arXiv subjects

J. LLorca

Publications and source records attributed to J. LLorca.

At least 37 records · Page 2Linked to original sources

arXiv2020

Nanomechanical characterization of the fracture toughness of Al/SiC nanolaminates

The fracture toughness of Al/SiC nanolaminates with different layer thicknesses (in the range 10 to 100 nm) was measured by means of micropillar splitting and bending of a notched beam. The crack plane was perpendicular to the layers in the former while notched beams with the notch parallel and perpendicular to the layers were milled in the latter. It was found that crack propagation parallel to the layers took place along the metal-ceramic interfaces and the toughness increased with the layer thickness due to the contribution of the plastic deformation of the Al layers. Crack propagation perpendicular to the layers showed evidence of crack deflection/arrest at the interface. The toughness in this orientation increased as the layer thickness decreased due to the higher density of interfaces except for the nanolaminates with 10 nm layer thickness. In the latter case, crack propagation took place along the weak columnar grain boundaries, leading to a marked reduction in toughness.

cond-mat.mtrl-sci↗

arXiv2020

High temperature mechanical properties and microstructure of hard TaSiN coatings

Room and high temperature mechanical properties of reactive magnetron sputtered TaSiN coatings were measured using nanoindentation (between 25C and 500C). Fracture toughness was also evaluated at a similar temperature range using the micropillar splitting method. The influence of the nitrogen concentration on the evolving phases and microstructure of the TaSiN coatings, before and after the high temperature testing, were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. XRD spectra showed broad peaks with hexagonal $γ$-Ta2N as the main phase, with the cubic $δ$-TaN phase emerging for higher N contents. Phase composition remained unchanged before and after the 500C tests. However, after the high temperature tests, TEM analysis showed the presence of an oxide surface layer, with a thickness that decreased (from 42 to 15 nm) with N content, due to residual oxygen diffusion, which replaces nitrogen to form amorphous SiOx. Beneath the oxide-rich surface layer, coatings exhibited a stable nanocrystalline columnar microstructure. Hardness and fracture toughness increased with N content, initially due to the formation of an amorphous Si-N tissue at grain boundaries, and for even higher N contents, due to the appearance of the hard cubic $δ$-TaN phase. Hardness at 500C decreased only by 15%, while fracture toughness followed the opposite trend, due to increased plasticity with temperature. The optimum composition turned out to be Ta55Si10N35, which retained a hardness of 30 GPa at 500C, being also the toughest. These observations make this system very interesting for high temperature applications.

physics.app-ph↗

arXiv2020

An analysis of the influence of the precipitate type on the mechanical behavior of Al-Cu alloys by means of micropillar compression tests

The influence of different types of precipitates (either Guinier-Preston zones, $θ''$ or $θ'$) on the critical resolved shear stress and strain hardening was determined by means of micropillar compression tests in an Al - 4 wt. \% Cu alloy. The size, shape and volume fraction of the precipitates were measured in each case. It was found that size effects were negligible for micropillars with diameter $\ge$ 5 $μ$m. Micropillars with Guinier-Preston zones showed strain localization due to precipitate shearing. The best mechanical properties were obtained with either a fine dispersion of the $θ''$ precipitates or a coarser dispersion of $θ'$. Both precipitate shearing and Orowan loops were observed around the $θ''$ precipitates and the micropillar strength was compatible with the predictions of the Orowan model. In the case of the alloy with $θ'$ precipitates, the strengthening contribution associated with the transformation strain around the precipitates has to be included in the model to explain the experimental results. Finally, the micropillar compression tests in crystals with different orientations were used to calibrate a phenomenological crystal plasticity. This information was used to predict the mechanical properties of polycrystals by means of computational homogeneization.

cond-mat.mtrl-sci↗

arXiv2020

Atomistic simulations of the interaction of basal dislocations with MgZn$_2$ precipitates in Mg alloys

