Application of combined EBSD and 3D‐SEM technique on crystallographic facet analysis of steel at low temperature

Electron backscatter diffraction has been increasingly used to identify the crystallographic planes and orientation of cleavage facets with respect to the rolling direction in fracture surfaces. The crystallographic indices of cleavage planes can be determined either directly from the fracture surface or indirectly from metallographic sections perpendicular to the plane of the fracture surface. A combination of electron backscatter diffraction and 3D scanning electron microscopy imaging technique has been modified to determine crystallographic facet orientations. The main purpose of this work has been to identify the macroscopic crystallographic orientations of cleavage facets in the fracture surfaces of weld heat affected zones in a well‐known steel fractured at low temperatures. The material used for the work was an American Petroleum Institute (API) X80 grade steel developed for applications at low temperatures, and typical heat affected zone microstructures were obtained by carrying out weld thermal simulation. The fracture toughness was measured at different temperatures (0°C, ?30°C, ?60°C and ?90°C) by using Crack Tip Opening Displacement testing. Fracture surfaces and changes in microstructure were analyzed by scanning electron microscopy and light microscopy. Crystallographic orientations were identified by electron backscatter diffraction, indirectly from a polished section perpendicular to the major fracture surface of the samples. Computer assisted 3D imaging was used to measure the angles between the cleavage facets and the adjacent polished surface, and then these angles were combined with electron backscatter diffraction measurements to determine the macroscopic crystallographic planes of the facets. The crystallographic indices of the macroscopic cleavage facet planes were identified to be {100}, {110}, {211} and {310} at all temperatures.     

Application of electron backscatter diffraction (EBSD) to fracture studies of ferritic steels

The application of electron backscatter diffraction (EBSD) to fracture studies has provided a new method for investigating the crystallography of fracture surfaces. The crystallographic indices of cleavage planes can be measured both directly from the fracture surface and indirectly from metallographic sections perpendicular to the plane of the adjoining fracture surfaces. The results of direct individual cleavage facet plane orientation measurements are presented for carbon–manganese (C–Mn) and low‐alloy Mn–Mo–Ni (similar to ASTM A553 type‐B). Pressure vessel steel weld metals, obtained from fracture surfaces of Charpy impact test specimens fractured at various test temperatures and for an ultra‐low carbon steel (Fe–0.002C–0.058P) fractured at –196 °C by impact. In addition to the direct measurement from the fracture surface, cleavage facet orientation measurements for the ultra‐low carbon steel were complemented by the results obtained from the metallographic sections. Fractographic observations revealed that cleavage fracture is accommodated by a microvoid coalescence fracture micromechanism, which was induced by decohesion of second phase particles (inclusions). The correlation between the direct and indirect methodologies shows that the cleavage facet planes are dominated by the {001} plane orientations, and indicated that even when information concerning the full five degrees of freedom is inaccessible, the cleavage facet plane could still be determined. Finally, the advantages and disadvantages of direct orientation measurements from the fracture surface and indirectly by a destructive sectioning technique are discussed.     

A specimen preparation technique for transmission electron microscopy of surface layers

A TEM specimen preparation method is described, with the aid of which electron transparent foils can be obtained across the external surface of a specimen. After careful pre-treatment, steel specimens have been electrolytically coated with nickel. Conventional thinning in a plane cutting the substrate-coating interface, gave thin foils displaying the internal structure as a function of depth under the initial free surface. The method has also been applied to minute metal particles, of dimensions too small to allow manipulating and foil preparation by conventional methods. Image examples are shown, and the applicability of the method is discussed.     

Preparation of cross-section specimens for transmission electron microscopy from alpha-Al2O3/Ni solid state bonded bicrystal interfaces.

The technique which is described combines the advantages of the techniques formerly proposed in the literature in each stage of the preparation of transmission electron microscopy (TEM) specimens including a single metal-ceramic interface. It allows easy handling of the thin foils in spite of their brittleness. Preferential thinning of the softer material in the two-phase foil is prevented, and both sides of the interface are thinned down to comparable thicknesses. The nickel-alumina bicrystal interface observed in TEM is neat and free from any reaction layer. This method is easily adaptable to other metal-ceramic systems.     

