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J. LLorca

Publications and source records attributed to J. LLorca.

53 records · Page 3Linked to original sources
arXiv2018

Tensile deformation and fracture mechanisms of Cu/Nb nanolaminates studied by in situ TEM mechanical tests

The mechanisms of deformation and failure of Cu/Nb nanolaminates manufactured by accumulated roll bonding were analysed using in-situ TEM mechanical tests. Dog-bone small-scale tensile specimens were prepared using a FIB-based technique with the layers parallel and perpendicular to the loading axis. Load, deformation and TEM micrographs were recorded during the in-situ TEM mechanical tests. The specimens deformed parallel to the layers presented very high strain hardening and ductility while those deformed in the perpendicular orientation were more brittle. These results were rationalized in terms of the deformation and fracture mechanisms observed during the tests.

cond-mat.mtrl-sci↗
arXiv2018

An atomistic investigation of the interaction of dislocations with Guinier-Preston zones in Al-Cu alloys

The interaction between edge dislocations and Guinier-Preston zones in an Al-Cu alloy was analyzed by means of atomistic simulations. The different thermodynamic functions that determine the features of these obstacles for the dislocation glide were computed using molecular statics, molecular dynamics and the nudged elastic band method. It was found that Guinier-Preston zones are sheared by dislocations and the rate at which dislocations overcome the precipitate is controlled by the activation energy, $ΔU$, in agreement with the postulates of the harmonic transition state theory. Moreover, the entropic contribution to the Helmholtz activation free energy was in the range 1.3-1.8$k_b$, which can be associated with the typical vibrational entropy of solids. Finally, an estimation of the initial shear flow stress as a function of temperature was carried out from the thermodynamic data provided by the atomistic simulations. Comparison with experimental results showed that the effect of the random precipitate distribution and of the dislocation character and dislocation/precipitation orientation has to be taken into account in the simulations to better reproduce experiments.

cond-mat.mtrl-sci↗
arXiv2018

Computational Homogenization of Polycrystals

This paper reviews the current state-of-the-art in the simulation of the mechanical behavior of polycrystalline materials by means of computational homogenization. The key ingredients of this modelling strategy are presented in detail starting with the parameters needed to describe polycrystalline microstructures and the digital representation of such microstructures in a suitable format to perform computational homogenization. The different crystal plasticity frameworks that can describe the physical mechanisms of deformation in single crystals (dislocation slip and twinning) at the microscopic level are presented next. This is followed by the description of computational homogenization methods based on mean-field approximations by means of the viscoplastic self-consistent approach, or on the full-field simulation of the mechanical response of a representative polycrystalline volume element by means of the finite element method or the fast Fourier transform-based method. Multiscale frameworks based on the combination of mean-field homogenization and the finite element method are presented next to model the plastic deformation of polycrystalline specimens of arbitrary geometry under complex mechanical loading. Examples of application to predict the strength, fatigue life, damage, and texture evolution under different conditions are presented to illustrate the capabilities of the different models. Finally, current challenges and future research directions in this field are summarized.

cond-mat.mtrl-sci↗
arXiv2018

Discrete dislocation dynamics simulations of dislocation-$θ'$ precipitate interaction in Al-Cu alloys

The mechanisms of dislocation/precipitate interaction were studied by means of discrete dislocation dynamics within a multiscale approach. Simulations were carried out using the discrete continuous method in combination with a fast Fourier transform solver to compute the mechanical fields. The original simulation strategy was modified to include straight dislocation segments by means of the field dislocation mechanics method and was applied to simulate the interaction of an edge dislocation with a $θ'$ precipitate in an Al-Cu alloy. It was found that the elastic mismatch has a negligible influence on the dislocation/precipitate interaction in the Al-Cu system. Moreover, the influence of the precipitate aspect ratio and orientation was reasonably well captured by the simple Orowan model in the absence of the stress-free transformation strain. Nevertheless, the introduction of the stress-free transformation strain led to dramatic changes in the dislocation/precipitate interaction and in the critical resolved shear stress to overcome the precipitate, particularly in the case of precipitates with small aspect ratio. The new multiscale approach to study the dislocation/precipitate interactions opens the possibility to obtain quantitative estimations of the strengthening provided by precipitates in metallic alloys taking into account the microstructural details.

cond-mat.mtrl-sci↗
arXiv2018

Influence of temperature on the strain rate sensitivity and deformation mechanisms of nanotwinned Cu

