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Minor addition of zirconium is common in polycrystalline nickel-based superalloys, where it is believed that it segregates at grain boundaries and contributes to increase the creep resistance. However, in superalloys produced by additive manufacturing, zirconium may become detrimental as it promotes hot-cracking during the fabrication stage. Here, we clarify the controversial role of this element by studying its distribution at near atomic scale in the as-built and heat-treated microstructures. In the as-built microstructure, zirconium is almost exclusively found at grain boundaries. However, after heat-treatment, zirconium is no longer found at grain boundaries. Instead, it partitions in γʹ precipitates and zirconium oxides particles. The formation of zirconia is shown to originate from the reduction of nano-particles of alumina by zirconium during heat-treatment. The absence of zirconium at grain boundaries in this state challenges the classic view often reported in the literature for superalloys.

The effect of increasing pre-straining on the corrosion behaviour of UNS S32304 duplex stainless steel has been studied using potentiodynamic tests in a highly acidic chloride-rich solution. The emergence of slip steps in the austenitic phase has been particularly addressed showing that the hence modified topographies had a significant impact on the kinetics of the Hydrogen Evolution Reaction (HER) that, in turn, tuned a non-monotonic behaviour of the corrosion rate. An attempt is made to link the variation of the Electron Work Function (EWF) of the surface with the electrochemical results by quantifying slip steps using Atomic Force Microscopy.

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, ( δ ) Al 2 O 3 alumina, (Cr,Mo) 3 B 2 and (Cr,Mo) 5 B 3 borides, and the intermetallic phase Ni 7 Zr 2 . The precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.

International audience; The mechanical behavior of micro-composites Duplex Stainless steels (DSS) produced by Accumulative re-Bundling and Drawing (ABD) has been investigated. We evidenced a systematic increase in the yield strength, as a function of the manufacturing step, independently from the phases' volume fraction. A simple rule of mixture (ROM) successfully predicted the first generation of composites presenting a simple microstructure. However, a modified ROM taking into account the contribution of each component and its grain size could not predict the mechanical response of the composites of higher generation. An in-depth analysis is conducted to investigate the microstructure-mechanical behavior relationship and to rationalize the resultant mechanical behavior from that of each constitutive phase. For this purpose, in-situ synchrotron High Energy X-ray Diffraction measurements during uniaxial tensile experiments have been carried out to calculate the strain partitioning within each phase of the composites.

An Al-Mg-Si aluminium alloy was shaped by using a two-pass equal-channel angular pressing (ECAP) process. This led to fragmentation of the coarse Fe-rich intermetallics (IMCs), a decrease in grain size and an increase in the high angle grain boundary (HAGB) density, with overconcentration of HAGBs around the IMCs. Corrosion tests in NaCl solution showed that, before and after ECAP, only pitting corrosion occurred. However, for ECAP samples, pits were more numerous due to the fragmentation of the IMCs; they were also larger and less deep, their propagation being strongly influenced by the presence of very small grains around the IMCs.

We investigate the hot cracking susceptibility and creep resistance of a nickel-based superalloy with three different contents of boron, carbon and zirconium fabricated by laser powder bed fusion. Crack-free and creep resistant components are achieved for the alloy version with boron, carbon and no zirconium. We then rationalize this result by evaluating how boron, carbon and zirconium are distributed at grain boundaries in the as-built and heat-treated microstructures of an alloy containing all these elements. Observations are conducted by scanning and transmission electron microscopy, and atom probe tomography. In the as-built microstructure, boron, carbon and zirconium segregate at high-angle grain boundaries as a result of solute partitioning to the liquid and limited solid-state diffusion during solidification and cooling. After heat-treatment, the amount of boron and carbon segregating at grain boundaries increases significantly. In contrast, zirconium is not found at grain boundaries but it partitions at the γ' precipitates formed during the heat treatment. The presence of zirconium at grain boundaries in the as-built condition is known to be susceptible to enhance hot cracking, while its absence in the heat-treated microstructure strongly suggests that this element has no major effect on the creep resistance. Based on our observations, we propose alloy design guidelines to at the same time avoid hot cracking during fabrication and achieve the required creep performance after heat-treatment.

