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International audience; The present study aims to understand the underlying mechanisms of macroscopic contraction that occurs in γ/γ′ nickel based superalloys submitted to isothermal annealing and resulting in stress relaxation retardation. Samples of the AD730TM alloy were characterized at different scales using dilatometry, X-Ray Diffraction (XRD) with in-situ heating, Electron Channeling Contrast Imaging (ECCI) in the Scanning Electron Microscope (SEM), conventional Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). APT analyses revealed chemical composition changes in both the γ matrix and the γ′ hardening precipitates during annealing at 760°C after sub-solvus solution heat treatment. These chemical alterations lead to a decrease of the lattice parameters of both phases, which was captured by XRD analyses. The macroscopic volume contraction is shown to be mainly governed by that decrease in the lattice parameters. In addition, from APT analyses, the chemical alterations seem to be promoted by diffusion of solutes such as Cr, Co and Fe, along dislocations occurring under relaxation testing conditions.

In this article, the Joule effect heating influence on recrystallization phenomena in the Inconel 718 nickel-based superalloy is investigated in details. On the one hand, static recrystallization kinetic studies at $$1020\,^\circ \mathrm{C}$$ after cold deformation up to $$\varepsilon =0.1$$ have been performed on a Gleeble 3800 machine, with Joule effect heating, and compared with conventional heating in a radiative furnace, reproducing exactly the same thermal paths in experiments. On the other hand, dynamic recrystallization kinetics have been compared between specimens deformed in the same conditions but varying the heating principle. Compression tests were thus performed on a MTS 250 (radiant heating) and on a Gleeble 3800 (Joule effect heating) at strain rates in the range $$\dot{\varepsilon }\in \left[{10}^{-3}; 1\right]{ \mathrm{s}}^{-1}$$ up to a macroscopic strain $$\varepsilon =0.7$$ reproducing exactly the same thermomechanical paths. Compression tests were performed at two different nominal temperatures $$T=1050\,^\circ \mathrm{C}$$ and $$980\,^\circ \mathrm{C}$$ in order to investigate dynamic recrystallization in either δ-phase supersolvus or subsolvus conditions, respectively. As a general trend, Joule effect heating significantly accelerates recrystallization kinetics, and this holds for both static and dynamic recrystallization. Results also suggest that Joule effect heating significantly impacts other metallurgical phenomena like dislocations recovery mechanisms and δ-phase precipitation. The observed differences could not be totally explained by experimental biases resulting from Joule effect heating like radial temperature gradients or differences in initial microstructures. Interactions between electrons and microstructural features such as crystal defects, generally evoked in the literature, are indeed the most probable origin of Joule effect heating impact on microstructural evolutions.

International audience; Grain growth in polycrystals is one of the principal mechanisms that take place during heat treatment of metallic components. This work treats the single boundary simplification of the anisotropic grain growth problem. By applying the first principles of thermodynamics and mechanics, an expression for the velocity field of a migrating grain boundary with an inclination-dependent energy density is expressed. This result is used to generate the first, to the authors’ knowledge, space-time analytical solution to the anisotropic grain growth problem. This new anisotropic benchmark is simulated in order to explore the convergence properties of the proposed level-set finite element numerical model. Convergence of the method being illustrated, another configuration, using a more general grain boundary energy density, is investigated in order to show the added value of the new formulation.

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001)alpha-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K. The fcc-phase persists while the original texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 micron wide grains. Only the grains having specific orientation relationships to the sapphire grow; as in the case of pure fcc metal (M) films 4 orientation relationships (OR) are found: OR1 (M(111)[1-10]//alpha-Al2O3 (0001)[1-100]) and OR2 (M(111)[1-10]//alpha-Al2O3 (0001)[11-20]) and their twin-related variants (OR1t and OR2t). Below 1000 K, the film microstructure stabilizes into 10 micron wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures., 6 figures and 3 tables

The development of overgrown grains during sub-solvus annealing of a polycrystalline γ − γ ′ nickel-base alloy is investigated, with special emphasis on the primary γ′ precipitates. Quantitative analysis conducted on a large three-dimensional electron back-scattered diffraction dataset reveals a high fraction of γ′ precipitates on the annealing twin boundaries in the large grains. When observed in-situ in a scanning electron microscope, the formation of annealing twins is correlated to the crossing of γ′ precipitates by moving grain boundaries. The role of precipitates in the occurrence of growth accidents is discussed.

Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values, and histograms. Two kinds of microstructures are considered: some are generated statistically from EBSD maps, and the others are generated by the immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, which are, respectively, defined as a function of the disorientation and both the GB misorientation and the GB inclination. In terms of mean grain size value and grain size distribution (GSD), both formulations provide similar responses. However, the anisotropic formulation better respects the experimental disorientation distribution function (DDF) and predicts more realistic grain morphologies. It was also found that the heterogeneous GB mobility described with a sigmoidal function only affects the DDF and the morphology of grains. Thus, a slower evolution of twin boundaries (TBs) is perceived.

International audience; Secondary-ion mass spectrometry (SIMS) is probably the most widely used chemical analysis technique in semiconductor science and in metallurgy because of its ultimate sensitivity to all elements and in particular to light elements providing semiquantitative information on the depth distribution of elements, for instance, doping elements (i.e., B, H), contaminants (i.e., C and O), chemical gradients, and segregation in thin films and at interfaces, etc. With the size shrinking of systems, high-resolution 3D chemical imaging is becoming a prerequisite for the development of new materials at the nanoscale and for the deep understanding of the correlation between their properties and functionalities. Herein, the development of an innovative analytical SIMS implemented in a focused ion beam (FIB) (using Ga source)/scanning electron microscope (SEM) is reported. The equipment enables to give elemental chemical mapping at very high resolution (

Double-cone samples offer the great advantage of drastically reducing the number of required tests to correlate between processing parameters and microstructural evolution, as their characteristic geometry results in an equivalent strain gradient after compression. Their usual drawback is a relatively poor control of the equivalent strain rate ( $$ \dot{\bar{\varepsilon }} $$ ), which varies along the radius and in time. To tackle this drawback, a parametric optimization of the geometry of these double-cone samples for compression tests was performed based on FE simulations. This optimization aims at minimizing the average equivalent strain rate standard deviation in time, among five numerical tracking points placed along the radius. The optimized geometry provides, for all cases tested, an average variation in time within a 10 pct range regarding the mean equivalent strain rate at the center of the sample. Another advantage is a homogeneous plastic strain path along the radius at the mid-height plane of the double-cone sample, as also throughout the deformation. The optimized geometry was compared with other found in literature and systematically showed improvement in terms of strain rate control, especially at the center of the sample, where the variation for the tested geometries rose up to 50 pct regarding the average strain rate value, compared to a 10 pct variation for the optimized geometry at the same conditions.

Transients in plastic flow behavior and the kinetics of dynamic dissolution of α particles were established via isothermal, hot compression testing of Ti-6Al-4V (Ti64) and Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242S). For this purpose, samples were preheated at a low subtransus temperature at which the volume fraction of α was ~ 0.90, heated at a fixed rate to one of two higher temperatures, held for a time between 0 and 900 seconds, and then upset to a 2:1 reduction using a strain rate of 0.01, 0.1, or 1 s−1. For a given alloy, test temperature, and strain rate, the flow stress decreased with increasing hold time. The observations were interpreted in terms of various models of plastic flow and microstructure evolution. The plastic-flow behavior of the two-phase microstructures was analyzed using approaches based on isostrain (upper-bound), self-consistent (SC), and isostress (lower-bound) approaches coupled with the measured (transient/non-equilibrium) phase fractions/phase compositions. The isostrain and SC methods both provided reasonable estimates of the observed flow stresses; the isostress method greatly under-predicted the measurements. Microstructure models comprised diffusion-based analyses of the dissolution of α particles into the β matrix both statically (during heating to test temperature and holding prior to deformation) and dynamically (during deformation). Static dissolution predictions showed good agreement with measurements. A comparison of static and dynamic dissolution behaviors revealed that concurrent deformation led to an enhancement of diffusion rates by a factor of approximately 8 or 4 for Ti64 and Ti6242S, respectively.

International audience; The formability of sheet metals is frequently characterized by the forming limit curve (FLC), which is determined by applying proportional loading paths until the onset of necking. However, metal forming processes usually involve strain-path changes that can affect the limit strains. The purpose of the present work is to study the consequence of strain path changes on the formability of a 0.65 mm thick Zn-Cu-Ti sheet, by means of tests carried out in bi-linear stages. For this propose, tensile limit strains were determined by uniaxial deformation of biaxially pre-strained specimens. The FLC was first determined for six strain paths, obtained from hydraulic bulge and uniaxial tests with diverse specimen geometries. For the bi-linear strain paths, the tensile limit strains of the biaxially pre-deformed zinc sheet are significantly higher than the FLC of non pre-strained samples for the three orientations studied, revealing interdependence between the material’s formability, the loading history and microstructure evolution.

