Glass Formation and Glass Forming Ability of Alloys Near Eutectic Composition
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Glass formation and glass forming ability of alloys near eutectic composition Y. Li Department of Materials Science, Science Faculty, National University of Singapore, Singapore 119260 Abstract Onset temperature, Tm and offset temperature (liquidus) Tl of melting of a series of bulk glass forming alloys based on La, Mg, and Pd have been measured by studying systematically the melting behaviour of these alloys using DTA or DSC. Bulk metallic glass formation has been found to be most effective at or near their eutectic points and less effective for off-eutectic alloys. Reduced glass transition temperature Trg given by Tg/Tl is found to show a stronger correlation with critical cooling rate or critical section thickness for glass formation than Trg given by Tg/Tm. INTRODUCTION Glass formation will occur if the alloy is cooled fast enough from the liquidus temperature Tl to a temperature below glass transition temperature (Tg) so that the crystallization can be suppressed and is avoided. Glass forming ability can be represented by many parameters [1]. The reduced glass transition temperature, Trg defined below is one of the widely used indicators of glass forming ability of alloys, Tg (1) Trg = Tl As the alloy concentration increases, Tg generally has a weak dependence on composition and while Tl often decreases more strongly. Thus, the interval between Tl and Tg generally decreases and the value of Trg increases with increasing alloying concentration so that the probability of being able to cool through the interval between Tl and Tg without crystallization is enhanced. i.e. the glass forming ability is increased [2]. Accordingly, alloy systems for which the glass forming ability is higher, are those with a deep eutectic or low lying Tl compared with the melting points of the host metals, thus leading to high Trg [3]. From this analysis it can be seen that it is the liquidus temperature Tl that is important, not the onset melting point Tm or the eutectic point Te, since these latter usually do not vary very much near a eutectic composition as is also the case for Tg. There are many reported values of Trg in the literature [4] , but unfortunately most of them were calculated using Tg/Tm with minimal report of Tg/Tl [4,5]. Here we summarize our recent studies on the determination of reduced glass transition temperature Trg of bulk metallic glasses. The significance of Trg based on Tg/Tm or Tg/Tl and the correlation between resulting Trg and the critical cooling rate for glass formation and glass forming ability of the bulk glass forming alloys is discussed. The preliminary results of forming new bulk metallic glasses based on the criteria of high reduced glass transition temperature are also reported.
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EXPERIMENTAL DETAILS All the alloys were produced either by arc-melting or by induction melting mixtures of constituent pure elements under an argon atmosphere. Pieces of resulting ingots were sealed in quartz tubings with external diameter of 3 mm to minimize the oxidation during the DTA melting process. These tu
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