J. Aldazabal

Simulation of the microstructural evolution during liquid phase sintering using a geometrical Monte Carlo model

The present paper proposes a Monte Carlo model to simulate the evolution of the microstructure during liquid phase sintering. The model mainly assumes that the system temperature is constant and that the composition of the liquid phase is completely homogeneous, but it is also capable of simulating the final cooling stage of the sintering process. It works with solidification, or melting, probabilities of volume elements (voxels) that are obtained accounting for the local geometry via the closest surrounding neighbours and avoiding the use of thermodynamic probabilities. This method reproduces an isotropic behaviour during solidification and melting and gives growth and melt rates proportional to surface curvatures. The proposed model provides realistic simulations of the microstructural evolution that takes place in real materials during liquid phase sintering.

Original article: Diffusional Monte Carlo model of liquid-phase sintering

Abstract: Liquid-phase sintering (LPS) is a consolidation process for metallic and ceramic powders. At given temperature conditions, the process occurs with constant amount of liquid phase. However, the evolution of solid-particle shape is observed, namely, the rounding of particles and the growth of big particles at the expense of the small ones, which is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS. The model considers the change of state of the discretising elements, namely voxels, of the system. The microstructural evolution proceeds accounting for both the geometrical characteristics of the particles, such as the number of solid neighbours, and the amount of solute contained in or surrounding a randomly chosen voxel. This has been implemented in terms of two probability distribution functions (PDFs). The diffusion of solute has also been considered by means of the implementation of a three-dimensional finite-difference algorithm. The diffusional MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, results match the case in which the process is limited by the diffusion of the solute in the liquid phase.

Plastic deformation by conservative shear-coupled migration of tilt boundaries with intergranular nano-cracks or precipitates

We present molecular dynamics (MD) simulations of the shear-coupled migration (SCM) behaviour of symmetrical tilt boundaries perturbed by the presence of nano-cracks or nano-precipitates lying on the boundary plane. The simulations have been performed for copper bicrystals at room temperature (300 K). The tilt boundary gets pinned by the crack tip or precipitates; shear-coupled migration occurs only ahead of the pinning points. Bulging of the tilt boundary reduces the shear stress on the boundary surface near the pinning points. In the case of cracks, the local deviation of the boundary from the crack plane close to the crack tip hinders mode II crack propagation; in fact, crack healing is observed in some cases. The applied stress grows until depinning of the boundary takes place by SCM bulging or by the combined action of SCM with another deformation mechanism (emission of dislocations from the pinning point vicinity, grain boundary sliding).

Geometrical Monte Carlo model of liquid-phase sintering

Liquid-phase sintering (LPS) is an industrial process used to consolidate materials composed of two different kinds of metallic and/or ceramic powders. At constant temperature, the amount of the present liquid-phase is constant. However, the shape of particles of solid phase changes over time. In general, the rounding of particles and the growth of big particles at the expense of the small ones are observed. This process is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS. The discretizing elements of the system, namely the voxels, change state between solid and liquid, according to previously defined melting and solidification probability distribution functions (PDFs). The generated PDFs take into account the geometrical characteristics of the system particles in terms of number of solid neighbours that surround a randomly chosen voxel. The geometrical MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, matches the case in which the process occurs limited by the attachment/detachment of the solid phase to/from the surface of the particle.

A mesoscopic plasticity model accounting for spatial fluctuations of plastic strains, internal stresses and dislocation densities

Abstract A very simple one-dimensional cellular automaton numerical model of plastic deformation that takes into account the local heterogeneity of strains, stresses and dislocation density has bee...

Couple stresses and the fracture of rock

An assessment is made here of the role played by the micropolar continuum theory on the cracked Brazilian disc test used for determining rock fracture toughness. By analytically solving the corresponding mixed boundary-value problems and employing singular-perturbation arguments, we provide closed-form expressions for the energy release rate and the corresponding stress-intensity factors for both mode I and mode II loading. These theoretical results are augmented by a set of fracture toughness experiments on both sandstone and marble rocks. It is further shown that the morphology of the fracturing process in our centrally pre-cracked circular samples correlates very well with discrete element simulations.

Rational design of artificial cellular niches for tissue engineering

Tissue Engineering is a promising emerging field that studies the intrinsic regenerative potential of the human body and uses it to restore functionality of damaged organs or tissues unable of self-healing due to illness or ageing. In order to achieve regeneration using Tissue Engineering strategies, it is first necessary to study the properties of the native tissue and determine the cause of tissue failure; second, to identify an optimum population of cells capable of restoring its functionality; and third, to design and manufacture a cellular microenvironment in which those specific cells are directed towards the desired cellular functions. The design of the artificial cellular niche has a tremendous importance, because cells will feel and respond to both its biochemical and biophysical properties very differently. In particular, the artificial niche will act as a physical scaffold for the cells, allowing their three-dimensional spatial organization; also, it will provide mechanical stability to the artificial construct; and finally, it will supply biochemical and mechanical cues to control cellular growth, migration, differentiation and synthesis of natural extracellular matrix. During the last decades, many scientists have made great contributions to the field of Tissue Engineering. Even though this research has frequently been accompanied by vast investments during extended periods of time, yet too often these efforts have not been enough to translate the advances into new clinical therapies. More and more scientists in this field are aware of the need of rational experimental designs before carrying out complex, expensive and time-consuming in vitro and in vivo trials. This review highlights the importance of computer modeling and novel biofabrication techniques as critical key players for a rational design of artificial cellular niches in Tissue Engineering.

Computer Simulation of C-N-V Precipitates Evolution Based on Local Concentration Fluctuations

From a “macroscopic” point of view, steel composition is assumed to vary smoothly along its microstructure. A closer look reveals that, on the atomic level the material composition does not change so smoothly. Single atoms jump randomly along the crystal lattice due to their thermal energy. These random jumps create sporadic zones of the crystal with higher concentration of certain species, and they are responsible for many phenomena, such as precipitation, Ostwald ripening, some phase transformations… This paper proposes a model to simulate the evolution of C-N-V precipitates in microalloyed steels heat treated in the range of warm temperatures (800-900 °C); when the matrix is austenite (fcc), thus taking into account for the local composition fluctuations. The model works by dividing the space into very small cells, containing a single atomic cell each. If during the random movement of atoms a cell that touches a precipitate reaches some critical composition, it is very easy to stick it to the precipitate by changing its “phase”. But it is also possible that some atoms escape from the precipitate by jumping to the austenitic matrix. Both processes happening simultaneously, and which one is leading depends on the atoms energy, i.e. system temperature.