The interaction between Mg edge basal dislocations and rod-shaped $β_1'$-MgZn$_2$ precipitates was studied by atomistic simulations using a new interatomic potential. The atomistic model was carefully built taking into account the experimental information about the orientation relationship between the matrix and the precipitate to ensure minimum energy interfaces. It was found that the dislocations initially overcame the precipitate by the formation of an Orowan loop that penetrated in the precipitate. The precipitate was finally sheared after several Orowan loops were piled-up. The number of loops necessary to shear the precipitate decreased as precipitate cross-section decreased and the temperature increased but was independent of the precipitate spacing. Precipitate shearing did not take place along well-defined crystallographic planes but it was triggered by the accumulation of the elastic energy in the precipitate which finally led to formation of an amorphous layer below and above the slip plane of the basal dislocations. The kink induced in the precipitate by this mechanism was in good agreement with transmission electron microscopy observations.

cond-mat.mtrl-sci↗

arXiv2020

Prediction of the Al-rich part of the Al-Cu phase diagram using cluster expansion and statistical mechanics

The thermodynamic properties of α-Al and other phases (GP zones, θ'', θ' and θ) in the Al-rich part of the Al-Cu system have been obtained by means of the cluster expansion formalism in combination with statistical mechanics. This information was used to build the Al-rich part of the Al-Cu phase-diagram taking into account vibrational entropic contributions for θ', as those of the other phases were negligible. The simulation predictions of the phase boundaries between α-Al and either θ'', θ' or θ phases as a function of temperature are in good agreement with experimental data and extend the phase boundaries to a wider temperature range. The DFT calculations reveal the presence of a number of metastable Guinier-Preston-zone type configurations that may coexist with α-Al and θ'' at low temperatures. They also demonstrate that θ' is the stable phase below 550K but it is replaced by θ above this temperature due to the vibrational entropic contribution to the Gibbs energy of θ'. This work shows how the combination of cluster expansion and statistical mechanics can be used to expand our knowledge of the phase diagram of metallic alloys and to provide Gibbs free energies of different phases that can be used as input in mesoscale simulations of precipitation.

cond-mat.mtrl-sci↗

arXiv2020

Multiscale modelling of precipitation hardening in Al-Cu alloys: dislocation dynamics simulations and experimental validation

The mechanisms of dislocation/precipitate interactions were analyzed in an Al-Cu alloy containing a homogeneous dispersion of $θ'$ precipitates by means of discrete dislocation dynamics simulations. The simulations were carried out within the framework of the discrete-continuous method and the precipitates were assumed to be impenetrable by dislocations. The main parameters that determine the dislocation/precipitate interactions (elastic mismatch, stress-free transformation strains, dislocation mobility and cross-slip rate) were obtained from atomistic simulations, while the size, shape, spatial distribution and volume fraction of the precipitates were obtained from transmission electron microscopy. The predictions of the critical resolved shear stress (including the contribution of solid solution) were in agreement with the experimental results obtained by means of compression tests in micropillars of the Al-Cu alloy oriented for single slip. The simulations revealed that the most important contribution to the precipitation hardening of the alloy was provided by the stress-free transformation strains followed by the solution hardening and the Orowan mechanism due to the bow-out of the dislocations around the precipitates.

cond-mat.mtrl-sci↗

arXiv2020

Interactions between basal dislocations and $β_1'$ precipitates in Mg-4Zn alloy: mechanisms and strengthening

The mechanisms of dislocation/precipitate interaction as well as the critical resolved shear stress were determined as a function of temperature in a Mg-4 wt. % Zn alloy by means of micropillar compression tests. It was found that the mechanical properties were independent of the micropillar size when the cross-section was $>$ 3 x 3 $μ$m$^2$. Transmission electron microscopy showed that deformation involved a mixture of dislocation bowing around the precipitates and precipitate shearing. The initial yield strength was compatible with the predictions of the Orowan model for dislocation bowing around the precipitates. Nevertheless, precipitate shearing was dominant afterwards, leading to the formation of slip bands in which the rod precipitates were transformed into globular particles, limiting the strain hardening. The importance of precipitate shearing increased with temperature and was responsible for the reduction in the mechanical properties of the alloy from 23C to 100C.

