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材料的界面與表面對相變化與塑性變形的理論研究

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The interface is a region in which two different phases are in contact with eachother. For example, it can be formed between two grains of the same material with different crystallographic orientations (grain boundary) or between twophases of the same material (inter-phase boundary). Surfaces can b

e classifiedas a particular type of interface between the solid and air. The type of interfacenot only influences the properties but also controls the transformation betweentwo phases. Also, the large surface area to volume ratio in nanomaterials suchas nanowires and nanorods is responsible for thei

r exceptional mechanical, electronic, and optical properties. This study is concerned with (a) role of interfaces inaustenite (γ) to ferrite/martensite (α) transformation in iron and (b) deformationbehavior of single and multi-component high entropy alloy (HEA) nanowires.This thesis focuses on explo

ring the role of interfaces during interface-controlledphase transformations using atomistic simulations. First, we compare the transformation mechanisms for the flat and ledged interface using an embedded atom method (EAM) potential. After that, we have explored the role of disconnectionson interfa

ce velocity and mobility for the ledged interface. At last, we study thedeformation behavior of Ag nanowires and CoCrFeMnNi HEA nanowires andexplore the synergistic sequence of the mechanisms responsible for their uniquedamage tolerance and other mechanical properties.The thesis begins with a genera

l introduction to solid-solid phase transformation related to iron systems in Chapter 1. We begin our discussion with a briefdescription of different types of solid-solid phase transformation, associated interface structure, and orientation relationships. This is followed by the review ofsome previo

us works related to the FCC-BCC phase transformation in iron. Atlast, we discuss the nanowires and their mechanical properties.Chapter 2, discusses different tools to simulate phase transformation, deformation behavior, and related material properties. We briefly introduce variousconcepts of molecul

ar dynamics (MD) simulation and density functional theory(DFT). We also discuss the interatomic potentials such as EAM and MEAM usedin the current study.In chapter 3, using MD and DFT based ab initio calculations, we determine thethermodynamic properties required for iron phase transformation and na

nowires’deformation behavior. We discuss calculating several thermodynamic properties,like the lattice parameter, enthalpy, melting temperature, Gibbs free energy, andstacking fault energy. These properties are in good agreement with the existing experimental and first-principle studies, which valid

ates the accuracy of thepotential used to describe the inter-atomic interactions. We calculate the stacking fault energy of the different elements using DFT-based ab initio calculations.We obtain the unstable stacking fault (USF), intrinsic stacking fault (ISF), unstable twinning fault (UTF), and ex

trinsic stacking fault (ESF) for all the given elements and demonstrate their respective generalized stacking fault energy (GSFE)curves. We use the approach used by Kibey et al. [‡] to get the input structuresfor different fault configurations.Chapter 4, shows how the interface morphology affects th

e phase transformation in iron by running MD simulations for the flat BCC-FCC interface in whichthe two phases are joined according to Nishiyama–Wasserman orientation relationship vs. a ledged interface having steps similar to the vicinal surface at different temperatures. We also characterize the a

tomic matching pattern, dislocationnetwork, and respective line and Burgers vector directions at the interface with the help of common neighbor analysis and Nye tensor analysis (NTA) for both theinterfaces. We identify the atomic displacements and the misfit dislocation network at the interface lead

ing to the phase transformation. Atomic structures ofthe inter-phase boundary and displacements leading to the phase transformationare also uncovered. Interestingly, interface mobility is found to follow Arrheniuslaw in case of ledged interfaces, while exactly opposite behavior is observed incase of

flat interfaces. We also demonstrate the role of structural ledges or stepsaffecting interface motion at the inter-phase boundary.Chapter 5, investigates the role of disconnections during the austenite to ferrite transformation in pure-Fe, using classical molecular dynamics simulations.We first cre

ate BCC-FCC-BCC interfaces based on Nishiyama–Wasserman orientation relationship and its derivatives. By rotating the FCC crystal, we vary thenumber of disconnections at the adjoining BCC-FCC interfaces. We find that thedisconnections present at the interphase boundary assist in growth of the ferrit

ephase. Small interface velocities (1.19–4.67 m/s) suggest a phase change via massive transformation mechanism. Boundary mobilities obtained in a temperaturerange of 1000 to 1400 K show an Arrhenius behavior, with activation energiesranging from 30–40 kJ/mol. Our study clearly shows that the disconn

ections located at the austenite-ferrite interface facilitate the growth of the α-Fe phase.In chapter 6, we study the deformation behavior of single element Ag nanowiresand CoCrFeMnNi HEA nanowires. We show that deformation mechanism is dependent on dislocation nucleation and propagation for both th

e nanowires. Thesimulation is carried out at a cryogenic temperature, room temperature, and elevated temperatures. Due to high surface energy at cryogenic temperatures, single element Ag nanowires transform into a more preferred phase via nucleationand propagation of partial dislocation across the n

anowire enabling superplasticity. In high entropy alloy CoNiCrFeMn nanowires, the motion of the partialdislocation is hindered by the friction due to the difference in the lattice parameter of the constituent atoms, which is responsible for the hardening and lowering the ductility. We demonstrate th

e temperature-dependent superplasticityand strengthening in both the nanowires. Interestingly, HEA nanowires can perform exceptional strength-ductility trade-offs at cryogenic temperatures. Evenat high temperatures, HEA nanowires can maintain good flow stress and ductility before failure. Mechanical

properties of HEA nanowires are better thanAg nanowires due to synergistic interactions of deformation twinning, FCC-HCPphase transformation, and the special reorientation of the cross-section. Furtherexamination reveals that simultaneous activation of twining-induced plasticity and transformation-

induced plasticity is responsible for the plasticity at differentstages and temperatures. The contribution of stacking fault energy in identifying deformation mechanisms is also discussed. These findings are beneficial fordesigning nanowires at different temperatures with high stability and superior

mechanical properties in the semiconductor industry.Finally, we summarize the main findings of our work in chapter 7, followedby a discussion of the future scope.