Papers by Ioannis Mastorakos
Bulletin of the American Physical Society, Mar 22, 2011
Physical review, Dec 1, 1993
We investigated numerically the number of sites visited SN by random walks on the backbone struct... more We investigated numerically the number of sites visited SN by random walks on the backbone structure of the percolation cluster at the critical threshold. This quantity can be predicted by the scaling conjecture in terms of the fractal and the random-walk dimensions (df and dw). Our results confirm this scaling with time, similarly to the critical cluster. The scaling exponent (spectral dimension) is numerically calculated, and it is found to be dBBs=1.23, while the scaling conjecture predicts a value of 1.19, suggesting that there are uncertainties in the dBBf and dBBw values. This value is also smaller (by about 5%) than ds, the spectral dimension on the full percolation cluster, suggesting that the walk is less efficient on the backbone. Previous estimates of the dBBw suggested that the walk should be more efficient on the backbone. We investigate this apparent contradiction by calculating and comparing the full distributions of SN for the backbone and the full percolating cluster. We investigated a few higher moments of this quantity and we found that they exhibit constant-gap scaling, similar to the percolation cluster. The backbone considerations help our understanding of the diffusion on the percolation cluster, especially the contribution of the dangling ends and the ramified parts of the structure, which are so characteristic of percolation at criticality.
Computational Materials Science, Feb 1, 2020
Pure metal nanofoams in the form of interconnected ligament networks have shown strong potential ... more Pure metal nanofoams in the form of interconnected ligament networks have shown strong potential over the last few years in areas such as catalysts, batteries, and optics. However, they are often fragile and therefore difficult to integrate into engineering applications. For these reasons, a new class of materials, composite metallic nanofoams made of ligaments coated with thin metallic layers, have been proposed to solve these issue. The mechanical properties of these nanofoams depend on their relative density and their internal geometrical structure. In this work, we combine molecular dynamics (MD) and finite element method (FEM) to investigate the compressive behavior of nanofoams made of copper ligaments coated with nickel. We also investigate how this behavior relates to their microstructures. For that purpose, we built different types of representative cell structures, and atomistic simulations of multiaxial compression tests were performed to produce yield surfaces. Then the generated yield surfaces were curve fitted into a normalized model to obtain the shape parameters. Last, a plasticity model was introduced to study and compare their mechanical behavior under compression. The results suggest that the coating of the ligaments can improve the mechanical behavior of the nanofoams. They also reveal the importance and the limitations of the microstructural geometry on the strengthening of these structures. These findings can be used for the design of metallic nanofoams with tailored mechanical properties.
MRS Proceedings, 2012
ABSTRACTStructural materials in the new Generation IV reactors will operate in harsh radiation co... more ABSTRACTStructural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production and will experience severe degradation of mechanical properties. Therefore, understanding of the physical mechanisms responsible for the microstructural evolution and corresponding mechanical property changes is critical. As the involved phenomena are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. In previous work, we used molecular dynamics simulations to develop critical rules for the mobility of dislocations in various iron alloys and their interaction with several types of defects that include, among others, helium bubbles and grain boundaries. In this work, Dislocation Dynamics simulations of iron alloys are used to study the mechanical behavior and the degradation under irradiation of large systems with high dislocation and defect densities.
EGU General Assembly Conference Abstracts, Apr 1, 2019
MRS Proceedings, 2010
Structural materials in the new Generation IV reactors will operate in harsh radiation conditions... more Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradation of the material under examination. In particular, information pertaining to the dislocation-defects (such as voids, helium bubbles and prismatic loops) interactions is obtained from MD simulations. Then this information is used by DD to simulate large systems with high dislocation and defect densities.
