Abstract Slip systems involving dislocations with Burgers vectors have been studied in hexagonal close packed Zr and Ti, by means of in situ straining experiments in a transmission electron microscope, at various temperatures and as a... more
Abstract Slip systems involving dislocations with Burgers vectors have been studied in hexagonal close packed Zr and Ti, by means of in situ straining experiments in a transmission electron microscope, at various temperatures and as a function of resolved shear stress. The results show that Zr and Ti are very similar in many respects. Prismatic slip is activated at rather low resolved shear stresses, and is controlled by the interaction between mobile dislocations and solute atoms (presumably oxygen). Pyramidal slip requires substantially higher resolved shear stresses and is characterized by straight screw dislocations moving by a kink-pair mechanism. Basal slip is activated at and above room temperature, for resolved shear stresses equal or higher than those in the prismatic planes. The slip traces are always wavy, presumably due to intensive cross slip from basal to prismatic planes. It also involves straight screw dislocations moving by a kink-pair mechanism. These microscopic observations are discussed in the light of some aspects of the mechanical behavior, in particular the increase of yield-stress at decreasing temperature and the discontinuity of activation area close to room temperature.
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ABSTRACT In this present work, the plasticity of a rod bar of a 14% Cr ferritic ODS steel is examined through a multiscale approach based on both macroscopic and microscopic results. This bar was elaborated at CEA by powder metallurgy and... more
ABSTRACT In this present work, the plasticity of a rod bar of a 14% Cr ferritic ODS steel is examined through a multiscale approach based on both macroscopic and microscopic results. This bar was elaborated at CEA by powder metallurgy and consolidated by hot extrusion. The microstructure of the material has been characterized.First, the tensile behavior of this material is studied in a wide range of temperatures. Thereafter, through in situ Transmission Electron Microscopy (TEM) straining experiments, dislocation/dislocation and dislocation/precipitates interactions are observed. The collapse of the tensile properties noticed from 400 °C can be explained by a change in the deformation mechanism. At lower temperatures, the hardening seems to be due to the precipitates, dislocations are pinned on oxides. At higher temperatures, the hardening role of the precipitates is still observed, but the dislocations seem to move in a more steady way, thermal activation of dislocations sources is observed and leads to formation of cavities at the grain boundaries.
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The microscopic mechanisms associated with non-compact slip in hexagonal close packed metals, face centered cubic metals and ordered intermetallics are described. Informations are obtained from in-situ straining experiments in the... more
The microscopic mechanisms associated with non-compact slip in hexagonal close packed metals, face centered cubic metals and ordered intermetallics are described. Informations are obtained from in-situ straining experiments in the electron microscope as well as macroscopic deformation tests on magnesium, beryllium, aluminium, copper and Ni 3 Al. Dislocation motion on non-compact planes occurs through various cross slip processes for screws. In some cases edge dislocations are slower than screws. The role of non-compact slip in high temperature deformation is discussed
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In the first part, dislocation mechanisms controlling individual dislocation glide in prismatic and basal planes were studied and compared with results of atomistic calculations. In this second part, the plasticity of Ti3Al is studied in... more
In the first part, dislocation mechanisms controlling individual dislocation glide in prismatic and basal planes were studied and compared with results of atomistic calculations. In this second part, the plasticity of Ti3Al is studied in situ at an intermediate scale. The critical resolved shear stress (CRSS) of prismatic slip is shown to be controlled by the easy glide process of
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In situ experiments have been performed on beryllium between 80 and 423 K, in order to find the exact origin of the anomalous increase in the critical resolved shear stress (CRSS) of prismatic glide, as the temperature is increased... more
In situ experiments have been performed on beryllium between 80 and 423 K, in order to find the exact origin of the anomalous increase in the critical resolved shear stress (CRSS) of prismatic glide, as the temperature is increased between 160 and 330 K. In the whole temperature range, including the domain of the stress anomaly, the results clearly show that the CRSS is controlled by the locking-unlocking mechanism as described in Part I of this study. As in part I, quantitative measurements are also performed at different temperatures. In particular, local stress measurements reproduce very well the shape of the macroscopic CRSS as a function of temperature. All these results show that the origin of the stress anomaly is at variance with all proposed models. It is an individual dislocation mechanism, corresponding to an increase in the difficulty of cross-slipping from basal to prismatic planes at increasing temperatures (entropy effect). This effect may be due to an intrinsic decrease in stacki...
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Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
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ABSTRACT In this work, we carry out a detailed study, by in-situ Transmission Electron Microscopy (in-situ TEM), focused on two single-crystals of Cu-Al-Ni shape memory alloys with different transformation temperatures. The first single... more
ABSTRACT In this work, we carry out a detailed study, by in-situ Transmission Electron Microscopy (in-situ TEM), focused on two single-crystals of Cu-Al-Ni shape memory alloys with different transformation temperatures. The first single crystal is in beta phase at Room Temperature (RT) and has been cycled under stress, by super-elastic effect, inside the TEM. Two different mechanisms for the nucleation of β'3 and γ'3 martensite phases were observed: a) Martensite can nucleate on dislocations during super-elastic tests and when withdrawing the stress, the reverse transformation takes place by the disappearance of the martensite plate on the dislocation. b) During mechanical cycling martensite plates nucleate in other plates. The second single crystal is in martensite phase at RT, and when the stress is applied different mechanisms are observed: a) Reorientation and interface motion of the plates under the external applied stress, b) nucleation of mobile dislocations inside the martensite. A quantitative analysis of the experimental results, having into account the images and the diffraction patterns, has been realized and different mechanisms have been proposed to explain the experimental results.
