DOMAIN STUDIES ON MECHANICALLY ALLOYED Fe-Zr-B - NANOCRYSTALLINE POWDER

摘要

Nanocrystalline iron-based ribbons of certain metallic glass alloys (f.e. Fe-Si-Cu-Nb-B [1] or Fe-Zr-B-Cu [2]) which are fabricated by rapid quenching and subsequent annealing exhibit excellent soft magnetic properties due to an averaging of the local crystal anisotropy [3]. The magnetic domains in such materials [4] strongly resemble the domains in amorphous ribbons. In [5] we investigated the structure and magnetic properties of mechanically alloyed nanocrystal-line Fe-Zr-B-Cu-powders. Ball milling proved well suited for the direct solid state processing of a nanocrystalline microstructure in such systems, consisting of a two-phase mixture of bcc-solid solution with typical grain sizes of some nanometers and a small amount of amorphous phase. However, the coercivity of the as-milled powders is about 4 orders of magnitude higher than known from nanocrystalline ribbons, caused mainly by internal strain induced by the milling process. Although the coercivity can be reduced by a factor of ten by annealing at elevated temperatures, accompanied with a reduction of atomic level strain and without significant grain growth, the soft magnetic properties of the annealed powder are still far away from those achievable in melt spun materials. In this paper some insight in the reasons for the diminished magnetic properties is given by domain observation on nanocrystalline powder.