The Formation Of Fe-B Based Bulk Metallic Glasses In Relation To Clusters

The bulk metallic glasses (BMGs) are special kinds of metastable multi-compositional complex metallic alloys, which have important potential applications, with many unique qualities of high elastic and super-plastic ability. Recently, Fe-based BMGs are drawing more attentions due to their superior mechanical and magnetic properties, so that finding out new kinds of Fe-based BMGs are becoming an impotant issue. The thesis applicated the cluster line rule in combination with minor alloying principle which raise by our research group into the composition design of Fe-B-Y based quaternary system. We found a new serious of BMGs, analysed their formation by (cluster)₁(glue-atom)₁ model and tried to design Fe-B based ternary bulk metallic glasses in relation to this model.Fe-based BMGs have relatively weak glass formation abilities (GFA) and are formed only in multi-component systems, generally containing five or more than five constituents, except a few ternary systems such as Fe-B-Y which can form BMGs rods with a diameter of 2 mm. Furthermore, Fe-based BMGs are very composition-sensitive and the BMGs-forming range is quite small. In the Fe-rich corner of Fe-B binary crystalline phases, there are two kinds of clusters which are all centered by a B atom: CN10 Archimedean octahedral antiprism Fe₈B₃, CN9 capped trigonal prisms Fe₈B₂ and Fe₉B. There exists an icosahedral cluster Fe₁₂Y centered by a Fe atom in Fe₆₂B₁₄Y₃ ternary crystalline phase. Thus, the cluster lines Fe₈B₃-Y, Fe₈B₂-Y, Fe₉B-Y and Fe₁₂Y-B are then constructed in the ternary Fe-B-Y system. The Fe-B cluster lines and the Fe₁₂Y-B cluster line intersect at three basic ternary compositions Fe_68.6B_25.7Y_5.7 (Fe₈B₃-Y), Fe₇₅B_18.8Y_6.2 (Fe₈B₂-Y) and Fe_83.7B_9.3Y₇ (Fe₉B-Y), respectively. These basic compositions are alloyed by small amounts of M (1-5 at. %), M=Ti,Hf,Nb,Mo,Ta,Ag, to study the BMGs formation. The formation ranges of BMGs with a diameter of 3 mm by using copper mould suction casting method are determined. Thermal analysis indicates that minor Nb,Mo,Ta added to the basic alloy compositions increase the GFA of Fe-B-Y system. The (Fe_68.6B_25.7Y_5.7)₉₇Nb₃, (Fe_68.6B_25.7Y_5.7)₉₇Mo₅, (Fe_68.6B_25.7Y_5.7)₉₇Ta₃ bulk metallice glasses have the largest GFA and the lengths of the fully amorphous rods are respectively 22 mm, 20 mm and 12 mm.Fe₈B₃ cluster has a decisive role on the glass formation. The R^*(the topological packing density) values are increased after minor alloying with Nb, Mo and Ta addition, so the GFA is increased by the Nb, Mo and Ta additions. All the (Fe_68.6B_25.7Y_5.7)_100-xNb_x,(Fe_68.6B_25.7Y_5.7)_100-xMo_x,(Fe_68.6B_25.7Y_5.7)_100-xTa_x(x=3-5 at. %) bulk metallic glasses are close to (Fe₈B₃)₁(Y, Nb)₁, (Fe₈B₃)₁(Y, Mo)₁ or (Fe₈B₃)₁(Y, Ta)₁, that is, the BMGs structure is composed of the Fe₈B₃ cluster plus the glue atom (Y, M(M=Nb, Mo, Ta)). So the best BMGs formation composition can be expressed with a unified composition formula (Fe₈B₃)₁(glue-atom)₁, where the Archimedean octahedral antiprism cluster Fe₈B₃ is topologically dense-packed structure. Finally, this model is also applicated in the composition design of Fe-B-M(M=Y, Nb, Ta, Si, Ag) ternary system and we get the Fe_66.7B₂₅Nb_8.3 bulk metallic glasses with a diameter of 2 mm.