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Precipitation hardening is the most effective strategy to enhance the mechanical properties of metals. Dislocation mechanisms to control strengthening during precipitation have been demonstrated extensively. However, owing to the complexity of different precipitates in alloys, variations in ductility caused by precipitation are very complex and have not been clarified so far. In this study, the effects of precipitation on ductility in precipitation hardening aluminium alloys are investigated based on a modified dislocation-based approach and experimental characterisation. The AlMgScZr alloy with spherical Al3(Sc, Zr) precipitates is used as a model alloy system to understand the effects of precipitation on ductility. Via heat treatment, shearable and non-shearable Al3(Sc, Zr) precipitates are introduced in the AlMg matrix. The results show that the ductility of AlMgScZr alloy decreases when shearable precipitates occur, while it increases during the shearable–non-shearable transition. The variation in ductility is mainly controlled by the dynamic recovery rate of the dislocations. This dislocation mechanism is supported by analysing the different precipitation–dislocation interactions and evaluating the dislocation density evolution during deformation. This study reveals the dislocation mechanism for controlling ductility during precipitation, which can provide a theoretical foundation for the design of high-performance structural materials.

cited By 0; International audience; Damage evolution during tensile straining of an AlSi12Ni alloy has been analyzed in situ at synchrotron source using microtomography and in the scanning electron microscope by surface imaging. It is shown that damage development in the analyzed alloy presenting an interconnected network of intermetallic phases is completely different from damage progression previously observed in materials with disperse distribution of particles. In the present material, which is typical for most eutectic structures, damage is dominated by the rupture of the brittle intermetallic phase while void growth is limited by a shielding effect of the intermetallic particles encasing the void. Primary voids exhibit a size close to the thickness of branches of the intermetallic phase. Final failure occurs by void coalescence, but without the formation of secondary voids. Damage analysis from tomographic scans was only possible by applying 3D image correlation to successive reconstructions, which thanks to its sub-voxel resolution, could satisfactorily detect the volume fraction of small-voids inaccessible by conventional thresholding. The presence of many small voids issued from the breakage of the intermetallic phase also was confirmed by scanning electron microscopy imaging performed at higher resolution. © 2019 Acta Materialia Inc.

International audience; Precipitation hardening is the most effective strategy to enhance the mechanical properties of metals. Dislocation mechanisms to control strengthening during precipitation have been demonstrated extensively. However, owing to the complexity of different precipitates in alloys, variations in ductility caused by precipitation are complex and have not been clarified so far. In this study, the effects of precipitation on ductility in precipitation hardening aluminium alloys are investigated based on a modified dislocation-based approach and experimental characterisation. The AlMgScZr alloy with spherical Al3(Sc, Zr) precipitates is used as a model alloy system to understand the effects of precipitation on ductility. Via heat treatment, shearable and nonshearable Al3(Sc, Zr) precipitates are introduced in the AlMg matrix. The results show that the ductility of AlMgScZr alloy decreases when shearable precipitates occur, while it increases with shearable precipitates being replaced by nonshearable precipitates. The variation in ductility of AlMgScZr alloy is mainly controlled by the dynamic recovery rate of the dislocations. Finally, by analysing the different precipitate-dislocation interactions and evaluating the dislocation density evolution during deformation, the dislocation mechanisms of 2 ductility during precipitation for AlMgScZr alloy are demonstrated. This study reveals the dislocation mechanism for controlling ductility during precipitation for AlMgScZr alloy which can provide a theoretical foundation for the design of high-performance structural materials.

International audience; Inkjet printing has a high potential for cost reduction in solar cell and thermoelectric industry. This study demonstrates that silicon thin films can be produced by inkjet-printing of silicon nanoparticles followed by subsequent drying and annealing steps. Ink formulation is crucial for the sintering of the silicon nanoparticles and control of the microstructure at low temperature. Upon heating, the microstructure is modified from porous layer made of juxtaposed silicon nanoparticles to denser layer with coarser grains. This evolution is monitored by scanning electron microscopy and by micro-Raman spectroscopy, which offer a fast and precise characterization of the microstructure and chemical composition of thin films. Above a threshold temperature of 800 degrees C cracks appear within thin film and substrate because of the stress induced by the oxidation of the surface. An innovative sintering method, photonic annealing, is studied in order to reduce both oxidation and stress in the thin films as well as reducing processing time. Evolution of the thermal conductivity is performed by micro-Raman spectroscopy and can be tailored in a large range between similar to 1 and similar to 100 W.m(-1).K-1 depending on the sintering method and atmosphere. Therefore control of the microstructure evolution with applied annealing process allows tailoring of both microstructure and thermal conductivity of the silicon thin films. (C) 2014 Elsevier B.V. All rights reserved.

