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International audience; In polycrystalline metallic materials, quantitative and statistical assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. Plastic deformation often localizes into physical slip bands at the sub-grain scale. Detrimental microstructural configurations that result in the formation and evolution of slip bands during loading require advanced strain mapping techniques for the identification of these atomically sharp discontinuities. A new discontinuity-tolerant DIC method, Heaviside-DIC, has been developed to account for discontinuities in the displacement field. Displacement fields have been measured at the scale of the physical slip bands over large areas in nickel-based superalloys by high resolution scanning electron microscopy digital image correlation (SEM DIC). However, conventional DIC methods cannot quantitatively measure plastic localization in the presence of discontinuous kinematic fields such as those produced by slip bands. The Heaviside-DIC technique can autonomously detect discontinuities, providing information about their location, inclination, and identify slip systems (in combination with orientation mapping). Using Heaviside-DIC, discontinuities are physically evaluated as sharp shear-localization events, allowing for the quantitative measure of strain amplitude nearby the discontinuities. Measurements using the new Heaviside-DIC technique are compared to conventional DIC methods for identical materials and imaging conditions.

The anisotropic lattice rotation of individual grains induced by plasma nitriding of 316L austenitic stainless steel has been analyzed with the aim of identifying correlations between the initial grain’s orientation and the rotation behavior. Due to the quite large nitriding-induced strains (up to 20%), the Taylor–Bishop–Hill model has been chosen for the simulation of the lattice rotations. The model predicts the overall rotations, both amplitude and direction, reasonably well over the entire stereographic triangle. The magnitude of the rotations is in agreement with the level of deformation induced by insertion of nitrogen atoms into an austenitic lattice. With regard to plasticity, parallels between the nitriding process and tensile elongation along the normal surface can be drawn.

The new capabilities of a FEG scanning electron microscope (SEM) equipped with a scanning transmission electron microscopy (STEM) detector for defect characterization have been studied in parallel with transmission electron microscopy (TEM) imaging. Stacking faults and dislocations have been characterized in strontium titanate, a polycrystalline nickel-base superalloy and a single crystal cobalt-base material. Imaging modes that are similar to conventional TEM (CTEM) bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the different accelerating voltages highlighted. Defect images have been simulated for the transmission scanning electron microscopy (TSEM) configuration using a scattering matrix formulation, and diffraction contrast in the SEM is discussed in comparison to TEM. Interference effects associated with conventional TEM, such as thickness fringes and bending contours are significantly reduced in TSEM by using a convergent probe, similar to a STEM imaging modality, enabling individual defects to be imaged clearly even in high dislocation density regions. Beyond this, TSEM provides significant advantages for high throughput and dynamic in-situ characterization.

The impact of plasma nitriding at 400 °C on the monotonic mechanical behaviour of 316L austenitic stainless steel at room temperature has been investigated. It is shown that the residual stresses in the nitrided layer lead to a tension–compression anisotropy whose magnitude depends on the residual stresses intensity and extension (i.e. thickness of the nitrided layer). Using the stress differential technique, average residual stresses in the nitrided layer ranging from −1.5 up to −3 GPa were measured. The local mechanical behaviour of the nitrided layer has also been investigated through SEM in situ tensile tests. A quasi-brittle mechanical behaviour of the nitrided layer is observed with first evidences of crack initiation for plastic strains below 1%, whatever the nitrided layer. By increasing the total applied strain up to 20%, a progressive segmentation of the layer occurs. The crack initiation location mainly depends on the local nitrided thickness.

This work describes an experimental procedure to measure the progressive strain localization and crystal lattice rotation within metallurgical grains. A digital image correlation software was implemented and associated with mechanical tests carried out inside a scanning electron microscope on specimens exhibiting nanometric grainy patterns. Cross-correlation analyzes between electron backscattering diffraction maps were also developed to quantify the corresponding local crystal rotation relative to the original structure. The microscale strain and rotation fields on the surface of a tensile-loaded specimen made of austenitic stainless steel 316L are presented as an illustration. Their direct spatial correlation between strain heterogeneities and the progressive activation of slip systems is put into evidence and discussed.

