Perricone, Matthew J., Accelerated Alloy Development: Nuclear Power Generation Technology Depends on the Reliability of High Performance Alloys to Operate in Severe Service Environments, Corrosion Solutions Conference, Lake Louise, Alberta, Canada, September 25-30, 2011.
Nuclear power generation technology depends on the reliability of high performance alloys to operate in severe service environments. Consideration of corrosion resistance, mechanical strength, and irradiation effects makes alloy selection a main concern. Optimization of the desired properties in the chosen alloy requires control of the microstructure, which depends on manufacturing (casting, forging, heat treating, etc.) and fabrication (welding, etc.) processes. This microstructural control is not a trivial task for high alloy stainless steel and nickel base alloys that have complex chemistries that can encourage the formation of unintended intermetallic phases during solidification, heat treatment or fabrication, often in structurally compromising microstructural locations like grain boundaries. Furthermore, the local redistribution of critical alloying elements during welding can reduce the local corrosion resistance of an alloy below that of the bulk material. Understanding microstructural development in an alloy system becomes critical in designing new alloy compositions or optimizing processes for manufacturing or fabrication. This can be time consuming and costly even by highly experienced metallurgists if done by trial and error, but modern computer technology has enabled more cost-effective ways to streamline this process. Fewer samples are required for validation than with an iterative approach, thereby maximizing the value of targeted experiments that are conducted.
Commercially available computational thermodynamic software offers the metallurgist with the tools required to reduce the time and expense of developing new alloys or optimizing existing ones. These programs can be used to calculate alloy-specific multi-component phase diagrams from databases of thermodynamic data from the refereed technical literature that consider all of the alloying elements that are present in the nominal composition. No longer is it necessary to start with the binary Fe-C phase diagram for steel and extrapolate to “real” alloys with seven or more elements in their composition. Knowledge of thermodynamic stability of phases at a given composition and temperature makes prediction and control of the microstructural development of an alloy possible. When combined with information about a particular process (temperature profiles, cooling rates, grain size, etc.), alloy-specific process-property-microstructure maps can be calculated to guide alloy selection and process optimization. The resulting diagrams can also provide useful support for technical inquiries regarding regulatory compliance.
This paper presents three case studies in which this approach is applied to real world challenges. First, the development of a filler metal for welding superaustenitic stainless steels will be discussed, with a focus on maintaining local corrosion resistance in and around the fusion zone. Second, the selection of welding parameters and control of fusion zone composition is optimized to avoid crack-susceptible sigma formation in superaustenitic stainless steels. Finally, the development of a gadolinium-containing nickel-base alloy for spent nuclear fuel storage will be discussed. These case studies are presented to demonstrate applications of this general methodology rather than a discussion of the merits of a particular brand or type of software. Instead, this paper is intended to illustrate just a few of the possibilities presented by the combination of metallurgical expertise, computer technology, and scientific problem solving when applied to the challenges facing industry.