Co-Cr-Mo-C-B metallic glasses with wide supercooled liquid region obtained by systematic adjustment of the metalloid ratio

We investigate the influence of the ratio between carbon and boron on glass-forming ability, thermal stability and mechanical properties of the Co65Cr13Mo22 ternary eutectic alloy modified by carbon and boron addition. Besides, the correlation between the extension of the supercooled liquid region (SCLR) and three bonding parameters, i.e. electronegativity, atomic size, and valence electron is investigated. The results reveal that carbon is more effective than boron to form a glassy phase in this system, and fully glassy alloys or metallic glass composites can be obtained when the metalloid contents are in the range of 20-25 at.%. Fully glassy alloys are only obtained when the carbon-to-boron ratio is 2:1. The glassy alloys and the glass composites exhibit high strength and extraordinarily wide SCLRs of > 120 K. The width of the SCLR of the Co-based BMGs can be articulated as a function of three bonding parameters, which is different to the relationship previously observed for Fe-, Mg- and Pd-based BMGs. Based on these findings we propose the optimum C/B ratio for glass phase formation and the new criterion for predicting the width of the SCLR of Co-based BMGs by the three bonding parameters.