Preparation of nanostructured iron by mechanical milling under cryogenic conditions

Abstract

Fabrication of nanostructured materials is of outmost importance for future advanced materials. Nanostructurization of materials with structure sizes in the micrometer range and below can significantly alter mechanical and physical properties while using the identical chemical composition as their counterparts. Up to now there are only few approaches on synthesizing large amounts of bulk nanostructured materials and micromechanics of the fabrication process are not completely understood and characterized. This work develops and investigates a new production process for nanostructured pure iron via cryomilling using a SPEX freezer/mill. Based on the creation of an experimental design and the definition of a process protocol multiple sample series were fabricated to investigate influences of individual parameters. Laser diffraction analysis and scanning electron microscopy (SEM) were used to characterize powder evolution with varied milling times, milling loads and milling rates. It was found that increasing milling times promote a homogenization of the particle size distribution and the creation of nanoparticles due to brittle fragmentation triggered by the employed cryogenic temperatures. Increasing milling loads augmented the probability to create micrometer-sized particles. Lower milling rates increased plastic deformation processes and particle agglomeration mechanics during the milling process. Fabricated iron nanoparticles were used to synthesis bulk specimens by a cold and subsequent warm consolidation process. Created microstructure samples were analyzed by Vickers hardness micro indentation tests, by using optical microscopy and SEM and by electron backscatter diffraction. Vickers hardness was found to increase with milling time up to a maximum of 569 HV with a testing load of 0,02 kg. SEM analysis proved that former nanoparticles were conserved during the consolidation process forming grains with minimum grain sizes of less than 20 nm. Larger particles showed a plastically deformed grain structure with micrometer sized flattened grains including low angle grain boundaries. Those and the formation of a few nanograins inside former large particles were accorded to a severe plastic deformation process. Micro tensile testing was performed after heat treatment of 30 min at 650°C. All samples showed a brittle fracture behavior that is most likely linked to compaction flaws like pores and other observed inhomogeneities.