International audience; A methodology is presented for the use of the oxide scale that develops in polycrystalline Ni-base super-alloys at service temperature, as a speckle pattern for μm-scale resolution strain measurements. Quantitative assessment of the heterogeneous strain field at the grain scale is performed by high-resolution SEM digital image correlation under monotonic and cyclic loading in polycrystalline Ni-base superalloys up to 650 • C. In the René 88DT superalloy, strain localization is observed near twin boundaries during low cycle fatigue (LCF) at intermediate temperatures, correlating with activation of {111}}110 and {111}}112 slip systems. A strong correlation between the microstruc-tural configuration that promotes strain localization during monotonic loading and crack initiation at 650 • C in low cycle fatigue was observed.

International audience; Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.

The nature of the near-surface γN phase produced by low-temperature (∼400 °C) plasma-assisted nitriding of an austenitic stainless steel 304L is studied. A combination of global probes (X-ray diffraction, nuclear reaction analysis, glow discharge optical emission spectroscopy) and local probes (field ion microscopy, conversion electron Mossbauer, X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopies) is employed to reveal the morphology, phase structure, atomic ordering and chemical state of the obtained γN phase. The results consistently reveal the heterogeneous nature of the nitrided layer consisting of nanometric CrN precipitates embedded in a Fe4N-like matrix. The size of the precipitates is found to be larger at the surface than at the nitrided layer–steel interface. The precipitates have irregular, sphere-like shapes. Moreover, X-ray spectroscopic investigation revealed three different intermetallic distances and different chemical environments for Fe, Cr and Ni, accompanied by a large static disorder. These findings suggest that the presence of the interstitial N destabilizes the homogeneous element distribution in 304L even at such low temperatures. This leads to the segregation into Cr-rich zones that are coherent with the Fe4N matrix. Possible atomistic decomposition mechanisms are discussed. Based on the heterogeneous nature of the γN phase revealed in 304L, an alternative view of its remarkable combination of properties such as large hardness, induced ferromagnetism and preserved corrosion resistance is considered.

Correlations between the grain orientations and elastic properties of plasma-nitrided polycrystalline 316L austenitic stainless steel are investigated. The grain orientations (h k l) in a delimited area were obtained from electron backscatter diffraction and related to hardness (Hhkl) and elastic modulus (Ehkl) maps obtained from large nanoindentation matrices. The influence of nitrogen concentration on the local mechanical properties has been studied by repeating these indentation matrices in the same area after successive partial removals of the nitrided layer. This nanoindentation tomography allowed the orientation, the shape and the surroundings of individual grains to be taken into account. The results show that plasma nitriding leads to a complete reversal of the elastic behaviour anisotropy: while the non-nitrided 316L austenitic stainless steel shows the typical elastic anisotropy of face-centred-cubic-type metals with a maximum of Ehkl for the 〈 1 1 1 〉 oriented grains, the maximum of Ehkl is observed for the 〈 0 0 1 〉 oriented grains in the nitrided layer. A similar anisotropy reversal is observed for the hardness Hhkl. These observations are discussed on the basis of the microstructural changes induced by the nitrogen incorporation.

Swelling of 316L austenitic stainless steel plasma nitrided at 400°C under floating potential has been investigated using electron back scattered diffraction and white-light interferometry. Swelling of individual grains strongly depends on their crystallographic orientation, similarly to the thickness of the nitrided layer. After 1 h of treatment, swelling is maximum for the 〈001〉 oriented grains and minimum for the 〈111〉 oriented grains. After 8 and 33 h of nitriding, the maximum of swelling is observed in the grains having their normal direction at about 15° from the 〈001〉 orientation. These results are discussed on the basis of plastic strain after comparison with calculated swellings of the 〈001〉 and 〈111〉 oriented grains, using the thickness of the nitrided layer deduced from the trapping–detrapping diffusion model and a rough estimation of the plastic strain. The satisfactory agreement between experimental and calculated swellings supports the idea that swelling results from the lattice expansion due to the incorporation of nitrogen plus an elastic strain and a plastic strain. For individual grains of the 316L matrix, nitriding leads to a tensile-like elongation of high magnitude (around 20%) and it might be the origin of the lattice rotations which were previously observed after nitriding.