The microstructure degradation and subsequent phase transformations in Waspaloy nickel-based superalloy during thermal exposure at 780 °C for 10,000 h were investigated. Two paths of η phase formation in the centre of extra-large γ’ (EL-γ’) following the formation of EL-γ’ were observed: (i) η phase directly precipitated within EL-γ’ when the coalescence of γ’ reached a critical stage; (ii) η phase precipitated at the interface of small size MC carbide and EL-γ’, with both MC and η embedded inside EL-γ’. The phase transformation process including the formation of EL-γ’ were experimentally observed and the formation sequences were schematically suggested. Two criteria of η formation and growth within EL-γ’ were established: (i) stacking faults formation in the nucleation site and (ii) sufficient atom diffusion during nuclei growth. The study of kinetics of η formation through two different paths revealed the critical role of small size carbides in promoting η nucleation and growth. It is concluded that η formation may be suppressed by controlling the size and density of MC carbides during materials processing.

The design of duplex stainless steels (DSS) with strain induced martensite (SIM) transformation relies on the optimization of austenite stability. The goal is to achieve a distribution of SIM over a wide range of strain so as to get the best combination of strength and ductility. A systematic study of plastic deformation mechanisms, of the link between phase chemistry, SIM kinetics and related stress–strain behavior has been carried out by combining advanced characterization: orientation mapping in the transmission electron microscope, in-situ high energy X-ray diffraction, and microscopic digital image correlation. The role of Ni/N balance on controlling the austenite stability has been investigated in medium-Ni lean DSS steels containing ∼4 wt% Ni. Results show that SIM occurs in the DSS by a two-steps transformation: first some austenite transforms into e-martensite and the latter subsequently transforms into α’-martensite at the intersections of e-bands and further grow into the austenite. It is found that SIM formation occurs at a slower kinetics in the DSS as compared to its fully austenitic counterpart, however with a similar relationship to flow stress. An optimal level of mechanical behavior is shown to be related to an optimal rate of SIM formation during tensile testing. The yield strength can be improved by rolling with a limited impact on the phase transformation potential.

International audience; The impact of microstructure evolution on mechanical properties in superduplex stainless steel UNS S32750 (EN 1.4410) was investigated. To this end, different thermomechanical treatments were carried out in order to obtain clearly distinct duplex microstructures. Optical microscopy and scanning electron microscopy, together with texture measurements, were used to characterize the morphology and the preferred orientations of ferrite and austenite in all microstructures. Additionally, the mechanical properties were assessed by tensile tests with digital image correlation. Phase morphology was not found to significantly affect the mechanical properties and neither were phase volume fractions within 13% of the 50/50 ratio. Austenite texture was the same combined Goss/Brass texture regardless of thermomechanical processing, while ferrite texture was mainly described by α-fiber orientations. Ferrite texture and average phase spacing were found to have a notable effect on mechanical properties. One of the original microstructures of superduplex stainless steel obtained here shows a strength improvement by the order of 120 MPa over the industrial material.

International audience; The influence of the martensitic transformation onmicroscale plastic strain heterogeneity of a duplexstainless steel has been investigated. Microscale strainheterogeneities were measured by digital image correlationduring an in situ tensile test within the SEM. Themartensitic transformation was monitored in situ duringtensile testing by high-energy synchrotron X-ray diffraction.A clear correlation is shown between the plasticity-induced transformation of austenite to martensiteand the development of plastic strain heterogeneities atthe phase level.

