Effects and mechanisms of Nb and Fe additions on glass-forming ability and magnetic properties of a Co71.5Gd3.5B25 metallic glass

Aiming to unraveling the effects and mechanisms of Nb and Fe additions on glass-forming ability (GFA) and magnetic properties, especially coercivity (Hc) on Co-based metallic glasses, 2-4 at% Nb and/or 7.15-28.6 at% Fe have been alloyed into a Co71.5Gd3.5B25 alloy by substitution of Co. The results show that alloying either Nb or Fe into the base alloy leads to enhanced stability of supercooled liquid and GFA, and bulk metallic glass (BMG) can be formed for the alloy with 4 at% Nb or 28.6 at% Fe. Combined addition of Nb and Fe further improves the thermal stability and GFA, and lowers viscosity in supercooled liquid state. A (Co0.6Fe0.4)67.5Gd3.5Nb4B25 BMG possess a wide supercooled liquid region (& UDelta;Tx) of 85 K, critical sample diameter of 3.0 mm, and minimum viscosity coefficient of 2 x 108 Pass. The enrichment of Fe increases saturation magnetic flux density (Bs), Hc, and saturation magnetostriction coefficient (& lambda;s) of the (Co, Fe)-Gd-B-(Nb) metallic glasses; the addition of Nb effectively inhibits the deterioration of magnetic softness caused by Fe while reducing the Bs. The (Co0.6-0.9Fe0.1-0.4)67.5Gd3.5Nb4B25 BMGs possess good soft magnetic properties with the Bs and Hc ranging 0.43-0.69 T and 2.3-5.0 A/m, respectively. Ab initio molecular dynamics simulations indicated that alloying Fe into the Co-based metallic glasses leads to enhanced magnetic anisotropy energy. The increased magnetic anisotropy arising from enlarged & lambda;s results in the increased Hc. The combination of high GFA, large & UDelta;Tx, low viscosity, and good soft magnetic properties makes the Co-based Co-Fe-Gd-Nb-B BMGs highly promising for applications as micro electromagnetic devices produced through thermoplastic forming.