arXiv subjects

Alexander M. Korsunsky

Publications and source records attributed to Alexander M. Korsunsky.

12 recordsLinked to original sources

arXiv2026

An Efficient Approach for Calculating Free Energy in Molecular Dynamics: Demineralization of Hydroxyapatite as a Case Study

Despite the strength of Molecular Dynamics simulations in providing insights into the microscopic details of phenomenon in many fields in materials science, physics and biology, the biggest barrier is its limited timescale which is several orders of magnitude lower than the timescale of the real-world processes and phenomena being modeled. Free energy calculations are designed as a remedy to this problem that in theory can overcome this barrier. This is particularly relevant for biomineralisation processes such as tooth mineral formation and dissolution, while reflecting a broader challenge in accurately modelling rare events and long-timescale phenomena across complex molecular systems. However, due to the novelty of the field, a number of questions remain outstanding pertaining to the best practice of applying this method. The non-equilibrium work approach based on the Jarzynski equation is one of the most promising free energy calculation methods. However, the biggest challenge for the broader use of this method is the question of how many simulations are required for an accurate free energy estimation. Comparing the free energy results with very long reversible pulling simulations of atom clusters from the surface, each taking 75 days on the 48-core Intel Xeon Platinum 8268 CPUs, in this study we showed that this question is irrelevant and higher quality and better equilibrated initial structures is the proper approach than simply based on the number of simulations. We designed a new adaptive free energy calculation methodology which combines high quality, high computational cost free energy values with lower quality, lower cost values to build up the entire free energy profile. In the best case scenario this method lowers manifold the computational cost required for the non-equilibrium work free energy calculations compared to both the regular method and the reversible simulation.

cond-mat.mtrl-sci↗

arXiv2026

Uncovering the role of ionic doping in hydroxyapatite: The building blocks of tooth enamel and bones

Hydroxyapatite (HAp) is the primary mineral component of various mineralized tissues in the human body, including bone and teeth, where it performs critical roles of structural support and load transmission. In the context of dental health, the two most crucial properties of HAp are mechanical stability, which ensures resistance to forces, and chemical stability, which preserves surface integrity in acidic environments. During early stages of human evolution, e.g. when teeth were used to crush uncooked food, mechanical stability was of paramount importance. However, with changes in diet and lifestyle, the principal origins of tooth damage and loss shifted towards bacterially mediated chemical attack, known as tooth decay, or caries. To enhance the chemical stability, ion doping has emerged as a particularly significant approach, and it lies at the focus of the present study. A Molecular Dynamics (MD) framework was developed to investigate the effects of ion doping on the chemical and mechanical stability of HAp and to identify optimal doping candidates. The framework combines conventional MD with Steered Molecular Dynamics (SMD), Thermodynamic Integration (TI) and uniaxial compression test simulations to provide comprehensive insights into the doping process. The findings revealed surface atoms as the most viable candidates for doping, as demonstrated by SMD and conventional MD simulations. Notably, TI calculations have identified magnesium ions as a better candidate among the ions considered here for enhancing the chemical stability of HAp. The results presented in this study offer valuable guidelines for synthesizing HAp-based substituent materials with properties tailored to meet the demands of modern dental applications such as implant coatings, enamel remineralization agents and restorative materials.

cond-mat.mtrl-sci↗

arXiv2024

Single-exposure elemental differentiation and texture-sensitive phase-retrieval imaging with a neutron counting micro-channel plate detector

Micro-channel plate (MCP) detectors, when used at pulsed-neutron-source instruments, offer the possibility of high spatial resolution and high contrast imaging with pixel-level spectroscopic information. Here we demonstrate the possibility of multimodal analysis including total neutron cross-section spectra measurements, quantitative material differentiation imaging and texture-sensitive in-line phase imaging, from a single exposure using an MCP detector. This multimodal approach operates in full-field imaging mode, with the neutron transmission spectra acquired at each individual detector pixel. Due to the polychromatic nature of the beam and spectroscopic resolving capability of the detector, no energy scanning is required. Good agreement with the library reference data is demonstrated for neutron cross-section spectra measurements. Two different images corresponding to two selected energy bandwidths are used for elemental differentiation imaging. Moreover, the presence of changes in texture, i.e., preferred grain orientation, in the sample is identified from our phase-retrieval imaging results.

cond-mat.mtrl-sci↗

arXiv2023

A numerical study of the effect of discretization methods on the crystal plasticity finite element method

