Metastability at the nanometer scale
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Under constraints and at the nanometer scale, transitory metastable states can be generated in multicomponents materials. Examples illustrating such specific states are presented. They concern i) The crystalline nucleation in a growing undercooled liquid droplet formed from a liquid parent phase. ii) The suppression of intermetallic nucleation in solid solutions or glasses subjected to sharp concentration gradients.iii) The nanocrystalline transitory state preceding amorphisation by ball milling. In connection with this latter example, a thermodynamic model for the nanocrytal to glass transition, based on an hypothesis of a topological disorder wetting at the nanograin boundaries, is proposed. Thermodynamics
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Nucleation - Phase transitions - nanocrystals - metallic glasses
INTRODUCTION
A metastable phase is a phase which has attained the lowest possible Gibbs energy consistent with the accessible ensemble of configurations which can be covered continuously along trajectories in phase space.The thermodynamic metastability is characterized by the existence of an energy barrier which, temporarily, prevents the evolution towards a phase of lower Gibbs energy. The largest thermodynamic fluctuations taken among a distribution of probability are able to overcome such an energy barrier leading the system either to another metastable phase or to the equilibrium phase. A glass, for example, is confined to visit a limited number of configurations. The scanning of the whole accessible phase space becomes impossible in the range of a reasonable macroscopic time scale. The resulting non ergocity character of a glass [ 1 ] would formally prohibit to qualify this state as a thermodynamic metastable state. This is why thermally frozen phases as glasses where atomic mobility is comparable to the crystal one are often designated as "kinetically metastable" [ 2 1. The persistence of a metastable phase depends strongly on the atomic mobility. Consequently, the time of life may differ considerably from one metastable phases to another. It may extend from geological time scale as for oxide glasses to microscopic time scale for highly ephemeral transitory metastable phases. 209
Mat. Res. Soc. Symp. Proc. Vol. 398 01996 Materials Research Society
An example of an intermediate time scale for the time of life of a metastable phase has been given recently from solidification experiments of metal and alloy droplets performed in the Grenoble drop tube [ 3 ]. The ultra high vacuum (4.10 10 mbar) maintained in this tube has allowed to attain a high amount of undercooling of liquid refractory metals and some of their binary alloys [ 4 ]. The recalescence thermal pic is detected by silicon photodiodes.The temperature is deduced from measurements of the free fall time before recalescence and by application of the radiation cooling law. A double recalescence has been detected in pure tantalum droplets ( 5).Very recently, such a double recalescence has been found in Ta Re droplets 15 at%/Re (3). The temperature at the end of the first recalesc
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