A further development of the rechargeable and long-lasting ion-batteries requires a comprehensive study of a plenty of processes undergoing on micro- and nano-scale in cathode active material during battery charge/discharge cycle. The discharge process results in significant transformations of the cathode’s structure while trying to accommodate the changing conditions, such as migration of transition metals, loss of oxygen, or rearrangements of the metal−oxygen polyhedra. Thus, an experimental information about structural changes occurring at the unit cell level becomes crucial for the evolution of the electrochemical performance and degradation routes of a cathode active material. In situ transmission electron microscopy (TEM) is one the most powerful analytical tools allowing to follow structural and chemical transformations in battery active materials at nanoscale with high spatial resolution in liquid high-vapor conventional electrolytes. For instance, very recently, the changes in the unit cell structure of lithium battery cathode materials (LFP) during electrochemical cycling in liquid electrolyte were determined for nano-particles using in situ electron diffraction tomography. The same compound was earlier comprehensively studied by means of 4D-STEM (ASTAR system from NanoMegas) and other techniques available in TEM (EELS). This work demonstrated that use of ASTAR system allowed to identify the LiFePO4 and FePO4 phases in individual grains and correlate them with the crystalline orientation and the facets of the individual crystals. ASTAR system is phase and crystalline orientation mappings developed from nanobeam electron diffraction. Precession of electron beam (to reduce dynamical effects) and relatively low acquisition/total scan time (to preserve the sensitive to electron beam samples) are one of the most valuable advantages of this system, particularly in in situ electrochemical liquid TEM studies. The growing demand of the modern industry to store an increasing amount of renewable energy in a sustainable way on the one hand and concerns about future Li availability on the other hand rekindled interest for sodium-ion battery technology, owing to the natural abundance of sodium. Sodium vanadium (III) fluorophosphate, Na3V2(PO4)2F3 (NVPF), attracting great interest as a potential positive electrode for Na-ion batteries due to its exceptional rate and electrochemical cycling capabilities was chosen as an object of our study. During charge process the leaving of two Na+ ions leads to the change of vanadium oxidation state (VIII → VIII/IV → VIV) and corresponding structural transformations (Amam → I4/mmm → Cmc21) which can be registered with help of ASTAR system. Our research is aimed to couple liquid electrochemical TEM studies with ASTAR system and, thus, to observe the evolution of the insertion/de-insertion zones of Na+ ions in individual positive electrode crystals and to correlate them with structural inhomogeneities, orientations and crystalline facets. The obtained in situ observations should resolve a dispute on the models proposed to account for insertion and phase separation mechanisms: mobile phase limits, intercalation waves and domino cascades. Here, we would like to present the first results of our TEM studies of NVPF. In our work we employed Poseidon in situ electrochemical liquid cell TEM holder (Protochips) and TECNAI G2 transmission electron microscope equipped with Oneview camera (GATAN) and ASTAR system. Thanks to 4D-STEM system we succeeded to follow the structural and, thus, phase transformations of NVPF grains during charge/discharge process in coin-cell battery. The results of obtained phase mapping are in a good agreement with corresponding galvanostatic curves and available X-ray diffraction synchrotron data.

The microstructure of a centrifugally cast HP alloy was studied in its as-received state and after ageing at 900 °C. A multi-scale approach combining X Ray Diffraction (XRD), advanced electron microscopy modes (scanning and transmission electron microscopies (SEM, TEM), together with focused ion beam/SEM nanotomography (FIB-nt)) has been carried out to characterize the evolution of niobium carbides during ageing. After thermal treatment, the carbides exhibit a complex microstructure, consisting of a core of untransformed NbC, an intermediate layer of G-phase (Ni16Nb6Si7) with embedded nanometric titanium carbide precipitates, and an outer shell of alternating chromium carbides Cr23C6 and G-phase. A simple diffusion model was used to explain the faster external growth of G-phase compared to the internal NbC dissolution, and to determine a diffusion coefficient of niobium in the G-phase at 900 °C.

International audience; The isothermal decomposition of austenite between BS and MS was investigated after carburizing and carbonitriding in low-alloy steel 23MnCrMo5. The microstructure of the products differed significantly between carburizing and carbonitriding. The carburized alloy produced an expected bainitic microstructure consisting mainly of bainitic ferrite with some cementite and retained austenite. In contrast, the microstructure of the nitrogen-containing sample showed finer, entangled ferrite plates, retained austenite and nitrides. The ferrite plates contain numerous transformation twins, which can be as thin as only a few nanometers. This original result proves that abundant twinning can be found in bainite as well as in martensite.

