Effect of composition on the formation and thermal stability of Ni72(Mo, Co)8B20 metallic glass

The effect of the addition of Co and Mo on the formation and stability of a Ni-B amorphous alloy has been investigated. Metal ribbons were prepared by melt-spinning in vacuum and characterized by X-ray diffraction and isochronal differential scanning calorimetry. When no Mo is present, only crystalline material containing the phases Co3B and Ni is produced. As the content of Mo is increased, the distance between metal-metal nearest neighbors increases, the glass transition and the crystallization temperatures increase, the enthalpy of crystallization decreases, and there is a marked increase in the activation energy associated with crystallization. The explanation for this pattern of glass formation and the increase in thermal stability induced by Mo has been explored by reference to similar systems and to a semi-empirical kinetic model of the thermal stability of amorphous alloys. This model predicts the dependence of crystallization temperature on composition in this quaternary alloy from the enthalpies of hole formation of its constituent elements with good success