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Deformation-free nanotwin formation in zirconium and titanium

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Molecular dynamics simulations of single crystal zirconium fracture were performed to study the deformation mechanisms active on the basal and prismatic planes. The effects of temperature (0 to 300 K) and strain rate (10⁸–10¹⁰ s⁻¹) were investigated. Crack tip orientation was found to strongly affect the fracture behaviour. On the basal plane twinning ({112̄1}<11̄26>) and emission of <c+a> type dislocations that then dissociated into partial dislocations around pyramidal I2 stacking faults were seen to occur during fracture. At higher strain rates (10⁹ and 10¹⁰ s⁻¹), twinning occurred. The emission of edge dislocations (13<12̄10> type) was prevalent on the prismatic plane and were found to be strongly affected by temperature. At higher temperature (150 and 300 K), the dislocation density increased. The crack grew further at 150–300 K than at 0 K and the shielding effect of dislocations was limited due to their movement away from the crack tip. The addition of iodine at basal I2, pyramidal I1 and I2 stacking faults was seen to decrease the energy of its formation whereas for the prismatic stacking fault it was found to increase it. The iodine also changed the order of favourability of the stacking faults with basal I2 and pyramidal I1 stacking faults becoming much more favourable and prismatic going from most to least favourable.
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In this work, a nanotwinned structure of a two-phase titanium (TC21) alloy was prepared via the martensitic phase transformation method. The TC21 alloy forms initially a single phase β when the temperature reaches 1050 °C. Then, it transitions into an orthorhombic martensite (α″) material when quenched in water. After tempering at 500 °C, the martensite transforms into a high-density nanotwinned α phase material. The hardness of the nanotwinned alloy is 25% higher than that of the TC21 raw material.
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New interatomic potentials describing defects, plasticity, and high temperature phase transitions for Ti are presented. Fitting the martensitic hcp-bcc phase transformation temperature requires an efficient and accurate method to determine it. We apply a molecular dynamics method based on determination of the melting temperature of competing solid phases, and Gibbs-Helmholtz integration, and a lattice-switch Monte Carlo method: these agree on the hcp-bcc transformation temperatures to within 2 K. We were able to develop embedded atom potentials which give a good fit to either low or high temperature data, but not both. The first developed potential (Ti1) reproduces the hcp-bcc transformation and melting temperatures and is suitable for the simulation of phase transitions and bcc Ti. Two other potentials (Ti2 and Ti3) correctly describe defect properties and can be used to simulate plasticity or radiation damage in hcp Ti. The fact that a single embedded atom method potential cannot describe both low and high temperature phases may be attributed to neglect of electronic degrees of freedom, notably bcc has a much higher electronic entropy. A temperature-dependent potential obtained from the combination of potentials Ti1 and Ti2 may be used to simulate Ti properties at any temperature.
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Diffusion of point defects, nucleation of dislocation loops, and the associated dimensional changes of pure and H-loaded hcp-Zr have been investigated by a combination of ab initio calculations and classical simulations. Vacancy diffusion is computed to be anisotropic with D vac,basal = 8.6×10-6 e-Q/(RT) (m2/s) and Dvac,axial = 9.9×10-6 e-Q/(RT) (m2/s), Q=69 and 72 kJ/mol for basal and axial diffusion, respectively. At 550 K vacancy diffusion is about twice as fast in the basal plane as fast as in a direction parallel to the c-axis. Diffusion of self-interstitials is found to be considerably faster and anisotropic involving collective atomic motions. At 550 K diffusion occurs predominantly in the a-directions, but this anisotropy diminishes with increasing temperature. Furthermore, the diffusion anisotropy is very dependent on the local strain (c/a ratio). Interstitial H atoms are found to diffuse isotropically with D H = 1.1×10-7 e-42/(RT) (m2/s). These results are consistent with experimental data and other theoretical studies. Molecular dynamics simulations at 550 K with periodic injection of vacancies and self-interstitial atoms reveal the formation of small nanoclusters, which are sufficient to cause a net expansion of the lattice in the a-directions driven by clusters of self-interstitials and a smaller contraction in the c-direction involving nanoclusters of vacancies. This is consistent with and can explain experimental data of irradiation growth. Energy minimizations show that vacancy c-loops can collapse into stacking-fault pyramids and, somewhat unexpectedly, this is associated with a contraction in the a-directions. This collapse can be impeded by hydrogen atoms. Interstitial hydrogen atoms have no marked influence on self-interstitial diffusion and aggregation. These simulations use a new Zr-H embedded atom potential, which is based on ab initio energies.