cond-mat.mtrl-sci↗

arXiv2019

A criterion for slip transfer at grain boundaries in Al

The slip transfer phenomenon was studied at the grain boundaries of pure Aluminum by means of slip trace analysis. Either slip transfer or blocked slip was analyzed in more than 250 grain boundaries and the likelihood of slip transfer between two slip systems across the boundary was assessed. The experimental results indicate that slip transfer was very likely to occur if the residual Burgers vector, $Δ$b, was below 0.35b and the Luster- Morris parameter was higher than 0.9, and that the ratio of the Luster-Morris parameter and the residual Burgers vector has a threshold above which slip transfer is probable.

cond-mat.mtrl-sci↗

arXiv2019

High fidelity simulation of the mechanical behavior of closed-cell polyurethane foams

The mechanical behavior of closed-cell foams in compression is analyzed by means of the finite element simulation of a representative volume element of the microstructure. The digital model of the foam includes the most relevant details of the microstructure (relative density, cell size distribution and shape, fraction of mass in the struts and cell walls and strut shape), while the numerical simulation takes into account the influence of the gas pressure in the cells and of the contact between cell walls and struts during crushing. The model was validated by comparison with experimental results on isotropic and anisotropic polyurethane foams and it was able to reproduce accurately the initial stiffness, the plateau stress and the hardening region until full densification in isotropic and anisotropic foams. Moreover, it also provided good estimations of the energy dissipated and of the elastic energy stored in the foam as a function of the applied strain. Based on the simulation results, a simple analytical model was proposed to predict the mechanical behavior of closed-cell foams taking into the effect of the microstructure and of the gas pressure. An example of application of the simulation tool is presented to design foams with an optimum microstructure from the viewpoint of energy absorption for packaging.

cond-mat.mtrl-sci↗

arXiv2019

Influence of the stress state on the cross-slip free energy barrier in Al: an atomistic investigation

The influence of the stress state on the cross-slip rate in Al was analyzed by means of molecular dynamics simulations and transition state theory. The activation energy barrier in the absence of thermal energy was determined through the nudged elastic band method while the cross-slip rates were determined using molecular dynamics simulations for different magnitudes of the Schmid stress on the cross-slip plane, and of the Escaig stresses on the cross-slip and glide planes. The enthalpy barrier and the effective attempt frequency were determined from the average rates of cross-slip obtained from the molecular dynamics simulations. It was found that the different stress states influence the cross-slip rate assuming harmonic transition state theory. Moreover, the theoretical contributions to the enthalpy barrier (configurational and due to the interaction of the applied stress with the local stress field created by the defect) were identified from the atomistic simulations while the entropic contribution to the activation energy could be estimated by the Meyer-Neldel rule. Based on these results, an analytical expression of the activation enthalpy for cross-slip in Al as a function of the different combinations of Schmid and Escaig stress states was developed and validated. This expression can be easily used in dislocation dynamics simulations to evaluate the probability of cross-slip of screw dislocation segments.

cond-mat.mtrl-sci↗

arXiv2019

Simulation of the Hall-Petch effect in FCC polycrystals by means of strain gradient crystal plasticity and FFT homogenization

