Relationship between the shear modulus and volume relaxation in high-entropy metallic glasses: Experiment and physical origin

We performed parallel measurements of the high-frequency shear modulus G and relative volume ΔV/V for high-entropy Ti₂₀Zr₂₀Hf₂₀Be₂₀Cu₂₀ and Ti₂₀Zr₂₀Hf₂₀Be₂₀Ni₂₀ glasses upon heating from room temperature up to the complete crystallization. The changes of these properties due to structural relaxation both below and above the glass transition temperature are singled out. It is shown that these changes for both initial and preannealed samples can be well described within the framework of the Interstitialcy theory. It is found that the whole relaxation process in the full temperature range of the experiments for both samples’ states can be characterized by a single dimensionless temperature-independent parameter K_i=dlnG/dlnV, which equals to −44 and −53 for the above glasses, respectively, and strongly points at interstitial-type defects as a source of the relaxation. We also show that relaxation of the relative volume linearly depends on the defect concentration. This behavior can be described by another dimensionless temperature-independent parameter, which is related with the relaxation volume of defects. It is shown that the obtained results provide resonable estimates of density changes upon crystallization. Possible contribution of vacancy-like defects into the relaxation is discussed.