Characteristic free volume change of bulk metallic glasses

Abstract

The free volume change ΔV[sub f](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 ΔV[sub f-sr], was identified quantitatively from the ΔV[sub f](T) curve. For a Fe-based BMG, it was found that ΔV[sub f-sr] increases with decreases in the sample diameter and heating rate. ΔV[sub f-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 ΔV[sub f-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 ΔV[sub f-sr] and LogD[sub c]² or LogD[sub c], where D[sub c] is the critical diameter. ΔV[sub f-sr] is thus sensitive to and has a close correlation with GFA. Furthermore, the ΔV[sub f-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, ΔV[sub f-sr] thus plays an important role given its comparability and convenience.