On the nitrogen-induced lattice expansion of a non-stainless austenitic steel, Invar 36®, under triode plasma nitriding

Chromium, as a strong nitride-forming element, is widely regarded to be an “essential” ingredient for the formation of a nitrogen-expanded lattice in thermochemical nitrogen diffusion treatments of austenitic (stainless) steels. In this article, a proprietary “chrome-free” austenitic iron-nickel alloy, Invar® 36 (Fe-36Ni, in wt pct), is characterized after triode plasma nitriding (TPN) treatments at 400 °C to 450 °C and compared with a “stainless” austenitic counterpart RA 330® (Fe-19Cr-35Ni, in wt pct) treated under equivalent nitriding conditions. Cr does indeed appear to play a pivotal role in colossal nitrogen supersaturation (and hence anisotropic lattice expansion and superior surface hardening) of austenitic steel under low-temperature (≤ 450 °C) nitrogen diffusion. Nevertheless, this work reveals that nitrogen-induced lattice expansion occurs below the nitride-containing surface layer in Invar 36 alloy after TPN treatment, implying that Cr is not a necessity for the nitrogen-interstitial induced lattice expansion phenomenon to occur, also suggesting another type of γN

Micro-Strain and Cyclic Slip Accumulation in a Polycrystalline Nickel-Based Superalloy

International audienceThis work provides a comprehensive characterization and analysis of deformation and fatigue damage mechanisms in a nickel-based superalloy during ambient temperature fatigue and points to a fundamental deformation mechanism that results in the onset of crack nucleation. Strain and slip irreversibility are investigated at the nanometer scale using high-resolution digital image correlation and high-resolution electron backscatter diffraction, highlighting distinct deformation mechanisms contributing to crack nucleation. It is observed during early fatigue cycling at relatively low applied stress, the formation of intense slip events that induce grain boundary shearing. This results in intense micro-scale strain in the neighboring grains, producing localized plasticity and stresses. Such stresses facilitate fatigue extrusion-intrusion mechanisms during subsequent cycling, resulting in preferred crack nucleation. Finally, the configurations within the microstructure that promote such deformation and damage mechanisms sequence are highlighted

Near-surface mechanical heterogeneities in a dissimilar aluminum alloys friction stir welded joint

International audienceThe local mechanical properties of a dissimilar friction stir welded AA-2024-T3/AA-2198-T3 joint were documented during a uniaxial tensile test. High-resolution digital image correlation was performed during monotonic tensile tests to capture the local in-plane strain fields of the heterogeneous macrostructure of the weld. In the shoulder-affected region, banded macrostructures with heterogeneous mechanical properties were found. They were related to pronounced textures regions, which can be associated to strain-rate gradient during one rotation of the tool. The banded macrostructures in the nugget region were observed to be responsible for early plasticity in the joint and ultimately to be the fracture location of the weld. The heterogeneous mechanical response of the joint was also investigated by microhardness measurements. Differences were found between hardness and local tensile properties, demonstrating microhardness measurements can be misleading and only direct high-resolution digital image correlation techniques can document the mechanical behavior of materials having complex and heterogeneous micro-/macrostructures

Sub-grain scale digital image correlation by electron microscopy for polycrystalline materials during elastic and plastic deformation

International audienceDamage during loading of polycrystalline metallic alloys is localized at or below the scale of individual grains. Quantitative assessment of the heterogeneous strain fields at the grain scale is necessary to understand the relationship between microstructure and elastic and plastic deformation. In the present study, digital image correlation (DIC) is used to measure the strains at the sub-grain level in a polycrystalline nickel-base superalloy where plasticity is localized into physical slip bands. Parameters to minimize noise given a set speckle pattern (introduced by chemical etching) when performing DIC in a scanning electron microscope (SEM) were adapted for measurements in both plastic and elastic regimes. A methodology for the optimization of the SEM and DIC parameters necessary for the minimization of the variability in strain measurements at high spatial resolutions is presented. The implications for detecting the early stages of damage development are discussed

Observation of bulk plasticity in a polycrystalline titanium alloy by diffraction contrast tomography and topotomography

International audienceThe mechanical properties of polycrystalline metals are governed by the interaction of defects that are generated by deformation within the 3D microstructure. In materials that deform by slip, the plasticity is usually highly heterogeneous within the microstructure. Many experimental tools can be used to observe the results of slip events at the free surface of a sample; however, there are only a few methods for imaging these events in the bulk. In this article, the imaging of bulk slip events within the 3D microstructure is enabled by the combined use of X-ray diffraction contrast tomography and topotomography. Correlative measurements between high-resolution digital image correlation, X-ray diffraction contrast tomography, topotomography and electron backscattered diffraction are performed during deformation of Ti-7Al to investigate the sensitivity of the X-ray topotomography method for the observation of slip events in the bulk. High-resolution digital image correlation and electron backscattered diffraction measurements uniquely provide slip and lattice rotation activities at the surface of the specimen during deformation and are used to corroborate the diffraction contrast tomography and topotomography measurements. Much larger neighborhoods of grains were mapped by topotomography measurements than in previous studies, enabling quantitative measurements of slip transmission. Significant differences were observed between surface and bulk grains, indicating the need for 3D observations of plasticity to better understand deformation in polycrystalline materials

Automated and quantitative analysis of plastic strain localization via multi-modal data recombination

International audienceA multi-modal data recombination method that enables the automated, quantitative and statistical assessment of strain localization as a function of the microstructure is presented. It consists of merging high resolution digital image correlation (HR-DIC) datasets collected in a scanning electron microscope (SEM), with crystallographic data obtained from electron back-scattered diffraction (EBSD). As the data is typically gathered over large areas (about 1 mm2), this method enables the quantitative assessment of plastic strain localization over hundreds to thousands of grains, yet with a spatial resolution of tens of nanometers. The data is treated in a hierarchical manner so that strain localization phenomena can be studied as a function of phases, texture and grain orientation. The use of discontinuity tolerant DIC codes, such as Heaviside DIC (H-DIC) in the present case, enables identification the active slip system associated with slip band discontinuities. Analyses conducted over thousands of bands in thousands of grains enable the quantitative assessment of fundamental plasticity laws. The capabilities of this method are shown through application to Ti-6Al-4V and Inconel 718 alloys

Slip localization in Inconel 718: a three-dimensional and statistical perspective

International audienceThe slip localization behavior of the polycrystalline nickel base superalloy Inconel 718 during monotonic tensile loading at room temperature, is investigated for the first time in relation to the 3D microstructure. Multi-modal data merging tools are used to recombine high resolution digital image correlation (HR-DIC) data with 3D electron back-scatter diffraction tomography (3D EBSD), over a wide region of interest. This procedure enables reconstruction of the slip band planes in the 3D microstructure. Statistical analyses conducted over 500 individual slip bands reveal strong correlations between their location and specific microstructure configurations. In particular, over half of the slip bands emanate from triple junction lines (3D lines defined by the junction of three crystals). Moreover, the most intense and longest slip bands, which would become critical fatigue crack nucleation sites during cyclic loading, are located close and parallel to particular annealing twin boundaries and are simultaneously connected to triple junction lines. Crystal plasticity finite elements calculations are performed on the experimental microstructure to identify the slip activity that results in the formation of high intensity slip bands (localized plasticity) or zones of high lattice rotation (non-localized plasticity) in these particular microstructure regions