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}.     

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.     

The role of misorientation and phosphorus content on grain growth and intergranular fracture in iron–carbon–phosphorus alloys

The relationship between the crystallography of intergranular fracture and phosphorus segregation has been investigated in a Fe?0.06wt%P?0.002wt%C alloy aged for 1 h at temperatures between 600 °C and 1000 °C. Two novel techniques were devised for the investigation: first, electron back‐scatter diffraction (EBSD) across the reconstructed fracture surface and, second, a combination of Auger electron spectroscopy, stereophotogrammetry and microscopy to measure phosphorus and carbon on fracture facets combined with EBSD measurements direct from the fracture surface. In total, 700 misorientations were measured from across the reconstructed fracture surface and in ‘control’ areas away from the fracture. It was found that Σ 3s were in general more resistant to brittle fracture than were random boundaries, and it was suggested that alloys of this type could be grain boundary engineered to improve fracture resistance by a short anneal in the austenite region to increase the final proportion of Σ 3s. Sixteen fracture facets yielded combined Auger/EBSD data. The combined Auger/EBSD methodology to acquire joint crystallographic and segregation information from facets was shown to be feasible, although laborious. There were significantly more {110} planes than any other type in the sample population of facets from which combined segregation/crystallography data had been collected. The data suggested that there was on average lower phosphorus segregation on fracture facets that were near {110} than on other intergranular fracture facets.     

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.     

不同温度下镍基单晶高温合金的低周疲劳性能

研究了Ni-Cr-Co-Mo-W-Ta-Nb-Re-Al-Hf-C系镍基单晶高温合金在不同温度(800,980℃)下的低周疲劳性能.结果表明:与800℃下相比,980℃下合金的塑性变形量更大,损伤更严重,疲劳强度更低,寿命更短;2种温度下合金的疲劳断裂均为类解理断裂;800℃时,裂纹萌生于疏松组织处,沿{111}平面扩...     

X80管线钢焊接接头TEM观察下的断裂行为对比

对X80管线钢焊接接头3个区域的试样在透射电子显微镜（TEM）下进行原位拉伸试验,通过观察各个区域裂纹的萌生与扩展过程,对其微观断裂行为进行了研究和分析。结果表明：母材区中,裂纹萌生扩展主要以多裂纹起裂为主,表现为沿晶扩展的断裂形式,最后造成在切应力作用下的剪切脆性断裂;热影响区内,主裂纹首先在切应力作用下以45°方向起裂,整个扩展过程是主裂纹钝化、位错发射、主裂纹前方无位错区形成、微裂纹形核、主裂纹与微裂纹连接扩展这一多尺度过程不断重复的过程,最后导致准解理穿晶断裂;焊缝区中,从主裂纹分支出多条裂纹,裂纹均以与拉伸轴成45°的方向起裂,表现为沿晶与穿晶混合的断裂模式,最后造成穿晶脆性解理断裂。     

厚板窄间隙埋弧焊焊缝阶梯形断口形成机理研究

为提高2.25Cr-1Mo-0.25V耐热钢埋弧焊接头的韧性,采用冲击试验和三点弯曲试验找到接头韧性的薄弱环节,并通过进一步的微观分析,试图揭示接头韧性的弱化机理。冲击试验显示,焊缝的韧性低于母材和热影响区,因此利用三点弯曲试验,对裂纹在焊缝中的扩展过程进行更深入的研究。在三点弯曲试验时观察到焊缝中出现阶梯形断口。通过扫描电镜观察到断口表面呈现出韧窝和准解理共存的混合形貌。金相观察发现,阶梯形断口出现在焊缝的柱晶区,其方向与柱晶生长方向平行或垂直。利用苦味酸腐蚀试样,观察到阶梯形断口与结晶方向平行的部分是沿原奥氏体晶界开裂,电子背散射衍射(Electron backscattered diffraction, EBSD)试验结果也证明了这一点,而垂直于结晶方向的部分是沿层状线开裂。因此,原奥氏体晶界和层状线是焊缝中需要特别关注的部位。通过调整焊丝成分和工艺参数,减少了焊缝中的层状线,焊缝韧性明显提高。     

Scanning electron microscopy and transmission electron microscopy in situ studies of grain boundary migration in cold-deformed aluminium bicrystals

The crystallography of recrystallization has been investigated in channel‐die deformed pure aluminium bicrystals with {100}<011>/{110}<001> orientations. The microstructural and microtextural changes during the early stages of recrystallization were followed by systematic local orientation measurements using scanning and transmission electron microscopes. In particular, orientation mapping combined with in situ sample heating was used to investigate the formation and growth of new grains at very early stages of recrystallization. Grain boundary migration and ‘consumption’ of the as‐deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation.     

Determination of the fatigue fracture planes of Co–Cr–Mo biomedical alloys using electron backscatter diffraction

Electron backscatter diffraction on a scanning electron microscope has been utilized to acquire crystal orientation information around faceted fatigue cracks in a Co–Cr–Mo alloy for medical implants. The faceted fracture planes are unambiguously determined as {111} planes.     