The mechanical behavior of nanotwinned Cu was studied through indentation creep and constant strain rate indentation tests from 25 $^\circ$C to 200 $^\circ$C. The results showed an enhanced strain rate sensitivity of nanotwinned Cu with temperature, which was higher than that found in coarse-grained Cu. Transmission electron microscopy revealed the same deformation mechanism in the whole temperature range: confined dislocation slip between coherent twin boundaries and formation of dislocation pile-ups at the coherent twin boundaries. The mechanisms responsible for large increase in strain rate sensitivity of nanotwinned Cu with temperature were discussed to the light of experimental observations.

cond-mat.mtrl-sci↗
arXiv2018

Crystal plasticity simulation of the effect of grain size on the fatigue behavior of polycrystalline Inconel 718

A microstructure-based model that accounts for the effect of grain size has been developed to study the effect of grain size on the fatigue life of Inconel 718 alloys. The fatigue behavior of two alloys with different grain size was determined by means of uniaxial cyclic deformation tests under fully-reversed deformation ($R_\varepsilon$ = -1) at 400$^\circ$C in the low cycle fatigue regime. The model was based in the determination of the fatigue indicator parameter (based on the local crystallographic strain energy dissipated per cycle) by means of computational homogenization of a representative volume element of the microstructure. The mechanical response of the single crystal within the polycrystal was modelled through a phenomenological crystal plasticity model which was modified to account for the effect of grain size on the monotonic and cyclic hardening/softening mechanisms. The microstructure-based crack initiation model parameters were calibrated from the experimental tests of the material with fine grain size. The results of the fatigue simulations were in good agreement with the experimental results in terms of the cyclic stress-strain curves and of the number of cycles for fatigue crack initiation. The model did not show any grain size effect on the fatigue life for the largest cyclic strain ranges while the predicted fatigue life predicted was considerably longer in the case of the microstructure with fine grain size for the lowest strain ranges, in quantitative agreement with experimental data. These differences were attributed to changes in the deformation modes between homogeneous plastic deformation at large cyclic strain ranges and localized plasticity in a few grains at low cyclic strain ranges.

cond-mat.mtrl-sci↗
arXiv2018

An analysis of the influence of grain size on the strength of FCC polycrystals by means of computational homogenization

The effect of grain size on the flow stress of FCC polycrystals is analyzed by means of a multiscale strategy based on computational homogenization of the polycrystal aggregate. The mechanical behavior of each crystal is given by a dislocation-based crystal plasticity model in which the critical resolved shear stress follows the Taylor model. The generation and annihilation of dislocations in each slip system during deformation is given by the Kocks-Mecking model, which was modified to account for the dislocation storage at the grain boundaries. Polycrystalline Cu is selected to validate the simulation strategy and all the model parameters are obtained from dislocation dynamics simulations or experiments at lower length scales and the simulation results were in good agreement with experimental data in the literature. The model is applied to explore the influence of different microstructural factors (initial dislocation density, width of the grain size distribution, texture) on the grain size effect. It is found that the initial dislocation density, $ρ_i$, plays a dominant role in the magnitude of the grain size effect and that dependence of flow stress with an inverse power of grain size ($σ_ y -σ_\infty \propto d_g^{-x}$) breaks down for large initial dislocation densities ($> 10^{14}$ m$^{-2}$) and grain sizes $d_g > $ 40 $μ$m in FCC metals. However, it was found that the grain size contribution to the strength followed a power-law function of the dimensionless parameter $d_g\sqrt{ρ_i}$ for small values of the applied strain ($<$ 2 \%), in agreement with previous theoretical considerations for size effects in plasticity.

cond-mat.mtrl-sci↗
arXiv2017

Microstructure-based fatigue life model of metallic alloys with bilinear Coffin-Manson behavior

A microstructure-based model is presented to predict the fatigue life of polycrystalline metallic alloys which present a bilinear Coffin-Manson relationship. The model is based in the determination of the maximum value of a fatigue indicator parameter obtained from the plastic energy dissipated by cycle in the microstructure. The fatigue indicator parameter was obtained by means of the computational homogenization of a representative volume element of the microstructure using a crystal-plasticity finite element model. The microstructure-based model was applied to predict the low cyclic fatigue behavior of IN718 at 400$^\circ$C which exhibits a bilinear Coffin-Manson relationship under the assumption that this behavior is triggered by a transition from highly localized plasticity at low cyclic strain amplitudes to more homogeneous deformation at high cyclic strain amplitudes. The model predictions were in very good agreement with the experimental results for a wide range of cyclic strain amplitudes and two strain ratios ($R_ε$ = 0 and -1) and corroborated the initial hypothesis. Moreover, they provided a micromechanical explanation for the influence of the strain ratio on the fatigue life at low cyclic strain amplitudes.