Combined application of electron backscatter diffraction and stereo-photogrammetry in fractography studies

The main aim of this paper is to report on recent experimental developments that have succeeded in combining electron back-scatter diffraction (EBSD) with stereo-photogrammetry, compared with two other methods for study of fracture surfaces, namely visual fractography analysis in the scanning electron microscope (SEM) and EBSD directly from facets. These approaches will be illustrated with data relating to the cleavage plane orientation analysis in a ferritic and C-Mn steel. It is demonstrated that the combined use of EBSD and stereo-photogrammetry represents a significant advance in the methodology for facet crystallography analysis. The results of point counting from fractograph characterization determined that the proportions of intergranular fracture in C-Mn and ferritic steels were 10.4% and 9.4%, respectively. The crystallographic orientation was determined directly from the fracture surface of a ferritic steel sample and produced an orientation distribution with a clear trend towards the {001} plane. A stereo-photogrammetry technique was validated using the known geometry of a Vickers hardness indent. The technique was then successfully employed to measure the macroscopic orientation of individual cleavage facets in the same reference frame as the EBSD measurements. Correlating the results of these measurements indicated that the actual crystallographic orientation of every cleavage facet identified in the steel specimens is {001}.     

A method for preparing cross sections of films on wear surfaces for transmission electron microscopy study

A method for preparing cross sections of surface layers which exist on bulk metal substrates for transmission electron microscopy (TEM) study is described. The surface layer or film is protected by a vacuum-deposited or sputtered coating of a suitable metal. A mask is placed over the surface and non-masked areas are subjected to ion beam etching until the substrate is exposed. A thick electroplated layer is then applied to the surface. This layer adheres well to the ion-etched substrate and seals the coated surface film against damage during the usual slicing and grinding steps which are required for the preparation from bulk materials of thin foils for TEM study. The method was developed specifically for the analysis of boundary and extreme pressure lubrication films on wear surfaces together with the near-surface region of the substrate. However, it is also applicable to the investigation of oxide, corrosion and other surface films.     

The stereology of projected images

A generalized, geometrical approach is adopted in reviewing the basic relationships of projected structures. Projections of features from a plane to a projection line, and from a three-dimensional foil to a projection plane are considered briefly. The relationships that apply to apparent and total projections are next assembled for lines, surfaces and bounded regions. The properties of convex bodies, the geometrical attributes, and the related projected quantities are enumerated. The inter-relationships of important microstructural parameters are expressed for convex bodies in space, in sections, and in the projection plane. Based on the important concept of total projection, expressions are obtained that correct for truncation and overlap of particles in foils. Approximations to the total projection are developed for lines obscured by surfaces, and for points hidden by surfaces in space. Finally methods for determining size distributions of particles in thin foils are reviewed and recent modifications to the original procedures are discussed.     

HRTEM of dislocation cores and thin-foil effects in metals and intermetallic compounds

Examples of the observation and analysis of dislocation cores and dislocation fine structure in metals and intermetallics using high resolution transmission electron microscopy are discussed. Specific examples include the 60 degrees dislocations in aluminum, a011 edge dislocations in NiAl, and screw dislocations in Ni3Al. The effect of the thin TEM foils on the structure and imaging of these dislocations is discussed in light of embedded atom method calculations for several configurations and coupled with image simulations. Some generalizations based on these calculations are discussed. These analyses enables determination of the spreading or decomposition of the edge component of the cores, both in and out of the glide plane, which can have significant implications for the modeling of macroscopic behavior.     

Stepped surfaces of sapphire (alpha-Al2O3) with low Miller indices.

Oxygen-annealed surfaces of sapphire with low Miller indices ((0001), [1010], [1120], [1011]) have been studied in both transmission electron microscopy (TEM) and reflection electron microscopy (REM) configurations. The significance of REM diffraction conditions for the determination of the nature of the step heights is discussed. The relationship between the TEM and REM images is explained. The structural features are those that might be expected from considerations of the atom arrangement in the low Miller index planes. The structural features on the surfaces varied with respect to annealing temperature and surface condition. Thermally stable structures that might appear from consideration of the equilibrium-annealing temperature are proposed.     

A method for the location of specific points on surfaces in the SEM

A method for precisely locating points on large surfaces during SEM examination and for finding corresponding points on matching surfaces (such as those resulting from the fracture of a solid body) is described. This method also allows for the repeated examination of specific points during subsequent SEM sessions after the specimen has been removed and replaced in the SEM. The method is based on a coordinate transformation between the SEM stage coordinates and specimen coordinate systems defined by arbitrarily chosen reference points on the specimen.     

