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.

Comparison of Grain-Growth Mean-Field Models Regarding Predicted Grain Size Distributions

Mean-field models have the ability to predict the evolution of grain size distribution that occurs through thermomechanical solicitations. This article focuses on a comparison of mean-field models under grain-growth conditions. Different microstructure representations are considered and discussed, especially regarding the consideration of topology in the neighborhood construction. Experimental data obtained with a heat treatment campaign on 316L austenitic stainless steel are used for the identification of material parameters and as a reference for model comparisons. Mean-field models are also applied to both mono- and bimodal initial grain size distributions to investigate the potential benefits of introducing neighborhood topology in microstructure prediction models. This article demonstrates that improvements in the predictions can be obtained in monomodal cases for topological models. In the bimodal test, no comparison with experimental data was performed as no data were available. But relative comparisons between models indicated few differences in the predictions. Although of interest, the consideration of neighborhood topology in grain-growth mean-field models generally results in only small improvements compared to classical mean-field models, especially in terms of implementation complexity.

Level-Set Modeling of Grain Growth in 316L Stainless Steel under Different Assumptions Regarding Grain Boundary Properties

Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values, and histograms. Two kinds of microstructures are considered: some are generated statistically from EBSD maps, and the others are generated by the immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, which are, respectively, defined as a function of the disorientation and both the GB misorientation and the GB inclination. In terms of mean grain size value and grain size distribution (GSD), both formulations provide similar responses. However, the anisotropic formulation better respects the experimental disorientation distribution function (DDF) and predicts more realistic grain morphologies. It was also found that the heterogeneous GB mobility described with a sigmoidal function only affects the DDF and the morphology of grains. Thus, a slower evolution of twin boundaries (TBs) is perceived.

Estimation of geometrically necessary dislocation density from filtered EBSD data by a local linear adaptation of smoothing splines

An implementation of smoothing splines is proposed to reduce orientation noise in electron backscatter diffraction (EBSD) data, and subsequently estimate more accurate geometrically necessary dislocation (GND) densities. The local linear adaptation of smoothing splines (LLASS) filter has two advantages over classical implementations of smoothing splines: (1) it allows for an intuitive calibration of the fitting versus smoothing trade-off and (2) it can be applied directly and in the same manner to both square and hexagonal grids, and to 2D as well as to 3D EBSD data sets. Furthermore, the LLASS filter calculates the filtered orientation gradient, which is actually at the core of the method and which is subsequently used to calculate the GND density. The LLASS filter is applied on a simulated low-misorientation-angle boundary corrupted by artificial orientation noise (on a square grid), and on experimental EBSD data of a compressed Ni-base superalloy (acquired on a square grid) and of a dual austenitic/martensitic steel (acquired on an hexagonal grid). The LLASS filter leads to lower GND density values as compared to raw EBSD data sets, as a result of orientation noise being reduced, while preserving true GND structures. In addition, the results are compared with those of filters available in theMTEXtoolbox.

Erratum: Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO2 on Polycrystalline Ti

[This corrects the article DOI: 10.1021/acsomega.6b00472.].

Electron backscatter diffraction study of orientation gradients at the grain boundaries of a polycrystalline steel sheet deformed along different loading paths

In a polycrystal, the heterogeneity of plastic deformation in a particular grain is greatly enhanced by adjacent grains that constrain the grain's local behavior, often imposing orientation gradients. This work aims to characterize and quantify the local orientation gradients near grain boundaries (GBs). Electron backscatter diffraction (EBSD) measurements were made on a 0.67 mm thick aluminium-killed drawing quality (AKDQ) steel sheet subjected to different loading paths that are typical of forming operations. A statistical analysis shows that a considerable fraction of the analyzed GB profiles can be described by an orientation profile with a constant slope near the GB. In order to quantify this behavior, as well as the degree of localization, two new parameters, based on the local orientation gradient assessed by EBSD, are proposed: BET (boundary effective thickness) and GAS (gradient average severity). These parameters should be considered together, the BET as an effective thickness of the GB zone where the orientation gradient takes place and the GAS as a measure of the magnitude or severity of the orientation gradient. Additionally, the GAS parameter shows a strong correlation with the accumulated macroscopic strain for the investigated deformation levels and loading paths, while the BET profile clearly reveals the influence of the GB on the misorientation profiles. Tension and biaxial stretching results lead to a BET value between 1.5 and 2 µm. Finally, it is shown that the local misorientation in the GB zone, on both sides of the GB line, is disperse and it does not correlate simply with misorientation or even the slip-transfer geometry across the GB. Moreover, the observed average local misorientation dispersions in GB zones are different for each loading condition.

Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO2 on Polycrystalline Ti

Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low- and intermediate-temperature proton-exchange membrane fuel cells. For applications in electrocatalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic heterogeneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO₂ films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 °C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications.

Fabrication of Ti substrate grain dependent C/TiO2 composites through carbothermal treatment of anodic TiO2

Composite materials of titania and graphitic carbon, and their optimized synthesis are highly interesting for application in sustainable energy conversion and storage. We report on planar C/TiO2 composite films that are prepared on a polycrystalline titanium substrate by carbothermal treatment of compact anodic TiO2 with acetylene. This thin film material allows for the study of functional properties of C/TiO2 as a function of chemical composition and structure. The chemical and structural properties of the composite on top of individual Ti substrate grains are examined by scanning photoelectron microscopy and micro-Raman spectroscopy. Through comparison of these data with electron backscatter diffraction, it is found that the amount of generated carbon and the grade of anodic film crystallinity correlate with the crystallographic orientation of the Ti substrate grains. On top of Ti grains with ∼(0001) orientations the anodic TiO2 exhibits the highest grade of crystallinity, and the composite contains the highest fraction of graphitic carbon compared to Ti grains with other orientations. This indirect effect of the Ti substrate grain orientation yields new insights into the activity of TiO2 towards the decomposition of carbon precursors.

Misorientations induced by deformation twinning in titanium

The misorientations induced by cold-rolling in commercially pure titanium by twinning and multiple twinning were investigated by electron backscatter diffraction. Three main types of twins occur: compressive twins (64.6° 〈1\overline 100〉) which are predominant, followed by tensile twins of type 1 (84.8° [11\overline 20]) and a few of type 2 (35.1° 〈1\overline 100〉). The main multiple twinning configurations are (i) several variants of a given type of twin inside a given grain and (ii) second-generation tensile twins inside primary compressive twins. All possible misorientations resulting from the combination of two out of the three main twinning systems have been calculated and compared with the experimentally observed twin configurations. This facilitated interpretation of the peak found at 41° 〈5\overline 1\overline 43〉 in the misorientation distribution as the result of double twinning (tensile twins inside primary compressive twins).

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.