Glass-Forming Ability and Crystallization Behavior in High-Density Bulk Metallic Glasses
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Glass-Forming Ability and Crystallization Behavior in High-Density Bulk Metallic Glasses Laszlo J. Kecskes, Samuel F. Trevino, and Robert H. Woodman U.S. Army Research Laboratory Aberdeen Proving Ground, MD 21005-5069 USA ABSTRACT Alloys of composition (Hfx,Zr1-x)57Ti5Cu20Ni8Al10 and (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10, where x = 0, 0.2, 0.4. 0.6, 0.8, and 1.0, were evaluated for glass-forming ability, and the devitrification behavior on heating of the (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10 glasses was examined. Glass-forming ability was determined by suction casting 3-mm-diameter rods, followed by neutron diffraction examination. Results show that substitution of Hf for Zr in these alloys degrades glass-forming ability, and that this effect is more pronounced in compositions of the (Hfx,Zr1-x)57Ti5Cu20Ni8Al10 type. To examine devitrification behavior, thermal analysis was used to identify temperatures at which exothermic events occurred on heating in the (Hfx,Zr1-x)52.5Ti5Cu17.9Ni14.6Al10 series. The amorphous alloys were then subjected to annealing treatments corresponding to the exothermic events. Crystal structures formed during annealing were probed by neutron diffraction, and the size scale of features developed examined by smallangle neutron scattering. Results show that the initial decomposition produces a structure with a characteristic length scale, but it is not yet possible to comment on the mechanism. INTRODUCTION Among the properties of bulk amorphous alloys which are of interest for engineering applications is mechanical failure by localized shear [1]. In applications where this property is useful, it would be advantageous to have as dense a glass as possible, as well as a glass that can be formed with a critical dimension of 15 mm or larger [2]. Failure by adiabatic shear in amorphous alloys which can be formed in bulk was reported in systems where Zr was the primary constituent in the alloy [1]. These glasses have densities of ~6.7 g/cm3 [3]. In an effort to produce a glass of increased density, a series of alloys substituting varying amounts of Hf for Zr has been prepared. The resulting alloys have densities ranging from 6.7 to 11.1 g/cm3 [3]. Determination of critical casting thickness is an established method of assessing the glassforming ability of an alloy [4]. In a similar manner, demonstrating that an alloy can be cast to millimeter dimensions has also been used [5]. Amorphous structure has long been verified by diffraction techniques [4,6], and by evidence of crystallization events (exotherms) during thermal analysis [4,7]. Small-angle neutron scattering (SANS) has been shown to provide information about the length scale of features formed during phase transformations [8]. To gain further insight into the stability of Hf-based glasses relative to those that are Zr-based, selected samples of glass were annealed at temperatures corresponding to their exothermic events. Following annealing, we examined them using neutron diffraction and SANS.
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EXPERIMENT Ingots of (Hfx,Zr1-x)52.5T
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