An atomistic study of the structural changes in a Zr–Cu–Ni–Al glass-forming liquid on vitrification monitored in-situ by X-ray diffraction and molecular dynamics simulation
Introduction
The high glass-forming ability (GFA) of some alloy compositions allowed production of bulk metallic glasses (BMGs) (also called glassy alloys) in the thickness range of 1–100 mm using various casting processes [1,2]. Zr-based BMGs are among the best metallic glass-formers known to date [[1], [2], [3]] which thermal stability is also good [4]. Formation of a glassy phase in both Zr–Cu and Zr–Ni alloys is strongly enhanced by the addition of Al thus forming ternary and quaternary bulk metallic glasses with good glass-forming ability [[1], [2], [3], [4], [5]]. The atomic structure of these BMGs at room temperature was studied in detail [6,7].
Liquids retain their volume as crystals but flow under the action of gravity [8,9]. They constantly undergo a restructuring of the atomic structure [10,11]. Above the equilibrium liquidus temperature (Tl) and slightly below it metallic melts show nearly Arrhenius-type temperature dependence of viscosity ():where η0 is a pre-exponential factor, R is the gas constant and Ea is an activation energy [12]. However, on cooling below the liquidus temperature, starting from a crossover temperature [13], they exhibit a non-Arrhenius temperature dependence on viscosity which is known as fragility of liquids [14,15].
Here one should mention that anomalous variation in a liquid viscosity in the vicinity of Tl and above it was observed for Fe- [16,17] and Zr- based glass-forming alloys [18] as well as for Al-based crystalline alloys [19]. The authors stated that liquid metallic alloys can have various structural states and significant overheating is required to dissolve clusters inherited from the solid state and to reach equilibrium liquid structure. The existence of liquid-liquid transitions in deeply supercooled state found for water [20], molecular liquids [21,22] (confirmed by computer simulation [23]) and even metallic glasses [24] should also be taken into account when describing fragility.
The inverse temperature - logarithmic viscosity plot [25,26] with some limitations [27,28] is a very useful illustration of the difference between so-called “strong” and “fragile” liquids [29,30]. Fragility is also related to the glass-forming ability of BMGs [31,32] (with some argumentation [33]) partly because stronger liquids, in general, have a higher viscosity in the entire temperature range from Tl down to Tg. There are also experimental results which indicate correlation between liquid fragility and vibrational properties of the glass below Tg [34]. The fragility index (m) [35,36] of the supercooled liquid is calculated slightly above the glass-transition temperature (Tg) as a derivative:m = dlog(η)/d(Tg/T)
Another way of representation of fragility is a famous Vogel-Fulcher-Tammann-Hesse equation [1]:η = η0exp[D*T0/(T-T0)]where η0, D (as an indicator of the fragility) and T0 are the fitting parameters. This equation is widely used but describes the temperature dependence of viscosity well only in an intermediate temperature region [37]. While some other equations make a better representation of the entire viscosity plot [38,39] they have a larger number of fitting parameters which physical meaning could be less clear.
On the other hand fragility can be expressed by the ratio of the activation energies for viscous flow in equation (1) at low (EL), (close to Tg) and high (EH) temperature region (slightly above Tl) [40,41]:RD = EL/EH
Other methods are also used to describe and separate thermodynamic and kinetic fragility [42].
Metallic glasses exhibit structural changes in the supercooled liquid state and such changes are found to be responsible for the liquid fragility. A relatively fragile Pd42·5Cu30Ni7·5P20 melt (its fragility index m is close to 60 [43]) during cooling was studied by using the real-space pair distribution function (G) (which is a reduced radial distribution function (RDF)) [44] and compared to the results obtained by the first-principles calculations. As a result strong correlation between the structural changes and the variation of viscosity was observed in the supercooled liquid [45]. The rate of structural rearrangements on cooling was enhanced in the supercooled liquid close to Tg. It was also demonstrated for Ni–Nb–Ta [46] and other [47,48] liquid alloys.
