SOFT-MAGNETIC PROPERTIES OF FE-HF-B TERNARY ALLOYS WITH NANOSCALE BCC STRUCTURE

A mostly single bcc phase with a grain size of about 15 nm was obtained by annealing amorphous Fe-Hf-B alloys ranging from 5 to 7 at%Hf and 1 to 8 at%B for 3.6 ks at temperatures between 773 and 923 K. High effective permeability (mu(e)) above 10000 at 1 kHz and high saturation magnetization (B(s)) above 1.5 T were simultaneously obtained for the nanocrystalline-bcc alloys. Mu(e) and B(s) reach 15000 and 1.65 T, respectively for Fe91Hf7B2 and 32000 and 1.59 T, respectively for Fe89Hf7B4. Saturation magnetostriction (lambda(s)) war, as small as -1 x 10(-6) for the nanocrystalline-bcc Fe89Hf7B4 alloy. Based on the data of the temperature dependence of magnetization and mu(e), the achievement of the soft magnetic properties is presumably due to the amorphous phase along the grain boundary of the bcc phase which has a Curie temperature (T(c)) high enough to maintain the magnetic coupling between bcc grains. The residual existence of the amorphous phase causes the suppression of grain growth of the bcc phase and the precipitation of compounds from the crystallization of the remaining amorphous phase results in the decrease in mu(e). It is therefore concluded that the residual existence of the amorphous phase with high T(c) plays an important role in the achievement of the good soft magnetic properties, in addition to the small grain size and low magnetostriction of the bcc phase.