arXiv subjects

J. LLorca

Publications and source records attributed to J. LLorca.

At least 19 recordsLinked to original sources

arXiv2024

Grain growth competition and formation of grain boundaries during solidification of hcp alloys

Grain growth competition during directional solidification of a polycrystal with hexagonal (hcp) symmetry (Mg-1wt%Gd alloy) is studied by phase-field modeling, exploring the effect of the temperature gradient G on the resulting grain boundary (GB) orientation selection. Results show that selection mechanisms and scaling laws derived for cubic (fcc, bcc) crystals also apply to hcp materials (within their basal plane), provided a re-estimation of fitting parameters and re-scaling to account for the sixfold symmetry. While grain growth competition remains stochastic with rare events of unexpected elimination or side-branching along the developing GBs, we also confirm an overall transition from a geometrical limit to a favorably oriented grain limit behavior with an increase of thermal gradient within the dendritic regime, and the progressive alignment of dendrites and GBs toward the temperature gradient direction with an increase of G during the dendritic-to-cellular morphological transition. Comparisons with original thin-sample directional solidification experiments show a qualitative agreement with PF results, yet with notable discrepancies, which nonetheless can be explained based on the stochastic variability of selected GB orientations, and the statistically limited experimental sample size. Overall, our results extend the understanding of GB formation and grain growth competition during solidification of hcp materials, and the effect of thermal conditions, nonetheless concluding on the challenges of extending the current studies to three dimensions, and the need for much broader (statistically significant) data sets of GB orientation selected under well-identified solidification conditions.

cond-mat.mtrl-sci↗

arXiv2023

Assessment of slip transfer criteria for prismatic-to-prismatic slip in pure Ti from 3D grain boundary data

Slip transfer and blocking across grain boundaries was studied in a Ti foil with a strong rolling texture deformed in tension. Prior to deformation, the shape of the grains and the orientation of the grain boundaries were quantified through laboratory scale diffraction contrast tomography (LabDCT). Mechanical deformation led to the activation of prismatic slip, and slip transfer/blocking was assessed in > 300 grain boundaries by means of slip trace analysis and electron backscatter diffraction. A categorical model was employed to accurately assess slip transfer, and the "F1 score" of various slip transfer criteria proposed in the literature was evaluated for the first time from 3D grain boundary information. Remarkably, for the prismatic-dominated slip transfer in the current Ti sample, the results show that the best predictions of slip transfer/blocking are provided by the angle \k{appa}, which is directly related to the residual Burgers vector, and by the Luster-Morris parameter m'. In contrast, metrics based on the twist angle {\theta} and on the LRB criterion were not able to predict accurately slip transfer/blocking. Thus, the extensive analysis of the 3D grain boundary data and the novel application of LabDCT was able to help clarify the role of grain boundary orientation on the mechanisms of plastic deformation in polycrystals with strong prismatic-dominated slip.

cond-mat.mtrl-sci↗

arXiv2023

Processing and mechanical properties of novel biodegradable poly-lactic acid/Zn 3D printed scaffolds for application in tissue regeneration

The feasibility to manufacture scaffolds of poly-lactic acid reinforced with Zn particles by fused filament fabrication is demonstrated for the first time. Filaments of 2.85 mm in diameter of PLA reinforced with different weight fractions of $\mu$m-sized Zn - 1 wt. \% Mg alloy particles (in the range 3.5 to 17.5 wt. \%) were manufactured by a double extrusion in method in which standard extrusion is followed by a precision extrusion in a filament-maker machine. Filaments with constant diameter, negligible porosity and a homogeneous reinforcement distribution were obtained for Zn weight fractions of up to 10.5\%. It was found that the presence of Zn particles led to limited changes in the physico-chemical properties of the PLA that did not affect the window temperature for 3D printing nor the melt flow index. Thus, porous scaffolds could be manufactured by fused filament fabrication at 190\textdegree C with poly-lactic acid/Zn composites containing 3.5 and 7 wt. \% of Zn and at 170\textdegree C when the Zn content was 10.5 wt. \% with excellent dimensional accuracy and mechanical properties.

physics.med-ph↗

arXiv2023

Phase-field modeling of pitting and mechanically-assisted corrosion of Mg alloys for biomedical applications

