Nucleation is the re-arrangement of a small number of atoms in the structure of a material leading to a new phase. According to the classical nucleation theory, a nucleus will grow if there is an energetically favourable balance between the stability of the newly formed structure and the energy costs associated to the formation of strains and new phase boundary. However, due to their atomic and dynamic nature, nucleation processes are difficult to observe and analyse experimentally. In this work, atomic mechanisms and thermodynamics of the homogeneous nucleation of BCC phase in FCC iron have been analysed by molecular dynamics simulations. The study shows that atomic system circumvents the high energy barrier for homogeneous nucleation that would occur according to the classical nucleation theory by opting for alternative, nonclassical nucleation processes, namely coalescence of subcritical clusters and stepwise nucleation. These observations show the potential of nonclassical nucleation mechanisms in metals.

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Molecular dynamics simulations are used to study the effects of tensile loading on nucleation and subsequent growth of bcc phase in pure fcc iron. The results show that orientation variant selection occurs during the stress-induced fcc-to-bcc transformation, which leads to the coalescence of neighbouring bcc platelets with identical orientation. The bcc phase nucleates mainly following Nishiyama–Wassermann and Kurdjumov–Sachs orientation relationships with the parent fcc phase. The present simulations contribute to a better understanding of mechanisms controlling mechanically induced martensitic transformation as well as coalescence of bcc platelets in steels.

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Molecular dynamics (MD) simulation has been used to study the martensitic transformation in iron at the atomic scale. The paper reviews the available interatomic interaction potentials for iron, which describe the properties of different phases present in that system. Cases on the fcc-to-bcc transformation in iron by MD simulations were included in the present paper. Factors affecting the fcc-to-bcc transformation in iron were analysed: (a) structural factors, such as grain/phase boundaries, grain sizes and stacking faults; (b) simulation conditions, such as the presence of free surfaces, external stress/strain and studied temperatures; (c) the interatomic interaction potential. The main emphasis of the present paper is on results giving insight on the mechanisms of the nucleation and growth of bcc phase in iron. This review was submitted as part of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.

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The aim of this PhD. thesis is to use molecular dynamics (MD) simulations to comprehend the mechanisms governing nucleation and growth of martensite phase in austenitic phases in iron. In view of this objective, the thesis is divided into five parts: Chapter 2 reviews previous investigations of the fcc-to-bcc transformation in iron by MD simulations; Chapter 3 performs a preliminary study of the nucleation and growth of bcc phase into fcc bulk in iron at existing bcc/fcc interfaces in the Nishiyama-Wassermann orientation relationship; Chapter 4 studies the mechanisms governing the growth of bcc phase at bcc/fcc interfaces in iron; Chapter 5 and 6 study the thermodynamics of the homogeneous and heterogeneous nucleation of bcc phase inside the fcc crystals, respectively; Chapter 7 illustrates the effects of external strain on the nucleation and growth of bcc phase in fcc iron with (and without) fcc/fcc grain boundaries. A more detailed description of each chapter are included below.@en

Molecular dynamics simulations have been used to study the effects of different orientation relationships between fcc and bcc phases on the bcc/fcc interfacial propagation in pure iron systems at 300 K. Three semi-coherent bcc/fcc interfaces have been investigated. In all the cases, results show that growth of the bcc phase starts in the areas of low potential energy and progresses into the areas of high potential energy at the original bcc/fcc interfaces. The phase transformation in areas of low potential energy is of a martensitic nature while that in the high potential energy areas involves occasional diffusional jumps of atoms.@en

Molecular dynamics simulations have been used to study the effect of fcc/bcc interfaces in the Nishiyama-Wasserman (N-W) orientation relationship on the fcc-to-bcc transformation at 300 K in pure iron. Simulations show the growth of the original bcc phase present in the initial configuration as well as the nucleation and growth of new bcc grains in the original fcc phase. During growth, heterogeneous and homogenous bcc nuclei both pin the propagation of the original bcc/fcc interface. In some locations, neighboring newly-nucleated bcc plates merge into a single bcc grain. The fcc phase transforms to bcc by a predominantly martensitic mechanism.@en