Mechanism analysis of metalloid elements affecting amorphous forming ability and magnetic properties of Co-Y-B alloy

Abstract

<sec>Co-based metallic glass (MG) is a new class of soft magnetic material and has promising applications in high-frequency fields due to its high magnetic permeability and low coercivity. However, this kind of MG has poor glass-formation ability (GFA) and relatively low saturated magnetic flux density, so its application scope is limited. The atomic size of metalloid element M (B, C, Si, and P) is small, which can easily enter into the gap between atoms, and there is a relatively large negative enthalpy of mixing between metalloid element and metal element. Therefore, alloying with metalloid element M is an effective method to improve the GFA while maintaining superior soft magnetic properties for Co-based MG. In this work, the formation process of Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>M<sub>10</sub> MG is simulated by <i>ab initio</i> molecular dynamics (AIMD) method, and the effects of the addition of metalloid elements C, Si, P on the GFA and magnetic properties of Co-Y-B MGs are investigated. It is devoted to analyzing the relationship between local atomic structure and property at an atomic level.</sec><sec>According to the results of the characterization parameters of local atomic structure (pair distribution function, coordination numbers, chemical short-range order, Voronoi polyhedron index, local five-fold symmetry, and mean square displacement), it is found that the GFA of the four alloys is different due to their different local atomic structures. Both Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>C<sub>10</sub> alloy and Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>P<sub>10</sub> alloy possess a higher fraction of prism structure, weaker solute segregation between B/C-C and B/P-P atoms, higher atomic diffusivity in the supercooled state (1100 K), and hence weakening the GFA of the alloys. The Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>Si<sub>10</sub> alloy has a higher fraction of icosahedral-like structure, stronger attraction between Co-Si atoms and the solute segregation between B/Si-Si atoms, lower atomic diffusivity in the supercooled state, thereby increasing the GFA. Therefore, the addition of Si is beneficial for enhancing the GFA, while the addition of C or P will reduce the GFA, that is, the GFA of the four alloys decreases in the order of Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>Si<sub>10</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>25</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>P<sub>10</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>C<sub>10</sub>. In terms of magnetic properties, with the addition of C, Si, P elements, the total magnetic moment of Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>M<sub>10</sub> (M = B, C, Si, P) alloy decreases in the following order: Co<sub>72</sub>Y<sub>3</sub>B<sub>25</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>Si<sub>10</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>C<sub>10</sub> > Co<sub>72</sub>Y<sub>3</sub>B<sub>15</sub>P<sub>10</sub>. The stronger p-d orbital hybridization between Co-Si atoms enhances the ferromagnetic exchange interaction, leading the total magnetic moment to be less affected by Si addition.</sec>