Enthalpy relaxation behaviour of (Fe, Co, Ni)75Si10B15 amorphous alloys upon low temperature annealing

Abstract

The effect of composition on the anneal-induced enthalpy relaxation for (Fe, Co, Ni)75Si10B15 amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures well belowT g (T g<T g−150 K), an excess endothermic reaction (enthalpy relaxation) occurs above the annealing temperature. The endothermic specific heat, ΔC p, evolves in a continuous manner with annealing time. The change in the magnitude of ΔC p withT a is divided into two stages: a low-temperature stage which peaks at aboutT g−150 K and a high-temperature peak just belowT g. The activation energy,E m, increases with the peak temperature of ΔC p,T m, from 2.29 to 3.15 eV for the low-temperature peak and from 4.40 to 4.96 eV for the high-temperature peak. The low-temperature endothermic reaction is attributed to local and medium range atomic rearrangements and the high-temperature reaction to the long-range cooperative atomic regroupings. The magnitude of the low-temperature ΔC p is most pronounced for (Fe-Co)75Si10B15, followed by (Fe-Ni)75Si10B15, (Co-Ni)75Si10B15, Fe75Si10B15, Co75Si10B15 and then Ni75Si10B15. The reason for such a significant compositional effect of the enthalpy relaxation was investigated, based on the concept of distribution in relaxation time, which had been previously derived from the percolation theory, and was interpreted as due to the difference in the degree of local structure and compositional disorder in the as-quenched state; i.e. the higher the degree of short-range disorder (the larger the fraction of short relaxation time) the larger is the magnitude of the enthalpy relaxation (ΔH σ, endo). There existed a strong correlation between (ΔH σ, endo) or ΔC p and anneal-induced embrittlement tendency; the larger the (ΔH σ, endo) and ΔC p the larger is the embrittlement tendency. Such a strong correlation was interpreted based on the assumption that the internal strain which generated in the transition from as-quenched disordered state to a relaxed equilibrium state causes an enhancement of embrittlement tendency.