The Design and Development of Precipitation Strengthened Fe- and Co- Alloys for High Temperature Applications

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In today's rapidly changing world, there is constant demand for the development of new, high performance materials. Fire resistant steels can provide greater safety in the event of a building fire, creep resistant stainless steels can allow for higher power plant efficiency, and cobalt based superalloys have potential for use in high performance turbine blades. In order to accelerate the design and development of new materials, we have combined computational thermodynamics, multimodal characterization and analysis, and mechanical testing to develop a design approach for the rapid creation of high performance alloys. We applied this comprehensive design approach to develop a series of structural steels requiring two-thirds yield strength retention at 600°C. The result is a series of low-carbon ferritic steels with small alloying additions of V, Nb, and Mo that maintain over 80% of room-temperature yield strength in compression, and nearly 70% in tension, after 2 hours exposure at 600°C. The best alloy possesses yield strengths of 442 ± 32 MPa (room temperature) and 368 ± 18 MPa (at 600°C) in compression and 409 ± 40 (room temperature) and 284 ± 33 MPa (600°C) in tension. The steels were air-cooled after normalization with no thermomechanical processing. Atom probe tomography demonstrates the formation of thermally stable, nanoscale MX (where M = V + Nb + Mo and X = C + N) monocarbonitride precipitates after exposure to 600°C. This design process was extended to include corrosion resistant stainless steels containing 10 wt. % Cr for use in steam-driven power plants. High-temperature Vickers measurements demonstrated that these steels maintain their hardness for 1000 hours at 700°C. The favorable mechanical properties of these steels are derived from slow growing and coarsening nanoscale semi-coherent MX precipitates providing strengthening at elevated temperatures. For some systems, phase information does not exist or is not available to the public. To extend this design process to include new alloy systems, we have developed a functional combinatorial synthesis procedure for ternary metal systems. This process combines the inherently high-throughput methods of confocal magnetron sputtering, photolithography, and high temperature vacuum annealing to efficiently synthesize materials libraries with a high degree of flexibility in composition. We probe the structures and precise compositions of these libraries using X-ray diffraction and X-ray fluorescence techniques. We have successfully synthesized and identified isothermal sections containing many compositions with stable binary alloys of the Co-Ni-Ta systems, such as hcp Co2Ta and rhombohedral Co3Ta. The composition and phases observed so far show good agreement with available information from existing ASM databases.

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  • 04/18/2018
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