On The Glass Forming Ability Criteria Of Bulk Metallic Glasses
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CC2.6.1
On The Glass Forming Ability Criteria Of Bulk Metallic Glasses Z. P. Lu and C. T. Liu Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6115 Abstract A new criterion γ, defined as Tx/(Tg+Tl) (where Tx is the onset crystallization temperature, Tg the glass transition temperature and Tl the liquidus temperature), has been proposed for glass formation in bulk metallic glasses (BMGs). The interrelationship between this new parameter and glass-forming ability (GFA) was elaborated and discussed in comparison with other established criteria. It was found that the new criterion γ had a much better correlation with GFA than other established parameters. An approximation of the critical cooling rate for glass formation was also formulated and evaluated in the light of this new parameter. Introduction Up to date, many criteria have been proposed to interpret why some alloy systems can be vitrified and others not, and of what determines the composition ranges over which glasses can be made [1-3]. Some of them have a good theoretical basis but have to employ rather crude estimates and approximations. Only few of them be can easily obtained on the basis of the characteristic temperatures measured by differential thermal analysis techniques, which include the reduced glass transition temperature Trg (=Tg/Tl) [4-5], supercooled liquid range ∆Txg (TxTg), Donald and Davies parameter ∆Tl* [=( Tlmix -Tl)/ Tlmix , Tlmix is the normalized melting point] [6], Hruby parameter Kgl (=(Tx-Tg)/(Tm-Tx) , Tm is the onset melting point) [7] and Saad and Poulain parameter S (=(Tp-Tx)(Tx-Tg)/Tg, Tp is the peak temperature of crystallization) [8]. Among these criteria, the most extensively used in bulk metallic glasses are the reduced glass transition temperature Trg and the supercooled liquid region ∆Txg. However, in some bulk glass neither Trg or ∆Txg is reliable enough to gauge the relative GFA [9-12]. Particularly, Thompson et al [13] found that both parameters failed to indicate the variation of glass forming ability in (Au100-y)Cuy)77Si9Ge14 (y=0~75) alloy system. Thus, it is necessary to seek a better and more precise criterion to reflect the GFA of BMGs. It is also of importance to determine the effectiveness of those established parameters serving as GFA gauge. In this paper, a new criterion has been proposed for glass formation in bulk metallic glasses, which can be expressed as γ=
Tx Tg + Tl
(1)
The correlation between this new parameter and GFA of BMGs will be presented and compared with those established criteria. The extent to which all established criteria can be used to represent the relative GFA in bulk glass forming alloys will also be examined. Results
CC2.6.2
Table 1 shows the glass transition temperature Tg, onset crystallization temperature Tx, peak temperature of crystallization Tp, onset melting temperature (solidus temperature) Tm and final melting temperature (liquidus temperature) Tl for representative based bulk metallic alloys. Since these temperatures are heating rate depen
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