CONSOLIDATION OF NANOSCALE IRON POWDERS

The consolidation behavior of two types of nanoscale iron powders- vac uum condensed (nanograins in nanoparticles) and ball-nulled (nanograin s BI 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 m icrostructure of the resulting compacts were found to depend on the or iginal powder type and its consolidation history. Significant differen ces were found between the microscale and nanoscale powders. AII addit ional reason, besides the dissimilarity in grain size, for the differe nces observed relates to the fact that the nanograin powders contained ! significant amounts of oxygen, which ultimately resulted in a distin ctly two-phase bulk microstructure. The vacuum condensed powder achiev ed satisfactory green strength on CIPing, and high hardness (440 H-V) oil low temperature sintering. While unnecessary for complete consolid ation, HIPing at 500 degrees C was found to be beneficial and compacts of this powder thus tl-eared were found to have a hardness of 520 Hv and high compressive yield strength (1800 MPa). For ball-milled powder s, HIPing was found to be essential for achieving effective consolidat ion: ball-milled material, which remained friable after CIPing and sin tering at 580 degrees C, achieved exceptionally high hardness (820 H-V ) when HIPed at 580 degrees C and 175 MPa. The ductility was greatly i mproved when HIPed at temperatures between 700 degrees C and 850 degre es C, while preserving its relatively high strength. The behavior of t hese nanoscale powders can be understood by invoking the usual densifi cation, particle bonding, and grain growth mechanisms. Optimization of these processes may result in unique mechanical properties of ball mi lled powders. (C) 1998 Acta Metallurgica Inc.