PNNL

Abstract

In September 2020, the U.S. government issued an executive order to address the threat to the domestic supply chain from its reliance on critical minerals (CMs) from foreign competitors and to support the domestic mining and processing industry. The Advanced Materials and Manufacturing Technology (AMMT) program is addressing this executive order by evaluating advanced manufacturing (AM) and its impact on the demands of CMs for energy production in general and how the deployment of AM in nuclear energy will support the projected goals of the Paris Accord and further a net-zero carbon economy (NZE) by 2050. Three strategic reports were previously prepared by the AMMT program to date and identified two areas for more detailed exploration: (1) the replacement of high-risk CMs such as cobalt and niobium by more abundant minerals and (2) the minimization and utilization of CM waste streams. The design of nuclear materials without critical elements as alloying elements, is a part of the nuclear materials strategy to overcome the critical minerals scarcity. In this report, two approaches are evaluated namely (1) replacement of critical elements as alloying elements in nuclear materials, and (2) the design of a new alloys that does not contain critical minerals as an alloying element. Inconel 617 (IN617) has been selected as an alloy system to substitute its high Co concentration using noncritical Mn as the alloy system is also ASME-code certified for high-temperature nuclear systems under Boiler and Pressure Vessel Code (US Office of Nuclear Energy, 2020) [1]. A computational feasibility study of compositional changes to IN617 is presented in this report. Compositional modifications were made by systematically substituting the Co concentration in IN617 for Mn generating a set of possible compositional modifications. For the mechanical properties, tensile tests on cubic simulation cells have been performed to obtain the stress strain curve that reveal the effect of Co replacement with Mn on the tensile strength. Phase diagrams were calculated for a few compositions that showed promising results in the simulation evaluations. The composition with the best combination of oxygen penetration and tensile strength was selected for experimental evaluation. Two different methods were used in this study to obtain alloy samples, 1) casting and 2) friction stir consolidation and alloying. The materials were then characterized using SEM-EDS, XRD (casting alloy only) and Vickers hardness. Multi objective Bayesian optimization (MOBO) techniques were employed to design new alloys that do not contain the critical minerals nickel and cobalt. The high entropy alloy (HEA) system within which new compositions were developed for this study is Fe-Cr-Cu-Al-Nb-Ta-Ti-V-Zr-Mo-W-Mn. MOBO was used to develop new material compositions without nickel and cobalt with maximum yield strength and hardness.