International audience; A new evaluation procedure is presented to characterize the orientation and position of single grains within the bulk of a polycrystalline sample. Considering the symmetry properties of Friedel-pairs the contributions to reflection spot positions arising from grain orientation and position could be clearly separated. The proposed method avoids simultaneous fitting of all grain parameters with the goal of a higher accuracy. Depending on the number of reflections considered the accuracy of grain orientation may be less than 0.1 degrees, and the position of the center of mass of the grains can be accurate within one-third of the pixel size.

Microstructure evolution during high-temperature creep of a CuZn40Pb2 brass alloy was characterized in situ by synchrotron microtomography. It is shown that tomographic data are sufficiently accurate for local material characterization. Imposing a constant temperature gradient along the gauge length of the sample allows the apparent activation energy of steady-state creep to be evaluated for a single specimen.

A new approach for the estimation of the effective properties of real particle-reinforced metal–matrix composites is presented. The methodology relies on statistical functions of the three-dimensional structure of the composites, as obtained from microtomographic investigations, and uses an ergodic principle for the calculation of the overall properties. Based on the symmetrical functional form of the local volume fraction and results of finite element calculations we develop the so-called “mean window technique”, which leads to the remarkable result that the effective linear or nonlinear mechanical properties of a composite can be obtained by averaging randomly selected windows from the real structure. Such windows should have a size at least equal to the correlation length of the microstructure and the volume fraction of the particles equal to the mean value of the distribution of the local volume fraction.

A mean-field and a multiscale model of a particle reinforced metal–matrix composite were developed based on the knowledge of its three-dimensional microstructure obtained by holotomography. Both the mean-field and the multiscale model accounting for the distribution of particles’ local volume fractions approximate well the experimental stress–strain curves.

X-ray line profile analysis is an efficient non-destructive technique to determine some key statistical properties of dislocation structures developing during plastic deformation. In the first part of the paper recent developments describing the asymptotic behavior of the line profiles are outlined. A new evaluation method is presented to determine the average dislocation density and its spatial variation. The method is demonstrated on measurements obtained on compressed Cu single crystals. After this, it is shown that the relative dislocation density fluctuation exhibits a sharp maximum at the stages II–III transition. A nanoindentation tests indicate that the fluctuations of other quantities behave similarly. The results give new insight into the dislocation phenomena taking place in the stage III deformation regime.

Local crystallographic orientation of tensile deformed copper single crystals was investigated by the electron backscattering technique. Statistical evaluation of the data reveals the presence of an increased crystallographic correlation at the transition point between stages II and III of work-hardening. The transition state has the lowest probability of finding geometrically necessary dislocations in circular regions of radius smaller than 8 μm. According to the present results and other data showing that the relative fluctuation of the dislocation density has a maximum at the transition point, we conclude that the transition from stages II to III of work-hardening is similar to a second-order phase transformation of the statistical dislocation system.

Intergranular strain and the associated stored energy were investigated by X-ray diffraction in two texture components of cold-rolled ultra high purity (UHP) iron. Laue peaks were measured on plastically deformed samples and the center of the lines was compared with reference peaks recorded from a well-annealed powder sample of the same material. The intergranular stored energy as evaluated from the residual strain tensor was found to be much higher for the [1 1 0](0 0 1) α fibre component than for the [1 2 1](1−1 1) γ fibre component. Experimental results show that the soft α grains of the polycrystal, containing fewer dislocations, are more strained by their surrounding than hard γ grains with more dislocations. Generally intergranular strains decrease with deformation.

The elastic–plastic response of a metal–matrix composite (MMC) consisting of a AA6061-T6 aluminium matrix reinforced with 20 vol.% Al 2 O 3 particles was modeled using the finite element method. As input for the model, a real three-dimensional (3D) microstructure obtained by holotomography was used. The macroscopic behavior of the composite was obtained by multi-scale modeling based on a window-averaging and extrapolation technique. Model compression stress–strain curves obtained for a cubical volume of edge length 9–10 times larger than the two-point correlation length of the matrix show satisfactory agreement with experimental data.