Mesoscale Models, 2019
This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete disl... more This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete dislocation dynamics (DDD) with continuum dislocation-based plasticity. In this framework, and guided by DDD, a continuum dislocation dynamics (CDD) plasticity model involving a set of spatio-temporal evolution equations for dislocation densities representing mobile and immobile species is developed. The evolution laws consist of a set of components each corresponding to a physical mechanism that can be explicitly evaluated and quantified from DDD analyses. In this framework, stochastic events such as cross-slip of screw dislocations and uncertainties associated with initial microstructural conditions are explicitly incorporated in the continuum theory based on probability distribution functions defined by activation energy and activation volumes. The result is a multiscale dislocation-based plasticity model which can predict not only the macroscopic material mechanical behavior but also the corresponding microscale deformation and the evolution of dislocation patterns, size and gradient-dependent deformation phenomena, and related material instabilities at various length and time scales.
MRS Communications, 2021
We investigated the effect of complexity on the mechanical behavior and recovery of metallic nano... more We investigated the effect of complexity on the mechanical behavior and recovery of metallic nanoarchitecture structures. We studied four different suggested geometries with various levels of complexity using molecular dynamics simulations. The structures exhibited multiple degrees of self-recovery under compressive loading conditions at three temperatures, 300 K, 400 K, and 500 K. A methodology to qualitatively measure the geometric complexity was used. The results revealed correlations between the complexity of the structures and their recovery ability and strength, and the geometric cell size and temperature. These findings can guide the design of novel nanoarchitecture geometries for specific applications with tailored properties.
This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete disl... more This chapter, outlines a multiscale dislocation-based plasticity framework coupling discrete dislocation dynamics (DDD) with continuum dislocation-based plasticity. In this framework, and guided by DDD, a continuum dislocation dynamics (CDD) plasticity model involving a set of spatio-temporal evolution equations for dislocation densities representing mobile and immobile species is developed. The evolution laws consist of a set of components each corresponding to a physical mechanism that can be explicitly evaluated and quantified from DDD analyses. In this framework, stochastic events such as cross-slip of screw dislocations and uncertainties associated with initial microstructural conditions are explicitly incorporated in the continuum theory based on probability distribution functions defined by activation energy and activation volumes. The result is a multiscale dislocation-based plasticity model which can predict not only the macroscopic material mechanical behavior but also the...
MRS Advances, 2019
ABSTRACTPure metallic nanofoams in the form of interconnected networks have shown strong potentia... more ABSTRACTPure metallic nanofoams in the form of interconnected networks have shown strong potentials over the past few years in areas such as catalysts, batteries and plasmonics. However, they are often fragile and difficult to integrate in engineering applications. In order to better understand their deformation mechanisms, a multiscale approach is required to simulate the mechanical behavior of the nanofoams, although these materials will operate at the macroscale, they will still be maintaining an atomistic ordering. Hence, in this work we combine molecular dynamics (MD) and finite element analysis (FEA) to study the mechanical behavior of copper (Cu) nanofoams. Molecular dynamics simulations were performed to study the yield surface of a representative cell structure. The nanofoam structure has been generated by spinodal decomposition of binary alloy using an atomistic approach. Then, the information obtained from the molecular dynamics simulations in the form of yield function i...
Journal of Materials Research, 2017
Computational Materials Science, 2017
Pre-existing dislocations (PED) are ubiquitous inside crystalline lattices which in turn affect t... more Pre-existing dislocations (PED) are ubiquitous inside crystalline lattices which in turn affect the yield stress and the process of plastic deformation. Hence, understanding the onset of dislocations motion and interaction is critical in modifying or designing new materials with advanced properties so that their mechanical behavior approach realistic conditions. One such family of new materials is the ceramic/ metallic nanolaminates. In this work, we have investigated the effect of pre-existing dislocations on the mechanical behavior of NbC/Nb nanolaminates using molecular dynamics simulations. Upon unloading at different strains from stress-strain curve of 3 nm NbC/7 nm Nb sample, we were able to generate structures with various pre-existing dislocation densities inside the layers. Uniaxial loadings parallel to the interface at two different temperatures (10 K and 300 K) were performed on each structure. Also, the yield locus was determined at 300 K by applying biaxial in-plane loading and fitted with a general flow potential to be used in macroscale analysis. Finally, the tension-compression asymmetry (TCA) was investigated for the structures with pre-existing dislocations along two different in-plane loading directions.