An implementation of smoothing splines is proposed to reduce orientation noise in electron backscatter diffraction (EBSD) data, and subsequently estimate more accurate geometrically necessary dislocation (GND) densities. The local linear adaptation of smoothing splines (LLASS) filter has two advantages over classical implementations of smoothing splines: (1) it allows for an intuitive calibration of the fitting versus smoothing trade-off and (2) it can be applied directly and in the same manner to both square and hexagonal grids, and to 2D as well as to 3D EBSD data sets. Furthermore, the LLASS filter calculates the filtered orientation gradient, which is actually at the core of the method and which is subsequently used to calculate the GND density. The LLASS filter is applied on a simulated low-misorientation-angle boundary corrupted by artificial orientation noise (on a square grid), and on experimental EBSD data of a compressed Ni-base superalloy (acquired on a square grid) and of a dual austenitic/martensitic steel (acquired on an hexagonal grid). The LLASS filter leads to lower GND density values as compared to raw EBSD data sets, as a result of orientation noise being reduced, while preserving true GND structures. In addition, the results are compared with those of filters available in theMTEXtoolbox.

International audience; In this study, four different finite element level-set (FE-LS) formulations are compared for the modeling of grain growth in the context of polycrystalline structures and, moreover, two of them are presented for the first time using anisotropic grain boundary (GB) energy and mobility. Mean values and distributions are compared using the four formulations. First, we present the strong and weak formulations for the different models and the crystallographic parameters used at the mesoscopic scale. Second, some Grim Reaper analytical cases are presented and compared with the simulation results, and the evolutions of individual multiple junctions are followed. Additionally, large-scale simulations are presented. Anisotropic GB energy and mobility are respectively defined as functions of the mis-orientation/inclination and disorientation. The evolution of the disorientation distribution function (DDF) is computed, and its evolution is in accordance with prior works. We found that the formulation called “Anisotropic” is the more physical one, but it could be replaced at the mesoscopic scale by an isotropic formulation for simple microstructures presenting an initial Mackenzie-type DDF.

International audience; VDM Alloy 780 is a new nickel-based superalloy developed for higher service temperatures up to 750°C. In addition to fine γ′-Ni3(Al,Ti) strengthening precipitates, grain boundary precipitates resembling δ-Ni3Nb and/or η-Ni3Ti phases are also observed in this alloy. Previous studies on Alloy 718Plus, with chemical composition close to VDM Alloy 780, revealed that phase discrimination between δ and η particles is not straightforward and requires the use of high-resolution characterization techniques such as HRTEM. This work aims at evaluating the use of EBSD technique with regards to phase identification of these plate-shaped particles. Classical indexing of diffraction patterns based on Kikuchi band detection in Hough space fails to differentiate between the two phases. However approaches based on the comparison of experimental and simulated patterns, at least partly, could overcome this issue. The advanced dictionary-based approach that involves the simulation of dynamic electron scattering events identified η phase as the main component of these grain boundary precipitates, consistent with the recently published HRTEM investigation on VDM Alloy 780. Refinement of the classical indexing method under kinematic assumptions implemented in the “Refined Accuracy” mode on the Oxford AZtec software package also led to appreciable improvement.

In numerous polycrystalline materials, grain size is controlled by second phase particles (SPPs) that hinder the grain boundaries (GBs) by pinning mechanisms. The Smith–Zener pinning (SZP) model describes the physical interaction between SPPs and GBs. Both of them can evolve when applying a heat treatment to the material. As industrial forging processes involve hot deformation steps near the solvus temperature, it is thus of prime importance to characterize the evolution of the SPPs due to their impact on the final microstructure, notably on the grain size. The level set (LS) method is classically used to describe the influence of SPPs on grain growth (GG) by considering the simulated particles as inert and represented by static holes in the used finite element (FE) mesh. A new formalism to model GG mechanism under the influence of the SZP phenomenon, able to take into account evolving particles is proposed. It involves the representation of SPPs by a LS function and a particular numerical treatment around the grain interfaces encountering SPP, making possible the modelling of SPPs evolution without altering the undergoing pinning pressure. Validation and comparison of the new method regarding previous FE-LS formulation in 2D and 3D simulations and an application on GG under the influence of dissolving particles are described.

International audience; The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by magnetron cosputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.

