Structural Relaxation of La 55 Al 25 Ni 10 Cu 10 Bulk Metallic Glass
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INTRODUCTION
BULK
metallic glass (BMG) systems[1–3] exhibit outstanding stability against crystallization. One can study their physical and structural properties at temperatures even exceeding the glass transition temperature Tg. The as-prepared BMG, which is usually in nonequilibrium, will relax to a more stable state. The relaxation process is important for BMG not only because the time-dependent variation of the physical properties of BMG, such as mechanical[4] and diffusion properties,[5] have to be considered in application, but also because one can obtain more information on the atomic-scale dynamics of their strongly temperaturedependent properties around Tg during this process. In order to understand the glass transition and the relaxation process of glassy solids, various theories have been proposed, such as the mode coupling theory,[6] energy landscape model,[7] and free volume model,[8] which satisfactorily answered one or some of the ten leading questions summarized by Angell.[9] Relaxation functions have been developed to describe the structural relaxation of a glass.[10] Ngai[11] found the nonexponential nature of the structural relaxation governing the dynamic properties of the glass-forming substances. This stretched exponential (Kohlrausch) relaxation function has been employed in both Pd-based BMGs and amorphous polymer.[12–14] La-based BMGs[15,16] are among those systems with large glass forming ability (critical cooling rate less than 30 K/s) and broad super cooled liquid range (~80 K), suggesting a good thermal stability above Tg. This makes it suitable for research TAO ZHANG, Graduate Student, F. YE and J.P. LIN, Professors, and Y.L. WANG, Associate Professor, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 100083 Beijing, PeopleÕs Republic of China. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online January 23, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
around Tg. Lu et al.[17] made thermodynamic analyses on La-based BMGs. Yet, little research has been done on the isothermal relaxation process of La-based BMG. In this article, we study the enthalpy relaxation of La55Al25Ni10Cu10 BMG using a stretched exponential relaxation function by isothermal annealing experiments well below the glass transition temperature. Low activation energy was derived from the structural relaxation process. The equilibrium free volume concentration is also calculated using the parameters D* and T0, obtained from the heating rate dependence of glass transition temperature.
II.
EXPERIMENTAL
We prepared the La55Al25Ni10Cu10 BMG sample by melt casting. The elements of purity ranging from 99.5 to 99.9 pct were mixed and arc melted in a Ti-gettered Ar atmosphere. After
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