73 research outputs found
Surface-Induced Phase Transition During Coalescence of Au Nanoparticles: A Molecular Dynamics Simulation Study
In this study, the melting and coalescence of Au nanoparticles were
investigated using molecular dynamics simulation. The melting points of
nanoparticles were calculated by studying the potential energy and Lindemann
indices as a function of temperature. The simulations show that coalescence of
two Au nanoparticles of the same size occurs at far lower temperatures than
their corresponding melting temperature. For smaller nanoparticles, the
difference between melting and coalescence temperature increases. Detailed
analyses of the Lindemann indices and potential energy distribution across the
nanoparticles show that the surface melting in nanoparticles begins at several
hundred degrees below the melting point. This suggests that the coalescence is
governed by the liquid-phase surface diffusion. Furthermore, the surface
reduction during the coalescence accelerates its kinetics. It is found that for
small enough particles and/or at elevated temperatures, the heat released due
to the surface reduction result in a melting transition of the two attached
nanoparticles.Comment: 15 pages, 4 figures, 1 table, full length articl
Thickening of T1 Precipitates during Aging of a High Purity Al−4Cu−1Li−0.25Mn Alloy
The age hardening response of a high-purity Al–4Cu–1Li–0.25Mn alloy (wt. %) during isothermal aging without and with an applied external load was investigated. Plate shaped nanometer size T1 (Al2CuLi) and θ′ (Al2Cu) hardening phases were formed. The precipitates were analyzed with respect to the development of their structure, size, number density, volume fraction and associated transformation strains by conducting transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies in combination with geometrical phase analysis (GPA). Special attention was paid to the thickening of T1 phase. Two elementary types of single-layer T1 precipitate, one with a Li-rich (Type 1) and another with an Al-rich (Defect Type 1) central layer, were identified. The results show that the Defect Type 1 structure can act as a precursor for the Type 1 structure. The thickening of T1 precipitates occurs by alternative stacking of these two elementary structures. The thickening mechanism was analyzed based on the magnitude of strain associated with the precipitation transformation normal to its habit plane. Long-term aging and aging under load resulted in thicker and structurally defected T1 precipitates. Several types of defected precipitates were characterized and discussed. For θ′ precipitates, a ledge mechanism of thickening was observed. Compared to the normal aging, an external load applied to the peak aged state leads to small variations in the average sizes and volume fractions of the precipitates.DFG, 237105621, SPP 1713: Stark gekoppelte thermo-chemische und thermo-mechanische Zustände in Angewandten Materialie
Precipitation of T<sub>1</sub> and θ′ Phase in Al-4Cu-1Li-0.25Mn During Age Hardening: Microstructural Investigation and Phase-Field Simulation
Experimental and phase field studies of age hardening response of a high purity Al-4Cu-1Li-0.25Mn-alloy (mass %) during isothermal aging are conducted. In the experiments, two hardening phases are identified: the tetragonal θ′ (Al2Cu) phase and the hexagonal T1 (Al2CuLi) phase. Both are plate shaped and of nm size. They are analyzed with respect to the development of their size, number density and volume fraction during aging by applying different analysis techniques in TEM in combination with quantitative microstructural analysis. 3D phase-field simulations of formation and growth of θ′ phase are performed in which the full interfacial, chemical and elastic energy contributions are taken into account. 2D simulations of T1 phase are also investigated using multi-component diffusion without elasticity. This is a first step toward a complex phase-field study of T1 phase in the ternary alloy. The comparison between experimental and simulated data shows similar trends. The still unsaturated volume fraction indicates that the precipitates are in the growth stage and that the coarsening/ripening stage has not yet been reached
Quantitative Shape-Classification of Misfitting Precipitates during Cubic to Tetragonal Transformations: Phase-Field Simulations and Experiments
The effectiveness of the mechanism of precipitation strengthening in metallic alloys de-pends on the shapes of the precipitates. Two different material systems are considered: tetragonal γ′′ precipitates in Ni-based alloys and tetragonal θ′ precipitates in Al-Cu-alloys. The shape formation and evolution of the tetragonally misfitting precipitates was investigated by means of experiments and phase-field simulations. We employed the method of invariant moments for the consistent shape quantification of precipitates obtained from the simulation as well as those obtained from the experiment. Two well-defined shape-quantities are proposed: (i) a generalized measure for the particles aspect ratio and (ii) the normalized λ2, as a measure for shape deviations from an ideal ellipse of the given aspect ratio. Considering the size dependence of the aspect ratio of γ′′ precipitates, we find good agreement between the simulation results and the experiment. Further, the precipitates’ in-plane shape is defined as the central 2D cut through the 3D particle in a plane normal to the tetragonal c-axes of the precipitate. The experimentally observed in-plane shapes of γ′′-precipitates can be quantitatively reproduced by the phase-field model. © 2021 by the authors. Licensee MDPI, Basel, Switzerland
Revealing in-plane grain boundary composition features through machine learning from atom probe tomography data
Grain boundaries (GBs) are planar lattice defects that govern the properties
of many types of polycrystalline materials. Hence, their structures have been
investigated in great detail. However, much less is known about their chemical
features, owing to the experimental difficulties to probe these features at the
atomic length scale inside bulk material specimens. Atom probe tomography (APT)
is a tool capable of accomplishing this task, with an ability to quantify
chemical characteristics at near-atomic scale. Using APT data sets, we present
here a machine-learning-based approach for the automated quantification of
chemical features of GBs. We trained a convolutional neural network (CNN) using
twenty thousand synthesized images of grain interiors, GBs, or triple
junctions. Such a trained CNN automatically detects the locations of GBs from
APT data. Those GBs are then subjected to compositional mapping and analysis,
including revealing their in-plane chemical decoration patterns. We applied
this approach to experimentally obtained APT data sets pertaining to three case
studies, namely, Ni-P, Pt-Au, and Al-Zn-Mg-Cu alloys. In the first case, we
extracted GB-specific segregation features as a function of misorientation and
coincidence site lattice character. Secondly, we revealed interfacial excesses
and in-plane chemical features that could not have been found by standard
compositional analyses. Lastly, we tracked the temporal evolution of chemical
decoration from early-stage solute GB segregation in the dilute limit to
interfacial phase separation, characterized by the evolution of complex
composition patterns. This machine-learning-based approach provides
quantitative, unbiased, and automated access to GB chemical analyses, serving
as an enabling tool for new discoveries related to interface thermodynamics,
kinetics, and the associated chemistry-structure-property relations
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