A phase-field model is developed to simulate the corrosion of Mg alloys in body fluids. The model incorporates both Mg dissolution and the transport of Mg ions in solution, naturally predicting the transition from activation-controlled to diffusion-controlled bio-corrosion. In addition to uniform corrosion, the presented framework captures pitting corrosion and accounts for the synergistic effect of aggressive environments and mechanical loading in accelerating corrosion kinetics. The model applies to arbitrary 2D and 3D geometries with no special treatment for the evolution of the corrosion front, which is described using a diffuse interface approach. Experiments are conducted to validate the model and a good agreement is attained against in vitro measurements on Mg wires. The potential of the model to capture mechano-chemical effects during corrosion is demonstrated in case studies considering Mg wires in tension and bioabsorbable coronary Mg stents subjected to mechanical loading. The proposed methodology can be used to assess the in vitro and in vivo service life of Mg-based biomedical devices and optimize the design taking into account the effect of mechanical deformation on the corrosion rate. The model has the potential to advocate further development of Mg alloys as a biodegradable implant material for biomedical applications.

physics.med-ph↗

arXiv2022

First-principles analysis of the Al-rich corner of Al-Li-Cu phase diagram

The phase diagram of Al-Li-Cu system in the Al-rich region was determined by means of first-principles calculations and statistical mechanics. The mixing enthalpies of many configurations for different lattices in the whole Al-Li-Cu system were determined by density functional theory simulations to find the stable phases in the convex hull. They were fitted with a cluster expansion to calculate the free energy of the configurations with different compositions as a function of temperature in the Al-rich region (Al content > 40 at. %) by means of Monte Carlo simulations. It was found that the ground state phases in the Al-rich part of the Al-Li-Cu phase diagram were {\alpha}-Al, {\theta}' (Al2Cu), {\delta}' (Al3Li), {\delta} (AlLi) and T1 (Al6Cu4Li3), while {\theta}'' (Al3Cu), T1' (Al2CuLi) and Al3Cu2Li were found on the lowest mixing enthalpy surfaces of their lattices and were metastable. {\alpha}-Al, {\delta} and T1 are stable phases in the whole temperature range while {\delta}' becomes metastable at very low temperature and {\theta} (Al2Cu) replaces {\theta}' as the stable phase at approximately 550 K due to the vibrational entropic contribution. In addition, the phase diagram in the Al-rich region was built and it was shown in isothermal sections from 100 K to 900 K. They were in good agreement with the limited experimental data in the literature and provided new information regarding the stability, solubility and stoichiometry of the different phases. This information is important to understand the precipitation mechanisms during high temperature aging.

cond-mat.mtrl-sci↗

arXiv2022

A generalized line tension model for precipitate strengthening in metallic alloys

A generalized line tension model has been developed to estimate the critical resolved shear stress in precipitation hardened alloys. The model is based in previous line tension models for regular arrays of either impenetrable or shearable spherical precipitates that were expanded to take into account the effect of the elastic mismatch between the matrix and the precipitates. The model parameters are calibrated from dislocation dynamics simulations that covered a wide range of precipitate diameters and spacing as well as of the mismatch in elastic constants. This model is extended to deal with random arrays of monodisperse spherical precipitates by changing the geometrical parameters of the model by the averaged ones corresponding to the random distributions. The model predictions were in good agreement with the critical resolved shear stresses obtained from dislocation dynamics simulations of random spherical precipitates distributions for both impenetrable and shearable precipitates, providing a fast and accurate tool to predict precipitate strengthening in metallic alloys.

cond-mat.mtrl-sci↗

arXiv2022

Icosahedral quasicrystal enhanced nucleation in commercially pure Ni processed by selective laser melting

This work provides unambiguous evidence for the occurrence of icosahedral quasicrystal (iQC) enhanced nucleation during selective laser melting of gas atomized commercially-pure Ni powders. This solidification mechanism, which has only been recently reported in a few alloys and has to date never been observed in pure metals, consists on the solidification of grains of the primary phase on the facets of iQCs formed due to the presence of icosahedral short range order in the liquid. The occurrence of iQC enhanced nucleation has been inferred from the observation in the SLM processed pure Ni samples of an excess fraction of partially incoherent twin boundaries and of clusters of twinned grain pairs sharing common <110> five-fold symmetry axes. This work further evidences that additive manufacturing methods may constitute an invaluable tool for investigating the fundamentals of solidification and for the design of unprecedented grain boundary networks.