The dislocation densities in individual texture components of a hot, plane strain compressed, Al–2.5%Mg alloy have been determined by high resolution X-ray diffraction using recent methods for peak broadening interpretation. After deformation at 400 °C and strains up to 1.5 dislocation densities range from 2 to 20 × 1013 m−2. Both high energy synchrotron (European synchrotron radiation facility, ESRF) transmission peak analysis and standard reflection line analysis show that the S {1 2 3}〈4 1 2〉 component develops significantly higher stored energies than the cube, brass {0 1 1}〈2 1 1〉 and Goss {0 1 1}〈1 0 0〉 components. After subsequent annealing to partial recrystallization, EBSD orientation maps over large areas demonstrate that the high energy S component is preferentially consumed by recrystallization. It is also shown that the development of the cube recrystallization texture is due to a nucleation frequency advantage, function of the applied strain.

The method of holotomography was applied to investigate the microstructure of a metal–matrix composite (MMC) consisting of an AA6061 aluminium matrix reinforced with 20 vol.% Al 2 O 3 particles. Various statistical functions characterizing the Al 2 O 3 particles and the matrix were evaluated, which facilitate a better understanding of the correlation between microstructural features and the overall physical properties of the composite. The shape of the particles is approximated by an ellipsoid and their spatial distribution is described in terms of the pair-distribution function of particle centers and the two-point probability functions of the component phases. The investigated statistical functions emphasize the existence of a short-range correlation in the microstructure, which has a correlation length of about two times the average nearest-neighbor distance. To understand better the influence of the particle shape on different statistical descriptors a comparison between the real microstructure and a system of random hard-spheres is presented.

The influence of the free surface on the load carried by elastic inclusions embedded in an elastic–plastic matrix was investigated by the finite element method. A 2D multi-particle model was developed based on the statistical evaluation of the microstructure of a commercial metal–matrix composite. The results of simulated tensile tests show that the average particle stresses are sensitive to the adopted boundary conditions. Inclusions near a free boundary carry in general smaller loads than the inclusions in the bulk. Damage predictions of the model are in good agreement with the experimental data obtained by X-ray microtomography.

A multi-particle 2D finite element model of a 20% particulate reinforced metal-matrix composite was developed on a statistical basis taking into account the correlations between the position, size and orientation of the ceramic particles in the matrix. The stress–strain curves in tension and compression given by the clustered multi-particle model are compared with the curves obtained from one-particle unit cell simulations. It is shown that clustering of particles increases the plastic strain accumulated in the matrix leading to a higher strain hardening and thus to a higher flow stress. The size of the representative volume element (RVE) should be at least equal to the correlation length of the geometrically relevant correlation functions, which was ∼2.4 times larger than the average interparticle distance for the experimentally studied case. Reasonable agreement is obtained between computed residual strains and data available in the literature.

Single and polycrystalline samples of Cu were deformed at a constant shear stress of 1.43×10 −3 G ( G , shear modulus). At room temperature deformation ceases owing to work hardening after formation of a cell structure; at 527 K a steady state of deformation is reached with formation of a subgrain structure which is frozen in by cooling to room temperature at constant load. The dislocation density and the internal stresses were determined by analyzing the 200 Bragg peaks measured with high resolution. Measurements under load show broadening of the peaks by deformation owing to an increase of dislocation density and a build-up of long-range internal stresses. Subsequent removal of the load causes the dislocation density to decrease by 17–30%, and the internal forward stress in the hard regions of concentrated stress to decrease by one-third. It is concluded that the factor of average stress concentration in hard regions is about 2 in the work hardened state at room temperature as well as in the steady-state deformation at 527 K.

Ball-milled WC powder has been compacted and sintered. The particle size has been determined by transmission and scanning electron microscopy and high resolution X-ray diffraction. The X-ray diffraction profiles were evaluated for particle size and dislocation densities by the recently developed procedures of modified Williamson-Hall plot and modified Warren-Averbach method. Log-normal particle size distributions have been obtained by a newly developed numerical method from the size parameters provided by X-ray line profile analysis. The size distributions obtained by TEM and X-rays are compared and discussed. The dislocation density was found to be 1.9 × 1017 m−2 in the ball-milled powder which decreases to 2.0 × 1015 m−2 after annealing.

Densities and arrangements of dislocations and mesoscopic long-range internal stresses in cold-rolled polycrystalline copper were determined in scanning mode by X-ray peak profile analysis with synchrotron radiation. The high brilliance enabled the use of a focal spot of less than 50 μm. The diffraction profiles of the 200 reflections were measured by scanning the specimen step by step in front of the beam. In this way, the heterogeneities of the deformation-induced microstructure within single grains were obtained. At small deformations including stage III, the dislocation densities and the internal stresses were uniform within the grain interior, but larger near the grain boundaries where the dislocations showed a tendency to form stress-intensive arrangements. At higher deformations towards and in stage IV, the dislocation density and the internal stresses increasingly fluctuated within the whole grain. The fluctuations were interpreted as the transformations of polarized dipolar dislocation walls (PDW) into polarized tilt walls (PTW) observed recently by macroscopic diffraction experiments on highly deformed specimens. These results are important for the current research of strengthening processes in stage IV.