cond-mat.mtrl-sci↗
arXiv2017

Effects of layer thickness on the mechanical behavior of oxidation-strengthened Zr/Nb nanoscale multilayers

The mechanical behaviour and deformation mechanisms of magnetron-sputtered Zr/Nb nanoscale multilayers were analysed as a function of the periodicity (L = 10 and 75 nm) and annealing time at 350C (2-336 hrs). The strength of the as-deposited multilayers was independent of the layer thickness and it was controlled by the co-deformation of the Zr and Nb layers. Annealing led to transformation of the Zr layers into ZrO2 after a few hours, while the Nb layers oxidised at a much slower rate. The volumetric expansion associated with the oxidation led to the formation of cracks at the interfaces and within the ZrO2 layers for the multilayers with L = 75 nm but not in the case of L = 10 nm. The nanoindentation hardness increased significantly after annealing due to the contribution of the residual stresses associated with the volume increase due to oxidation and to the higher strength of the oxides. The strength increase after annealing, as measured by micropillar compression tests, was smaller than that measured by nanoindentation, as it did not include the contribution of the residual stresses, which were relieved during micropillar fabrication. The nanolaminate with L = 10 nm presented the highest strength and toughness as damage during oxidation was suppressed, and a deformation mechanism controlled by the formation of shear bands.

cond-mat.mtrl-sci↗
arXiv2017

Deformation mechanism map of Cu/Nb nanoscale metallic multilayers as a function of temperature and layer thickness

The mechanical properties and deformation mechanisms of Cu/Nb nanoscale metallic multilayers (NMMs) manufactured by accumulative roll bonding (ARB) are studied at 25C and 400C. Cu/Nb NMMs with individual layer thicknesses between 7 and 63 nm were tested by in-situ micropillar compression inside a scanning electron microscope Yield strength, strain-rate sensitivities and activation volumes were obtained from the pillar compression tests. The deformed micropillars were examined under scanning and transmission electron microscopy in order to examine the deformation mechanisms active for different layer thicknesses and temperatures. The analysis suggests that room temperature deformation was determined by dislocation glide at larger layer thicknesses and interface-related mechanisms at the thinner layer thicknesses. The high temperature compression tests, in contrast, revealed superior thermo-mechanical stability and strength retention for the NMMs with larger layer thicknesses with deformation controlled by dislocation glide. A remarkable transition in deformation mechanism occurred as the layer thickness decreased, to a deformation response controlled by diffusion processes along the interfaces, which resulted in temperature-induced softening. A deformation mechanism map, in terms of layer thickness and temperature, is proposed from the results obtained in this investigation.

cond-mat.mtrl-sci↗
arXiv2017

Multiscale modelling of the morphology and spatial distribution of θ' precipitates in Al-Cu alloys

A multiscale approach based on the phase-field model is developed to simulate homogeneous and heterogeneous formation of θ' precipitates during high temperature ageing in Al-Cu alloys. The model parameters that determine the different energy contributions (chemical free energy, interfacial energy, lattice parameters, elastic constants) were obtained from either computational thermodynamics databases or from first-principles density functional theory and molecular statics simulations. From the information, the evolution and equilibrium morphology of the θ' precipitates is simulated in 3D using the phase-field model. The model was able to reproduce the evolution of the different orientation variants of plate-like shaped θ' precipitates with orientation relationship (001)θ'//(001)α and [100]θ'//[100]α during homogeneous nucleation as well as the heterogeneous nucleation on dislocations, leading to the formation of precipitate arrays. Heterogeneous nucleation on pre-existing dislocation(s) was triggered by the interaction energy between the dislocation stress field and the stress-free transformation strain associated to the nucleation of the θ' precipitates. Moreover, the mechanisms controlling the evolution of the morphology and the equilibrium aspect ratio of the precipitates were ascertained. All the predictions of the multiscale model were in good agreement with experimental data.

cond-mat.mtrl-sci↗
arXiv2017

Structural composites for multifunctional applications: current challenges and future trends

