Molekulardynamik-Simulationen von strukturellen Phasenumwandlungen in Festkörpern, Nanopartikeln und ultradünnen Filmen
In this work structural properties of bulk, nano-particles, and thin films were investigated by using molecular-dynamics simulations. The focus was on the investigation of martensitic transformations in those systems, mainly consisted of Fe and Ni. For the describtion of the interatomic forces the Embedded-Atom Method was used. The calculation of the free energy as a function of temperature gave insight into the thermodynamics of the system, and led to a correct interpretation of the structural transformation from a closed packed structure (face-centered-cubic, hexagonal-closed-packed) to the body-centered-cubic structure and vice versa. Pre-existing lattice defects turned out to be the dominant factor for the martensitic transformation at low temperatures, whereby the austenitic transformation at high temperatures is less affected by defects. The explanation of the different behavior of the martensitc transformation process and the austenitic transformation process could be given by a detailed examination of the free energy along the Bain-path. The study of the crystallographic orientational relationships of the austenitic and martensitic phases gave insight into the transformation process on the atomic-scale. The transformation process observed in the molecular-dynamics simulations can be described in terms of the Wechsler-Lieberman-Read theory as a combination of Bain-transformation, rotation, and lattice invariant shear due to stacking faults and twinning. Simulations of very large supercells containing up to eight million atoms facilitated the study of the homogeneous and heterogeneus nucleation process of structural phase transitions within the solid state. Molecular-dynamics simulations of shock-induced austenitic transformations gave valueable insight into the grain-boundary dynamics of the developing austenitic grains. The heterogeneous nucleation process at different types of defects in nano-particles was studied. The burst-type growth of the martensitc phase starts at pre-existing defects with further growth of the twinned martensitic phase into the austenite-matrix. With decreasing size of the nano-particle, transition temperatures decreased as revealed in the few experiments that exist. In the framework of the used Embedded-Atom Method-potential, this effect is due to the different surface energies of the austenitic and the martensitic phases. The interplay between the structure of films and an underlying substrate was intensively studied for the well known Fe on Cu-system. Experimental observations, like the increasing tendency for a structural transformation from the face-centered-cubic structure to the body-centered-cubic structure with increasing film thickness and decreasing temperature, were confirmed. Simulations of the growth process gave insight into recently performed experiments of the dependence of the structural stability of face-centered-cubic Fe-films on Cu(111)-substrates as a function of the deposition process like thermal deposition and pulsed laser deposition.
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