Evolution of defect concentration in Zr50–xCu34Ag8Al8Pdx (x = 0, 2) amorphous alloys derived using shear modulus and calorimetric data

Abstract

Amorphous alloys exhibit unique physical and mechanical properties, which are closely connected with their microstructural heterogeneity. The correlation between structural heterogeneity and mechanical properties is one of the important issues of amorphous alloys. Micro-alloying is an effective way to tune the mechanical and physical properties of amorphous alloys. In the present study, Zr_50–<i>_x</i>Cu₃₄Ag₈Al₈Pd<i>_x</i> (<i>x</i> = 0 and 2) amorphous alloys with ability to form excellent glass are chosen as model alloys. The evolutions of heat flow and shear modulus in different states (as-cast, relaxed and crystalline) with temperature of Zr_50–<i>_x</i>Cu₃₄Ag₈Al₈Pd<i>_x</i> (<i>x</i> = 0 and 2) glass system are studied by differential scanning calorimetry (DSC) and electromagnetic-acoustic transformation (EMAT) technique, respectively. The experiment demonstrates that a decrease of the shear modulus is accompanied by the endothermic heat flow and vice versa. The correlation between the heat flow and shear modulus is investigated according to the interstitialcy theory. The calculations of the interstitialcy defect concentration and activation energy spectra suggest that the microstructure remains stable at relatively low temperatures. When temperature increases, the interstitialcy defect structure is activated. Compared with that in the as-cast state, the interstitialcy defect concentration in the relaxed state is reduced by structural relaxation, indicating that temperature-dependent shear modulus softening is inhibited. At temperatures above glass transition temperature, a rapid growth of interstitialcy defect concentration results in the accelerated shear softening, which is accompanied by significant endothermic heat flow. It is noted that the minor addition of palladium reduces the interstitialcy defect concentration in the Zr_50–<i>_x</i>Cu₃₄Ag₈Al₈Pd<i>_x</i> (<i>x</i> = 0 and 2) metallic glass systems. It is suggested that the introduction of Pd reduces the atomic mobility and increases the characteristic relaxation time. In parallel, the change of shear modulus as a function of the aging time (below the glass transition temperature) is studied by using EMAT equipment. The results indicate that the interstitialcy defect concentration decreases in the physical aging process, which is accompanied by an increase of shear modulus. The interstitialcy defect concentration and shear modulus change towards the quasi-equilibrium state with aging time increasing. A reduction of the interstitialcy defect concentration leads to a decrease of the shear modulus change upon microalloying by Pd into Zr_50–<i>_x</i> Cu₃₄Ag₈Al₈Pd<i>_x</i> (<i>x</i> = 0 and 2) metallic glass system.