Study of curtaining effect reduction methods in Inconel 718 using a plasma focused ion beam.

The curtaining effect is a common challenge in focused ion beam (FIB) surface preparation. This study investigates methods to reduce this effect during plasma FIB milling of Inconel 718 (nickel-based superalloy). Platinum deposition, silicon mask and XeF2 gas injection were explored as potential solutions. These methods were evaluated for two ion beam current conditions; a high ion beam intensity condition (30 kV-1 µA) and a medium one (30 kV-100 nA) and their impact on curtaining reduction and resulting cross-section quality was assessed quantitatively thanks to topographic measurements done by atomic force microscopy (AFM). XeF2 assistance notably improved cross-section quality at medium current level. Pt deposition and Si mask individually mitigated the curtaining effect, with greater efficacy at 100 nA. Both methods also contributed to reducing cross-section curvature, with the Si mask outperforming Pt deposition. However, combining Pt deposition and Si mask with XeF2 injection led to deterioration of these protective layers and the reappearance of the curtaining effect after a quite short exposure time.

Discrimination of dynamically and post-dynamically recrystallized grains based on EBSD data: application to Inconel 718.

The importance of angular resolution in EBSD analyses is discussed based on an Inconel 718 sample containing several populations of recrystallized grains, with subtle differences in dislocation contents. Classical EBSD analyses (with angular resolution in the range of 0.5-1°) do not allow for distinguishing recrystallized grains grown dynamically or post-dynamically. The angular resolution of EBSD orientation and misorientation data can be significantly improved (down to about 0.1-0.2°) either using more sophisticated Kikuchi pattern indexing methods and/or using the recently proposed LLASS denoising filter (Local Linear Automatic Smoothing Splines). Then the coexistence of both dynamically and post-dynamically recrystallized grains in the sample can be confirmed and quantified. ECCI images unambiguously confirm the conclusions drawn from the analysis of improved angular resolution EBSD data, and furthermore reveal the presence of thermal stress induced dislocations with typical patterns in water quenched Inconel 718 recrystallized grains. LAY DESCRIPTION: EBSD is widely used to study recrystallization phenomena. Conventional EBSD is nevertheless not able to distinguish dynamic recrystallized grains from post-dynamic recrystallized grains which differ by subtitle differences in dislocation contents. In this paper, we show that improving the orientation precision of EBSD data by means of different methods allows distinguishing these two recrystallized grains populations. Analyses and discussion are based on an Inconel 718, a famous Nickel-based superalloy in aeronautic.

Statistical analysis of dislocations and dislocation boundaries from EBSD data.

Electron BackScatter Diffraction (EBSD) is often used for semi-quantitative analysis of dislocations in metals. In general, disorientation is used to assess Geometrically Necessary Dislocations (GNDs) densities. In the present paper, we demonstrate that the use of disorientation can lead to inaccurate results. For example, using the disorientation leads to different GND density in recrystallized grains which cannot be physically justified. The use of disorientation gradients allows accounting for measurement noise and leads to more accurate results. Misorientation gradient is then used to analyze dislocations boundaries following the same principle applied on TEM data before. In previous papers, dislocations boundaries were defined as Geometrically Necessary Boundaries (GNBs) and Incidental Dislocation Boundaries (IDBs). It has been demonstrated in the past, through transmission electron microscopy data, that the probability density distribution of the disorientation of IDBs and GNBs can be described with a linear combination of two Rayleigh functions. Such function can also describe the probability density of disorientation gradient obtained through EBSD data as reported in this paper. This opens the route for determining IDBs and GNBs probability density distribution functions separately from EBSD data, with an increased statistical relevance as compared to TEM data. The method is applied on deformed Tantalum where grains exhibit dislocation boundaries, as observed using electron channeling contrast imaging.

Evidence of multimicrometric coherent γ' precipitates in a hot-forged γ-γ' nickel-based superalloy.

