EFFECT OF COMPOSITION ON THE FORMATION AND THERMAL-STABILITY OF NI-72(MO,CO)(8)B-20 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 p repared by melt-spinning in vacuum and characterised by X-ray diffract ion and isochronal differential scanning calorimetry. When no Mo is pr esent, only crystalline material containing the phases Co3B and Ni is produced. As the content of Mo is increased, the distance between meta l-metal nearest neighbours increases, the glass transition and the cry stallisation temperatures increase, the enthalpy of crystallisation de creases, and there is a marked increase in the activation energy assoc iated with crystallisation. 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 kine tic model of the thermal stability of amorphous alloys. This model pre dicts the dependence of crystallisation temperature on composition in this quaternary alloy from the enthalpies of hole formation of its con stituent elements with good success.