cond-mat.mtrl-sci↗

arXiv2021

Criteria for slip transfer across grain and twin boundaries in pure Ni

Slip transfer at grain boundaries and annealing twin boundaries was studied in polycrystalline Ni by means of slip trace analysis. Slip transfer or blocking was assessed in $>$ 200 boundaries and was related with geometrical criteria that establish the alignment between the active slip systems across the boundaries. It was found that slip transfer mainly occurs at low-angle regular grain boundaries and that a Luster-Morris parameter $>$ 0.8 stands for the best criterion to assess slip transfer. In the case of coherent or incoherent twin boundaries, slip transfer occurs when the residual Burgers vector is close to zero through dislocation cross-slip.

cond-mat.mtrl-sci↗

arXiv2021

On the effect of slip transfer at grain boundaries on the strength of FCC polycrystals

The effect of slip transfer on the flow strength of various FCC polycrystals was analyzed by means of computational homogenization of a representative volume element of the microstructure. The crystal behavior was governed by a physically-based crystal plasticity model in the framework of finite strains where slip transfer at grain boundaries was allowed between slip systems suitably oriented according to geometrical criteria. Conversely, slip transfer was blocked if the conditions for slip transfer were not fulfilled, leading to the formation of dislocation pile-ups. All the model parameters for each material were identified from either dislocation dynamics simulations or experimental data from the literature. Slip transfer led to a reduction in the flow stress of the polycrystals (as compared with the simulations with opaque grain boundaries) which was dependent on the fraction of translucent and transparent grain boundaries in the microstructure. Moreover, dislocation densities and Von Mises stresses were much higher around opaque grain boundaries, which become suitable places for damage nucleation. Finally, predictions of the Hall-Petch effect in Al, Ni, Cu and Ag polycrystals including slip transfer were in better agreement with the literature results, as compared with predictions assuming that all grain boundaries are opaque, particularly for small grain sizes ($<$ 20 $\mu$m).

cond-mat.mtrl-sci↗

arXiv2021

First principles prediction of the Al-Li phase diagram

The phase diagram of the Al-Li system was determined by means of first principles calculations in combination with the cluster expansion formalism and statistical mechanics. The ground state phases were determined from first principles calculations of fcc and bcc configurations in the whole compositional range while the phase transitions as a function of temperature were ascertained from the thermodynamic grand potential and the Gibbs free energies of the phases. Overall, the calculated phase diagram was in good agreement with the currently accepted experimental phase diagram but the simulations provided new insights that are important to optimize microstructure of these alloys by means of heat treatments. In particular, the structure of the potential GP zones, made up of Al0.5Li0.5 (001) monolayers embedded in Al matrix, was identified. It was found that Al3Li is a stable phase although the energy barrier for the transformation of Al3Li into AlLi is very small (a few meV) and can be overcome by thermal vibrations. Moreover, bcc AlLi was found to be formed by martensitic transformation of fcc configurations and Al3Li precipitates stand for favorable sites for the nucleation of AlLi because they contain the basic blocks of such fcc ordering. Finally, polynomial expressions of the Gibbs free energies of the different phases as a function of temperature and composition were given, so they can be used in mesoscale simulations of precipitation in Al-Li alloys.

cond-mat.mtrl-sci↗

arXiv2021

Icosahedral quasicrystal-enhanced nucleation in Al alloys fabricated by selective laser melting

Selective laser melting (SLM) is rapidly evolving to become a mainstream technology. However, the fundamental mechanisms of solidification and microstructure development inherent to the non-equilibrium conditions of this additive manufacturing method, which differ largely from those typical of conventional processing techniques, remain widely unknown. In this work, an in-depth characterization of the microstructure of Al7075 SLM processed samples, built from powder mixtures containing ZrH2 microparticles, demonstrates the occurrence of icosahedral quasicrystal-enhanced nucleation during laser fabrication. This solidification mechanism, only observed to date in cast Al-Zn and yellow gold alloys containing minute additions of Cr (Kurtuldu et al., 2013) or Ti (Chen et al. 2018), and Ir (Kurtuldu et al., 2014), is evidenced by the presence of an abnormally high fraction of twin boundaries and of five-fold orientation symmetry between twinned nearest neighbors lying within a matrix of equiaxed, randomly textured, ultrafine grains. This research attests to the wide range of possibilities offered by additive manufacturing methods for the investigation of novel physical metallurgy phenomena as well as for the design of advanced metals.

