Synthesis and stability of nanocrystalline Fe alloys produced by high energy ball milling

In the first portion of the study, the synthesis of a nanocrystalline Fe-B-Si alloy by mechanical alloying in a SPEX shaker mill is investigated. Electron probe microanalysis and precise lattice parameter measurements indicate that 64 hours of milling of elemental powders is insufficient to produce the complete dissolution of either Si or B in Fe, while milling to 128 hours results in dissolution of the majority of Si, but only partial dissolution of B. Second, the volume of debris incurred due to vial and ball wear is found to be large, representing 38 wt.% of the powders after 128 hours of milling. Milling of pre-alloyed Fe-7at.%B-10at.%Si powders produced by spray atomization, resulted in the precipitation of Fe{dollar}sb2{dollar}B. This precipitation process was essentially complete after 50 hours of milling, resulting in a nanocrystalline Fe(Si) solid solution with homogeneously distributed Fe{dollar}sb2{dollar}B precipitates. This result indicates that the extent of supersaturation which may be achieved by ball milling is not unlimited, but instead approaches a metastable equilibrium limit which is related to the equilibrium solid solubility limit.; In the second segment of the study, the thermal stability of nanocrystalline Fe alloys is addressed. Nanocrystalline Fe-xAl (x = 0, 2.6 or 10 wt.%) powders are prepared using cryogenic mechanical alloying (cryomilling). The extremely low rate of diffusion of Al in Fe at the process temperature ({dollar}{lcub}-{rcub}196spcirc{dollar}C) effectively limits dissolution of Al in the nanocrystalline {dollar}alpha{dollar}Fe grains. Grain growth of the nanocrystalline Fe is rapid, leading to an average grain size of approximately 100 nm after 1 hour at 700{dollar}spcirc{dollar}C. Thermal stability of the cryomilled Fe-Al materials, in contrast, is significantly higher. For example, the average grain size of cryomilled Fe-10wt.%Al is less than 20 nm after 1 hour at 700{dollar}spcirc{dollar}C. Transmission electron microscopy demonstrates that heat treatment of the cryomilled Fe-Al materials results in a highly inhomogeneous distribution of grain size and microchemistry. The observed thermal stability is evaluated in terms of Zener pinning, solute drag and chemical ordering mechanisms. Hot isostatic pressing of the stable Fe-Al powders at 900{dollar}spcirc{dollar}C produces compacts of greater than 99% density with an average grain size of less than 50 nm.