of Ni-base alloys in a nuclear waste repository environment.
I. INTRODUCTION
NI-BASE alloys such as Alloy 22 (Ni-21.2 Cr-15.5 Mo-4 Fe-3W in wt pct. UNS N06022) are being considered for use as the outer container of the waste package for the disposal of high-level nuclear waste.1 During fabrication processes and long-term storage. Ni-hase alloy outer containers can undergo microstruclural changes due to the formation of ordered Ni^sub 2^(Cr. Mo) and topologically close-packed (TCP) phases. The relevant multicomponent phases include those of Ni^sub 2^(Cr. Mo). Ni^sub 2^(Cr,W), and TCP phases in the Ni-Cr-Mo, Ni-Cr-W, and Ni-Cr-Mo-W systems. The Ni^sub 2^(Cr, Mo) and Ni^sub 2^(Cr,W) phases are orthorhomhic (oI6). while the TCP phases have hexagonal (μ phase), orthorhombic (P phase and δ phases), and tetragonal (σ phase) crystallographic structures.2-5 These phases contain substantial amounts of Cr and Mo whose depletion from the matrix can impact negatisely on the corrosion resistance of the Ni-base alloys. Since the TCP phases are brittle, their formation can also exert an adverse effect on the mechanical properties of the Ni-base alloys.
The precipitation temperatures of TCP phases and the ordering temperature of Ni^sub 2^(Cr,Mo) are in the range of 500 °C to 1200 °C and both reactions are sluggish.2,3,4 The energy and entropy of Ni^sub 2^Cr formation at [asymptotically =]500 °C were determined experimentally by Hirabayashi et al.2 The phase boundaries of the Ni^sub 2^Cr-type long-range-order structure in Ni-Cr-Mo alloys were reported by Karmazin et al.3 The formation of TCP phases in Ni alloys was reported in an article by Gozlan et al.4 Because of slow reaction kinetics, the formation, morphological evolution, and properties of the Ni^sub 2^(Cr,Mo) and TCP phases cannot be measured confidently using short-term tests...