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 <111> 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 < 50nm, contradicting results of mechanical alloying by Hays et al. (14).