The particle size and the dislocation structure in inert gas condensed nanocrystalline copper were determined by high-resolution X-ray diffraction profile analysis. Well-behaved smooth curves were obtained in the modified Williamson–Hall plot and the modified Warren–Averbach plot through knowledge of the variation in dislocation contrast with Bragg reflection and the effect of twinning on particle size. The particle size was between 14 and 30 nm, in agreement with TEM results. The root-mean-squared strains were explained by the presence of dislocations, with a dislocation density of about 5×1015 m−2. The dislocations were found to have screw character probably related to the particle growth mechanism.

Polycrystalline samples of Fe-0.005%-C were deformed by torsion at 300 K far into stage IV of deformation and investigated by X-ray peak profile analysis (XPA) for the microstructural evolution. The long-range internal stresses Δτ w −δτ c (τ w , τ c are the shear stresses in the cell wall and cell interior regions) pass through a maximum at the onset of stage IV, but reincrease within stage IV at higher deformation. A similar maximum is observed in the formal dislocation density derived directly from XPA which, however, is not seen in residual electrical resistivity. These results can be consistently explained by the assumption that in stages II and III the cell walls are formed as polarized dipole walls (PDW) which in stage IV change into polarized tilt walls (PTW), similarly to recent findings in cold rolled Cu. The significant constancy of total volume fraction of cell walls as well as of specific internal stresses in stage IV confirm the importance of the PTW s for stage IV strengthening.

Copper single-crystals were wire drawn far into stage III and into the onset of stage IV. TEM investigations revealed the well known dislocation cell structure. High resolution X-ray diffraction line profile analysis was performed on the deformed specimens. The diffraction experiments have provided characteristically asymmetric line profiles which were evaluated for the dislocation densities and arrangements and for the residual long-range internal stresses prevailing in a dislocation cell structure. The formal dislocation densities were found to be spatially anisotropic and were interpreted in terms of anisotropic Burgers vector populations. The residual long-range internal stresses were found to form plane-stress states in conformity with the applied stress conditions. A microscopic model has been suggested accounting for the density and arrangement of dislocations and for the residual long-range internal stresses, at the same time, in a self consistent manner.

Long-range internal stresses in dislocation cell and subgrain structures were investigated experimentally. The transition of the dislocation structure from cells to subgrains was achieved by deforming copper polycrystals in compression creep tests at constant stress normalized by the shear modulus in the temperature range from 298 K to 633 K. The long-range internal stresses were investigated by two methods. The first one was the evaluation of characteristically asymmetric X-ray line profiles. The internal stresses are the result of the analysis of the X-ray line profiles. The second one was the measurement of local lattice parameters by convergent beam electron diffraction. The internal stresses can be determined from the changes in the local lattice parameters. The results obtained from both methods show that long-range internal stresses of the same type exist in the cell as well as in the subgrain structures.

The microstructure of ball-milled iron powder has been investigated by high resolution X-ray line profile analysis. Analysis of line breadths suggested that the contrast factors related to dislocations have to be taken into account in the Williamson-Hall procedure. This concluded to a modified Williamson-Hall plot which provided, in a straightforward manner, the grain size refinement of nanocrystals ball-milled for different periods of time. At the same time it has been shown that strain broadening, even in these nanoscale small-grain particles, is caused by the presence of dislocations. The line profiles were also studied by the method of Fourier analysis, which gave the absolute values of dislocation densities.

X-ray line profiles and line positions were measured on single grains of cyclically deformed polycrystalline copper specimens unloaded from characteristic points of the stress-strain hysteresis loop. A high-resolution double crystal diffractometer was used to obtain local lattice parameter changes, from which long-range internal stresses and mean stresses were evaluated. In the case of samples unloaded from the load-reversal points the asymmetry of the characteristically broadened X-ray line profiles indicated forward and backward internal stresses prevailing in the dislocation cell wals and cell interiors of the dislocation cell structure, respectively. The changes of the long-range internal stresses after unloading from different points of the stress-strain hysteresis loop can be explained in terms of the composite model. In addition, mean strains and stresses, varying from grain, could be determined from the shifts of the Bragg reflections. A detailed microstructure-based discussion of the elastic stored energy, evaluated from the analysis of the stress-strain hysteresis loop, is given.