International audience; The movement of grain boundaries in pure metals and alloys with a low concentration of dislocations has been historically proved to follow curvature flow behavior. This mechanism is typically known as grain growth (GG). However, recent 3D in-situ experimental results tend to question this global picture concerning the influence of the curvature on the kinetics of interface migration. This article explains, thanks to 2D anisotropic full-field simulations, how the torque effects can complicate these discussions. It is then illustrated that neglecting torque effects in full-field formulations can have a significant effect on the overall rate of GG, the shape of grains and grain boundaries as well as their local kinetics. The apparent reduced mobility can be much more complex than expected without necessarily questioning the influence of the curvature on the local kinetic equation.

International audience; A new β-metastable Ti-alloy is designed with the aim to obtain a TWIP alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated regime. The designed alloy exhibits a large ductility combined with an elevated and stable work-hardening rate. Deformation occurring by formation and multiplication of {332} twins is evidenced and followed by in-situ electron microscopy, and no primary stress induced martensite is observed. Since microstructural investigations of the deformation mechanisms show a highly heterogeneous deformation, the reason of the large ductility is then investigated. The spatial strain distribution is characterized by micro-scale digital image correlation, and the highly deformed regions are found to stand at the crossover between twins, or at the intersection between deformation twins and grain boundaries. Detailed electron back-scattered imaging in such regions of interest finally allowed to evidence the formation of thin needles of stress induced martensite. The latter is thus interpreted as an accommodation mechanism, relaxing the local high strain fields, which ensures a large and stable plastic deformation of this newly designed Ti-alloy.

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050°C and cooled at 5°C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

In this work the recently proposed full field approach to model dynamic recrystallization [1] is applied to model 304L steel. The framework couples a CPFEM (crystal plasticity finite element method) model with a LS-FE (level-set finite element method) for grain boundary migration and phenomenological laws. 304L steel samples are subjected to thermomechanical tests and their flow behaviour is characterized, additionally Electron Back Scattered Diffraction (EBSD) is used to study microstructure evolutions. Part of the experimental data is used to calibrate the model parameters and describe their evolution as a function of the thermomechanical conditions. The calibrated model is used to predict the microstructural evolution of 304L steel, the results are compared with the remaining experimental measurements. The comparison shows that the model correctly predicts the flow behaviour and recrystallization fraction evolution. However the results also show that the use of classical phenomenological models limit the model capability to predict grain size evolution. Different approaches to improve the model grain size prediction are presented and compared, the results show significant improvements when compared with experimental data.

The need of developing new high temperature materials has increased significantly in the last decades owing to the demand of higher engine operating temperatures. This requires improved microstructural stability of the polycrystalline nickel-based superalloys used for turbine disks. The microstructure of VDM Alloy 780 consists of γ′ strengthening precipitates in addition to the needle/plate-shaped particles (of δ and/or η phase) to pin the grain boundaries. The present article aims at discussing the recrystallization behavior of the new VDM Alloy 780 in the supersolvus domain. Both dynamic and post-dynamic microstructural evolutions are reported. The dynamic recrystallization (DRX) kinetics was found to be rather sluggish. For a plastic strain of 1.3 at 1050 °C applied at a strain rate of 0.01 s−1, the microstructure of VDM Alloy 780 is only 50% recrystallized. The DRX grain sizes are quite close for the two applied strain rates—0.01 and 0.1 s−1. Despite slow DRX kinetics, the fast post-dynamic evolutions allowed to achieve fully recrystallized microstructures with a grain size of 26 μm within 5 min of post-deformation holding at 1050 °C. The post-dynamically recrystallized grain sizes were predominantly temperature dependent and were not sensitive to strains and strain rates within the applied range. Deformation followed by 5 min holding at temperatures below 1050 °C but still in the single-phase domain could eventually generate finer grain sizes (

Performing the double aging thermal treatment on Inconel 718 forged components directly after forging instead of the standard thermal treatment sequence including a solution annealing step before the double aging is known to be beneficial to mechanical properties. In this work, this so-called direct aging (DA) effect has been assessed for tensile properties at room temperature and for creep properties at 650 °C/750 MPa. Mechanical characteristics obtained on direct aged specimens are compared to those obtained on specimens submitted to the standard thermal treatment sequence (i.e., including a solution heat treatment) and tested in the same conditions. The classical effect of direct aging thermal treatment is obtained; better tensile and creep properties are reached for DA specimens. However, contrary to what is generally assumed, fine microstructural characterizations in specimen threads reveal that the direct aging effect cannot be attributed to a higher residual strain hardening level preserved in the microstructure. It is actually shown that its beneficial effect on mechanical properties is due to a lower δ phase content obtained in microstructures having undergone such a treatment rather than the standard sequence. On the one hand, it has indeed been demonstrated that, counter-intuitively, the δ phase content actually increased during the “solution” treatment performed at 955 °C for 1 h. On the other hand, the relationship between mechanical properties and microstructural features (grain size, δ phase content, residual forging strain hardening level) has been analyzed in detail, and differences in mechanical behavior are mainly controlled by this increased amount of δ phase which deteriorates the mechanical properties. On the contrary, no significant effect of residual forging strain hardening on tensile and creep properties has been found for the investigated conditions.

