Processing of bulk nanostructured Ni-Fe materials

Nanocrystalline (nc) Ni-Fe alloys are promising soft magnetic materials that exhibit low coercivity and high permeability (Suryanarayana, 1995; Qin et al., 1999). Especially, the retention of the metastable nanograined structure during the process is mandatory for preserving the superior properties of such a nc Ni-Fe alloy. In recent years Lee and his colleagues (Lowe and Valiev, 2000; Segal, 1995); Langdon et al., 2000) have attempted to find a new processing route for fabricating bulk nc gamma-Ni-Fe materials by conventional (pressureless) sintering of mechano-chemical processed gamma-Ni-Fe nanoalloy powder from metal oxides. It was found that the inhomogeneity of the initial pore size distribution in nano-agglomerate gamma-Ni-Fe powder played a significant role for the full densification process accompanying the nanograined structure (Knorr et al, 2000). Namely, as inhomogeneity in pore size distribution enlarges, full density is hardly achieved. This study attempts to find out how to optimize a full density processing of nc gamma-Ni-Fe alloy powder that effectively controls grain growth during the consolidation process. The densification mechanism has been explained on the basis of the microstructural development and the diffusion process. (CSA)