Modelling and Simulation in Materials Science and Engineering

Wdowik, Urszula D; Wasik, Magdalena; Twardowska, Agnieszka

doi: 10.1088/0965-0393/24/2/025001pmid: N/A

Density functional theory studies on the Ti5Si3Cxsystems with various concentrations of carbon impurities () are reported. The effects of interstitial carbon atoms on crystal and electronic structures, and on the elastic and vibrational properties of the Ti5Si3Cxcompound are analysed and discussed. The results of the present investigations indicate not only strong bonding between carbon atoms and their neighbouring titanium atoms, but also the effects of carbon impurities on the atomic bonds beyond the immediate proximity of the dopants. These determine to a great extent the electron densities of states, and the structural and elastic properties of the Ti5Si3CxNowotny phases. Although carbon atoms tend to stabilise Ti5Si3Cxphases, they also have a negative effect on their ductile properties. The strong impact of carbon impurities on the lattice dynamics of Ti5Si3Cxcompounds is revealed by the phonon and Raman spectra, which remain sensitive to changes in the interatomic distances. In C-doped systems the phonon bands originating from the vibrations of carbon impurities appear at high frequencies and remain well-separated from the lower lying phonon bands dominated by the vibrations of Ti and Si sublattices. The lower frequency phonon bands also experience changes due to the incorporated dopants. Impurities occupying the interstitials of the Ti5Si3 lattice are responsible for the appearance of new infrared active and optically inactive modes of A2u, E1uand E2usymmetries, leaving the number of Raman active modes unchanged. Modifications to the dynamical properties of ternary Ti5Si3Cxphases manifest themselves via shifts and the suppression of phonon peaks as well as the emergence of new phonon peaks which are absent in the binary Ti5Si3 system. The observed effects become enhanced with an increased concentration of carbon impurities.

He, An-Min; Wang, Pei; Shao, Jian-Li

doi: 10.1088/0965-0393/24/2/025002pmid: N/A

Large-scale molecular dynamics simulations are performed to study the fragmentation of microsheets produced from single crystal Cu with a wedged surface groove. Heterogeneous fragmentation of the microsheet is observed and the ejecta size distribution in the large size limit obeys a bilinear exponential dependence, consistent with the geometric fragmentation model with statistical heterogeneity. The underlying mechanisms governing the statistical heterogeneity are illustrated within the frameworks of geometric fragmentation models.

Jespersen, Freja N; Hattel, Jesper H; Somers, Marcel A J

doi: 10.1088/0965-0393/24/2/025003pmid: N/A

Nitriding of stainless steel causes a surface zone of expanded austenite, which improves the wear resistance of the stainless steel while preserving the stainless behaviour. During nitriding huge residual stresses are introduced in the treated zone, arising from the volume expansion that accompanies the dissolution of high nitrogen contents in expanded austenite.An intriguing phenomenon during low-temperature nitriding is that the residual stresses evoked by dissolution of nitrogen in the solid state, affect the thermodynamics and the diffusion kinetics of nitrogen dissolution. In the present paper solid mechanics was combined with thermodynamics and diffusion kinetics to simulate the evolution of composition-depth and stress-depth profiles resulting from nitriding. The model takes into account a composition-dependent diffusion coefficient of nitrogen in expanded austenite, short range ordering (trapping) of nitrogen atoms by chromium atoms, and the effect of composition-induced stress on surface concentration and diffusive flux. The effect of plasticity and concentration-dependence of the yield stress was also included.

Pan, S P; Feng, S D; Qiao, J W; Dong, B S; Qin, J Y

doi: 10.1088/0965-0393/24/2/025004pmid: N/A

We proposed a scheme to describe the spatial correlation between two atoms in metallic glasses. Pair distribution function in a model iron was fully decomposed into several shells and can be presented as the spread of nearest neighbor correlation via distance. Moreover, angle distribution function can also be decomposed into groups. We demonstrate that there is close correlation between pair distribution function and angle distribution function for metallic glasses. We think that our results are very helpful understanding the atomic structure of metallic glasses.

Rajan, V P; Warner, D H; Curtin, W A

doi: 10.1088/0965-0393/24/2/025005pmid: N/A

A family of interatomic potentials is constructed for which the intrinsic ductility can be tuned systematically. Specifically, the elastic constants and critical energy release rate for Griffith cleavage, , are held constant, while the critical energy release rate for dislocation emission, , can be varied. This behavior is achieved by modifying a standard near-neighbor pair potential; the new potential is applicable to either 2D (hexagonal lattice) or 3D (FCC/HCP). Analytical expressions are provided for and , enabling a potential with a desired intrinsic ductility to be easily developed. Direct atomistic simulations are used to demonstrate that the new potentials control the intrinsic material ductility, i.e. crack tip dislocation emission versus brittle cleavage, under quasi-static loading. For the 2D potential, the mode I crack tip behavior can be tuned from brittle to ductile; for the 3D potential, such tuning is only possible for certain crack orientations. More generally, the new potentials are expected to be useful in a wide range of physical problems in which behavior is controlled by the ability of the material to nucleate dislocations, including problems involving crack tips, grain boundaries, contact and friction, and bi-material interfaces.

