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New materials: metallic glasses

last modified Mar 23, 2016 12:27 PM
Interatomic repulsion softness directly controls the fragility of supercooled metallic melts
New materials: metallic glasses

Dr Alessio Zaccone

Researchers have developed a theoretical model which demonstrates the effect of alloy composition in liquid metals on their flow and mechanical properties. The results are published in the current early edition of the journal PNAS, Proceedings of the National Academy of Sciences of the United States of America.

By mixing different metallic elements together at high temperatures, new materials called metallic glasses can be formed by rapidly quenching the liquid to room temperatures. This technology enables the formation of solid materials with a disordered atomic-level structure (thus similar to glasses) with outstanding mechanical properties. For example, metallic glasses are as stiff as steel but they can sustain much larger loads prior to yielding, a key property which is going to revolutionize several fields of engineering where materials that can sustain large loads under extreme conditions are required. 

The viscosity of these metallic alloys in the liquid state is the crucial parameter to control the solidification process (glass transition) to form the new materials. At the glass transition, the viscosity increases by many orders of magnitude within a narrow range of temperature. This phenomenon has puzzled scientists for decades and no first-principle theory is able to capture this dramatic increase of viscosity.

By considering peculiar motions in disordered structures, called nonaffine motions, which are absent in perfect crystals, Dr Alessio Zaccone (head of the Statistical Physics Group in our Department) and his PhD student Johannes Krausser have been able to relate the effective interatomic repulsion of the alloy, which sensibly depends on the elemental composition, to the viscosity and elastic modulus close to the glass transition. The new theory predicts a double-exponential dependence of the viscosity on temperature in perfect agreement with experimental data obtained at the University of Goettingen, in Germany (as shown in Fig.1).

Fig1: viscosity as a function of temperature for various metallic alloys; symbols are experimental data, continuous line are theoretical fits.

Previous theories such as e.g. the Mode-Coupling Theory of the glass transition, predict power-law dependence of the viscosity which cannot capture the data. Furthermore, the theory can be used to extract or back-engineer the effective interatomic potential in complex metallic alloys with many components from experimental measurements of viscosity and shear modulus (using ultrasound techniques), see Fig.2.

Fig2: interatomic effective potential estimated from the theoretical fits of viscosity as a function of temperature. The total potential (continuous line) is the sum of a Thomas-Fermi component (dashed-dotted line) representing the ion-ion Coulomb repulsion screened by the electron gas, and of a Born-Mayer component (dashed line) stemming from the Pauli repulsion of overlapping electron shells.

Using this framework it will now be possible to improve the control over the flow and mechanical properties of these advanced materials, which is still a bottleneck for the scale-up of their industrial production.

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