Jung Gyu Nam

Synthesis and microstructureal characterization of nanostructured γ-Ni-Fe powder

Abstract The microstructures and their relation to synthesis conditions of nanostructured γ-Ni-Fe alloys, synthesized by a mechano-chemical process, were investigated by using various techniques which include EDS, infrared and optical emission spectroscopy, XRD, TEM, BET and Laser particle analyzer as well as field emission SEM. The results indicate that nanostructured γ-Ni-xFe alloys with x ≈ 32, 46, 55, and 64wt% were successfully synthesized. The impurity contents in these γ-Ni-Fe alloys were very low. Typically, the microstructures of nano γ-Ni-46Fe and their variations with synthesis conditions were focused on. It was documented that γ-Ni-46Fe alloy with a minimum average grain size of 20nm and minimum average particle size ∼70nm can be obtained under specific synthesis conditions. Micro-pore analysis combined with BET and XRD experimental results indicated that the particles usually consisting of less than one hundred grains were actually small (sub-micrometer) polycrystals with no micro-pores within them. Experiments also showed that there were large hard-agglomerates which had an average size of ∼100nm in the nano γ-Ni-46Fe powders obtained at temperatures below 800°C. Above this temperature both the grain growth and the sintering process were significant. Moreover, it was revealed that upon annealing at temperatures below 800°C, the grains of nano γ-Ni-46Fe have an elongated shape with their long axis being in direction and with an aspect ratio of 1.37–1.25. Annealing at temperatures above 800°C caused their grains to change shape gradually to a disk-like form. In addition, the lattice of the nano γ-Ni-46Fe was found to be in a shrunken state with an average grain size of d (14) .

Densification and microstructural development of nanocrystalline γ-Ni-Fe powders during sintering

Abstract The sintering behavior of nanocrystalline (nc) γ-Ni-Fe powders was investigated by laser-photo-dilatometry. The sinterability of nc powders was found to depend crucially on the state of agglomeration. The compacted γ-Ni-Fe powders exhibited a bimodal pore distribution comprised of nanoscale intra-agglomerate pores and large, microscale inter-agglomerate pores. The results are discussed on the basis of the microstructural evolution during densification which was followed by optical microscopy and BET specific surface area measurements.

Mechano-chemical synthesis of nanosized stainless steel powder

Abstract The present investigation has attempted to fabricate nanosized stainless steel powder of Fe-18Cr-8Ni(wt%) by mechano-chemical synthesis. The synthesis process was conducted by hydrogen reduction of a high energy ball milled mixture of Fe 2 O 3 -NiO-Cr(NO 3 ) 3 ·9H 2 O. In-situ alloying of Ni-Fe-Cr nanophase simultaneously occurring during reduction process is presumably responsible for the formation of nanophase stainless steel powders. The powder characteristics were examined by X-ray diffractometry and SEM observation. The kinetic phenomena in association with the oxide reduction and the alloying process were investigated by thermogravimetry, hygrometry and X-ray diffractometry, and discussed in relation to powder characteristics.

Synthesis and consolidation of γ-Ni-Fe nanoalloy powder

The present work studies the synthesis and consolidation of γ-Ni-Fe nanoalloy powder by the mechano-chemical process comprising high-energy ball-milling of NiO-Fe2O3 powder and a subsequent hydrogen reduction process. To examine the formation mechanism of the nanoalloy powder, the effect of the oxide powder char-acteristics on the reduction process and alloying was investigated by varying the ball-milling time. The reduction process and the alloying of the γ-Ni-Fe phase proved to accelerate as the ball-milling time increased. However, prolonged milling (for 30 hours) retarded the reduction of Fe2O3 as well as the alloying process. The densification process of the Ni-Fe nanoalloy powder strongly depended on the degree of agglomeration which results in enhancing homogeneous sintering. The limited densification of the nanoalloy powder originates from the high degree of particle agglomeration. While intra-agglomerate porosity is easily eliminated in the course of sintering, it is found to resist densification. The limitation of the sintered density could be overcome by increasing the green density of the powder compacts. Full density was achieved by starting with a green density of 72% theoretical density.