Fine microstructural analyses have been performed to identify the microstructural mechanisms controlling stress relaxation during aging heat treatment of AD730TM disk superalloy. Morphological evolution of the hardening γ′ precipitates and plastic activity occur during relaxation tests. For a 500 MPa initial stress, the relaxation test shows atypical behavior with sluggish relaxation in the first hours and then a faster one. To understand this atypical behavior, isothermal dilatometry tests were used to decouple the effects of stress and temperature. The latter revealed a contraction of the specimen when subjected to a constant temperature. This contraction induces a tendency for an increase in stress during the relaxation test to meet the imposed condition of constant total deformation. Relaxation is then controlled by the competition between the classical relaxation mechanisms (vacancy diffusion and/or dislocation gliding) which tend to lower the stress and the contraction of the specimen which tends to increase the stress during the test.

This work presents an analytical mean field model of agglomeration (equivalent agglomeration model, EAM) by contact between growing precipitates. The principle of the EAM is to add a 2-steps task to existing precipitation models: at each time-step, (1) calculate the number of agglomerations per unit volume, then (2) use it to update the numbers and radii of classes in order to take agglomeration into account. The first step is validated by full-field simulations of growing spheres randomly placed in a cube. The EAM can be added as extension to existing precipitation models and be applied to the mean radius, multi-class Lagrangian and multi-class Eulerian representations of the precipitate population. The EAM is incorporated to a mean field precipitation model based on the classical nucleation theory and Zener’s law for size evolution, and catches the rarefaction of γ″ precipitates agglomerating as they grow during an isothermal annealing in a nickel-based superalloy.

Nano-crystalline Ni and Cu films deposited on the r-plane of sapphire (-Al 2 O 3) develop a fiber-texture upon annealing, in which grains grow up to 300 to 500 times larger than the film thickness. Most of the largest grains, which have grown at the expense of others, display one of four preferred orientation relationships (ORs) to the substrate. The four preferred ORs are OR1r = Me(111) //-Al 2 O 3 , OR2r = Me(111) //-Al 2 O 3 (Me = Ni or Cu), and their twins, which are rotated 60° about the axis perpendicular to the substrate. For these ORs, one of the densest atomic rows that lies within the Me {111} interfacial plane, aligns with the direction of the step edges that form at the intersection of the r-plane with one of its neighboring facets on the equilibrium shape of sapphire (i.e. the c-(0001), p-or s-planes). One of the ORs is most preferred when the steps at the interface are such that a {100}-type ledge of the Me {111} interfacial plane faces the ledge of the sapphire step. This observation provides a useful insight into the origin of the preferred ORs, and confirms the important role of surface steps in texture development. The evolution of Ni films of different thicknesses, ranging from 100 to 560 nm, was analyzed. Grain boundary grooving inhibits grain boundary motion and favors hole formation and dewetting in the thinnest films (100 nm). The crystals left behind after dewetting display ORs which may differ from the preferred ones. Large grains with the preferred ORs can grow at the expense of others when the film thickness is greater than 300 nm.

International audience; Postdynamic recrystallization and γ′ precipitation during cooling from γ′ supersolvus temperature after hot compression were studied in the AD730 (AD730 is a trademark of Aubert et Duval Company in Paris, France) γ-γ′ nickel-based superalloy. Emphasis was on not only both phenomena as distinct mechanisms but also on their mutual influence in terms of physical mechanisms. The growth of γ′ precipitates is hastened in the unrecrystallized grains compared to the recrystallized ones. This could possibly be attributed to the higher dislocation content acting as high-diffusivity paths. Postdynamic recrystallization is not prevented by Smith–Zener pinning of the recrystallization front by the γ′ precipitates. Instead, the recrystallization front dissolves γ′ precipitates, which then reprecipitate discontinuously or continuously and coherently with the surrounding γ matrix in the recrystallized grains.