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Scanning (SEM) and cross-sectional transmission (TEM) electron microscopy analyses have been performed to study the transformations induced on the surface of titanium implants by a sequence of chemical treatments having as goal to induce... more
Scanning (SEM) and cross-sectional transmission (TEM) electron microscopy analyses have been performed to study the transformations induced on the surface of titanium implants by a sequence of chemical treatments having as goal to induce the nucleation and growth of hydroxycarbonated apatite (HCA). In the first step, an acid etching forms a rough titanium hydride layer, which remains unchanged after subsequent treatments. In a second step, soaking in a NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In a third step, soaking in a simulated body fluid transforms sodium titanate into calcium and phosphorus titanate, by ion exchange in the monoclinic structure. Then, HCA grows and embodies the tangled structure. The interfaces between the different layers are shown to be strong enough to prevent from interfacial decohesion. The role of the titanate structure in the nucleation of HCA is finally discussed.
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Research Interests: Engineering, Technology, Electron Microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, and 12 moreElectrochemical Impedance Spectroscopy, Physical sciences, Equivalent Circuit, Boric Acid, Microstructures, Thin Solid Films, Phosphoric acid, Phosphoric Acid, Aluminium Alloy, Elsevier, Anodic Oxidation, and Structural model
Simulated body fluid (SBF) solutions are widely used for in vitro bioactivity tests and to coat bioinert materials with biomimetic calcium phosphates. In this study SBF solutions with varying HCO3 - content were used to precipitate... more
Simulated body fluid (SBF) solutions are widely used for in vitro bioactivity tests and to coat bioinert materials with biomimetic calcium phosphates. In this study SBF solutions with varying HCO3 - content were used to precipitate hydroxy carbonated apatite (HCA) on a bioactive titanium surface. XRD as well as cross-sectional TEM analyses revealed that the biomimetically derived crystallites show a preferred growth orientation in direction of their c-axis and perpendicular to the surface of the substrate. FTIR and Raman analyses revealed that, as long as the HCO3 - concentration in the testing solutions is below 20 mmol/l, only B-type HCA precipitates. Using SBF with a HCO3 - concentration equal to human blood plasma (27 mmol/l) leads to a AB-type substitution where, in accordance with bone mineral, CO3 2- substitutes PO4 3- as well as OH-.
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Research Interests: Materials Science, Technology, Biomimetics, Hydroxyapatite, Scanning Electron Microscopy, and 15 moreMedicine, Biological Sciences, Carbonates, Biomolecular Engineering, Calcium Phosphate, Crystallite Size, Particle Size, Cross Section, Surface Properties, Microstructures, Apatites, Materials Testing, Body Fluids, Biomimetic, and Elemental composition
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The transformation of titanium into several hydride phases has been studied by transmission electron microscopy at the surface of an acid-etched titanium substrate designed for biomedical applications. Four different orientation... more
The transformation of titanium into several hydride phases has been studied by transmission electron microscopy at the surface of an acid-etched titanium substrate designed for biomedical applications. Four different orientation relationships have been found between face-centred cubic TiH2−ε (ε≪1), face-centred tetragonal TiH and hexagonal close-packed Ti. A fast method based on the alignment of diffraction spots perpendicular to interfaces (Δg
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Research Interests: Materials Science, Carbon, Hydroxyapatite, Scanning Electron Microscopy, Titanium, and 15 moreNanoparticles, Medicine, Multidisciplinary, Humans, Nanostructures, Cross Section, Ion Exchange, Oxides, Sodium Hydroxide, Bone Substitutes, Materials Testing, Durapatite, Simulated Body Fluid, Hydrochloric Acid, and Chemical Treatment
Defects in quasicrystals are simpler than what could be anticipated from the rather exotic quasicrystalline structure. For instance, dislocations exhibit properties (contrast in TEM, possible modes of motion under stress) that are close... more
Defects in quasicrystals are simpler than what could be anticipated from the rather exotic quasicrystalline structure. For instance, dislocations exhibit properties (contrast in TEM, possible modes of motion under stress) that are close to those in crystals. The aim of this talk is to introduce these properties using simple concepts, and avoiding complex calculations in high-dimensional space.
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In-situ straining experiments have been carried out on NiAl single crystals, along soft orientations, at 143 and 300 K, in order to analyse the glide properties of 001 dislocations and the fracture process. The results show that... more
In-situ straining experiments have been carried out on NiAl single crystals, along soft orientations, at 143 and 300 K, in order to analyse the glide properties of 001 dislocations and the fracture process. The results show that dislocations slip either in {110} planes or in wavy slip surfaces as the result of intensive crossslip, in agreement with atomistic calculations of the dislocation core structure along the screw orientation. Cross-slip also leads to the nucleation of high densities of prismatic loops. Dislocations exhibit kinked shapes with almost no screw portion, in agreement with results of atomistic and anisotropic elastic calculations. The local stress deduced from dislocation curvature is close to that measured in macroscopic experiments. Peierls frictional forces are low at 300 K, but higher at 144 K on edge segments in planes close to {100}. The dislocations emitted at a crack tip are analysed and compared with those formed in atomistic simulations under similar conditions.
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ABSTRACT After a predeformation in the constant strain rate stage, electron microscope observations - weak beam images at 100 kV, dynamic experiments at high voltages - are used to describe the microstructure: the three dimensional... more
ABSTRACT After a predeformation in the constant strain rate stage, electron microscope observations - weak beam images at 100 kV, dynamic experiments at high voltages - are used to describe the microstructure: the three dimensional network and the subboundaries. Numerous microscopic processes take place and the predominant ones appear to be dislocation multiplication inside some of the cells, dislocation crossing of subboundaries, dislocation insertion in the subboundaries. The latter phenomenon may lead to annihilation but could not be observed directly. Strong evidences of cross-slip are observed at dislocation subboundary-interactions.