In this work we propose a top-down strategy in which an austenitic stainless steel (type AISI 316L) and a ferritic stainless steel (type AISI 430LNb) are mechanically alloyed by Severe Plastic Deformation (SPD) to elaborate an architectured duplex stainless steel. This proposed strategy serves two main objectives: i) enhancing the properties by microstructure refinement down to sub-micron scale, and ii) elaborating a model material for understanding the behavior of Duplex Stainless Steel (DSS) obtained by the conventional metallurgical methods. The Accumulative Drawing and re-Bundling (ADB) technique has been successfully implemented for these specific materials, allowing to obtain multi-scale micro-composites of 316L/430LNb steels. The limits of this process in terms of microstructure refinement, have been identified as due to the complete regression of the micro-scale austenitic phase during annealing. Keywords: Architectured duplex stainless steel, Severe plastic deformation, Micro-composites, Accumulative drawing and bundling

International audience; A ternary beta-metastable titanium Ti-9Mo-6W (wt.%) was designed. A very high work hardening rate close to 2100 MPa and a uniform deformation larger than 35% were recorded, thanks to combined transformation-induced plasticity and twinning-induced plasticity effects. In this paper, detailed microstructural analysis was performed to understand the deformation process. Various mechanisms, {332} < 113 > mechanical twinning, stress-induced omega phase and stress-induced alpha '' martensite were identified after mechanical testing, resulting in a complex network of deformed microstructures with very special synergetic features.

International audience; The paper describes an automated crystal orientation and phase mapping technique that allows nanoscale characterization of crystalline materials with a transmission electron microscope. The template matching strategy used to identify the diffraction patterns is detailed and the resulting outputs of the technique are illustrated. Some examples of applications are used to demonstrate the capability of the tool and potential developments are discussed.

International audience; MgH2-based nanocomposites are excellent candidates for hydrogen storage applications due to their high hydrogen gravimetric capacities, reversibility of the absorption/desorption reactions and low cost of Mg. A conventional route to obtain MgH2 is high-energy ball milling which involves processing in controlled atmosphere due to the poor resistance of the nanometric powder for air contamination (O-2, H2O, etc.). In the present work we evaluated the H-sorption properties of bulk Mg samples produced by different severe plastic deformation (SPD) routes and we associated the results with their microstructural characteristics. The SPD techniques included high pressure torsion (HPT) and cold rolling (CR), applied to process commercial Mg and fine-grained Mg ribbons, which were obtained by rapid solidification. Enhanced H-sorption properties were observed for the sample processed by melt-spinning followed by cold-rolling and these properties were associated with refined microstructure and stronger (0001) texture, (C) 2013 Elsevier B.V. All rights reserved.

International audience; A transmission electron microscopy (TEM) based orientation mapping system was employed to investigate the twinning behavior during hot rolling process of a magnesium alloy. Conducting scanning with the tilting series method, both {10 (1) over bar1) {1012} and {10 (1) over bar2} twinning types could be unambiguously identified. It is also found that for {10 (1) over bar1) {10 (1) over bar2) twinning the compositive influences of both twinning Schmid factor and the ease of potential basal slip within twinned area should be considered as the twinning selection criterion. (C) 2013 Elsevier B.V. All rights reserved.

A recently developed technique based on the transmission electron microscope, which makes use of electron beam precession together with spot diffraction pattern recognition now offers the possibility to acquire reliable orientation/phase maps with a spatial resolution down to 2 nm on a field emission gun transmission electron microscope. The technique may be described as precession-assisted crystal orientation mapping in the transmission electron microscope, precession-assisted crystal orientation mapping technique-transmission electron microscope, also known by its product name, ASTAR, and consists in scanning the precessed electron beam in nanoprobe mode over the specimen area, thus producing a collection of precession electron diffraction spot patterns, to be thereafter indexed automatically through template matching. We present a review on several application examples relative to the characterization of microstructure/microtexture of nanocrystalline metals, ceramics, nanoparticles, minerals and organics. The strengths and limitations of the technique are also discussed using several application examples.