This paper proposes a new approach for mean field modeling of dynamic recrystallization. The main advantage of the presented model compared to the state of the art is based on a more precise description of the immediate vicinity and of the shape of each grain to describe microstructural evolution all along the hot deformation process. Results provided by the new model are compared to those of a former mean field formulation and those of a full field model with an explicit description of the microstructure. The predictions of the new model in terms of recrystallization kinetics and grain size distributions are satisfactory and the progress when compared to former mean field models is obvious. Furthermore, the limitation of mean field models concerning the non-realistic shape of grain size distributions has been solved in this new formulation. Keywords: Microstructure, Mean field modeling, Topology, Recrystallization

In a polycrystal, the heterogeneity of plastic deformation in a particular grain is greatly enhanced by adjacent grains that constrain the grain's local behavior, often imposing orientation gradients. This work aims to characterize and quantify the local orientation gradients near grain boundaries (GBs). Electron backscatter diffraction (EBSD) measurements were made on a 0.67 mm thick aluminium-killed drawing quality (AKDQ) steel sheet subjected to different loading paths that are typical of forming operations. A statistical analysis shows that a considerable fraction of the analyzed GB profiles can be described by an orientation profile with a constant slope near the GB. In order to quantify this behavior, as well as the degree of localization, two new parameters, based on the local orientation gradient assessed by EBSD, are proposed: BET (boundary effective thickness) and GAS (gradient average severity). These parameters should be considered together, the BET as an effective thickness of the GB zone where the orientation gradient takes place and the GAS as a measure of the magnitude or severity of the orientation gradient. Additionally, the GAS parameter shows a strong correlation with the accumulated macroscopic strain for the investigated deformation levels and loading paths, while the BET profile clearly reveals the influence of the GB on the misorientation profiles. Tension and biaxial stretching results lead to a BET value between 1.5 and 2 μm. Finally, it is shown that the local misorientation in the GB zone, on both sides of the GB line, is disperse and it does not correlate simply with misorientation or even the slip-transfer geometry across the GB. Moreover, the observed average local misorientation dispersions in GB zones are different for each loading condition. Fil: Signorelli, Javier Walter. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Roatta, Analía. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: de Vincentis, Natalia Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Schwindt, Claudio Daniel. Universidad Nacional del Sur. Departamento de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Avalos, Martina Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Bolmaro, Raul Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Bozzolo, N.. Ecole Des Mines de Paris; Francia

International audience; In a previous article (Fausty et al., 2018) a new level-set finite element formulation for pure grain growth with heterogeneous grain boundary energies (i.e. one energy per grain interface) was developed and validated for simple configurations. In this work, the authors apply this new tool to the simulation of two dimensional grain growth of polycrystals using different disorientation dependent grain boundary energy functions. The results of these full-field calculations are assessed using the time dependent evolution of the following criteria: grain size, grain number, total interface energy, grain boundary disorientation distribution, grain boundary energy distribution and number of neighboring grains distribution. Of particular interest is the relationship between the grain boundary energy function and the evolution of the grain boundary network in the sense of both its morphology and its constitution. Some notable results are that the disorientation distribution evolution is inversely correlated to the grain boundary energy function itself and that the kinetics of grain growth are heavily effected by the heterogeneity of the system.

The mechanism of inverse precipitation of γ-like shells forming around primary γ' leading to heteroepitaxial recrystallization after thermomechanical processing was studied. The wrought nickel-based AD730™ alloy was subjected to a sub-solvus heat treatment at 1050 o C and cooled at 5 o C/min in order to form heteroepitaxial γ-like shells on primary γ'. Electron backscattered diffraction analysis confirmed no measurable crystal misorientation between the γ-like shells and primary γ'. Atom probe tomography revealed segregation of Cr, Co and Fe at dislocations within the primary γ', while the γ-like shell was found enriched in these solutes compared to the γ matrix. A mechanism rationalizing the inverse precipitation of γ-like shells is proposed aiming to better understand the nucleation process of heteroepitaxial recrystallization in polycrystalline Ni-based superalloys.

Thermomechanical processing routes are used to produce microstructures that minimize plastic strain localization at the sub-grain scale in a polycrystalline \(\gamma -\gamma \)’ nickel-based superalloy. This novel approach is made possible by the use of innovative experimental tools and statistical data analysis that capture slip events over large representative fields of view. Results are correlated to conventional observations of fatigue crack initiation and early stage of propagation. The effect of coherent twin boundaries and primary \(\gamma '\) precipitates on fatigue properties and plastic localization is detailed.

