The influence of the stress annealing on the reversible structural relaxation of a Ni-Si-B amorphous ribbon was studied. Creep-induced structural changes in the amorphous structure were derived from anisothermal DSC and dilatometric experiments. It is demonstrated that considerable enthalpy and specimen length variations associated with the reversible structural relaxation are observed after previous creep at higher temperatures.Keywords: structural relaxation, inelastic strain, Ni-based amorphous alloys.
Introduction.Metallic glasses represent a class of metallic materials with unique physical and mechanical properties. From a thermodynamic point of view, they are unstable and annealing them at elevated temperatures leads to structural relaxation. Many papers dedicated to structural changes, which manifest themselves through changes in various physical properties, have been published up to now [1][2][3][4]. They show that, besides irreversible structural relaxation accompanied by annealing-out and quenching-in effects below the glass transition temperature, there are reversible structural changes brought about by thermal, magnetic, or mechanical effects. Atomic disorder and defects of various levels also play an important role in magnetic properties of metallic glasses [5,6].Glassy alloys with a high Ni content were found to exhibit a glass transition before crystallization. The combination of high glass-forming ability and good mechanical and soft magnetic properties of the Ni-based glassy alloys indicates their perspective applications [7]. The homogeneous deformation ability is influenced by the structure of amorphous alloys, namely by the amount and mobility of defects. Creep and creep recovery experiments can help understanding these phenomena [8]. For this purpose, creep-induced structural changes in the Ni-Si-B amorphous alloy were studied by means of differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) methods. The aim of the present work is to find out how the creep applied at different temperatures influences the reversible structural relaxation of the Ni-rich amorphous alloy.Experimental Details. An amorphous metallic ribbon of the nominal composition Ni 77.5 Si 7.5 B 15 (at.%) with a thickness of 18.8 µm prepared at the Institute of Physics of the Slovak Academy of Sciences in Bratislava by rapid quenching of the melt on a rotating disc was used in experiments. The amorphous structure of the samples was checked by X-ray diffraction. The crystallization temperature of the amorphous ribbon determined by differential scanning calorimetry for an as-quenched sample at the temperature of the first crystallization peak onset was T x =°492 C.The specimens of 5 mm width were cut from as-received ribbon and initially heated up to a preannealing temperature of 380°C and kept for 30 min to eliminate further irreversible structural relaxation processes. After cooling down, they were annealed at temperatures of 300, 325, and 350°C for about 18 h under an external tensile stress of 282 MPa. A...