Usually niobium is added in ferritic stainless steels to avoid chromium carbides precipitation and then to improve corrosion resistance and to avoid embrittlemet. This study shows that a low Nb stabilization makes recrystallization nucleation much faster and prevents incomplete recrystallization. A qualitative interpretation, based on interaction with precipitates, is proposed and explains the main features of the softening kinetics as well as the microstructures obtained. Above a specific magnetizing frequency, the deformed state led to smaller losses than the recrystallized state. These results are believed to be attributed to a grain size effect. This leads to soft magnetic properties that makes 17%CrNb ferritic stainless steels a very interesting solution for the market of electromagnetic injection. Improving response-time of fuel injection valves is a great challenge for automotive industry in order to enhance car engine efficiency and to limit noxious gas emission. Normalmente o nióbio é adicionado aos aços inoxidáveis ferríticos para se evitar a precipitação de carbonetos de cromo e, em seguida, para se melhorar a resistência à corrosão e para se evitar a fragilização. Esse estudo mostra que a estabilização com baixo Nb baixo promove a nucleação da recristalização muito mais rapidamente e evita a recristalização incompleta. Uma interpretação qualitativa, baseada na interação com precipitados, é proposta. Nessa interpretação, explicam-se as principais características da cinética de amolecimento, bem como as microestruturas obtidas. Acima de uma frequência específica de magnetização, o estado deformado leva a perdas menores do que o estado recristalizado. Acredita-se que esses resultados podem ser atribuídos a um efeito de tamanho de grão. Isto leva a propriedades magnéticas macias, que faz dos aços inoxidáveis ferríticos 17% CrNb uma solução muito interessante para o mercado de injeção eletromagnética. Melhorar o tempo de resposta das válvulas de injeção de combustível é um grande desafio da indústria automobilística, a fim de se aumentar a eficiência dos motores, quanto às emissões de gases tóxicos.

High-temperature ductility of dual-phase stainless steels is investigated using a micromechanics approach of damage and fracture. Two different microstructures are studied with either a lamellar or a globular morphology of the ferrite phase, the latter being twice as ductile as the former at 1150 degrees C. The high-temperature damage evolution is characterized at different loading rates using notched round cylindrical bars producing different stress triaxialities, supplemented by fractographic analysis. The experimental observations have generated an advanced elasto-viscoplastic micromechanical damage model for both microstructures. With a detailed account of the process of nucleation, growth and coalescence of voids, the model properly captures the effect of stress triaxiality, strain rate, and morphology of the ferritic phase on the high-temperature ductility. The key factor controlling the loss of ductility in the lamellar microstructure is the constraint induced by the harder austenite layer on the softer ferrite zones. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; The strain induced martensite transformation in austenitic stainless steels is of considerable interest, because it results in materials with attractive combinations of strength and ductility. The present work examines the mechanical response for a variety of strain and temperature paths, and relates these to microstructural observations. New evidence of the detailed transformation sequence is presented, along with direct evidence of codeformation of the austenite and martensite. Using different deformation temperature sequences enables the transformation to be changed from one that is heterogeneous to one that propagates axially along the sample. The strain hardening that occurs due to combined plasticity and martensitic transformation results in high kinematic hardening that is revealed by microstructural observations here, and which are linked directly to the mechanical response of these materials described in Part II of the present work.

International audience; In the present work we report a modified configuration of a multiply twinned decahedral barrel-like nanoparticle. The nanoparticle is stabilized by a selective chemical etching, which yielded a truncated shape with multiple high-index planes on the surface. The surface planes and the shape of the nanoparticle have been characterized using electron microscopy techniques, including scanning and transmission electron microscopies and electron diffraction. Coherent electron diffraction revealed streaked Bragg reflections which are in agreement with the high-index facets measured from electron microscope projected images. Crystal orientation mapping assisted by precession electron diffraction was performed under nanobeam conditions giving information about the structure from different orientations. The combination of these techniques allowed us to resolve the 3D structure of the modified nanoparticle and dismiss any possible ambiguities from the interpretation of the projected images in direct space. (C) 2015 Elsevier B.V. All rights reserved.

International audience; To further understand graphite growth mechanisms in cast irons, this study focuses on the crystal structure of a graphite spheroid in the vicinity of its nucleus. A sample of a graphite spheroid from a commercial cast iron was characterised using transmission electron microscopy. The chemical composition of the nucleating particle was studied at the local scale. Crystal orientation maps of the graphite spheroid revealed misorientations and twist boundaries. High resolution lattice fringeimages showed that the basal planes of graphite were wavy and distorted close to the nucleus and very straight further away from it. These techniques were complementary and provided new insights on spheroidal graphite nucleation and growth.