Structural changes, thermal expansion and volume changes (structural relaxation) in Zr-based metallic glasses upon heating [49,50] and cooling [51] were studied by synchrotron X-ray radiation diffraction and transmission electron microscopy [52,53]. Recently it was shown that even scanning tunneling microscopy allows achieving atomic scale resolution for metallic glasses [54].
Zr–Cu(Ni)–Al alloys are among the most studied bulk metallic glasses [55]. A detailed study on vitrification of a ternary Zr60Cu30Al10 alloy was performed recently in-situ by high energy synchrotron radiation X-ray diffraction from above the liquidus Tl to room temperature [56]. Short (SRO) and medium range order (MRO) develop significantly during cooling the liquid phase to the glassy state. Significant glassy structure and volume changes are found during severe plastic deformation [57] and rolling [58]. Structural changes in Cu-Zr [59,60] and Cu–Zr–Al glasses [61] were also studied by MD computer simulation.
Here we study the atomic structure changes in a relatively strong Zr55Cu30Ni5Al10 glass-forming liquid (one of the best glass-formers [62]) on vitrification and compare the results with those obtained for fragile metallic glasses. Its measured fragility parameter m for this liquid varies from about 45 [63,64] to 29 [65] but it is lower than that obtained for the Pd–Cu–Ni–P one studied earlier.
Section snippets
Experimental procedure
An ingot of the Zr55Cu30Ni5Al10 alloy (the composition is given in nominal atomic percentage) was prepared from pure metals of more than 99.9 mass% purity using the arc-melting method. Thermal stability of the samples was tested using a differential scanning calorimeter (DSC).
In-situ X-ray diffraction experiments were carried out at the European Synchrotron Radiation Facility (ESRF). The incident beam wavelength was 0.1245 Å (100 keV). Diffraction spectra were acquired in transmission mode by a
Results and discussion
The alloy sample was heated up, melted and then cooled down at about 85 K/s (average cooling rate) from 1335 K (Tl ~ 1150 K [68]) to 388 K and the alloy vitrified [72]. The measured intensity and the structure factor profiles obtained at two temperatures are shown in Fig. 1a. The glass-transition temperature measured by DSC at the heating rate of 0.67 K/s was about 680 K. This value is close to those found in the earlier works [68,73].
Three typical G(R) graphs are shown in Fig. 1b. Pair
Conclusion
According to the results, a strong chemical ordering forming Zr–Cu,Ni, Zr–Al and Zr–Zr atomic pairs takes place in the Zr55Cu30Ni5Al10 supercooled liquid alloy on cooling below the liquidus temperature. However, here the change in the Zr–Cu,Ni peak area to other peaks area ratio (APR and APRn) is smaller than in case of the Pd–Cu–Ni–P alloy studied earlier (Cu,Ni–P to other peaks) in accordance with a lower fragility index of the Zr55Cu30Ni5Al10 melt. It is concluded that fragility is a sign of
CRediT authorship contribution statement
D.V. Louzguine-Luzgin: Supervision, Formal analysis, Writing - review & editing. K. Georgarakis: Supervision, Data curation, Formal analysis, Writing - review & editing. J. Andrieux: Methodology, Data curation. L. Hennet: Supervision, Methodology, Writing - review & editing. T. Morishita: Formal analysis, Writing - review & editing. K. Nishio: Formal analysis, Writing - review & editing. R.V. Belosludov: Formal analysis, Writing - review & editing.
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This paper is dedicated to the memory of our wonderful colleague Alain Resa Yavari who started this research but passed away before it was completed. D.L. gratefully acknowledges financial support received from the World Premier International Research Center Initiative (WPI), MEXT, Japan. R.V.B. is grateful to the crew of Center for Computational Materials Science and E-IMR center at the Institute for Materials Research, Tohoku University, for continuous support.
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