1. Thermal Stabilization and Mechanical Properties of nc Fe-Ni-Cr Alloys Ronald O. Scattergood, North Carolina State University, DMR 1005677 A study was completed on the high-temperature stabilization of grain size by addition of Zr in Fe-Ni alloys (Journal of Materials Science, in review 2013). Emphasis was placed on understanding the effects of Zr on microstructural evolution and grain growth at temperatures near and above the bcc-to-fcc phase transition (close to 700˚C). We showed previously that the fcc grains will produce abnormal grain growth in Fe-Ni alloys (Journal of Materials Science, 48(5) 2251 2013). Precipitation of Fe-Ni-Zr intermetallics at high temperatures can suppress grain growth by Zener pinning. Figure 1 illustrates this with TEM BF/DF/SAD images (left to right) for Fe 91 Ni 8 Zr 1 (top row) and Fe 88 Ni 8 Zr 4 (bottom row) annealed at 1000˚C. Eventually at higher temperatures the retention of nanocrystallinity is lost. Analysis of the hardness data in Figure 2a, based on combined solid solution, grain size (Hall Petch) and intermetallic precipitation (Orowan) strengthening models, leads to the results in Figure 2b for the substantial contribution of solid solution and precipitation to the hardness. The trends observed in Figure 2b result from an interplay of precipitation and overaging kinetics as annealing temperature is increased. Modeling (Slide 2) shows that segregation of Zr to grain boundaries can stabilize grain size in 4% Zr alloys up to 700˚C, but grain size stability at higher temperatures results from Zener pinning in these alloys. Figure 2 Hardness after isochronal annealing Figure 1 TEM results for 1000˚C annealing

2. Thermal Stabilization and Mechanical Properties of nc Fe-Ni-Cr Alloys Ronald O. Scattergood, North Carolina State University, DMR 1005677 Modeling thermodynamic stabilization of grain size (by solute segregation to grain boundaries) as a function of alloy composition and temperature is an integral part of this research. A new regular solution model for two component alloys, which incorporates both the elastic and chemical enthalpies, was developed (Journal of Applied Physics, 113, 063515 2013). The model has recently been extended to include ternary alloys (Journal of Applied Physics, in review 2013). A schematic of the grain boundary configuration is shown in Figure 3 (top). This applies to binary base alloys CA with a ternary solute addition B for stabilization, as opposed a binary model with elemental C and solute addition B. Solutions can be obtained using standard numerical analysis programs. Sample solution templates are provided with the submitted manuscripts. A broader impact of our research is to make the modeling/computational methods available as a tool for the materials research community. An example of ternary model results is shown in Figure 3 (bottom) for Fe-Ni-Zr and Fe-Cr-Zr alloys where Zr is the solute addition for stabilization. The grain size vs. temperature curves are shown for selected alloy compositions. These represent metastable grain size equilibrium states for simultaneous minimization of the excess Gibbs free energy with respect to solute segregation and grain boundary volume fraction. It is seen that Fe-Ni base alloys cannot be stabilized above about 700˚C with 4% Zr additions. This relates directly to the experimental results shown in Slide 1. It was concluded there that thermodynamic stabilization does not contribute to nanocrystalline grain sizes observed above 700˚C. This is confirmed by the model predictions. In contrast, Fe-Cr base alloys can be stabilized up to 1000˚C. Figure 3 Thermodynamic stabilization model