Relaxation and Crystallization Behavior of the Zr<SUB>50</SUB>Cu<SUB>40</SUB>Al<SUB>10</SUB> Metallic Glass

Abstract

We examined relaxation and crystallization behavior of the ternary Zr 50 Cu 40 Al 10 metallic glass by using positron annihilation and transmission electron microscopy (TEM). Observed changes in positron annihilation lifetime of the alloy annealed isothermally at 673 K, which is below the glass transition temperature, correlate well with observed density changes; while coincidence Doppler broadening (CDB) spectra exhibit no significant change. These observations demonstrate that free volume decreases without a rearrangement of atoms during structural relaxation. On the other hand, CDB spectrum has exhibited considerable changes when the same alloy was annealed at 773 K. TEM observations suggested that crystallization occurs via growth of spherulites of several hundred nm in diameter, which themselves are composed of radially grown grains. Chemical analysis revealed that Cu atoms are partitioned out during the growth of the spherulites. Diffraction study indicated that the unit cell of the crystalline phase belong to an orthorhombic system with a ¼ 0:892, b ¼ 0:550, and c ¼ 1:060 nm; while remaining interspherulite regions are found to crystallize into an fcc phase with a ¼ 1:28 nm, which is probably isostructural to the 3 phase.

Free volume in Zr-based bulk glassy alloys studied by positron annihilation techniques

“…As shown in this figure, the difference of positron lifetime comparing with as-prepared sample increases with increasing annealing temperature. This result agrees with our previous results [4,9,10]. As can be seen from this figure, density increases, and absolute value of positron lifetime decreases with increasing annealing temperature.…”

Relaxation of Free Volume in Zr<SUB>50</SUB>Cu<SUB>40</SUB>Al<SUB>10</SUB> Bulk Metallic Glasses Studied by Positron Annihilation Measurements

“…This tendency is also observed at 673 K, which is just below the T g . 9) These results demonstrate that the local chemical environment around the free volume remains essentially the same during the relaxation process; namely, no long-range diffusion around the free volume takes place in spite of the decrease in the free volume. Saida et al, have reported that the icosahedral local structure exists in Zr-based ternary and binary metallic glasses, and they does not indicate significant change in local structure during nanoquasicrystallization.…”

Morphology of Spherulites in Rapidly Solidified Ni3Ge Droplets

“…Numerous further studies [21][22][23][24] have also observed the formation of nano-crystalline spherulites during the devitrification of both binary and multicomponent component metallic glass systems. In virtually all cases the spherulites are homogenously distributed within the amorphous matrix, with a radial growth originating from the centre of nucleation sites.…”

“…Yano et al [21] observed the formation of spherulites in a Zr 50 Cu 40 Al 10 bulk metallic glass forming alloy via TEM and positron annihilation lifetime measurements. Upon annealing at 773 K, crystallisation proceeded through the formation of spherical agglomerates of radially developing crystallites, about 600 nm in diameter.…”