بررسي ترموديناميكي پايداري فازي در آلومينيوم نانو ساختار با استفاده از شبيه سازي ديناميك مولكولي

چكيده انگليسي :

Thermodynamic Study of Phase Stability in Nanostructured Al by Molecular Dynamics Simulation Zahra Noori z noori@ma iut ac ir Date of submission October 29 2012 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 8311 Iran Degree M Sc Language Farsi Supervisors Masoud Panjepour panjepour@cc iut ac ir Mehdi Ahmadian ahmadian@cc iut ac ir Abstract Nanocrystalline materials compared to conventional coarse grained polycrystalline materials exhibit unique behavior and properties such as melting in lower temperatures The main source of this difference in behavior with respect to conventional coarse grained polycrystalline materials is high volume fraction of grain boundaries in nanocrystalline materials Since this boundary regions have different structure and energy states with respect to grains the properties of nanocrystalline materials can not be accurately describe by classical relationships which certainly have been applied for coarse grained polycrystalline materials Howevere in this study using the thermodynamic theory models for nanocrystalline materials including equation of state EOS quasi debye approximation QDA and Song model the behavior of the phase stability of nanostructured materials was discussed But the obtained results suggested that these models can not correctely predict the melting temperature of the nanocrystalline materials Although this models were widely used in studying of themodynamic properties of nanocrystalline materials but so far has not checked the accuracy of their relationships in terms of thermodynamic principles Hence in continue the basis of thermodynamic relations was evaluated and errors in these models were identified The most important of these errors can be assumed to be suspicious of negative pressure of grain boundaries incorrect terms dependent on the temperature the theory relations of the heat capacity and some simplifying approximations These errors indicated that the description of the thermodynamic behavior of nanocrystalline materials as an ambiguous point requires further study Infact to study the melting behavior of nanocrystalline aluminum as an metallic example molecular dynamics simulations were used Simulation results showed that for the nanocrystallines with average grain size 14 nm and higher melting temperature is constant and the same as melting temperature of coarse grained polycrystalline But as the average grain size decreases the melting temperature reduces so that for the nanocrystalline aluminum with average grain size 2 nm the melting point than the melting point of coarse grained polycrystalline is 30 down Moreover by further reducing the grain size to about three times the lattice parameter values for aluminum the melting temperature is same as the melting temperature of amorphous phase Also it was observed that by reducing the grain size of the nanocrystalline samples the latent heat of melting was reduced Since this behavior appears to be related to the volume fraction of grain boundaries in nanocrystalline materials in continue using the outward layering method the atoms in different regions of the nanocrystalline samples have been separated and melting behavior in different regions of grain boundaries separately was studied Results showed that disorders in grain boundary regions create a liquidlike structure at temperatures lower than the melting temperature of nanocrystalline and thus premelting of this regions Snapshot images of nanocrystalline samples simulated at different temperatures indicate that the solid state structural disorders in the boundary regions has led to localized melting It was found that in the nanostructured materials the melting process begins from the vertex nodes VN and triple junctions TJ discontinuously and with expanding into the grain boundaries GB and grains the melting of nanocrystalline is completed Therefore it can be concluded that the molecular dynamics simulations as well as a powerful

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بررسي ترموديناميكي پايداري فازي در آلومينيوم نانو ساختار با استفاده از شبيه سازي ديناميك مولكولي

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