Thermal Stabilization and Mechanical Properties of Nanocrystalline Fe-Cr-Ni Alloys

North Carolina State University, Raleigh NC

Investigators

Abstract

TECHNICAL SUMMARY: It has been proposed that small additions of appropriate alloying elements (dopants) can stabilize a nanocrystalline grain size to high temperatures by means of a thermodynamic mechanism. The concept is based on the idea that solute segregation to grain boundaries can reduce the effective grain boundary energy to zero. Hence the driving force for grain growth is eliminated. Alloy additions of carefully selected solutes that segregate to grain boundaries in Fe-Ni-Cr alloys will be used in conjunction with ball milling to produce nanocrystalline alloys in powder form. Short term annealing with suitable microstructure characterization will be done to identify a regime of temperatures where the alloy powders can be consolidated by hot compaction without grain growth. An initial evaluation of the strength and ductility properties will follow. Investigation of the grain growth mechanisms that govern long-term thermal stability will be accomplished by a comprehensive and fundamental approach. The annealing kinetics and microstructure evolution will be determined using isothermal annealing experiments, with microstructure characterization techniques that have resolution down to the atomic scale, so that mechanisms can be identified and model-based extrapolations can be developed to verify long-term stability. The modeling results will be used to tailor Fe-Ni-Cr nanocrystalline alloys for optimum combinations of thermal stability, high strength and good tensile ductility. NON-TECHNICAL SUMMARY: The strength of metals can be increased to very high levels if the grain size is reduced to nanometers. However, these nanocrystalline metals are normally unstable at elevated temperature because the grains grow in size and the strength is lost. The proposed research will produce new nanocrystalline steels and stainless steels (Fe-Ni-Cr alloys) that have very high strength and the grain size is stable at high temperatures. These materials can be used to improve the efficiency of power generation systems, engines, and other applications where high strength-to-weight ratios and elevated temperature performance are required. An equally important aspect of the research is the development of intellectual resources in the form of science and engineering graduates who can develop new technology and transfer that knowledge to US industry.