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We investigate deformation in high quality Au nanowires under both tension and bending using in-situ transmission electron microscopy. Defect evolution is investigated during: (1) tensile deformation of 〈110〉 oriented, initially defect-free, single crystal nanowires with cross-sectional widths between 30 and 300 nm, (2) bending deformation of the same wires, and (3) tensile deformation of wires containing coherent twin boundaries along their lengths. We observe the formation of twins and stacking faults in the single crystal wires under tension, and storage of full dislocations after bending of single crystal wires and after tension of twinned wires. The stress state dependence of the deformation morphology and the formation of stacking faults and twins are not features of bulk Au, where deformation is controlled by dislocation interactions. Instead, we attribute the deformation morphologies to the surface nucleation of either leading or trailing partial dislocations, depending on the Schmid factors, which move through and exit the wires producing stacking faults or full dislocation slip. The presence of obstacles such as neutral planes or twin boundaries hinder the egress of the freshly nucleated dislocations and allow trailing and leading partial dislocations to combine and to be stored as full dislocations in the wires. We infer that the twins and stacking faults often observed in nanoscale Au specimens are not a direct size effect but the result of a size and obstacle dependent transition from dislocation interaction controlled to dislocation nucleation controlled deformation.
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The stability properties of vacancy clusters in hexagonal close-packed Zr—both cavities and dislocation loops—are investigated at the atomic scale, with a modeling approach based on density functional theory and empirical potentials. By considering the vacancy–vacancy interactions and the stability of small vacancy clusters, we establish how to build larger clusters. A study of extended vacancy clusters is then performed using continuous laws for defect energetics. Once validated with an empirical potential, these laws are parameterized with ab initio data. Our work shows that the easy formation of 〈a〉〈a〉 loops can be explained by their thermodynamic properties.
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Twin boundary migration is usually observed during annealing or plastic deformation. We report on in situ observation of Kr ion irradiation-induced microstructure evolution in epitaxial nanotwinned Ag films with an average twin thickness of 70 nm. Kr ion irradiation-induced defect clusters are absorbed by coherent and incoherent twin boundaries. Frequent interactions between defect clusters and twin boundaries lead to continuous migration of twin boundaries. The potential mechanisms of twin boundary migration are discussed. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
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Although it is well known that Fe–Mn–C TWIP steels exhibit high work-hardening rates, the elementary twinning mechanisms controlling the plastic deformation of these steels have still not been characterized. The aim of the present study is to analyse the extended defects related to the twinning occurrence using transmission electron microscopy. Based on these observations, the very early stage of twin nucleation can be attributed to the pole mechanism with deviation proposed by Cohen and Weertman or to the model of Miura, Takamura and Narita, while the twin growth is controlled by the pole mechanism proposed by Venables. High densities of sessile Frank dislocations are observed within the twins at the early stage of deformation, which can affect the growth and the stability of the twins, but also the strength of these twins and their interactions with the gliding dislocations present in the matrix. This experimental evidence is discussed and compared to recent results in order to relate the defects analysis to the macroscopic behaviour of this category of material.