The present report describes a big data numerical study of crystal plasticity finite element (CPFE) modelling using static and grain-based meshing to investigate the dependence of the results on the discretization approach. Static mesh refers to the integration point-based representation of the microstructure in which the integration points (IPs) within a finite element may belong to different grains, while in the grain-based meshing the minimum discretization unit is an element that may only belong to one grain. The crystal plasticity constitutive law was coded using UMAT subroutine within commercial finite element software Abaqus. Multiple sets of RVEs were investigated under strain-controlled loading and periodic boundary conditions. The stress and strain contour maps obtained from RVEs with static mesh and grain-based mesh were compared. The simulation results reveal that both discretization methods provide reliable predictions of the stress-strain curves and the stress/strain localization points in polycrystalline alloys. Static mesh tends to smooth the stress/strain profile at the grain boundary, whilst stress/strain discontinuities are present in the grain-based mesh results. The above findings remain valid when the number of grains within an RVE increases from 34 to 1250. To quantify the difference between static and grain-based meshing, a relative measure of deviation is defined. The deviations of global stress were found to be relatively small, within 0.5%, while local deviations were significant up to 50%. Static mesh has the advantage of reducing both the preprocessing procedures and computational time compared to grain-based mesh. It is concluded that static mesh is preferred when investigating the material's macroscopic behaviour, whilst grain-based mesh is recommended for the study of the local response using CPFEM.

cond-mat.mtrl-sci↗

arXiv2022

The creep deformation of a new nickel-base alloy-by-design studied using synchrotron X-ray diffraction

Understanding the creep mechanisms and deformation response at different stresses and temperatures is crucial for design using nickel-base superalloys for high-temperature applications. In this study, the creep behaviour of a newly designed superalloy (nominated Alloy 11) at different stress and temperature was systematically investigated using SEM, STEM, EBSD and synchrotron X-ray diffraction. Material properties and mechanical response were analysed by considering such properties as lattice parameters and misfit, phase volume fraction, microstrain in reference and crept samples. Detwinning and dynamic recrystallization were observed for the crept samples. STEM characterization of deformed materials reveals that multiple deformation mechanisms and defects could be identified in crept samples, namely, dislocations in the gamma matrix channels and in gamma prime precipitates, along with continuous and isolated stacking faults. The highest values of lattice misfit, microstrain and lattice parameter change were observed at the centre of dogbone-shaped crept samples. This was associated with the hump shaped temperature distribution profile during creep testing. The significant spatial variation detected in terms of lattice parameters, misfit and microstrain supports the conclusion that it is important to perform spatially resolved measurements instead of considering each sample as a single measurement point when investigating creep response. It is also worth noting that the statistics of the lattice-parameter distribution for the gamma and gamma prime phases obey a gaussian distribution. In conclusion, a discussion is given for the meaning and implications of these findings for future research.

cond-mat.mtrl-sci↗

arXiv2022

Direct TEM observation and quantification of the Gibbs-Thomson effect in a nickel superalloy

Gibbs-Thompson effect is the general term referring to the influence of interfaces on the course of phase transformations such as precipitation or solidification. Whilst attention is most often focused on the Gibbs-Thomson effect on nucleation, growth and coarsening, the present study considers the reverse process of precipitate dissolution in a nickel-base superalloy during in situ TEM observation. The presence of several distinct populations of gamma-prime precipitates (primary, secondary, tertiary and grain boundary) allows the differences due to particle size to be quantified and interpreted. Important implications arise for the selection of heat treatment schedules for nickel-base superalloys and other alloy systems.

cond-mat.mtrl-sci↗

arXiv2022

The Yield Volume Fraction approach to the description of the stress-strain curve of a nickel-base superalloy

The stress-strain curves of most metallic alloys are often described using the relatively simple Ramberg-Osgood relationship. Whilst this description captures the overall stress-strain curve under monotonic tensile loading with reasonable overall accuracy, it often presents significant errors in the immediate post-yield region where the interplay between the elastic and plastic strains is particularly significant. This study proposes and develops a new approach to the description of the tensile stress-strain curve based on the Yield Volume Fraction (YVF) function. The YVF description provides an excellent match to experimental stress-strain curves based on a physically meaningful parameter that corresponds to the cumulative volume fraction of the polycrystal that undergoes yielding during monotonic deformation. The statistical nature of the polycrystal yield phenomenon is highlighted by the fact that the YVF model achieves good agreement with observations when the lognormal and extreme value distributions are employed to express the cumulative density function for the total yield volume fraction, and the probability density function for the incremental yield volume fraction, respectively. This proposed approach is compared with crystal plasticity finite element (CPFE) simulations and the Ramberg-Osgood model, along with experimental observations. The results highlight the potential of more extensive use of statistical methods in the description of material deformation response for improved design.

cond-mat.mtrl-sci↗

arXiv2022

Recovering the second moment of the strain distribution from neutron Bragg edge data

