Bulk nanostructured and ultrafine-grained binary Al–Fe alloys have been studied in the past for their remarkable strength, hardness, and thermal stability. These properties have been attributed to a combination of solid solution strengthening, precipitate strengthening, and grain boundary strengthening. However, to date, no systematic investigation has been performed to address the factors that govern the evolution of the various metastable and equilibrium precipitates that form as a result of thermal exposure. In this study, Al–2at.%Fe and Al–5at.%Fe powders were synthesized via helium gas atomization and argon gas atomization, respectively. Cooling rates upwards of 106 K s−1 were achieved resulting in an intermetallic-free starting structure, and a map of the structure as a function of cooling rate was established. Electron backscatter diffraction analysis revealed the presence of a larger number of low-angle grain boundaries relative to high-angle grain boundaries, which influenced nucleation and precipitation of the metastable Al6Fe phase. Cryomilling of the atomized powder was subsequently performed, which led to grain refinement into the nanometer regime, dispersion of the Fe-containing phases, and forcing of 2at.%Fe into solution within the Al matrix compared to negligible Fe in solution in the as-atomized state. Finally, differential scanning calorimetry was utilized to elucidate the metastable Al6Fe precipitation temperature (~300 °C) and subsequent phase transformation to the equilibrium Al13Fe4 phase (~400 °C). An activation energy analysis utilizing the Kissinger method revealed three important factors, in order of importance, for ease of Al6Fe precipitation: segregated regions containing iron, availability of nucleation sites, and the number of diffusion pathways.
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The authors acknowledge helpful EBSD discussions with Scott Sitzman of Oxford Instruments. Dr. Baolong Zheng of UC Davis is thanked for assistance with gas atomization experiments. The assistance with cryomilling by Hanry Yang is greatly appreciated. The authors would like to acknowledge financial support provided by the Office of Naval Research (Grant No. ONR N00014-12-1-0237) with Dr. Lawrence Kabacoff as the program officer.
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