Effects of open-volume regions on relaxation time scales and fracture behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass
Introduction
Metallic glasses are essentially configurationally frozen liquids that exhibit various structural disorders. Among them, atomic scale open-volume regions, or so-called free volume, have been shown to play an important role in a wide range of physical and mechanical properties [1]. These include density, enthalpy, electrical resistivity, internal friction, diffusivity, viscosity, elastic modulus and ductility [1], [2], [3]. The most notable effects occur on transport properties such as viscosity and diffusivity, and resistance to crack extension including ductility, ductile to brittle temperature and fracture toughness.
Positron annihilation spectroscopy (PAS) is an ideal technique for the examination of atomic-scale open-volume regions in materials [4], [5]. The technique is based on the tendency of positrons to become localized at open-volume defects such as vacancies, vacancy agglomerates, voids and dislocations before they annihilate with electrons producing annihilation parameters characteristic of the defect site. By examining these annihilation characteristics, detailed information on the defect such as its nature, size, concentration and chemical environment can be revealed [6], [7].
In the present study, the effects of atomic-scale open-volume regions on the relaxation time scale for viscous flow and fracture behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass were investigated. The atomic-scale open-volume regions were changed by annealing at a temperature below the glass transition temperature. PAS was used to examine changes in open-volume regions as a result of annealing. The objective of the present paper is to understand the interrelationship between atomic-scale open-volume regions and macroscopic properties such as viscous flow and fracture toughness by systematically examining the effects of metallic glass structure on flow and fracture processes in a Zr-based bulk metallic glass.
Section snippets
Experimental procedure
The material selected for the study was a commercial-grade Zr41.25Ti13.75Ni10Cu12.5Be22.5 (at.%) plate that was vitrified through water quenching. Annealing was performed on specimens machined from the plate for selected times at 300 °C. The annealing temperature and times were carefully chosen so that decomposition and subsequent nanocrystallization did not occur as a result of the annealing treatment.
Atomic-scale open-volume regions of the metallic glass structure were examined using PAS.
Results
The effect of annealing at 300 °C on KIC is shown in Fig. 1. As shown in the figure, the plane strain fracture toughness was significantly degraded with annealing. The KIC value was degraded by ∼50% only after 0.75 h of annealing and decreased to 2 MPa√m after 30 h of annealing. Scanning electron microscope (SEM) examination of resulting fracture surfaces revealed a complete change of fracture morphology consistent with the degradation of crack extension resistance. While the as-received
Discussion
Previously reported studies by the authors on relaxation time scales over a wide range of temperatures showed that the high temperature process that occurs above Tg is the viscoelastic relaxation associated with a large-scale cooperative motion of groups of atoms which ultimately leads to viscous flow [9]. As shown in Fig. 6, such relaxation time scales were significantly increased with annealing time at elevated temperatures indicating retarded atomic arrangement processes for viscous flow.
Conclusions
The plane strain fracture toughness of a Zr–Ti–Ni–Cu–Be bulk metallic glass was found to be significantly degraded after annealing at 300 °C. The fracture morphology was changed from characteristic vein patterns to cleavage-like features with little evidence of plasticity. PAS results revealed that atomic scale open-volume regions were reduced by annealing. A dynamic mechanical analysis showed that the atomic arrangement process for viscous flow was retarded as a result of annealing. The
Acknowledgments
This work was supported by the MRSEC program of the National Science Foundation under award no. DMR-0080065. Materials were provided by Howmet Research Corporation. The PAS experiments were performed at Lawrence Livermore National Laboratory by Drs P. Asoka-Kumar, P.A. Sterne and R.H. Howell. One of the authors (D.S.) is grateful to the Kimball Stanford Graduate Fellowship for the financial support.
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