This paper demonstrates the existence of large γ' precipitates (several micrometres in diameter) that are coherent with their surrounding matrix grain in a commercial γ-γ' nickel-based superalloy. The use of combined energy dispersive X-ray spectrometry and electron backscattered diffraction (EBSD) analyses allowed for revealing that surprising feature, which was then confirmed by transmission electron microscopy (TEM). Coherency for such large second-phase particles is supported by a very low crystal lattice misfit between the two phases, which was confirmed thanks to X-ray diffractograms and TEM selected area electron diffraction patterns. Dynamic recrystallization of polycrystalline γ-γ' nickel-based superalloys has been extensively studied in terms of mechanisms and kinetics. As in many materials with low stacking fault energy, under forging conditions, the main softening mechanism is discontinuous dynamic recrystallization. This mechanism occurs with preferential nucleation on the grain boundaries of the deformed matrix. The latter is then being consumed by the growth of the newly formed grains of low energy and by nucleation that keeps generating new grains. In the case of sub-solvus forging, large γ' particles usually pin the migrating boundaries and thus limit grain growth to a size which is determined by the distribution of second-phase particles, in good agreement with the Smith-Zener model. Under particular circumstances, the driving force associated with the difference in stored energy between the growing grains and the matrix can be large enough that the pinning forces can be overcome, and some grains can then reach much larger grain sizes. In the latter exceptional case, some intragranular primary γ' particles can be observed, although they are almost exclusively located on grain boundaries and triple junctions otherwise. In both cases, primary precipitates have no special orientation relationship with the surrounding matrix grain(s). This paper demonstrates the existence of high fractions of large γ' precipitate (several micrometres in diameter) that are coherent with their surrounding matrix grain, in a commercial γ-γ' nickel-based superalloy. Such a configuration is very surprising, because there is apparently no reason for the coherency of such particles.

EBSD coupled to SEM in situ annealing for assessing recrystallization and grain growth mechanisms in pure tantalum.

An in situ annealing stage has been developed in-house and integrated in the chamber of a Scanning Electron Microscope equipped with an Electron BackScattered Diffraction system. Based on the Joule effect, this device can reach the temperature of 1200°C at heating rates up to 100°C/s, avoiding microstructural evolutions during heating. A high-purity tantalum deformed sample has been annealed at variable temperature in the range 750°C-1030°C, and classical mechanisms of microstructural evolutions such as recrystallization and grain coarsening phenomena have been observed. Quantitative measurements of grain growth rates provide an estimate of the mean grain boundary mobility, which is consistent with the value estimated from physical parameters reported for that material. In situ annealing therefore appears to be suited for complementing bulk measurements at relatively high temperatures, in the context of recrystallization and grain growth in such a single-phase material.

EBSD for analysing the twinning microstructure in fine-grained TWIP steels and its influence on work hardening.

A 22 Mn-0.6 C twinning induced plasticity steel with an average grain size of 2.6 mum was deformed in tension at room temperature. The electron backscattered diffraction technique was used to characterize the twinning structure in relation with the local texture evolution. For nanoscale analysis, additional transmission electron microscopy analysis was performed. Nanotwins were activated in the largest grains from the beginning of the deformation. They interacted with a well-developed dislocation structure that induced detectable intragranular orientation variations. With increasing deformation, dense bundles of nanotwins preferentially developed in grains oriented close to the <111>//tensile direction fibre (promoted by the deformation) as well as medium to high angle sub-boundaries. These key features of the twinned microstructure were finally related to the remarkably high strain hardening, which evolved according to different stages.

Accuracy of orientation distribution function determination based on EBSD data-A case study of a recrystallized low alloyed Zr sheet.

The question of the statistical accuracy of EBSD data for global texture calculation was re-explored on the basis of a very large grain population (83 000 grains measured on a recrystallized low-alloyed Zr sheet). Previous works aimed mainly at identifying and quantifying the main texture components and were based on much smaller data sets. The present work attempts to quantify the accuracy of the complete texture, including low-density regions of the orientation space. For that purpose, a new statistical parameter, V(Delta), based on the calculation of texture difference functions is proposed. This parameter has two main advantages: it is equally sensitive to both high and low peaks of the orientation density function (ODF), and it has a physical interpretation because it is the material volume fraction corresponding to the difference between a given ODF and a reference ODF (considered, or known to be close to the truth). Two main variables were studied: the number of grains taken into account and the peak width phi(0) of Bunge's 'Gaussian' model density used as kernel for the actual analysis. The orientation distribution functions were computed by nonparametric kernel density estimation with harmonics up to the order of 34. Minimizing the value of V(Delta) serves as the objective function for optimizing the peak width phi(0) as a function of the number of grains. The properties of the V(Delta) parameter also allows for the definition of a method for estimating the accuracy of a given texture that has been obtained from a limited number of grains, without knowing the true texture of the investigated material.