Consolidation of nanoscale iron powders

Abstract

The consolidation behavior of two types of nanoscale iron powders-vacuum condensed (nanograins in nanoparticles) and ball-milled (nanograins in microparticles), was studied. The consolidation of two microscale powders, atomized and ground, was also characterized for comparison. Consolidation techniques investigated were cold closed die-compaction, cold isostatic pressing (CIPing), and after CIPing, sintering or hot isostatic pressing (HIPing). The mechanical properties, density, and microstructure of the resulting compacts were found to depend on the original powder type and its consolidation history. Significant differences were found between the microscale and nanoscale powders. An additional reason, besides the dissimilarity in grain size, for the differences observed relates to the fact that the nanograin powders contained significant amounts of oxygen, which ultimately resulted in a distinctly two-phase bulk microstructure. The vacuum condensed powder achieved satisfactory green strength on CIPing, and high hardness (440 Hv) on low temperature sintering. While unnecessary for complete consolidation, HIPing at 500 °C was found to be beneficial and compacts of this powder thus treated were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powders, HIPing was found to be essential for achieving effective consolidation: ball-milled material, which remained friable after CIPing and sintering at 580 °C, achieved exceptionally high hardness (820 Hv) when HIPed at 580 °C and 175 MPa. The ductility was greatly improved when HIPed at temperatures between 700 °C and 850 °C, while preserving its relatively high strength. The behavior of these nanoscale powders can be understood by invoking the usual densification, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball milled powders.