Microstructural development of iron powder during attritor ball-milling in nitrogen

Iron powders were mechanically processed in an ATTRITOR ball-mill. Mossbauer spectroscopy and X-ray diffraction were used to characterize development of the nanocrystalline microstructure(s) as a function of processing time and environment. Processing in an inert gas environment resulted in a continuous decrease in grain size and after 150 hours of processing, produced 5-10 mu m particles with 5-10 nm grains. When processing in a nitrogen gas environment, the nitrogen concentration alloyed into the powder increased linearly with processing time. After approximately 50 hours of processing, and a nitrogen concentration of 0.5 wt%, a second 1-5 nm phase began to develop. The microstructure of this phase was determined to be bct-fe that developed on the surface of the grains as a result of cold-work induced stress and the presence of nitrogen at the grain boundary. Lattice d-spacing analysis and the components present in the Mossbauer spectra indicated the majority of the grain matrix was nitrogen free. The Mossbauer analysis also indicated that the majority of the alloyed nitrogen was distributed in a bct-Fe lattice. TEM examination of the mechanically processed powder was consistent with the X-ray diffraction and Mossbauer findings.