Relationship between glass transition temperature and Debye temperature in bulk metallic glasses
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The Debye temperature and glass transition temperature of a variety of bulk metallic glasses (BMGs) were determined by acoustic measurement and differential scanning calorimetry, respectively. The relationship between the Debye temperature and glass transition temperature of these BMGs was analyzed, and their observed correlation was interpreted in terms of the characteristics of the glass transition in BMGs.
A lot of efforts have been made to understand glass transition, which is ubiquitous phenomenon existing in almost all substances.1,2 The studies of glass transition mostly concern chainlike or network forming materials, such as polymer oxide glasses. Metallic glass, with dense random packed microstructure, is considered rather easily describable in contrast to nonmetallic glass. However, the glass transition in metallic alloys has received little attention until recently because for most alloys their supercooled liquid state is very unstable, and the glass transition can be realized only with a very high critical cooling rate (about 106 K/s). Recently, a variety of bulk metallic glasses (BMGs) exhibiting obvious glass transition and large super cooled liquid region were developed.3–8 These bulky glass-forming alloys are regarded as ideal model systems for study the glass transition and the nature of glass. The common features underlies all amorphization process, such as volume change and elastic softening of the lattice,9 have also been observed in the formation of BMGs.10 The softening phenomenon is similar to the melting process; the similarity implies the existence of intrinsic connection between Debye temperature D, melting temperature Tm, and glass transition temperature Tg.9,11 Tg depends on cooling rate and the process by which the glass is formed. However, in practice, the dependence of Tg on the cooling rate is weak (Tg changes by 3–5 K when the cooling rate changes by an order of magnitude3,12), and the transition scope is narrow, so Tg is an important material characteristic.12 D, which is another important characteristic of a material, represents the temperature at which nearly all modes of vibrations in a solid are excited. The Debye model was developed for crystal lattices; it seems inapplicable to the amorphous structure, yet the phenomena to be described in crystalline and amorphous alloys are very similar. For a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 12, Dec 2003
instance, the specific heat of the bulk metallic glasses and other glasses at low temperature follows the same path in temperature as that of their crystalline states.13 Under the circumstance, the metallic glasses can be treated as a monatomic lattice with an average cellular volume, by using acoustic data obtained by ultrasonic method D of the BMG can be obtained.14–16 The value of D from acoustic measurements agrees with that obtained by lowtemperature specific heat measurement.10,13 It expects a close correlation between Tg and D of the amorphous solids.17 There h
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