Effects of Ni substitution for Fe/Co on mechanical and magnetic properties of Co-based bulk metallic glasses

Abstract The influences of Ni substitution for Fe/Co on the thermal, mechanical and magnetic properties of (Co1-x-yNixFey)68B21.9Si5.1Nb5 (x = 0, y = 0.3; x = 0.1, y = 0.2; x = 0.2, y = 0.1; x = 0.3, y = 0; x = 0.4, y = 0; x = 0.5, y = 0) metallic glasses are investigated. A new (Co0.5Ni0.5)68B21.9Si5.1Nb5 bulk metallic glass (BMG) with a high strength of 4070 MPa and a large plasticity of 3.8% is successfully prepared. The higher Poisson’s ratio (ν) increases the effective free volume in the alloy, which promotes plastic flow before fracture. Besides, the lower glass transition temperature indicates lower shear flow barrier, and the narrower supercooled liquid region can easily improve the structural heterogeneity, both of which enhance the plastic strain of the BMG. Furthermore, the effects of the addition of elements that have large ν (Ni) or positive mixing enthalpy with major elements (Cu), as well as their co-addition, on the mechanical properties of CoFeBSiNb BMG system are compared. The substitution of Fe/Co with Ni leads to the decrease of both saturation magnetization and coercivity of the metallic glasses, and the material even changes from ferromagnetic to paramagnetic after Fe is completely replaced by Ni because of the low atomic magnetic moment of Ni. This work not only confirms the universal critical Poisson’s ratio for plasticity in Co-based BMGs, but also provides design guidelines for Co-based BMGs with large plasticity.