Softening kinetics of two 17% chromium (Cr) stainless steel grades that differ in niobium (Nb) content are compared. In the experiments, we observed that a low Nb stabilization makes recrystallization nucleation much faster and prevents incomplete recrystallization. A qualitative interpretation, based on interaction with precipitates, is proposed and explains the main features of the softening kinetics as well as the microstructures obtained. For the Nb stabilized grade, magnetic losses were measured in the deformed state and after recrystallization. Above a specific magnetizing frequency, the deformed state led to smaller losses than the recrystallized state. These results are believed to be attributed to a grain size effect.

The high temperature tearing resistance of dual phase ferritic stainless steel has been characterized by the essential work of fracture method. This method proved to be very well adapted to high temperature cracking. Two microstructures were tested to assess the method which turns out to be very discriminating. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Beta titanium alloys are increasingly the best choice for automotive and aerospace applications due to their high performance-to-density ratio. Among these alloys, the TIMETAL Ti-LCB is already used in the automotive industry because it presents excellent mechanical properties and a lower cost compared with other Ti alloys. The current study deals with the characterization of the nucleation and growth of the alpha phase in several thermomechanical processes, because the distribution and size of the alpha phase strongly influence the mechanical properties of the resulting microstructures. Several heat treatments were conducted after either cold rolling or annealing. The resulting microstructures were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, or electron backscatter diffraction. It was observed that the morphology and the volume fraction of the alpha phase are strongly dependent on the holding temperature, on the heating or cooling rate, and on the beta grain size.

International audience; Experimental investigations of the microstructure and physical properties of Fe/Pt multilayers and the hard magnetic FePt (L10) phase are reported. Accumulated cold rolling of a stack of pure Fe and Pt foils was used to produce a series of bulk multilayers of various internal scales (10 mm–10 nm). Annealing treatments were carried out to transform the multilayer via interfacial reactions into the ordered FePt (L10) phase. By combining different experimental techniques, it was shown that scale reduction operates by plastic co-deformation of both phases without any interfacial product. Crystallographic texture refinement, a strong increase in mechanical strength and a lowering of the transformation temperature occur with scale reduction. Kinetics and phase transformations were studied. Magnetic domain structure images were correlated to the grain and defect structure of the FePt phase.

The strengthening that results from the low-temperature formation of strain-induced martensite in austenitic stainless steel was studied. Specifically, the work hardening behaviour was characterized, as well as the spatial distribution of the martensite as a function of prior strain. Neutron diffraction measurements revealed the degree of elastic strain partitioning between the austenite and martensite. It was found that a sufficiently high initial dislocation density leads to a localization of the martensite transformation in the form of a L?ders front. The martensite acts as an elastic reinforcing phase as it supports a higher stress than the austenite tensile loading, even though the martensite co-deforms plastically with the austenite. A model was developed that predicts the volume fraction of martensite formed as a function of plastic strain., 13th International Conference on the Strength of Materials (ICSMA-13), Budapest, Hungary, 25-30 August 2003

The preparation of FePt hard magnetic foils by an original procedure is described in this paper. The process associates cyclic co-deformation of Fe and Pt foils down to the nanometer scale (total thickness of multilayer ≈100 μm) followed by heat treatment in the temperature range 450–550°C. The formation of the high-anisotropy L1 0 FePt phase results from controlled diffusion and an ordering phase transformation. Coercivities as high as 0.9 T were measured in a VSM at room temperature following annealing at 450°C for 48 h. The coercivity of this sample was decreased by half when measured at 600 K while its energy product decreased from 100 kJ/m 3 at 300 K to 25 kJ/m 3 at 600 K.

International audience; A new family of metastable beta type titanium alloys has recently been designed for ductility improvement with large strain-hardening effect. Superior plastic properties were obtained due to collective effects of both phase transformation induced plasticity and twinning induced plasticity. A series of stress induced phase transformations, including stress/strain-induced beta to omega transformation and stress-induced martensitic beta to α” transformations, play particular roles at each deformation stage. The SIM transformations were found to be closely related to the strain-hardening behavior of the material. In this work, the relationship between SIM transformations and strain-hardening effect were studied by microstructural investigations.