This review paper summarizes the current state-of-art and challenges for the future developments of fiber-reinforced composites for structural applications with multifunctional capabilities. After a brief analysis of the reasons of the successful incorporation of fiber-reinforced composites in many different industrial sectors, the review analyzes three critical factors that will define the future of composites. The first one is the application of novel fiber-deposition and preforming techniques together with innovative liquid moulding strategies, which will be combined by optimization tools based on novel multiscale modelling approaches, so fiber-reinforced composites with optimized properties can be designed and manufactured for each application. In addition, composite applications will be enhanced by the incorporation of multifunctional capabilities. Among them, electrical conductivity, energy storage (structural supercapacitors and batteries) and energy harvesting (piezoelectric and solar energy) seem to be the most promising ones.

cond-mat.mtrl-sci↗
arXiv2016

Selective oxidation-induced strengthening of Zr/Nb nanoscale multilayers

The paper presents a new approach, based on controlled oxidation of nanoscale metallic multilayers, to produce strong and hard oxide/metal nanocomposite coatings with high strength and good thermal stability. The approach is demonstrated by performing long term annealing on sputtered Zr/Nb nanoscale metallic multilayers and investigating the evolution of their microstructure and mechanical properties by combining analytical transmission electron microscopy, nano-mechanical tests and finite element models. As-deposited multilayers were annealed at 350 C in air for times ranging between 1 - 336 hours. The elastic modulus increased by approx. 20% and the hardness by approx. 42% after 15 hours of annealing. Longer annealing times did not lead to changes in hardness, although the elastic modulus increased up to 35% after 336 hrs. The hcp Zr layers were rapidly transformed into monoclinic ZrO2 (in the first 15 hours), while the Nb layers were progressively oxidised, from top surface down towards the substrate, to form an amorphous oxide phase at a much lower rate. The sequential oxidation of Zr and Nb layers was key for the oxidation to take place without rupture of the multi-layered structure and without coating spallation, as the plastic deformation of the metallic Nb layers allowed for the partial relieve of the residual stresses developed as a result of the volumetric expansion of the Zr layers upon oxidation. Moreover, the development of residual stresses induced further changes in mechanical properties in relation to the annealing time, as revealed by finite element simulations.

cond-mat.mtrl-sci↗
arXiv2016

Computational study of atomic mobility in HCP Mg-Al-Zn ternary alloys

The experimental data in the literature for the hcp phase of the Mg-Al-Zn ternary system have been critically reviewed. Based on the concentration profiles from the literature, the diffusion coefficients have been re-extracted using the Hall method for the impurity diffusion, and the Sauer-Freise and the Whittle-Green strategies for interdiffusion coefficients in binary and ternary systems, respectively. Moreover, extra interdiffusion coefficients were obtained from the Darken-type couples, which present relative maxima or/and minima at the concentration profiles. This information was assessed to obtain an atomic mobility database, by means of DICTRA software in conjunction with the CALPHAD thermodynamic description that is able to reproduce the diffusion couple experiments. Comprehensive comparisons between the calculated results and experimental values show an excellent agreement not only for the diffusion coefficient data, but also for the concentration profiles and the diffusion paths.

cond-mat.mtrl-sci↗
arXiv2016

Landau modeling of dynamical nucleation of martensite at grain boundaries under local stress

The dynamical nucleation of martensite in polycrystals is simulated by means of Lagrange-Rayleigh dynamics with Landau energetics, which is capable of obtaining the local stress as a result of the interplay of the potential of transformation and external loadings. By monitoring the patio-temporal distribution of the strain in response to the local stress, we demonstrate that the postcursors, high angle grain boundaries and triple junctions act as favorable heterogeneous nucleation sites corresponding to different loading and cooling conditions, and predict the phase diagram of the nucleation mode of martensite.

cond-mat.mtrl-sci↗
arXiv2016

A 3D dislocation dynamics analysis of the size effect on the strength of [111] LiF micropillars at 300K and 600K

The mechanical behavior in compression of [111] LiF micropillars with diameters in the range 0.5 $μ$m to 2.0 $μ$m was analyzed by means of discrete dislocation dynamics at ambient and elevated temperature. The dislocation velocity was obtained from the Peach-Koehler force acting on the dislocation segments from a thermally-activated model that accounted for the influence of temperature on the lattice resistance. A size effect of the type "smaller is stronger" was predicted by the simulations, which was in quantitative agreement with previous experimental results by the authors \cite{SWC14}. The contribution of the different physical deformation mechanisms to the size effect (namely, nucleation of dislocations, dislocation exhaustion and forest hardening) could be ascertained from the simulations and the dominant deformation mode could be assessed as a function of the specimen size and temperature. These results shed light into the complex interaction among size, lattice resistance and dislocation mobility in the mechanical behavior of $μ$m-sized single crystals.

cond-mat.mtrl-sci↗