Effects of chemical composition on the structural relaxation in ternary Zr-Cu-Al bulk glassy alloys studied by EXAFS and
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Effects of chemical composition on the structural relaxation in ternary Zr-Cu-Al bulk glassy alloys studied by EXAFS and positron annihilation techniques. A Ishii1, S Mineno1, A Iwase1, Y Yokoyama2, T J. Konno2 and F Hori1 1 Department of Materials Science, Osaka Pref. Univ., 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan 2 Institute for Materials Research, Tohoku Univ., 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ABSTRACT Mechanical properties and thermal stability of bulk glassy alloys depend on their chemical composition ratios, although their detailed local structures especially around free volume have not been clarified yet. In order to know the origin of property dependence on alloy composition in Zr-Cu-Al ternary bulk glassy alloys in a view point of atomic scale, positron annihilation lifetime, coincidence Doppler broadening (CDB) and EXAFS (extended X-ray absorption fine structure) measurements have been employed for eutectic Zr50Cu40Al10 and hypoeutectic Zr60Cu30Al10 bulk glassy alloys before and after structural relaxation by annealing below glass transition temperature Tg. The result of CDB experiment, which represents the electron momentum distribution around free volume, shows that significant atomic reordering around free volume does not take place by the annealing in each alloy. Besides, CDB ratio profiles for each alloy suggest that the fraction of Zr atom around free volume does not match the chemical composition of each alloy system. Change in positron lifetime, which is proportional to the size of free volume, during annealing for hypoeutectic alloy almost remains unchanged. INTRODUCTION Bulk glassy alloys (BG alloys) undergo structural relaxation below glass transition temperature (Tg). Associated with the relaxation, many mechanical properties, such as fracture toughness [1], viscosity [2], mechanical ductility [3], Young’s modulus [3] and Charpy impact value [3], change. On the other hand, recently, Y. Yokoyama et al have reported no degradation of ductility and toughness for hypoeutectic Zr60Cu30Al10 BG alloy [3]. However, XRD pattern for Zr60Cu30Al10 BG alloy exhibits a broad halo pattern with no Bragg peaks as well as Zr50Cu40Al10 BG alloy, so that it is important to study the origin of the alloy composition dependence from a viewpoint of the free volume or local structure. Recently, some methods of analysis of free volume and local structure, such as positron annihilation spectroscopy [4-6], pair distribution function analysis [7] or XAFS measurements [8,9], have been employed. Positron annihilation lifetime can estimate the size of free volume, and positron annihilation coincidence Doppler broadening (CDB) measurement provides the information on the detailed electron distribution around the free volume. Therefore, the local atomic element analysis around free volume has become possible by this technique. So far, we have reported that the chemical composition around free volume for Zr50Cu40Al10 BG alloy does not present appreciable change during relaxation below Tg [4]. Besides,
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