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.
The grain size in metals and alloys is typically in the range of 10 – 20 micrometers. Current processing methods can reduce this by several orders of magnitude to produce grain size less than 100 nanometers, ie., nano-grain alloys. In this case very high strength can be achieved along with improvements in wear and corrosion resistance. These can be very beneficial to applications in energy production and fuel efficient transportation leading to less pollution and resource depeletion. The downside is that nano-grain size will grow to much larger sizes as temperature increases due to the unstable grain boundaries between individual grains. The NSF research investigates mechanisms to stabilize grain size at much higher temperatures using a small amount of a stabilizer atom added to selected base alloys. Zr atoms were used as the stabilizer for Fe-Cr and Fe-Ni base alloys. These will be important prototypes for commercial alloys such as advanced alloy steels suitable for higher temperature applications. The principal investigators, Professors R. O. Scattergood (PI) and C. C. Koch (coPI) have extensive experience and expertise in the synthesis of nano-grain alloys and the determination of their microstructure and properties. Fe-Ni and Fe-Cr base alloys were combined with small additions of Zr (0.5 to 4%). Based on prelimary models that were available, this was identified as a possible thermodynamic stabilizer for the alloys, but with no clear distinction between the critical Zr bond-energy interactions with Ni vs. Cr. The research team consisted of the PI and CoPI and 2 graduate PhD research students. The initial results documented the performance for Fe-Cr-Zr alloys (Figure 1) and Fe-Ni-Zr alloys (Figure 2) using hardness test measurements on samples that had been heat treated after high-energy ball milling that refines the grain size to the nano-range. This is an indirect but convenient measure of grain size; higher hardness means smaller grain size. X-ray diffraction and electron microscopy revealed the true grain sizes. The effectiveness of Zr additions is clearly observed as shown by the microstructure in Figure 3. The dark-light regions indicate individual grains that are present after annealing Fe-10Cr-2Zr at 900?C. If Zr was not added, a single grain would encompass the entire frame in the Figure 3 image. Fe-Ni-Zr showed noticeably poorer performance compared to the Fe-10Cr-Zr alloys at comparable levels of Zr. This was attributed in part to phase transformations that occur in the Fe-Ni base alloys. However, this does not explain the trends over the entire range of temperatures investigated. One of the key stabilization factors for Zr (or other possible elements) is expected to be the segregation of Zr atoms to the grain boundaries (between grains), a process termed thermodynamic stabilization. Previous rationalization is inconclusive due to the lack of a comprehensive model for the thermodynamic interactions. This was taken up in the later stage of the research, concurrent with the experimental work. The newly developed modeling approach was very successful compared to previous models, and a notable achievement of the research work. Figure 4 shows model predictions of grain size vs. temperature for the Fe-Ni-Zr, Fe-Zr and Fe-Cr-Zr alloys. It is revealed that Fe-Ni-Zr performs poorly compared to the Fe-Cr-Zr and even the base Fe-Zr alloys. The new model results show this to be related to difference in the Cr-Zr and Ni-Zr bond energies and their tendency to phase separate. This is not obvious or predicable prior to the modeling results. The research has provided a modeling tool that can be used to target appropriate stabilizer atoms for all practical applications of model binary alloys. In addition to the intellectual outcomes described, the research has advanced intellectual capital through the development and training graduate students. Results have been disseminated through publications, presentations and press releases. From the beginning, and throughout the duration of the NSF program, the research students have matured to become very capable practitioners of techniques and analysis needed to address complex research problems. There is a need for such talent today and it will only grow as technology pervades the future of our society that is forthcoming.
