Free Volume Relaxation Process in Zr 50 Cu 40 Al 10 Bulk Metallic Glass Studied by Positron Annihilation Techniques
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1048-Z08-19
Free Volume Relaxation Process in Zr50Cu40Al10 Bulk Metallic Glass Studied by Positron Annihilation Techniques Akito Ishii1, Fuminobu Hori1, Akihiro Iwase1, Yoshihiko Yokoyama2, and Toyohiko J Konno2 1 Department of Materials Science, Osaka Prefecture University, 1-1 Gakuen-cho,Nakaku, Sakai, 599-8531, Japan 2 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 9808577, Japan ABSTRACT Structural relaxation around free volume in Zr50Cu40Al10 bulk metallic glass (BMG) during isothermal annealing at 473, 573 and 673 K which are below glass transition temperature Tg =675 K have been investigated by positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements. The trend of change in positron lifetime, which corresponds to the size of free volume at each annealing temperature, has a good correlation with their density change. These annealing processes obey a stretched exponential relaxation function (KWW: Kohlrausch-Williams-Watts law). The relaxation time t0 and β, used as adjustable parameters in the KWW function, were determined at each temperature. These relaxation parameters depend on the annealing temperature, suggesting the distribution of activation energy for structural relaxation. Moreover, the profile of the electron momentum distribution around free volume derived from CDB spectra during annealing showed no appreciable change at each temperature. These facts suggest that long range chemical ordering, particularly around the free volume, dose not take place essentially. INTRODUCTION It is well known that the excess quenched-in free volume in the bulk metallic glass (BMG) affects their physical and mechanical properties, such as viscosity, atomic diffusivity and strength [1-3]. Zr-based bulk glassy alloy, which has a good glass forming ability, allows us to study the mechanism of structural relaxation during isothermal annealing. The structural relaxation below glass transition temperature (Tg) has been recognized to depend strongly upon change in the free volume. Nevertheless, the atomic behavior around the free volume during relaxation has not been clarified yet. According to the free volume theory [4], the origin of the relaxation for amorphous materials can be deduced from the change in the free volume so that the nature of the free volume is important. Positron annihilation spectroscopy (PAS) is a powerful tool to detect the vacancy type defects. This technique is possible to detect the free volume in the BMG [5]. Recently, C. Nagel et al. reported that the relaxation process of the free volume far below Tg in the Johnson glass (Zr46.7Ti8.3Cu7.5Ni10Be27.5) is well described by the stretched exponential function called Kohlrausch-Williams-Watts (KWW) function with an exponent β by positron annihilation measurements [6]. Gallino et al. also reported the enthalpy relaxation behavior, which can be fitted well with KWW function for Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG just below Tg [7]. On the other hand, G. J. Fan et al. reported that the
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