MRS Advances, 2017
In this work, we demonstrate that the phase field crystal (PFC) method can be applied to identify... more In this work, we demonstrate that the phase field crystal (PFC) method can be applied to identify and predict the microstructure evolution during the solidification of the BCC metals in additive manufacturing. The results reveal the columnar structure of the grains, which matches the grain growth observed in real samples produced with the additive manufacturing technique. The effect of ambient temperature, seed-seed distance and seed-seed misorientation on the grain growth has been investigated. In addition, the formation of crystal defects during the process is recorded and the resulted long-range stresses are calculated using the eigenstresses theory.
MRS Proceedings, 2010
Structural materials in the new Generation IV reactors will operate in harsh radiation conditions... more Structural materials in the new Generation IV reactors will operate in harsh radiation conditions coupled with high levels of hydrogen and helium production, thus experiencing severe degradation of mechanical properties. The development of structural materials for use in such a hostile environment is predicated on understanding the underlying physical mechanisms responsible for microstructural evolution along with corresponding dimensional instabilities and mechanical property changes. As the phenomena involved are very complex and span in several length scales, a multiscale approach is necessary in order to fully understand the degradation of materials in irradiated environments. The purpose of this work is to study the behavior of Fe systems (namely a-Fe, Fe-Cr and Fe-Ni) under irradiation using both Molecular Dynamics (MD) and Dislocation Dynamics (DD) simulations. Critical information is passed from the atomistic (MD) to the microscopic scale (DD) in order to study the degradati...
Journal of Power Sources, 2012
h i g h l i g h t s < A mechanistic-based two-stage model was developed to study the self-healing... more h i g h l i g h t s < A mechanistic-based two-stage model was developed to study the self-healing glass materials. < Experimental healing measurements were conducted to characterize model parameters. < Finite element-based healing analyses are presented for various healing conditions.
Applied Surface Science, 2017
Exceptional mechanical and physical properties of transition metal carbides and nitrides make the... more Exceptional mechanical and physical properties of transition metal carbides and nitrides make them good coating-material candidates for extreme corrosive environments such as oil and natural gas wells. However, existence of small pores, pinholes and columnar structures of these ceramics significantly affect their resistance to corrosion, as pore sites would accelerate the diffusion of corrosive media into the substrate. In this research, molecular dynamics atomistic simulations are employed to investigate the effects of the isolated vacancies and the columnar structure on the diffusion rate of H atoms in NbC single crystal at various temperatures. Diffusion coefficient (D) of H atoms in NbC increased with C vacancy concentration. At elevated temperatures, the trapping effect of Nb vacancies is less effective when C vacancies are also present, as H atoms gain enough energy to jump back and forth between the C vacancies. Atomistic simulations also showed a jump in diffusion coefficient for cylindrical pore size of larger than 3 Å radius. Furthermore, D increased monotonically with temperature up to 1000 K in the presence of cylindrical pores. Further increase in temperature resulted in a drop in the diffusion coefficient for small pores while the large pores only showed a lower increasing trend in diffusion coefficient with the temperature.
Journal of Materials Research, Feb 6, 2019
MRS Advances, 2018
<jats:title>Abstract</jats:title><jats:p>A polycrystalline Cu foam with sub-mic... more <jats:title>Abstract</jats:title><jats:p>A polycrystalline Cu foam with sub-micron ligament sizes was formed by creating a non-woven fabric via electrospinning with a homogeneous mixture of polyvinyl alcohol(PVA)-and copper acetate(Cu(Ac)<jats:sub>2</jats:sub>). Thermogravimetric measurement of the electrospun fabric of the precursor solution is reported. Oxidizing the precursor fabric at 773K formed an oxide nano-foam; subsequent heating at 573K with a reducing gas transformed the CuO nano-foam to Cu with a similar ligament and meso-scale pore size morphology. A cross-section prepared by focused ion beam lift-out shows the polycrystalline structure with multi-scale porosity. The mechanical property of the Cu nano-foam is measured by nano-indentation. The load-depth curves and deduced mechanical properties suggest that additional intra-ligament pores lead to unique structure-property relations in this non-conventional form of metal.</jats:p>
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Papers by Ioannis Mastorakos