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Atomistic Simulations For Thermodynamic Properties Of Nanostructured Metals And Alloys

Posted on:2008-02-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:S F XiaoFull Text:PDF
GTID:1101360215979776Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
In the present thesis, with the analytic embedded atom method (EAM), thethermodynamic properties, such as melting behavior, thermal expansion, atomicthermal vibration and atomic self-diffusion of typical nanostructured metals andalloys are studied systematically. Some of the results have a good agreement withexperimental data and theoretical predictions.According to the theory that a stable system has a minimum energy, themean lattice contraction of nanoparticles have been calculated with a static method.It is found that the lattice contraction ratio decreases linearly with the reciprocalof particle size. The size and temperature effects on lattice distortion of Ag andPt nanoparticles have been investigated in terms of atomic mean bond length us-ing molecular dynamics simulations. The average values of lattice contractionover the whole system are larger than that of the experimental data, and the av-erage value of lattice contraction in the inner core has a better agreement withthe experiment results. This phenomenon is mainly resulted by inhomogeneouslattice distortion. The surface distortion and the size effect on the inner core dis-tortion are remarkable. As the grain size increases to a certain degree, the inho-mogeneous surface lattice distortion is mainly localized to the outer shell with athickness of 2-3 lattice parameters.Molecular dynamics has been performed to study the melting evolution,atomic diffusion and vibrational behavior of BCC metal vanadium nanoparticleswith number of atoms ranging from 537 to 28475 (diameters around 2-9 nm). Theresults reveal that the melting temperature of nanoparticle is in inverse propor-tion to the reciprocal of nanoparticle size linearly, and in good agreement with theprediction of thermodynamic liquid-drop model. For the vanadium nanoparticle,the melting process can be described as two stages, firstly the stepwise premelt-ing on the surface layer with a thickness of 2-3 perfect lattice constant, and thenthe abrupt overall melting of the whole cluster. The heat of fusion of nanoparticleis also in linearly inverse proportion to the reciprocal of nanoparticle size. Thediffusion is mainly localized to the surface layer at low temperature and increasewith the reduction of nanoparticle size at the same temperature. The radial meansquare vibration amplitude (RMSVA) is developed to study the anharmonic effect on surface shells. The simulation of solidification of nanodroplets indicates thatthe effective supercooled degree decreases with the reduction of droplet size. Thenucleation in the process of solidification occurs on the surface layer of droplet.Affected by close packed structure and surface effect, the small droplets of FCCmetals exhibit structural characteristic of icosahedron after solidification.The thermal expansion of nanocrystalline Ag is investigated by calculatingatomic Voronoi volume. Affected by grain boundary structure, the atoms ongrain boundary exhibit excess volume against atoms in the interior of grain. Thethermal expansion of grains in nanocrystal is slightly higher than that of a perfectlattice, while the thermal expansion of grain boundaries in nanocrystal is lowerthan that of a perfect lattice. The thermal expansion of nanocrystals is almostindependent on mean grain size.The molecular dynamics simulations have been performed to study the vi-brational properties of nanocrystalline nickels. The obtained results reveal thatthe similar phonon softening as in the perfect lattice mainly focuses on grainsin nanocrystalline materials, and the partial vibrational density of states of grainboundary phase is almost insensitive to temperature from 300 K to 900 K, espe-cially within the range of low frequency. The nanocrystalline materials have ahigher specific heat and vibrational entropy, and lower vibrational free energyrelative to the conventional crystals. Supposing the nanocrystalline material canbe treated as a composite constituted by grain and grain boundary (GB) phases,the vibrational thermodynamic properties can be well determined from the pro-portion of GBs and the corresponding thermodynamic properties of grain andGB phases. The Debye temperature of nanocrystalline materials is quantitativelylower than that of the conventional crystals and decreases with the reduction ofmean grain size.In the atomic scale, the melting behaviors of nanocrystalline Ag with themean grain size ranging from 1.21 to 12.12 nm have been investigated with themolecular dynamics simulations, and a method to determine the melting temper-atures of the infinite polycrystalline nanostructured materials are presented. It isfound that the melting in the nanostructured polycrystals starts from their grainboundaries, and the relative numbers of the three typical bonded-pairs, (1551),(1431) and (1541) existing in the liquid phase increase rapidly with the evolve-ment of melting. The grain size variation of melting temperature exhibits twocharacteristic regions. As mean grain size above about 4 nm for Ag, the melt- ing temperatures decrease with decreasing grain size, and it can be estimatedfrom the size dependent melting temperature of the corresponding nanoparti-cles. However, with grain size further shrinking, the melting temperatures almostkeep a constant. This is because the dominant factor on the melting temperatureof nanocrystal shifts from grain phase to grain boundary. By extrapolating themean grain size of nanocrystal to an infinitesimal value, an amorphous phasehas been obtained from the Voronoi construction. As a result of fundamentaldifference in structure, the amorphous phase has a much lower solid-to-liquidtransformation temperature than that of nanocrystal.Molecular dynamics simulations have been performed to obtain the atomic-scale details of crystallization from supercooled liquid. The radial distributionfunction and common neighbor analysis provide a visible scenario of structuralevolution in the process of phase transition. The crystallization from supercooledliquid is characterized by three characteristic stages: nucleation, rapid growth ofnucleus and slow structural relaxation. The homogeneous nucleation occurs at alarger supercooling temperature, which has an important effect on the process ofcrystallization and the subsequent crystalline texture. The kinetics of transitionfrom liquid to solid is well described by Johnson-Mehl-Avrami equation.Isothermal grain growth behaviors, including the effect of temperature andmean grain size, of nanocrystalline Ag are investigated using the molecular dy-namics simulations. The small grain size and high temperature accelerate thegrain growth, it is the same as the conventional polycrystalline materials. Thegrain growth processes of nanocrystalline Ag are well characterized by an expo-nent growth curve, followed by a linear relaxation stage. Beside grain boundarymigration and grain rotation mechanisms, the dislocations (or stacking faults)sever as the intermediate role in the grain growth process.The surface and size effects on the alloying ability and phase stability of im-miscible alloy nanoparticles have been studied with calculating the heats of for-mation of Au-Pt alloy nanoparticles from the single element nanoparticles of theirconstituents (Au and Pt) with a simple thermodynamic model and an analyticembedded atom method. The results indicated that, besides the similar composi-tional dependence of heat of formation as in bulk alloys, the heat of formation ofalloy nanoparticles exhibits notable size-dependence, and there exists a competi-tion between size effect and compositional effect on the heat of formation of im-miscible system. Contrary to the positive heat of formation for bulk-immiscible alloys, a negative heat of formation may be obtained for the alloy nanoparticleswith a small size or dilute solute component, which implies a promotion of thealloying ability and phase stability of immiscible system on a nanoscale. Thesurface segregation results in an extension of the size range of particles with anegative heat of formation. The molecular dynamics simulations have indicatedthat the structurally and compositionally homogeneous AuPt nanoparticles tendto form a core-shell structure with temperature increasing. The melting tempera-ture of FeAl alloy nanoparticles decreases linearly with the reciprocal of particlesize. In the solidification of alloy nanodroplets, the alloy component has an im-portant effect on the process of solidification and final structure.Only by fitting the bulk properties of metals, the analytic EAM can predictthermodynamical properties of nanostructured materials without any parameterabout nanostructure. This work is helpful to establish the theory of materials de-sign in the atomic scale systematically.
Keywords/Search Tags:EAM, Nanoparticle, Nanocrystalline, Thermal expansion, Melting temperature, Diffusion coefficient, Grain growth, Molecular dynamics
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