The changes in anisotropic hardness and indentation modulus induced by plasma nitriding at 400 °C of a 316L polycrystalline austenitic stainless steel are analyzed. The dependence of hardness and elastic modulus modifications on the crystallographic orientation is investigated through instrumented indentation and electron backscattering diffraction. Both hardness and indentation modulus exhibit an inverted anisotropy compared to the untreated 316L, likely associated with the presence of the N atoms in interstitial sites.

The introduction at moderate temperature of nitrogen in the 316L austenitic stainless steel by plasma nitriding modifies the crystallographic texture in the very near surface region. The evolution of texture components has been quantitatively characterized by electron backscattered diffraction. The analysis of these experiments shows that the amplitude of the lattice rotation as well as the direction of rotation are directly related to the initial orientation of each grain. The retexturing behaviour is consistent with the lattice rotation upon tensile elongation of polycrystalline materials predicted by the Taylor model.

AISI 316L austenitic stainless steel is widely used in nuclear and chemical industries for structural components that are submitted to cyclic deformation and stresses. The influence of low temperature (~ 400 °C) plasma nitriding on the crystallographic texture and fatigue durability of 316L has been investigated. Electron back-scattering diffraction measurements show that nitriding enhances the and texture components while the one vanishes. Series of fatigue tests carried out in air at room temperature in the low cycle fatigue range show a significant improvement of the fatigue life. Results are discussed taking into account the residual compressive stresses in the nitrided layer.

Linear friction welded Ti–6Al–4V was investigated in fatigue at various stress amplitudes ranging from the high cycle fatigue (HCF) to the low cycle fatigue (LCF) regime. The base material was composed of hot-rolled Ti–6Al–4V plate that presented a strong crystallographic texture. The welds were characterized in terms of microstructure using electron backscatter diffraction and hardness measurements. The microstructural gradients across the weld zone and thermomechanically affected zone of the linear friction welds are discussed in terms of the crystallographic texture, grain shape and hardness levels, relative to the parent material. The location of crack nucleation under fatigue loading was analyzed relative to the local microstructural features and hardness gradients. Though crack nucleation was not observed within the weld or thermomechanically affected zones, its occurrence within the base material in LCF appears to be affected by the welding process. In particular, by performing high resolution digital image correlation during LCF, the crack nucleation site was related to the local accumulation of plastic deformation in the vicinity of the linear friction weld.

Plasma-assisted nitriding is an attractive surface treatment for metallurgical surface modification to improve wear, hardness and fatigue resistance of austenitic stainless steels. However, this technique requires low temperature processing in order to avoid chromium nitride precipitation and hence the degradation of corrosion properties. This paper presents a low temperature high rate plasma nitriding process and will emphasis on the consequences of nitrogen incorporation on the metallographic and crystallographic properties of the sample surface.

Crystallographic structure and lattice rotation of the ‘expanded’ austenite produced by means of low temperature plasma nitriding is investigated. The microstructure of the nitrogen enriched layer has been investigated by means of X-ray diffraction (XRD) while the lattice rotations in the nitrided zone were assessed by electron backscattered diffraction (EBSD). The nitrogen depth profiles have been determined by means of glow discharge optical emission spectroscopy and nuclear reaction analysis. XRD shows the presence of the ‘expanded’ austenite or γN phase in all the nitrided samples characterized by average larger lattice spacing in relation to non-nitrided steel matrix. EBSD investigation demonstrates that in addition to the lattice expansion nitrogen incorporation into the stainless steel matrix induces significant lattice rotations. The amount and direction of these crystallographic plane rotations are function of the initial orientation. An unusual evolution of the 220 γN line as a function of the nitriding duration is observed together with an anomalously high intensity ratio of the 111 and 200 γN and matrix lines. The XRD results are interpreted on the basis of the lattice rotations of diffracting planes, nitrogen concentration gradient, nitrogen diffusion anisotropy and residual stress. It shows that these rotations are a pertaining feature for the understanding of the γN microstructure.

