Enhancement of the fracture strength and glass-forming ability of CoFeTaB bulk glassy alloy.

CoFeTaB bulk glassy alloy has the best glass-forming ability (GFA) among the Co-based glassy alloys, and the highest strength (the compressive true strength σ = 5185 MPa) among all known bulk crystalline and glassy alloys. With the aim of synthesizing new Co-based bulk glassy alloys with much higher strength and much larger GFA, we investigated the effect of Mo and Si additions on the enhancement of σ and GFA in the Co-(Fe, Mo, Ta)-(B, Si) system. The small amount of 2 at.% Mo added to the Co-Fe-Ta-B glassy alloy resulted in obtaining an ultrahigh true fracture strength of 5545 MPa and high Young's modulus (E) of 282 GPa. By further adding 1 and 2 at.% Si, Co-(Fe, Mo, Ta)-(B, Si) bulk glassy alloys were synthesized in the diameter range up to 3 mm, and they exhibited σ of over 4450 MPa and E of over 227 GPa. In addition, the ultrahigh-strength glassy alloys simultaneously exhibited excellent soft magnetic properties, i.e., saturation magnetization of 0.32-0.35 T, low coercive force of 0.7-1.1 A m, and high effective permeability of 3.9-4.77 × 10 at 1 kHz. The improvement of GFA and σ is interpreted to result from the enhanced atomic bonding nature by adding Mo and Si.