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Doppler-broadening measurements of the positron annihilation characteristics of both well-annealed and electron irradiated Zr and Ti have been made. Data obtained with the use of both a “hard” (Ge) and a “soft” (Na) positron-emitting source were used to yield information about the defect depth distributions of the irradiated materials. Good agreement between these results and theoretical defect profiles, based on electron energy loss calculations, were found for one set of irradiated samples. A striking disagreement between experimental and theoretical defect depth profiles for another set of samples, irradiated under different conditions, was resolved when a hitherto unsuspected H component of the electron beam was taken into account.
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We propose a three-dimensional model for twin nucleation in hcp materials based on the nonplanar dissociation of the leading dislocation in a pile-up of ⟨a⟩ slip dislocations. Continuum linear elastic dislocation theory is used to calculate the change in free energy with extension of the dissociated configuration, consisting of a stair rod and glissile twinning dislocation loops. The model is applied to Mg, which deforms primarily by basal slip, and to Zr, which deforms primarily by prismatic slip. It is found that dissociations from an isolated ⟨a⟩ slip dislocation are energetically unable to produce a stable twin fault loop, at least larger than 2r0, the core width of the initial ⟨a⟩ slip dislocation. For some reactions, dissociations of the lead dislocation in a basal or prismatic dislocation pile-up can, however, lead to a stable and sizable twin loop. In these, the loop size is found to increase with decreasing twin boundary energy and increasing number of dislocations in the pile-up.
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Crystal-melt interfacial free energies (γ) are computed for hcp Mg by employing equilibrium molecular-dynamics (MD) simulations and the capillary-fluctuation method (CFM). This work makes use of a newly developed embedded-atom-method (EAM) interatomic potential for Mg fit to crystal, liquid, and melting properties. We describe how the CFM, which has previously been applied to cubic systems only, can be generalized for studies of hcp metals by employing a parametrization for the orientation dependence of γ in terms of hexagonal harmonics. The method is applied in the calculation of the Turnbull coefficient (α) and crystalline anisotropies of γ. We obtain a value of α=0.48, with interfacial free energies for different high-symmetry orientations differing by approximately 1%. These results are compared to those obtained in previous MD-CFM studies for cubic EAM metals as well as experimental studies of solid-liquid interfaces in hcp alloys. In addition, the implications of our results for the prediction of dendrite growth directions in hcp metals are discussed.
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We discuss the nucleation of deformation twins in Mg from a fundamental perspective. Atomistic simulations reveal twinning mechanisms and suggest that twin nucleation most likely occurs at grain boundaries (GBs). We observe twin nucleation from symmetrical tilt grain boundaries using molecular dynamics and reveal that the nucleation pathway depends on the tilt angle and the GB defect state. In particular, twin nucleation is preferred at GBs with low misorientation angles, in agreement with electron back-scattering diffraction (EBSD) analyses. A probabilistic description of twin nucleation is then proposed with the aim of linking atomic-scale information with meso-scale EBSD statistical analyses.
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An in-depth statistical analysis using electron backscatter diffraction (EBSD) is carried out to expose statistical correlations between {} twinning and grain size, crystallographic orientation, grain boundary length, and neighbor misorientation in high-purity polycrystalline zirconium strained to 5% and 10% at 77 K. A strong correlation was found between the active twin variant and crystallographic orientation. The propensity of a grain to twin or not was found to be only weakly dependent on grain area and diameter. Within the population of grains containing twins the number of twins per grain noticeably increases with grain area, and twin thickness is found to be rather insensitive to grain size and orientation. A weak preference for twinning was found for smaller grain boundary misorientation angles. These and the other statistical results reported can improve theoretical treatments for twin nucleation in polycrystal models. The statistical methodology presented has general applicability for all twin types in a wide range of metals.
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The Open Visualization Tool (OVITO) is a new 3D visualization software designed for post-processing atomistic data obtained from molecular dynamics or Monte Carlo simulations. Unique analysis, editing and animations functions are integrated into its easy-to-use graphical user interface. The software is written in object-oriented C++, controllable via Python scripts and easily extendable through a plug-in interface. It is distributed as open-source software and can be downloaded from the website http://ovito.sourceforge.net/.
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