Point by point strain scanning is often used to map the residual stress (strain) in engineering materials and components. However, the gauge volume and hence spatial resolution is limited by the beam defining apertures and can be anisotropic for very low and high diffraction (scattering) angles. Alternatively, wavelength resolved neutron transmission imaging has a potential to retrieve information tomographically about residual strain induced within materials through measurement in transmission of Bragg edges - crystallographic fingerprints whose locations and shapes depend on microstructure and strain distribution. In such a case the spatial resolution is determined by the geometrical blurring of the measurement setup and the detector point spread function. Mathematically, reconstruction of strain tensor field is described by the longitudinal ray transform; this transform has a non-trivial null-space, making direct inversion impossible. A combination of the longitudinal ray transform with physical constraints was used to reconstruct strain tensor fields in convex objects. To relax physical constraints and generalise reconstruction, a recently introduced concept of histogram tomography can be employed. Histogram tomography relies on our ability to resolve the distribution of strain in the beam direction, as we discuss in the paper. More specifically, Bragg edge strain tomography requires extraction of the second moment (variance about zero) of the strain distribution which has not yet been demonstrated in practice. In this paper we verify experimentally that the second moment can be reliably measured for a previously well characterised aluminium ring and plug sample. We compare experimental measurements against numerical calculation and further support our conclusions by rigorous uncertainty quantification of the estimated mean and variance of the strain distribution.

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arXiv2021

On the Hill and Kolmogorov statistics and the K-function representation of judiciously modified Avrami kinetics

This study concerns the phase transformation kinetics and pertains to the processes of crystallisation of amorphous polymers, ceramics, and bulk metallic glasses (BMG), as well as solid state transformations by the nucleation and growth mechanism. It is also likely to be relevant to the description of dissolution and precipitation processes, and more generally chemical reaction kinetics, as well as biochemical processes such as ligand binding, haemoglobin oxygenation, etc. The mathematical framework is examined of the of the 'knee' function, or K-function, and its implications are explored.

cond-mat.mtrl-sci↗

arXiv2005

Reconstruction of Residual Stress in a Welded Plate Using the Variational Eigenstrain Approach

We present the formulation for finding the distribution of eigenstrains, i.e. the sources of residual stress, from a set of measurements of residual elastic strain (e.g. by diffraction), or residual stress, or stress redistribution, or distortion. The variational formulation employed seeks to achieve the best agreement between the model prediction and some measured parameters in the sense of a minimum of a functional given by a sum over the entire set of measurements. The advantage of this approach lies in its flexibility: different sets of measurements and information about different components of the stress-strain state can be incorporated. We demonstrate the power of the technique by analysing experimental data for welds in thin sheet of a nickel superalloy aerospace material. Very good agreement can be achieved between the prediction and the measurement results without the necessity of using iterative solution. In practice complete characterisation of residual stress states is often very difficult, due to limitations of facility access, measurement time or specimen dimensions. Implications of the new technique for experimental analysis are all the more significant, since it allows the reconstruction of the entire stress state from incomplete sets of data.

cond-mat.mtrl-sci↗

arXiv2005

The Principle of Strain Reconstruction Tomography: Determination of Quench Strain Distribution from Diffraction Measurements

Evaluation of residual elastic strain within the bulk of engineering components or natural objects is a challenging task, since in general it requires mapping a six-component tensor quantity in three dimensions. A further challenge concerns the interpretation of finite resolution data in a way that is commensurate and non-contradictory with respect to continuum deformation models. A practical solution for this problem, if it is ever to be found, must include efficient measurement interpretation and data reduction techniques. In the present note we describe the principle of strain tomography by high energy X-ray diffraction, i.e. of reconstruction of the higher dimensional distribution of strain within an object from reduced dimension measurements; and illustrate the application of this principle to a simple case of reconstruction of an axisymmetric residual strain state induced in a cylindrical sample by quenching. The underlying principle of the analysis method presented in this paper can be readily generalised to more complex situations.

cond-mat.mtrl-sci↗

arXiv2005

Power Law Multi-Scaling of Material Strength

Power law is one of the the simplest forms of the relationship between different variables of a system. It leads naturally to the introduction of compound parameters describing physical properties of the system. Often one of the variables of interest is the object dimension, or time. The prevalence of a simple power law over the entire range of dimensions or times can be helpfully interpreted as size or time independence of the corresponding compound physical parameter of the system. However, it is also often found that a simple power law only persists for some extreme values, e.g. for very large and/or small sizes, or very short or long times. Transitions between regimes of different power law asymptotic behaviour are encountered frequently in the description of a wide variety of physical systems. While asymptotic power law behaviour may often be readily predicted, e.g. on dimensional grounds, the evaluation of the relationship between system parameters in the transition range usually requires laborious detailed solution. To obviate this difficulty we introduce, on rather general basis, something we refer to as the merging, or 'knee' function. The function possesses sufficient flexibility to describe transitions of various sharpness. To demonstrate its usefuness, the merging function is applied to a variety of well-known scaling laws in the mechanics and strength of materials and structures.

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