Over the last two decades, the use of aluminum sheets in automotive applications has increased. Aluminum sheets are currently produced from direct-chill (DC) cast plates. The need for low-cost aluminum sheets is a challenge for the development of new materials produced by twin-roll continuous (TRC) casting and cold rolling. It is expected that the sheets produced from these different casting procedures can differ in their microstructure. Therefore, they can exhibit different formability behavior. The paper presents the results of the microstructural characterization and texture evaluation of aluminum sheets produced by both technologies. Sheets produced from twin-roll cast materials have much finer and more numerous second-phase particles, the grain structures of both types of materials are similar. Electron backscatter diffraction (EBSD) and X-ray diffraction techniques were used for texture evaluation and both confirmed the presence of stronger cube texture in the strips produced from DC-cast plates.

La fragilisation par les metaux liquides est illustree par des exemples sur deux alliages industriels d'aluminium. La penetration des joints de l'alliage 7010 par le gallium permet d'etudier l'influence de la microstructure et le role de la plasticite. La fragilisation a chaud de l'alliage 5083 est un exemple de clivage induit par la fragilisation par le metal liquide.

The fundamental mechanisms of the deformation of heavily deformed multiphase materials are not clearly understood despite the vast literature and the amount of data on this subject. This paper presents a new plastic flow mechanism observed by in-situ transmission electron microscopy deformation on Cu-Nb continuous multifilamentary nanostructured composites. The observed mechanism is interpreted in terms of dislocation loops nucleating in closely spaced parallel planes in the Cu channels. This behaviour explains many previous observations made on such composites such as the semicoherency of the interface and the 4° disorientation between the two phases. The modelling of the loop mechanism predicts the strength of the Cu-Nb nanocomposites. The very good agreement between the model and the experimental data suggests that the assumptions based on the in-situ observations are valid. The basic aspects of co-deformation of fcc-bcc nanostructured systems are reviewed in this new context.

A diametrical section of a graphite spheroid from a ductile iron sample was prepared using the focused ion beam-lift out technique. Characterization of this section was carried out through automated crystal orientation mapping in a transmission electron microscope. This new technique automatically collects electron diffraction patterns and matches them with precalculated templates. The results of this investigation are crystal orientation and phase maps of the specimen, which bring new light to the understanding of growth mechanisms of this peculiar graphite morphology. This article shows that mapping the orientation of carbon-based materials such as graphite, which is difficult to achieve with conventional techniques, can be performed automatically and at high spatial resolution using automated crystal orientation mapping in a transmission electron microscope.

The lattice mismatch between the γ matrix and the γ′ precipitates in the AM1 superalloy was measured in situ during a creep experiment up to fracture (150 MPa, 1050 and 1080 °C) by high energy XRD at the ID 15 beamline of the ESRF. The results are coherent with experiments done post mortem on specimens just annealed, and a specimen strained at 1050 °C, 150 MPa up to 1.03%. From the evolution of the mismatch, it is proposed that the elastic interfacial energy varies strongly during a creep test, and might put a limit on the high temperature ductility of superalloys.

Deformation mechanism in high strength Cu/Nb nanocomposites have been investigated through in situ TEM observations in tension. The results explain the particular interface microstructure that was previously observed by high resolution TEM. Moreover, the observed loops mechanism could be modeled, and the simulation of the very high mechanical properties of this type of composites is in good agreement with experimental data. Some criteria for higher strength composites are proposed.

Grâce a une machine d'essais mecaniques a haute temperature developpee a Nancy et aux caracteristiques de la ligne a haute energie de l'ESRF, nous avons mesure in situ l'evolution du desaccord parametrique du superalliage monocristallin AM1 au cours d'une deformation en fluage conduite jusqu'a la rupture. Ces mesures montrent une forte correlation entre cette evolution et les stades caracteristiques de la courbe de fluage. De plus, on observe que, durant le stade II qui correspond a la phase d'utilisation du materiau, le desaccord parametrique evolue de facon continue et monotone. Sa mesure permet donc de caracteriser l'etat d'endommagement de l'eprouvette et d'estimer sa duree de vie restante. La possibilite d'utiliser ce type de mesures comme methode de controle non destructif de l'endommagement est suggeree.

