Investigation of the free volume change of Fe _(41)Co _7Cr _(15)Mo _(14)C _(15)B _6Y _2 bulk metallic glass using the cyclic thermal dilatation test

摘要

In heating process, the free volume of bulk metallic glass (BMG) changes with temperature. This change, however, is too small to be directly investigated by thermal dilatation (DIL) test. In cyclic heating, the free volume change results in a difference between the thermal expansion during heating and reheating processes, i.e., Δ(δV(T)/V _0), which can be obtained by the cyclic DIL test. In this paper, the sample diameter and pre-annealing effect on the free volume change of Fe _(41)Co _7Cr _(15)Mo _(14)C _(15)B _6Y _2 BMG are investigated by the Δ(δV(T)/V _0) and ΔδV/ ~(V0?), where ΔδV/ ~(V0?) is the average value of Δ(δV(T)/V _0) in the low temperature range and represents the total free volume released in the cyclic DIL test. The as-cast samples with a smaller diameter have a larger ΔδV/ ~(V0?). After pre-annealing below the glass transition temperature for 2 hours, a peak that is attributed to the free volume generation in glass transition process occurs in the Δ(δV(T,t _a)/V _0) curve and becomes sharper as the pre-annealing time t a is prolonged. ΔδV(ta)/ ~(V0?) is fitted by a stretched exponential relaxation function with a characteristic relaxation time τ of 14194 ± 2808 s and a Kohlrausch exponent β kww of 0.34 ± 0.04. The β _(kww) of Fe _(41)Co _7Cr _(15)Mo _(14)C _(15)B _6Y _2 BMG is markedly lower than that of Pd- and Zr-based BMGs, and deserves a further investigation.