The influence of grain size on the flow stress of various FCC polycrystals (Cu, Al, Ag and Ni) has been analyzed by means of computational homogenization of a representative volume element of the microstructure using a FFT approach in combination with a strain gradient crystal plasticity model. The density of geometrically necessary dislocations resulting from the incompatibility of plastic deformation among different crystals was obtained from the Nye tensor, which was efficiently obtained from the curl operation in the Fourier space. The simulation results were in good agreement with the experimental data for Cu, Al, Ag and Ni polycrystals for grain sizes > 20 microns and strains < 5% and provided a physical explanation for the higher strengthening provided by grain boundaries in Al and Ni, as compared with Cu and Ag. The investigation demonstrates how the combination of FFT with strain gradient crystal plasticity can be used to include effect of grain boundaries in the mechanical behavior of polycrystals using realistic representative volume elements of the microstructure.

cond-mat.mtrl-sci↗

arXiv2019

Effect of slip transmission at grain boundaries in Al bicrystals

The effect of slip transfer on the deformation mechanisms of Al bicrystals was explored using a rate-dependent dislocation-based crystal plasticity model. Three different types of grain boundaries (GBs) were included in the model by modifying the rate of dislocation accumulation near the GB in the Kocks-Mecking law, leading to fully-opaque (dislocation blocking), fully-transparent and partially-transparent GBs. In the latter, slip transmission is only allowed in pairs of SS in neighbour grains that are suitably oriented for slip transfer according to the Luster-Morris parameter. Modifications of the GB character led to important changes in the deformation mechanisms at the GB. In general, bicrystals with fully-opaque boundaries showed an increase in the dislocation density near the GB, which was associated with an increase in the Von Mises stress. In contrast, the dislocation pile-ups and the stress concentration were less pronounced in the case of partially-transparent boundaries as the slip in one grain can progress into the next grain with some degree of continuity. No stress concentrations were found at these boundaries for fully-transparent boundaries, and there was continuity of strain across the boundary, which is not typical of most experimentally observed GBs. Simulations of ideal bicrystals oriented for favorable slip transfer on the most highly favored slip system in grains with high Schmid factors for slip transfer depends on the number of active SS in operation in the neighborhood and that most boundaries will lead to nearly opaque conditions while some boundaries will be transparent. Finally, the model was applied to a particular experimentally observed GB in which slip transfer was clearly operating indicating that the model predicted a nearly transparent GB.

cond-mat.mtrl-sci↗

arXiv2019

Mechanical behavior of InP twinning superlattice nanowires

Taper-free InP twinning superlattice (TSL) nanowires with an average twin spacing of ~ 13 nm were grown along the zinc-blende close-packed [111] direction using metalorganic vapor phase epitaxy. The mechanical properties and fracture mechanisms of individual InP TSL nanowires in tension were ascertained by means of in situ uniaxial tensile tests in a transmission electron microscope. The elastic modulus, failure strain and tensile strength along the [111] direction were determined. No evidence of inelastic deformation mechanisms was found before fracture, which took place in a brittle manner along the twin boundary. The experimental results were supported by molecular dynamics simulations of the tensile deformation of the nanowires that also showed that the fracture of twinned nanowires occurred in the absence of inelastic deformation mechanisms by the propagation of a crack from the nanowire surface along the twin boundary.

cond-mat.mtrl-sci↗

arXiv2019

Strengthening of Al-Cu alloys by Guinier-Preston zones: predictions from atomistic simulations

A scale bridging strategy based in molecular statics and molecular dynamics simulations in combination with transition state theory has been developed to determine the flow stress of Al-Cu alloy containing Guinier-Preston zones. The athermal contribution to the flow stress was determined from the Taylor model, while the thermal contribution was obtained from the obstacle strength and the free energy barrier. These two magnitudes were obtained by means of molecular statics and molecular dynamics simulations of the interaction of edge dislocations with Guinier-Preston zones in two different orientations. The predictions of the model were compared with experimental data and were in reasonable agreement, showing the potential of atomistic simulations in combination with transition state theory to predict the flow stress of metallic alloys strengthened with precipitates.

cond-mat.mtrl-sci↗

arXiv2019

Effect of solute content and temperature on the deformation mechanisms and critical resolved shear stress in Mg-Al and Mg-Zn alloys

