Characteristic free volume change of bulk metallic glasses

Abstract

The free volume change ΔVf(T) of bulk metallic glasses (BMGs) relative to a hypothesized amorphous reference state was measured using the thermal dilatation method. The characteristic free volume change, i.e., the free volume released in structural relaxation ΔVf-sr, was identified quantitatively from the ΔVf(T) curve. For a Fe-based BMG, it was found that ΔVf-sr increases with decreases in the sample diameter and heating rate. ΔVf-sr measured under the same sample diameter and heating rate conditions allowed the convenient comparison of different BMGs. The comparison revealed that the glass-forming ability (GFA) enhancement of each of two Pd-, Mg-, Cu-, Zr-, Ti-, and Fe-based BMGs can be sensitively reflected in the decrease in ΔVf-sr and the narrowing of the difference between the peak temperature of the thermal expansion coefficient and the end temperature of the glass transition process. In addition, for these twelve typical BMGs, there is a good linear relationship between ΔVf-sr and LogDc2 or LogDc, where Dc is the critical diameter. ΔVf-sr is thus sensitive to and has a close correlation with GFA. Furthermore, the ΔVf-sr measurement results are in good agreement with the free volume change measured with the specific heat capacity, room temperature density, and positron annihilation lifetime methods. In the study of the relationship between the structure and properties of BMGs, ΔVf-sr thus plays an important role given its comparability and convenience.