Maresca, F; Kouznetsova, V G; Geers, M G D

doi: 10.1088/0965-0393/24/2/025006pmid: N/A

Metallic composite phases, like martensite present in conventional steels and new generation high strength steels exhibit microscale, locally lamellar microstructures characterized by alternating layers of phases or crystallographic variants. The layers can be sub-micron down to a few nanometers thick, and they are often characterized by high contrasts in plastic properties. As a consequence, fracture in these lamellar microstructures generally occurs along the layer interfaces or within one of the layers, typically parallel to the interface. This paper presents a computational framework that addresses the lamellar nature of these microstructures, by homogenizing the plastic deformation at the mesoscale by using the microscale response of the laminates. Failure is accounted for by introducing a family of damaging planes that are parallel to the layer interface. Mode I, mode II and mixed-mode opening are incorporated. The planes along which failure occurs are captured using a smeared damage approach. Coupling of damage with isotropic or anisotropic plasticity models, like crystal plasticity, is straightforward. The damaging planes and directions do not need to correspond to crystalline slip planes, and normal opening is also included. Focus is given on rate-dependent formulations of plasticity and damage, i.e. converged results can be obtained without further regularization techniques. The validation of the model using experimental observations in martensite-austenite lamellar microstructures in steels reveals that the model correctly predicts the main features of the onset of failure, e.g. the necking point, the failure initiation region and the failure mode. Finally, based on the qualitative results obtained, some material design guidelines are provided for martensitic and multi-phase steels.

Ferrer, M M; Gouveia, A F; Gracia, L; Longo, E; Andrés, J

doi: 10.1088/0965-0393/24/2/025007pmid: N/A

Essentially, the exposed crystal planes of a given material, which primarily determine their morphology, tremendously affect its behavior. First principle calculations, based on the Wulff construction model and broken bonding density index, have been performed to calculate the equilibrium and their transformations for different metal oxides: Co3O4, α-Fe2O3, and In2O3. Present results point out that starting by surface thermodynamics is a helpful approach to predict and assess the morphology transformations of these materials. These complete set of morphologies may serve as a guide for researchers, when analyzing the images from electron microscopies, to gain further understanding of how to control crystal shape synthetically by tuning the surface chemistry and by controlling the relative values of surface energies.

Teijeiro, Carlos; Hammerschmidt, Thomas; Seiser, Bernhard; Drautz, Ralf; Sutmann, Godehard

doi: 10.1088/0965-0393/24/2/025008pmid: N/A

The modeling of materials at the atomistic level with interatomic potentials requires a reliable description of different bonding situations and relevant system properties. For this purpose, analytic bond-order potentials (BOPs) provide a systematic and robust approximation to density functional theory (DFT) and tight binding (TB) calculations at reasonable computational cost. This paper presents a formal analysis of the computational complexity of analytic BOP simulations, based on a detailed assessment of the most computationally intensive parts. Different implementation algorithms are presented alongside with optimizations for efficient numerical processing. The theoretical complexity study is complemented by systematic benchmarks of the scalability of the algorithms with increasing system size and accuracy level of the BOP approximation. Both approaches demonstrate that the computation of atomic forces in analytic BOPs can be performed with a similar scaling as the computation of atomic energies.

Pavlů, J; Vřešťál, J; Šob, M

doi: 10.1088/0965-0393/24/2/025009pmid: N/A

We analyse, from first-principles, the energetics and magnetic ordering of sigma phases in Co–Mo and Fe–Mo systems. Total energy differences between the sigma phase and Standard Element Reference (SER) structures are calculated in the whole concentration range at equilibrium volumes by means of the linear muffin-tin orbitals method in the atomic-sphere approximation (LMTO-ASA), the full-potential linearised augmented-plane waves (FLAPW) method and the pseudopotential approach. They are compared with the enthalpy of formation of sigma phase obtained from the phase equilibria calculations at higher temperature based on the semiempirical CALPHAD (CALculation of PHAse Diagram) method. It turns out that the binary sigma phases are more stable than the weighted average of the sigma phase of elemental constituents and that this stability for Fe–Mo is higher than for Co–Mo. On the other hand it was found that the binary sigma phases do not exhibit any stability with respect to the weighted average of the SER structures. The magnetic configurations in all systems are investigated and the stabilizing effect of magnetic order in sigma phase at 0 K is presented. It turns out that the atomic magnetic moment strongly depends on the type of occupied sublattice and total composition of the alloy.

Kim, Jinwon; Kim, Sung S

doi: 10.1088/0965-0393/24/2/025010pmid: N/A

The phase diagram in the UO2–ThO2 system has been assessed by thermodynamic modeling with existing data from the literature. The subregular solution model was used to represent the Gibbs free energies of the liquid and the solid phases. By considering the liquidus, solidus and miscibility gap data, the interaction parameters of the liquid and the solid phases were optimized through a multiple linear regression method. A consistent set of interaction parameters were derived for describing the miscibility gap as well as the liquidus/solidus. The phase diagram calculated in the present work is in good agreement with experimental data in the literature.