The microstructural changes induced by the incorporation of large quantities of nitrogen in austenitic stainless steel are analyzed. Phase and texture modification as well as grain rotation are investigated by X-ray and electron backscatter diffraction. A quantitative dependence of the rotation on the grain orientation is determined by means of depth profiling and diffraction techniques. Correlation between local nitrogen interstitial content, associated crystallographic rotation and degree of texturing is established.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

International audience; To develop predictive models for monotonic and cyclic loading of polycrystalline metallic alloys, there is a need for quantitative assessment of plastic localization at the nano- and micro- scales. The development of two sets of advanced experiments for measurements of plastic localization will bediscussed. First, the combination of SEM and DIC has emerged as a robust method for quantification of the strain fields at the plastic localization scale.The use of the Heaviside-DIC method provides a unique opportunity to capture quantitatively the plastic localization on large representative fields of view. The second set of experiments employ in-situ Transmission Scanning Electron Microscopy measurements on targeted microstructural configurations to elucidate the effect of microstructure on the dislocation dynamics that accompany strain localization. The combination of these two sets of experiments coupled with the characterization of the 3D microstructure of metallic alloys provides critical information for predictive models of cyclic loading.

International audience; Damage mechanisms in polycrystalline metallic alloys involve the accumulation of plastic strain at the sub-grain level. Measurements of the heterogeneous strain field that results from the complex grain boundary networks present in engineering alloys are combined with microstructural and orientation information to extract quantitative data on the deformation mechanisms. Digital image correlation (DIC) has been extended to higher resolution for analysis of discontinuities that evolve during fatigue and to measure strains at the sub-grain scale and below, where plastic strain localization can be directly correlated with physical slip bands. Strain fields have been measured in the polycrystalline nickel-base superalloy René 88DT to assess the deformation processes under monotonic and cyclic loading at room and intermediate temperature. A new analysis approach is presented to extract, for the first time, physical values of the magnitude of plastic localization and strain accumulation locally at the scale of the slip bands.

International audience

International audience; In polycrystalline metallic materials, assessment of the plasticity in relation to the microstructure is necessary to understand the deformation processes during mechanical loading. High resolution digital image correlation (HR-DIC) techniques coupled with novel DIC codes have emerged as a quantitative tool to assess heterogeneous strain field at the microscale. In-situ and ex-situ scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) tensile testing were developed to characterize the continuous and discrete kinematics fields at the sub-grain level associated with non-localized deformation and slip events at room temperature. In-situ HR-DIC under SEM was capable to evidence elastic anisotropy of nickel-based superalloys at the grain scale. When irreversible deformation is concerned, {111} slip systems identification using in-plane HR-DIC under SEM was possible. HR-DIC under LSCM added the possibility to measure the out-of-plane displacements and to fully quantify the slip activity, i.e. the local shear strain induced by individual slip bands.The validation of the HR-DIC technique at room temperature prompted its application at temperatures representative of service conditions for polycrystalline nickel-based superalloys. Ex-situ intermediate temperature HR-DIC tests under SEM were thus conducted to evaluate the local slip activity under macroscopic cyclic deformation in reactive atmosphere. Local oxide spikes were found to form under cyclic loading in grains where plastic deformation was observed to occur in the first cycles.HR-DIC clearly demonstrates the synergy existing between local slip activity at the microstructure scale and environment induced surface degradation.