Thermal Expansion of Glass-Forming Zr-based Alloys in the Melt, the Undercooled Liquid and the Different Solid States
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Thermal expansion of glass-forming Zr-based alloys in the melt, the undercooled liquid and the different solid states
K. Samwer, B. Damaschke, Univ. Göttingen, I. Physikalisches Institut, Göttingen, Germany; M. Krause, P. Ryder, Univ. Bremen, Institut für Festkörperphysik, Bremen, Germany.
Abstract The thermal expansion coefficients of glass-forming Zr-based alloys were measured in the melt, the undercooled liquid and the glassy/crystalline state. Due to the high reactivity of the liquid material the experiments were performed containerlessly in an electrostatic levitator. We used an optical method where the samples were imaged with a high-resolution CCDcamera and the volume of the samples was evaluated by digital image processing. The coefficients of thermal expansion in the liquid and in the solid state could be determined from the volume versus temperature curves. The results can be compared with measurements in the electromagnetic levitation facility TEMPUS performed under microgravity conditions in the mission MSL-1 and ground based DMA-measurements. The thermal expansion data can be interpreted in terms of the free volume model.
Introduction The difference between glass forming metallic alloys and conventional metallic systems is the absence of long range order in the atomic arrangement. This peculiarity leads to a new class of materials with interesting physical and technical properties. These alloys can be prepared by rapid cooling of their melts where the critical cooling rate depends on the special character of the system. Some of the most prominent metallic glass formers are the Zr-based systems Zr-Cu-Al, Zr-Ni-Al [1] and Zr-Ti-Ni-Cu-Be [2]. The physical nature of the glass transition is not yet fully understood. A key question is whether there is a thermodynamical phase transition of the system or only a drastic change of the kinetics. Important model descriptions of the glass transition are for example the free volume model [3, 4] and the mode coupling theory [5]. As an input, the free volume model needs the detailed knowledge of the thermal properties of the system. The measurement of the thermal properties at the glass transition, in the undercooled state and in the stable melt is complicated because of the high reactivity of the materials at high temperatures. Further, the solidification is influenced by convective currents in the melt. Former experiments were made in the TEMPUS facility under reduced gravity. The TEMPUS facility [6] allowed investigations of metallic samples using electromagnetic levitation. Thermal properties such as temperature, specific heat, volume, viscosity, surface tension and electrical resistance could be measured using contactless methods in the mission MSL-1 on board the space orbiter Columbia [7]. The volume of the samples and the thermal expansion coefficients of Zr57Cu15.4Ni12.6Nb5Al10, Zr65Cu17.5Al7.5Ni10, Zr11Cu47Ti34Ni8, Zr60Cu18Al10Ni9Co3, Pd78Cu6Si16 and Pd82Si18 were measured using an optical method [8, 9]. Measurements were also performed with an electrostati
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