Influence of strain rate on dynamic and post-dynamic recrystallization kinetics of Inconel 718 is investigated by performing hot compression tests at constant strain rate in the range [ 0.001 ; 1 ] s − 1 in the δ -subsolvus domain, with or without post-deformation holding at the deformation temperature. Dynamically and post-dynamically recrystallized grains are distinguished based on their internal misorientations, using EBSD data with enhanced angular resolution. For the applied deformation conditions ( T = 980 ° C and e = 0.7 ), dynamic recrystallization is inhibited at e ˙ > 0.1 s − 1 . On the other hand, very fast post-dynamic recrystallization is promoted by high strain rates, with characteristic times which can be as short as few seconds to achieve full recrystallization. Most of previous works on the effect of strain rate on dynamic recrystallization kinetics were done by quenching samples right after deformation, without discriminating dynamically and post-dynamically recrystallized grains. Those works led to the conclusion that increasing strain rate beyond a critical value leads to an increase in dynamic recrystallization kinetics. Experimental quenching delays cannot be shorter that few seconds, which is shown here to be sufficient to get a significant increase in recrystallized fraction by post-dynamic mechanisms. Based on the present work, post-dynamic evolutions are actually very likely to be responsible for the apparent increase in dynamic recrystallization kinetics at high strain rates which has often been reported previously.

A new Ti-Cr based alloy has been developed to reach a TWIP (TWinning Induced Plasticity) effect as the main deformation mechanism. This new composition, involving Fe addition, was derived from a classical TRIP/TWIP alloy Ti-8.5Cr-1.5Al (wt%) (TCA). The main objective is to achieve an optimized strength/hardenability combination by limiting the TRIP (TRansformation Induced Plasticity) effect whose critical resolved shear stress lowers the plasticity threshold. This new alloy Ti-7Cr-1Al-xFe (wt%) (TCAF) displays excellent mechanical properties, with an increased yield strength (with respect to TCA alloy), a very high work-hardening rate and an extremely high fracture strength (UTS=1415MPa), while maintaining an excellent ductility (ε=0.38 at fracture). Both mechanical (tensile tests) and microstructural characterization at different scales (EBSD, XRD) have been performed, evidencing a dense network of fine {332} mechanical twins as well as the presence of stress-induced martensite plates at twins intersections, as a secondary mechanism.

International audience; Microstructural predictions on multi-pass processes are very challenging because the grains topology evolves in a very wide range, due to the coupled action of hardening, recovery, grain growth (GG), discontinuous dynamic and post dynamic recrys-tallization (DRX and PDRX). Classical models reach their limits. More physical approaches taking into account the complexity of the topology becomes necessary. In this article, DIGIMUQ®, the first commercial full field (FF) simulation software dedicated to grain growth and recrystallization modeling will be presented. Its ability to model industrial processes will be illustrated on an industrial-like 2D plate rolling process.

International audience; The fragmentation of α lamellae and the subsequent spheroidization of α laths, in α/β titanium alloys, are well known phenomena, occurring during and after deformation. We will illustrate the development of a new finite element methodology to model these phenomena. This new methodology is based on a level set framework modeling the deformation and the ad hoc concurrent or subsequent interfaces kinetics. In the current paper, we will focus on the modeling of the surface diffusion at the α/β phase interfaces and the motion by mean curvature at the α/α grain interfaces.

International audience; Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low-and intermediate-temperature proton-exchange membrane fuel cells. For applications in electro-catalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic hetero-geneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO 2 films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications.

International audience; In the last decades, many numerical models have been proposed to simulate thermomechanical treatments and their related effects on the microstructure. The present study deals with a relatively recent full field model using the level set method within a finite element framework. The ability of this approach to consider static recrystallization in two and three dimensions with nucleation has been demonstrated in previous studies (Bernacki et al., 2008, 2009). Although accurate, this model lies on a numerical formalism which is rather inefficient from a numerical point of view and do not permit to consider complex 3D aggregates in reasonable computation times. The present paper introduces a new efficient implementation of the static recrystallization (SRX) model which aims to overcome this limitation by taking full advantage of recent numerical developments (Shakoor et al., 2015; Scholtes et al., 2015). Its efficiency is evaluated through large scale 3D simulations of SRX with several thousand of grains. Acceleration factors of up to 40 are obtained, compared with the existing implementation. The predictions in terms of evolution of the recrystallized fraction are also confronted with classical analytic models and experimental results from literature, showing good agreement.

