Understanding Fundamental Material Degradation Processes in High Temperature Aggressive Chemomechanical Environments
The objective of this research project was to address major materials performance and methodology issues for the design and construction of high-temperature and very high-temperature nuclear systems. This work provided a synergy between the development of simplified, but robust, design rules for high-temperature systems and materials testing, along with performance and improvement of these systems. Such systems would have to deal with time-dependent materials properties (creep, creep-fatigue,
... gh-temperature corrosion) in components with complex stress states, long intended service lives, and aggressive operating environments. Because routine mechanical properties data and current high-temperature design methodology does not provide adequate information for long-term, robust system design, this project investigated these issues with the focus on long-term material property degradation analysis. In addition, high-temperature materials testing in relevant corrosive environments (such as low oxygen, partial pressure with substantial carbon activities) has been performed to support further code qualification of existing alloys and the development of emerging alloys. Alloy 617 and Alloy 230 are the two lead alloys for very high temperature applications, and these two alloys were studied extensively in this research effort. The work in this program concentrated heavily on identifying and quantifying materials degradation mechanisms at high temperatures, including mechanical properties performance, corrosion resistance, and the synergy between the two. The materials design issues center primarily on a "damage mechanisms" approach rather than on the more tradiational approach of quantifying limitations on loads and operational times (e.g. isochronis stress-strain curves). It was found that in the temperature range studied in this program, 800 to 1000°C, a major issue with materials mechanical properties degration was the evolution of the internal carbide structure. The initial strengthening that the matrix carbide structure provides quickly dissipates and the migration of carbon to the grain boundaries results in significant weakening of the grain boundary strength during mechanical loading. This results in an internal damage mechanism that results in intergranular fracture. The materials response was also examined after long term aging, up to 10,000 hours at elevated temperature. Long term aged materials also showed the effects of materials degradation, principally through the redistribution of the carbide structure. Corrosion studies in high oxidizing, low oxidizing, and carburizing atmospheres indicate that Alloy 230 performs better than Alloy 617 in oxidizing conditions, but is more prone to degradation in carburizing atmospheres. The creep-rupture properties of the two alloys were studied in detail using pressurized tubes. In most cases, the elevated creep response showed a very short period of secondary creep and an extended period of tertiary creep. The pressurized tubes produce a biaxial stress state. The post test microstructure showed that grain boundary "damage," again associated with the carbide structure, was the major controlling feature in the creep-rupture response.