Direct observation of a Ti/Cu interface with atomic displacement under electron irradiation

A Ti film was deposited onto a Cu substrate by means of a radio frequency magnetron sputtering method. Cross-sectional thin foils for TEM observation were prepared using a focused ion beam. Electron irradiation was carried out using a high-resolution high-voltage electron microscope operated at 1.25 MV . The Cu/Ti interface of the foils was irradiated at 623 K. In-situ observation images during electron irradiation were recorded by a CCD camera with a digital video cassette. The (020)_Cu plane on the Cu/Ti interface preferentially moved towards the Ti film with irradiation. Composition analysis of the diffused region showed that its composition corresponded to Ti₃Cu₂.     

A practical method to detect and correct for lens distortion in the TEM

A practical, offline method for experimental detection and correction for projector lens distortion in the transmission electron microscope (TEM) operating in high-resolution (HR) and selected area electron diffraction (SAED) modes is described. Typical TEM works show that, in the simplest case, the distortion transforms on the recording device, which would be a circle into an ellipse. The first goal of the procedure described here is to determine the elongation and orientation of the ellipse. The second goal is to correct for the distortion using an ordinary graphic program. The same experimental data set may also be used to determine the actual microscope magnification and the rotation between SAED patterns and HR images. The procedure may be helpful in several quantitative applications of electron diffraction and HR imaging, for instance while performing accurate lattice parameter determination, or while determining possible metrical deviations (cell edges and angles) from a given symmetry.     

Novel techniques of preparing TEM samples for characterization of irradiation damage

Focus ion beam preparation of transmission electron microscopy (TEM) samples has become increasingly popular due to the relative ease of extraction of TEM foils from specific locations within a larger sample. However the sputtering damage induced by Ga ion bombardment in focus ion beam means that traditional electropolishing may be a preferable method. First, we describe a special electropolishing method for the preparation of irregular TEM samples from ex‐service nuclear reactor components, spring‐shaped spacers. This method has also been used to prepare samples from a nonirradiated component for a TEM in situ heavy ion irradiation study. Because the specimen size is small (0.7 × 0.7 × 3 mm), a sandwich installation is adopted to obtain high quality polishing. Second, we describe some modifications to a conventional TEM cross‐section sample preparation method that employs Ni electroplating. There are limitations to this method when preparing cross‐section samples from either (1) metals which are difficult to activate for electroplating, or (2) a heavy ion irradiated foil with a very shallow damage layer close to the surface, which may be affected by the electroplating process. As a consequence, a novel technique for preparing cross‐section samples was developed and is described.     

Burgers vector determination in deformed perovskite and post-perovskite of CaIrO3 using thickness fringes in weak-beam dark-field images

The thickness-fringe method [Ishida et al., Philosophical Magazine 42 (1980) 453] for complete determination of the character of a dislocation Burgers vector has been performed in CaIrO₃ perovskite and post-perovskite deformed at high pressures and high temperatures. By selecting several main zone axes and determining the number of terminating thickness fringes at the extremity of a dislocation from a wedge-shaped thin-foil specimen in weak-beam dark-field transmission electron microscope (TEM) images, the Burgers vectors were unambiguously determined. The results demonstrate that [1 0 0] screw and edge dislocations on the (0 1 0) slip plane are dominant in the post-perovskite phase. Curved [1 0 0] and [0 1 0] dislocations and straight 〈1 1 0〉 screw dislocations on a potential (0 0 1) slip plane were identified in the perovskite phase as well as a high density of {1 1 0} twins. Low-angle tilt boundaries consisting of different groups of parallel edge dislocations on the {1 1 0} and (0 0 1) planes indicate diffusion-assisted climb in perovskite at high temperatures. The differences in dislocation microstructures could be due to activations of limited numbers of slip systems for post-perovskite and of a large number of multiple slip systems for perovskite, which may result in the strong crystallographic preferred orientation (CPO) in post-perovskite and the lack of CPO in deformed perovskite.     

Combining moiré patterns and high resolution transmission electron microscopy for in-plane thin films thickness determination

This paper reports the coupling of HRTEM and moiré pattern observations, allowing the determination of the thickness ratio of two superimposed crystals. Pseudo-lattice fringes are observed using identical TEM experimental conditions as for observing moiré patterns. The pseudo-lattice spacing is first calculated in the dynamical theory framework in two beam conditions. This approach shows a linear behavior of the spacing as a function of the thickness ratio of the two crystals. The roles of sample crystallographic orientation and sample thickness on the thickness ratio determination are discussed from multi-beam simulations. Finally, the method is applied on a bimetallic CuAg core-shell nanoparticle of a known structure. It is demonstrated that for this particle, the thickness ratio of Cu and Ag can be determined with an error that results in a precision less than 0.75 nm on the Cu and Ag thicknesses. The advantages of the technique are the use of an in-plane sample configuration and a single HRTEM image.     