International audience; In the present study, mean field models of grain growth (Hillert and Burke–Turnbull models) are compared with 3D full field simulations considering an isotropic grain boundary energy and mobility and under the absence of second-phase particles. The present 3D full field simulations are based on a level set description of the grain interfaces within a finite element framework. The digital initial microstructures are generated using a coupled “Voronoï–Laguerre/dense sphere packing” algorithm. Based on full field simulation results, new formulations of Burke–Turnbull and Hillert models are proposed. In contrast with classical formulations, the new ones account for the possible heterogeneity of the initial grain size distribution.

International audience; We analyze measurements of dislocation densities carried out independently by several teams using three different methods on orientation maps obtained by Electron Back Scattered Diffraction on commercially pure tantalum samples in three different microstructural states. The characteristic aspects of these three methods: the Kernel average method, the Dillamore method and the determination of the lattice curvature-induced Nye’s tensor component fields are reviewed and their results are compared. One of the main features of the uncovered dislocation density distributions is their strong heterogeneity over the analyzed samples. Fluctuations in the dislocation densities, amounting to several times their base level and scaling as power-laws of their spatial frequency are observed along grain boundaries, and to a lesser degree along sub-grain boundaries. As a result of such scale invariance, defining an average dislocation density over a representative volume element is hardly possible, which leads to questioning the pertinence of such a notion. Field methods allowing to map the dislocation density distributions over the samples therefore appear to be mandatory.; Sur l'évaluation des densités de dislocations dans le tantale pur à partir de données d'orientation EBSD. On analyse les mesures de densités de dislocations effectuées indépendamment par plusieurs équipes en utilisant des méthodes différentes sur des cartes d'orientations obtenues par Electron Back Scattered Diffraction sur des échantillons de tantale commercialement pur dans trois états métallographiques différents. Les trois méthodes utilisées : la méthode du Kernel Average, la méthode de Dillamore et la méthode de détermination des composantes du tenseur de Nye induites par les courbures de réseau sont rappelées et analysées, et les résultats comparés. L'un des aspects essentiels des distributions de densités de dislocations, obtenues dans l'analyse des échantillons, est leur forte hétérogénéité. Des fluctuations de densité de dislocations s'élevant à plusieurs fois leur niveau de base, et s'exprimant en lois-puissance de leur fréquence spatiale sont observées le long des joints de grains et à un moindre degré le long des sous-joints de grains. Il résulte d'une telle absence d'échelle caractéristique que la définition d'une densité de dislocation moyenne sur un élément de volume représentatif pose problème, ce qui conduit à s'interroger sur la pertinence de cette notion. L'emploi de méthodes à champ complet permettant de dresser des cartes de densités de dislocations sur les échantillons analysés semble donc inévitable.

Grain growth is a ubiquitous thermally activated mechanism by which the microstructures of crystalline materials coarsen at relatively high temperatures. Individual grain boundaries in a material microstructure have their own structure and their own behavior and, as such, uniform grain boundary energy modelling approaches arrive at their predictive limits when it comes to certain types of local phenomena (abnormal grain growth, thermal twinning, etc.). This work presents a new heterogeneous grain boundary energy formulation for grain growth built on the thermodynamics of the phenomenon that can handle high grain boundary energy gradients. Using a full field finite element numerical framework it verifies the precision and convergence of this new formulation. Keywords: Grain growth, Grain boundary energy, Finite element, Level-set, Triple junction, Modelling

International audience; Ni-based single crystalline superalloys are used for high-pressure parts of aero-engines due to their superior mechanical properties and very good oxidation resistance at high temperature. However, shocks or unexpected mismatch in thermal contraction between molds and castings can occur during casting process and subsequent heat treatments, inducing plastic deformation of the alloy at low temperature. To mimic such events, a tensile plastic deformation is applied at room temperature on solutioned AM1 specimens and followed by standard aging heat treatments. Faster growth of the γ′ precipitates inside plastically deformed bands is obtained after full heat treatment with no lattice rotation or recrystallization. It has however been evidenced that the applied deformation has a detrimental impact on the creep properties, especially at high temperature (above 950 °C). It partly results from a highly localized failure process along former slip bands in which recrystallization is observed. The evolution of the microstructure during creep tests of prior deformed and nondeformed specimens has been thoroughly investigated to better identify under which conditions recrystallization occurs inside the bands during a creep test and by which mechanism.