Effect of compositional order upon ductile shear failure and structural relaxation of Fe(80−x)NixB20 metallic glass ribbons

Experimental results of the compositional dependence of the critical shear stress at low temperatures and of several structural relaxation characteristics at elevated temperatures are compared and examined with respect to singularities of the behavior at certain ‘stoichiometric’ compositions in a series of Fe80−xNixB20 (x=10, 20, 30, 40, 50 and 60) metallic glass ribbons. The shear stress for ductile failure has been determined in uniaxial tension; structural relaxation has been followed by the change of the vibration frequency during annealing with a constant heating rate up to crystallisation in a vibrating-reed set-up. Minima in all measured characteristics are observed for x=40 (and possibly for x=60). This is explained in the framework of the polycluster model of amorphous solids by small values of the critical stresses for local plastic shear along intercluster boundaries in the Fe40Ni40B20 metallic glass due to the low concentration of coincident nodes at these boundaries in comparison with those in metallic glasses of nearby compositions.