Effects of boron content on the glass-forming ability and mechanical properties of Co-B-Ta glassy alloys

The effects of boron content on the glass-forming ability, mechanical and elastic properties in Co92−xBxTa8 (at.%, x = 30, 32.5, 35, and 37.5) bulk glassy alloys were investigated. Among these alloys studied, the Co59.5B32.5Ta8 alloy exhibits the highest glass-forming ability, which could form a glassy rod at least 3 mm in diameter by the copper mold casting method. It is found that the plasticity of those glassy alloys increases with decreasing boron content. The plastic strains for the Co59.5B32.5Ta8 and Co62B30Ta8 BMGs are up to ∼2.1% and ∼2.5%, respectively. The decrease of shear modulus with decreasing boron content results in the increase of Poisson's ratio, which is responsible for the improved plasticity. In addition, the strength of the ductile glassy alloys remains very high at above 5000 MPa, although enhancing plasticity is accompanied by the loss of some strength. The present results show that Co-B-Ta bulk metallic glass with good glass-forming ability and significant plasticity as well as ultrahigh strength can be synthesized by adjusting alloy composition.