MEASUREMENT OF FOIL AND CABLE THICKNESSES BY A TRANSMISSION METHOD

X-ray fluorescence analysis techniques can be applied to determine sample thickness by either absolute or relative methods. An absolute method for thickness determination by x-ray fluorescence analysis has been devised, based on two types of independent measurements of the fluorescence intensity of the constituents of the sample and performance of transmission and reflection irradiation setups.

In the present work, a method for determination of the average thickness of material between a gamma-ray source and a detector is presented. The thicknesses of Au, Ag, and Cu foils, and Cu cables have been calculated by a transmission method. An Am-241 radioisotope source and a Si(Li) detector have been used.

The method has high accuracy and is easy to use, it is non-destructive towards the sample, and it allows one to the control the sample thickness. To assess the reliability of the method, the results obtained are compared with the results obtained with a micrometer. The results are in good agreement with each other, within the estimated experimental error     

Comparison of TEM and APFIM in microstructural characterization and interpretation: An overview

A comparison of transmission electron microscopy (TEM) and atom probe field-ion microscopy (APFIM) is presented with respect to the interpretation of complex microstructures, phase identification, determination of crystallographic order, and analysis of interfaces. The capabilities, spatial resolutions, and limitations of each technique are discussed with examples taken from combined analytical electron microscopy (AEM) and APFIM studies. Both techniques are extremely powerful for routine characterization of a wide range of materials, although care must be exercised in experimentation and interpretation. The combined use of TEM and APFIM is synergistic and extends their individual capabilities from the macro scale to the atomic level.     

不同应力比下U75V轨道钢疲劳裂纹扩展行为研究

对高速铁路常用轨道钢U75V进行了不同应力比下的疲劳裂纹扩展门槛值测试和疲劳裂纹扩展试验。试验结果表明,应力比对U75V轨道钢的裂纹扩展行为有着显著影响。利用分级降载法测得的疲劳裂纹扩展门槛值随应力比的增大而减小,裂纹扩展速率随应力比的增大而增大。对裂纹断口微观形貌进行了细致观察分析,发现整个疲劳断口分布有凸条纹棱线。疲劳门槛值附近出现沿晶体学平面的穿晶小刻面;低扩展速率区凸条纹棱线平行排列,棱线方向与珠光体片层方向一致;高扩展速率区的棱线出现弯曲及碎裂,同时伴有解理断面及二次裂纹。     

Fundamental study of ductile-regime diamond turning of single crystal gallium arsenide

Gallium arsenide (GaAs) components, ranging from the planar substrate to those possessing complicated shapes and microstructures, have attracted extensive interest regarding their applications in photovoltaic devices, photodetectors and emerging quantum devices. Single point diamond turning (SPDT) is regarded as an excellent candidate for an industrially viable mechanical machining process, as it can generate nano-smooth surfaces, even on some hard-to-machine brittle materials such as silicon and silicon carbide, with a single pass. However, the extremely low fracture toughness and strong anisotropic machinability of GaAs makes it difficult to obtain nano-smooth, crack-free machined surfaces. To bridge the current knowledge gaps in understanding the anisotropic machinability of GaAs, this paper studied the mechanical material properties of (001)-oriented GaAs through indentation tests, assuming the diagonals of the indenter acted in the similar way of the cutting edge of a diamond tool with a negative rake angle. The results showed that the (001) plane of the GaAs material displayed harder and more brittle when indented along direction I (one diagonal of indenter parallel to the <110> orientation) compared to direction II (one diagonal of indenter parallel to the <100> orientation), which coincides with anisotropic machined surface quality by SPDT. This finding reveals, for the first time, that the crystallographic orientation dependence of both hardness and fracture toughness represents the underlying mechanism for the anisotropic machinability of GaAs. The paper presents a novel approach to evaluate the critical depth of cut under a high cutting speed comparable to SPDT and to determine the maximum feed rate for ductile-regime diamond turning. The 26.57 nm critical depth of cut was obtained for the hardest cutting direction using a large negative rake angle diamond tool. Finally, a nano-smooth surface was successfully generated along all the orientations in ductile-regime diamond turning, in which the material removal mechanism is considered as plastic deformation caused by high-density dislocations. The subsurface layer remains to its single crystal structure and no cracks are found under a transmission electron microscope (TEM). The results proves the proposed evaluation approach for the critical depth of cut and the maximum allowed feed rate is highly effective for guiding the ductile-regime machining of brittle materials.