Effect Mechanism Of Co Element On The Glass-forming Ability Of Fe-based Bulk Metallic Glasses

The glass-forming ability (GFA) of Fe-based bulk metallic glasses (BMGs) can be dramatically improved by adding proper amount of cobalt. The alloy with the optimum composition containing 7% Co,i.e., Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂, exhibits higher GFA than Fe_48-xCo_xCr₁₅Mo₁₄C₁₅B₆Y₂ (x=0,3,5,and 9) alloys and could be cast into a 16-mm-diameter amorphous ingot. In this dissertation, the effect mechanism of Co element on the GFA of Fe-Co-Cr-Mo-C-B-Y BMGs will be discussed in views of dynamics, atomic structure and electronic structure.Studies about dynamic factors have shown that, among the Fe-based alloy system studied, the alloy containing 7% Co is supposed to has the fewest free volume. Therefore, the alloy possesses the densest atomic structure and the lowest atomic diffusion, making it the most resistant to crystallization than other alloys. The Fe-based BMG containing 7% Co has the fewest"relaxation centers", which are the special defects inducing the atomic flow in metallic glasses. The fewer the relaxation centers, the more difficult the atomic rearrangement, inhabiting the crystallization and giving enough time for alloy system to transfer into amorphous state, and thus enhancing the GFA.The electronic structure of the Fe-based alloy studied were examined by using conductivity measurements. Among the Fe-based alloy studied, the alloy containing 7 % Co has the lowest electronic conductivity, suggesting the greatest amount of localized valence electrons and thus the strongest directional bonds. Stronger directional bonds lead to the stiffer atomic binding and more difficult atomic rearrangement, inhibiting the crystallization and thus enhancing the GFA. Therefore, the alloy containing 7 % Co is the best glass former in the Fe-Co-Cr-Mo-C-B-Y alloy system.The atomic structure of the Fe-based alloy studied were examined by using Raman spectrum and X-ray photoelectron spectroscopy (XPS). The alloy containing 7% Co possesses the highest degree of short-range orders (SROs) and medium-range orders (MROs). SROs and MROs have been generally considered as the basic structure units of amorphous metals. The atomic-scale heterogeneity caused by SROs and MROs provides more opportunities to form atomic combinations with both efficient atomic packing and energy minimization for individual clusters, thereby improving the GFA of the multicomponent alloy. On the other hand, for the alloy containing 7 % Co, its average coordination number of C/B and Fe/Cr/Mo reaches minimum. The change in the coordination number reveals the change in topological structure. During the transformation of the topological structure, the internal stress releases. Therefore, the topological structure becomes more stable, improving the GFA of Fe-based BMG.