The influence of solute atoms (Al and Zn) on the deformation mechanisms and the critical resolved shear stress for basal slip in Mg alloys at 298 K and 373 K was ascertained by micropillar compression tests in combination with high-throughput processing techniques based on the diffusion couples. It was found that the presence of solute atoms enhances the size effect at 298 K as well as the localization of deformation in slip bands, which is associated with large strain bursts in the resolved shear stress ($τ_{RSS}$)-strain ($ε$) curves. Deformation in pure Mg and Mg alloys was more homogeneous at 373 K and the influence of the micropillar size on the critical resolved shear stress was much smaller. In this latter case, it was possible to determine the effect of solute content on the critical resolved shear stress for basal slip in Mg-Al and Mg-Zn alloys.

cond-mat.mtrl-sci↗

arXiv2019

An analysis of (the lack of) slip transfer between near-cube oriented grains in pure Al

Slip transfer across grain boundaries was studied in annealed polycrystalline Al foils deformed in uniaxial tension by means of the analysis of the slip traces on the specimen surface. Grain orientations and selected grain boundary misorientations were measured on both surfaces of the sample using electron back-scattered diffraction mapping. It was found that most of the grains were within 15° of a cube orientation and approximately half of the grains percolated through the specimen thickness. The Luster-Morris m' parameter (that can be computed from the surface grain orientation) was used to assess the likelihood of slip transfer across boundaries. It was found that transfer across grain boundaries was rare in near-cube oriented grains, and convincing evidence was only found when m' > 0.97, which corresponds to low-angle boundaries with <15° misorientation. This behavior was explained by the presence of many active slip slips in near-cube oriented grains that favor self-accommodation of the grain shape to the evolving boundary conditions imposed by neighboring grains instead of promoting slip transfer across the boundary. These results indicate that the alignment between slip planes and slip directions across the boundary is not the only important metric to determine the threshold for slip transfer, as the particular details of deformation in each grain (such as the number of available slip systems) also must be considered.

cond-mat.mtrl-sci↗

arXiv2019

Grain boundary strengthening of FCC polycrystals

The effect of grain size on the flow strength of FCC polycrystals was analyzed by means of computational homogenization. The mechanical behavior of each grain was dictated by a dislocation-based crystal plasticity model in the context of finite strain plasticity and takes into the account the formation of pile-ups at grain boundaries. All the model parameters have a clear physical meaning and were identified for different FCC metals from dislocation dynamics simulations or experiments. It was found that the influence of the grain size on the flow strength of FCC polycrystals was mainly dictated by the similitude coefficient $K$ that establishes the relationship between the dislocation mean free path and the dislocation density in the bulk. Finally, the modelling approach was validated by comparison with experimental results of the effect of grain size on the flow strength of Ni, Al, Cu and Ag.

cond-mat.mtrl-sci↗

arXiv2019

Precipitation during high temperature aging of Al-Cu alloys: a multiscale analysis based on first principles calculations

Precipitation during high temperature aging of Al-Cu alloys is analyzed by means of the integration of classical nucleation theory and phase-field simulations into a multiscale modelling approach based on well-established thermodynamics principles. In particular, thermal stability of $θ''$, $θ'$ and $θ$ precipitates was assessed from first principles calculations of the Helmholtz free energy while homogeneous and heterogeneous nucleation of $θ''$ and $θ'$ was analysed using classical nucleation theory. Precipitate growth was finally computed by means of mesoscopic phase-field model. The model parameters that determine quantitatively the driving forces for each transformation were obtained by means of first principles calculations and computational thermodynamics. The predictions of the models were in good agreement with experimental results and provided a comprehensive understanding of the precipitation pathway in Al-Cu alloys. It is envisaged that the strategy presented in this investigation can be used in the future to design optimum microstructures based on the information of the different energy contributions obtained from first principles calculations.

cond-mat.mtrl-sci↗