Single crystal nickel base superalloys are used for the high-temperature parts of aircraft engines like turbine blades. Their good mechanical properties at high temperature are related to the precipitation of an ordered {gamma}{prime} phase which induces a structural hardening of the material. The {gamma}{prime} phase has an ordered L1{sub 2} structure while the {gamma} matrix is disordered and has a FCC structure. The volume fraction of f{gamma}{prime} of the {gamma}{prime} phase evolves with the temperature and a complete solutionizing occurs above 1,280 C in the AM1 superalloy. The {gamma}{prime} phase of Ni based superalloys is usually analyzed through its prototype Ni{sub 3}Al. As the Ni{sub 3}Al structure remains totally ordered up to temperature very close to the melting point, it is commonly assumed in superalloys that the {gamma}{prime} phase precipitates are fully ordered up to their solutionizing and that the volume fraction of the precipitates is equivalent to the volume fraction of the ordered phase. However, in superalloys, it is difficult to separate experimentally the effects related to the solutionizing of the precipitates from those due to a possible partial disordering of the {gamma}{prime} phase and this assumption has not been verified yet. The aim of this paper is tomore » study the structural order in the {gamma}{prime} phase of a superalloy.« less

Diffraction profiles of single crystal Ni-based superalloy samples with different microstructures were measured in situ up to the complete solutionizing of the {gamma}{prime} phase, using a high resolution triple crystal diffactometer and high energy synchrotron radiation (150 keV, {lambda} = 0.08 {angstrom}). A comparison between an undeformed sample and creep-deformed specimens with various resultant microstructures evidenced a relation between the lattice parameter distribution, the {gamma}{prime} precipitate microstructure and the sign of the connectivity. It was shown that a deformation induces a change in the relative volume cell of {gamma} and {gamma}{prime} phases. Moreover, the high resolution of the experimental set-up allows in many cases the {gamma}{prime} phase volume fraction to be measured with a good accuracy.

International audience; Bright- and dark-field images are reconstructed by extracting the intensities of selected spots from a succession of digitalized electron diffraction patterns collected using a transmission electron microscope by scanning the focused beam over the area of interest. The procedure is similar to the generation of the scanning-transmission electron microscopy images. Several examples are shown to illustrate the flexibility and potentiality of such numerical off-line reconstruction

Due to its importance for industrial applications, the microstructural behavior of single crystal nickel base superalloys as a function of the thermo-mechanical history of the material is the subject of many studies. However, some controversies remain concerning parameters which are driving the coarsening of {gamma}{prime} precipitates. In particular the role of the lattice parameter mismatch between the {gamma} and {gamma}{prime} phases (usually defined as {Delta}d/d = (a{gamma}{prime} {minus} a{gamma})/ where a{gamma}{prime} and a{gamma} represent respectively the lattice parameter value of the {gamma}{prime} and {gamma} phases) and of the internal stresses at the interfaces has to be clarified. An experiment was performed on a creep deformed sample using high energy synchrotron radiation and a Triple Crystal Diffractometer set-up (TCD) which allow nondestructive measurements and probe the bulk of the sample. With this method the superstructure reflection (100) was measured with a good accuracy and a reasonable statistics.

High-energy X-ray synchrotron radiation was used to measure internal strains inside the bulk of creep deformed AM1 single crystal superalloy specimen. A sample creep deformed at 1050 °C with γ′ raft-like precipitates was studied ‘in situ’ in temperature. The superstructure reflections (100) and (001) as well as fundamental reflections (200) and (002) were measured up to the temperature of deformation using a high-resolution triple crystal diffractometer. The lattice parameter distribution which is related to internal stress distribution was determined for each phase in parallel and perpendicularly to the rafts. It was shown that these parameters depend strongly on the thermomechanical history of the analysed material and on the temperature at which measurement is performed.

Using the small angle neutron scattering technique, we have observed rafting in nickel based single crystal superalloys. Kinetics of morphological evolution of the precipitates have been studied in situ. Therefore we used a special furnace designed for the ageing of prestrained specimens under a neutron beam. The evolution of both the precipitate aspect ratio and the distance between precipitates confirms the importance of strain in the directional coarsening process. Results are presented and discussed regarding kinetics and microstructural aspects. In such conditions, rafts seem to be different from those obtained after a creep test.