cond-mat.mtrl-sci↗

arXiv2021

Phase-field modeling of microstructure evolution: Recent applications, perspectives and challenges

We briefly review the state-of-the-art in phase-field modeling of microstructure evolution. The focus is placed on recent applications of phase-field simulations of solid-state microstructure evolution and solidification that have been compared and/or validated with experiments. They show the potential of phase-field modeling to make quantitative predictions of the link between processing and microstructure. Finally, some current challenges in extending the application of phase-field models within the context of integrated computational materials engineering are mentioned.

cond-mat.mtrl-sci↗

arXiv2021

Effect of ZrH2 particles on the microstructure and mechanical properties of IN718 manufactured by selective laser melting

The influence of Zr additions (in the form of ZrH2 particles) on the microstructure and mechanical properties of IN718 Ni-based superalloy manufactured by selective laser melting was explored. Fully dense alloys could be obtained by careful selection of the processing parameters. The addition of ZrH2 did not modify the microstructure of the alloy but introduced a dispersion of Zr-rich globular particles of approximately 50 nm in diameter which increased the strength of the as-printed material at ambient (23C) and high temperature (550C). Heat treatments of solubilisation followed by aging led to a fine dispersion of gamma" precipitates that controlled the strength of the alloy, which was independent of the addition of Zr. Moreover, dynamic strain ageing was found in the heat treated materials when deformed at 550C. Finally, the strength of the IN718 deformed perpendicular to the building direction was higher than that along the building direction, regardless of the heat treatment and temperature. Computational homogenization simulations showed that the anisotropy was associated with the strong texture.

cond-mat.mtrl-sci↗

arXiv2021

Dislocation dynamics prediction of the strength of Al-Cu alloys containing shearable $\theta''$ precipitates

The critical resolved shear stress of an Al 4 wt. \% Cu alloy containing a homogeneous distribution of $\theta''$ precipitates was determined by means of dislocation dynamics simulations. The size distribution, shape, orientation and volume fraction of the precipitates in the alloy were obtained from transmission electron microscopy observations while the parameters controlling the dislocation/precipitate interactions (elastic mismatch, transformation strains, dislocation mobility and cross-slip probability, etc.) were calculated from atomistic simulations. The precipitates were assumed to be either impenetrable or shearable by the dislocations, the latter characterized by a threshold shear stress that has to be overcome to shear the precipitate. The predictions of the simulations in terms of the critical resolved shear stress and of the dislocation/precipitate interaction mechanisms were in good agreement with the experimental results. It was concluded that the optimum strength of this alloy is attained with a homogeneous distribution of $\theta''$ precipitates whose average size ($\approx$ 40 nm) is at the transition between precipitate shearing and looping. Overall, the dislocation dynamics strategy presented in this paper is able to provide quantitative predictions of precipitate strengthening in metallic alloys.

cond-mat.mtrl-sci↗

arXiv2021

Multiscale prediction of microstructure length scales in metallic alloy casting

In this article, we combine casting experiments and quantitative simulations to present a novel multiscale modeling approach to predict local primary dendritic spacings in metallic alloys solidified in conditions relevant to industrial casting processes. To this end, primary dendritic spacings were measured in instrumented casting experiments in Al-Cu alloys containing 1\,wt\% and 4\,wt\% of Cu, and they were compared to spacing stability ranges and average spacings in dendritic arrays simulated using phase-field (PF) and dendritic needle network (DNN) models. It is first shown that PF and DNN lead to similar results for the Al-1\,wt\%Cu alloy, using a dendrite tip selection constant calculated with PF in the DNN simulations. PF simulations cannot achieve quantitative predictions for the Al-4\,wt\%Cu alloy because they are too computationally demanding due to the large separation of scale between tip radius and diffusion length, a characteristic feature of non-dilute alloys. Nevertheless, the results of DNN simulations for non-dilute Al-Cu alloys are in overall good agreement with our experimental results as well as with those of an extensive literature review. Simulations consistently suggest a widening of the PDAS stability range with a decrease of the temperature gradient as the microstructure goes from cellular-dendrites to well-developed hierarchical dendrites.

