Composition Design And Properties Investigation For Fe-B-Si Based Amorphous And Nanocrystalline Alloys

Fe-B-Si based amorphous and nanocrystalline alloys exhibit excellent soft magnetic properties, which are potentially useful as transformer or motor core materials. The Fe-B-Si based amorphous alloys, however, are associated with a limited amorphous forming ability, and the ease of amorphous state formation by melt-quenching occurs within narrow composition ranges, which constrains their practical applications. Meanwhile, the compositional characteristic of Fe-B-Si based nanocrystalline alloys remains unclear. In this work, the so-called "cluster plus glue atom" model has been used to design Fe-B-Si based bulk amorphous alloys and to optimize the compositions of Finemet nanocrystalline alloys as well. The main results obtained in this study are summarized as follows:1. Cluster formulas, namely, [B-B2Fe8]Fe=Fe75B25 and [Si-B2Fe8]Fe (Fe75B16.67Si8.33) are derived from the eutectic related phase Fe2B, and the ideal amorphous compositions for Fe-B binary and Fe-B-Si ternary amorphous alloys are reached by using a 24-valence electron criterion for electronic structure stability. Experimental result shows that the crystallization temperature and amorphous-forming ability indicator (a) parameter are the highest at the ideal compositions.2. Considering the stability of cluster structures of [Si-B2Fe8-xETMx]Fe and [Si-B2Fe7.6ETMo.4/REMo.4]Fe (ETM:early transition metal, ETM= Zr, Hf, Nb and Ta, REM: rare earth metal, REM= Y, Dy, Ce, Nd, Pr, Sm and Gd), cluster formulas are derived by introducing a fourth element Si which has strongly negative heat of mixings with the basic constituents. The experimental results show that bulk amorphous alloys are obtained only in the ETM alloys, which exhibit a critical glass formation diameter as large as 2.5 mm, along with excellent mechanical properties (a tensile strength exceeding 4000 MPa) and soft magnetic properties (a saturation magnetization range of 1.14-1.57 T and a coercive force range of 1.5-11.0 A/m). Starting from the optimal compositions of Zr-and Hf-bearing amorphous alloys, and by increasing the number of glue atoms Fe, the saturation magnetization of the amorphous alloys can be further increased. Two series of alloy compositions, namely, [Si-B2Fe7.6Zro.4]Fex and [Si-B2Fe7.7Hfo.3]Fex are thus designed. The experimental results show that the high Fe content bulk amorphous alloys exhibit a high saturation magnetization (>1.5 T) and a low coercivity (< 3 A/m). Based on the Nb-bearing compositions with the best amorphous forming ability, a serial compositions of [(Si]-yBy)-B2Fe8-xNbx]Fex is obtained by assuming a dense atomic packing state of the clusters. The amorphous forming ability and mechanical properties are both improved through this kind of composition optimization for Fe-B-Si-Nb amorphous alloys. It can be shown from a local structure model that the intra-and inter-cluster correlations play crucial roles in understanding the variation tendencies of thermal glass stability, the accessibility and location change of glass transition and amorphous forming ability in the ETM, REM and Fe75B16.7Si8.3 amorphous alloys.3. In view of the fairly large composition range for amorphous state formation in the binary Fe-B system, covering the solid solution phase and intermetallic phase forming ranges in the equilibrium phase diagram, a double cluster structure model has been proposed to explain the composition range of Fe-B amorphous alloys. In light of this, two double-cluster formulas have been proposed for the Finemet nanocrystalline alloys, one being [Si-Fe14](Si12/13Cu1/13)3 cluster formula from the Fe3Si phase, and the other [(Si1-yBy)-B2Fe8-xNbx]Fe cluster formula from the ideal compositions of Fe-B-Si-Nb alloys. By combining the two kinds of clusters formulas with the 1:1 ratio, a series of nanocrystalline alloys compositions are obtained. The experimental results show that a nanocrystalline-amorphous diphase structure can be made in a Fe74B7.33Si15.23Nb2.67Cuo.77 system, which delivers good soft magnetic properties (a high saturation magnetization of 1.26 T, a low coercivity of 0.5 A/m and a high effective permeability of 8.5×105 at 1 KHz frequency) after annealing at 813 K for 60 min. The soft magnetic properties of this alloy have surpassed Finemet nanocrystalline alloys with Fe73.5Si13.5B9Cu1Nb3 composition.