One of the most challenging tasks in the design code development for Next Generation Nuclear Plant (NGNP) is the design of intermediate heat exchanger (IHX), whose operation temperature may range from 800°C–950°C (1472°F–1742°F). The ASME design code does not have any design provisions for any material at this temperature range. Hence, different candidate materials are under consideration for IHX and one of the leading candidate materials is Ni based Alloy 617. The operating temperature of IHX will be in the creep regime of Alloy 617 and low-cycle creep-fatigue and creep-ratcheting failure mechanisms of Alloy 617 need to be understood. This study is developing a unified constitutive model (UCM) for Alloy 617 based on a broad set of uniaxial and multiaxial creep-fatigue and creep-ratcheting experiments at high temperatures. The experiments were conducted at different temperatures, loading rates and strain ranges prescribing different loading histories. The unified constitutive model was developed based on the Chaboche viscoplastic model framework. Model improvement was performed by including cyclic hardening/softening, dynamic and static recovery, strain rate and strain range dependence, nonproportional loading parameter and multiaxial ratcheting features. The simulated responses of the modified UCM were compared against the broad range of experimental data to demonstrate the robustness of the improved model. The strengths and shortcomings of the model are discussed.

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