cond-mat.mtrl-sci↗

arXiv2020

Microstructure, mechanical properties, corrosion resistance and cytocompatibility of WE43 Mg alloy scaffolds fabricated by laser powder bed fusion for biomedical applications

Open-porous scaffolds of WE43 Mg alloy with a body-center cubic cell pattern were manufactured by laser powder bed fusion with different strut diameters. The geometry of the unit cells was adequately reproduced during additive manufacturing and the porosity within the struts was minimized. The microstructure of the scaffolds was modified by means of thermal solution and ageing heat treatments and was analysed in detail by means of X-ray microtomography, optical, scanning and transmission electron microscopy. Moreover, the corrosion rates and the mechanical properties of the scaffolds were measured as a function of the strut diameter and metallurgical condition. The microstructure of the as-printed scaffolds contained a mixture of Y-rich oxide particles and Rare Earth-rich intermetallic precipitates. The latter could be modified by heat treatments. The lowest corrosion rates of 2-3 mm/year were found in the as-printed and solution treated scaffolds and they could be reduced to ~0.1 mm/year by surface treatments using plasma electrolytic oxidation. The mechanical properties of the scaffolds improved with the strut diameter: the yield strength increased from 8 to 40 MPa and the elastic modulus improved from 0.2 to 0.8 GPa when the strut diameter increased from 275 \mu m to 800 \mu m. Nevertheless, the strength of the scaffolds without plasma electrolytic oxidation treatment decreased rapidly when immersed in simulated body fluid. In vitro biocompatibility tests showed surface treatments by plasma electrolytic oxidation were necessary to ensure cell proliferation in scaffolds with high surface-to-volume ratio.

cond-mat.mtrl-sci↗

arXiv2020

First principles analysis of precipitation in Mg-Zn alloys

Precipitation in Mg-Zn alloys was analyzed by means of first principles calculations. Formation energies of symmetrically distinct hcp Mg1-xZnx (0 < x < 1) configurations were determined and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (Mg4Zn7, MgZn2 cubic, MgZn2 hexagonal, Mg21Zn25 and Mg2Zn11) were calculated. The formation energies of Mg4Zn7, MgZn2 cubic, and MgZn2 hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that \beta_1^' precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high temperature stability. It was found that the MgZn2 hexagonal Laves phase is the most stable phase at high temperature and, thus, \beta_1^' precipitates tend to transform into the \beta_2^' (MgZn2 hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium \beta phase (Mg21Zn25) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase \beta_2^' to \beta .

cond-mat.mtrl-sci↗

arXiv2020

High temperature strength retention of Cu/Nb nanolaminates through dynamic strain ageing

The mechanical properties of Cu/Nb metallic nanolaminates with different layer thickness (7, 16, 34 and 63 nm) were studied by means of micropillar compression tests from room temperature to 400 C. Both strain-rate jump and constant strain rate tests were carried out and they showed evidence of dynamic strain ageing in the nanolaminates with 7, 16 and 34 nm layer thickness deformed at 200 C. Dynamic strain ageing was accompanied by a reduction of the strain rate sensitivity to 0, high strength retention at 200 C and the development of shear localization of the deformation at low strains (5%-6%) that took place along the Nb layers in the nanolaminates. Atom probe tomography of the deformed specimens revealed the presence of O in solid solution in the Nb layers but not in the Cu layers. Thus, diffusion of O atoms to the mobile dislocations in Nb was found to be the origin of the dynamic strain ageing in the Cu/Nb nanolaminates around 200 C. This mechanism was not found at higher temperatures (400 C) because deformation was mainly controlled by stress-assisted diffusion in the Cu layers. This discovery shows a novel strategy to enhance the strength retention at high temperature of metallic nanolaminates through dynamic strain ageing of one the phases.

cond-mat.mtrl-sci↗