Electron backscatter diffraction characterization of inlaid cu lines for interconnect applications

Automated electron backscatter diffraction (EBSD) techniques have been used to characterize the microstructures of thin films for the past decade or so. The recent change in strategy from an aluminum‐based interconnect structure in integrated circuits to one based on copper has necessitated the development of new fabrication procedures. Along with new processes, complete characterization of the microstructures is imperative for improving manufacturability of the Cu interconnect lines and in‐service reliability. Electron backscatter diffraction has been adopted as an important characterization tool in this effort. Cu microstructures vary dramatically as a function of processing conditions, including electroplating bath chemistry, sublayer material, stacking sequence of sublayers, annealing conditions, and line widths and depths. Crystallographic textures and grain size and grain boundary character distributions, all of which may influence manufacturability and reliability of interconnect lines, are ideally characterized using EBSD. The present discussion presents some results showing structural dependence upon processing parameters. In addition, the authors identify an in‐plane orientation preference in inlaid Cu lines {111} normal to the line surface and 〈110〉 aligned with the line direction. This relationship tends to strengthen as the line width decreases.     

Deformation behaviour of soft-brittle polycrystalline materials determined by nanoscratching with a sharp indenter

Polycrystalline zinc selenide (p-ZnSe) is a typical soft brittle material with important optical applications. In this work, single and repeated nanoscratching tests were performed using a Berkovich indenter along the face-forward (FF) and edge-forward (EF) directions. The morphological features of the scratched grooves and the subsurface microstructural changes in the material were characterised by scanning electron microscopy, Raman spectroscopy, and electron backscatter diffraction (EBSD). Material removal in the ductile mode was obtained in the EF scratching direction; this was accompanied by the slip lines, and the radial cracks generated along grain boundaries. In contrast, brittle fractures occurred in the FF scratching direction, resulting in radial and lateral cracks which are responsible for generating the peeling of the material. The EBSD results demonstrated that the {111} planes are the primary slip plane and secondary cleavage plane, whereas the {110} planes are the primary cleavage plane and secondary slip planes. Tensile residual stress was detected in the subsurface region of the grooves scratched along the FF direction, whereas compressive residual stress was detected in the EF scratching direction. Fishbone-like patterns were observed in the scratched grooves under all conditions, while no phase transformation was detected. This study provides insights into the fundamental material removal mechanisms of soft brittle crystals in various abrasive machining processes, such as grinding, lapping, and polishing.     

Single-section plane assessment in grain boundary engineered brass

The present paper reports a comparative analysis of Σ3 (in the coincidence site lattice notation) grain boundary types, in two grain boundary engineered brass specimens, by use of electron backscatter diffraction (EBSD) data coupled to the measurement of boundary traces in a single section. Although most of the data were analysed using the new single‐section technique, an analysis of boundary plane orientations in three dimensions was made in a subset of the data in order to validate the single section methodology. The single‐section trace analysis procedure, coupled with EBSD, is a viable and robust tool for analysis of Σ3 grain boundary planes. The procedure provides data which suggest that part of the enhanced strain‐to‐failure in specimen B compared to specimen A is the result of an increased proportion of mobile Σ3 boundaries, i.e. those which are displaced from the {111} symmetrical tilt configuration.     

Microstructure evolution and strain localization in Cu and Cu-8Al single crystals subjected to channel-die compression

Single crystals of pure Cu and Cu‐8%Al with two initial orientations, {112}〈111〉 and {112}〈110〉, were subjected to monotonic compression in channel‐die at room temperature (293 K). The dislocation microstructure and local crystallography were investigated by transmission electron microscopy after different amounts of deformation. Various factors, such as initial single crystal orientation, chemical composition and amount of plastic deformation, were analysed in order to determine their influence on the microstructure evolution, local orientation variations and strain localization phenomena.     

Grain size measurement by EBSD in complex hot deformed metal alloy microstructures

The measurement of grain size by EBSD has been studied to enable representative quantification of the microstructure of hot deformed metal alloys with a wide grain size distributions. Variation in measured grain size as a function of EBSD step size and noise reduction techniques has been assessed. Increasing the EBSD step size from 5% to 20% of the approximate mean grain size results in a change in calculated arithmetic mean grain size of approximately 15% and standard noise reduction techniques can produce a further change in reported size of up to 20%. The distribution of measured grain size is found not to be log‐normal, with a long tail of very small sizes in agreement with a computer simulation of linear intercept and areal grain size measurements through randomly oriented grains. Comparison of EBSD with optical measurements of grain size on the same samples shows that, because of the ability of EBSD to distinguish twins and resolve much smaller grains a difference of up to 50% in measured grain size results.     

The application of a hot deformation SEM stage, backscattered electron imaging and EBSD to the study of thermomechanical processing

The technique of combining in situ hot‐deformation and high resolution electron backscattered diffraction (EBSD) has been applied to study the mechanisms operating during the thermomechanical processing of metals. A simple hot tensile‐straining stage is installed in a field emission gun scanning electron microscope equipped with an EBSD system and has been used successfully for a number of preliminary investigations. These investigations include substructure formation, dynamic subgrain and grain growth, superplastic deformation in aluminium alloys, and dynamic recrystallization in copper. Despite the surface topography, which inevitably increases during plastic deformation, channelling contrast backscattered electron micrographs have been successfully obtained after strains of up to ～50%. Good quality EBSD maps have been obtained after strains of up to 100%. Most observations and measurements from the in situ experiments are consistent with what is known about the mechanisms occurring in the bulk. The microstructures revealed in the centre of the in situ samples after later repolishing are generally similar to those at the surface.     

EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre‐existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non‐cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub‐boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub‐boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.     

The analysis of quartz c-axis fabrics using a modified optical microscope

A new fully automated microfabric analyzer (MiFA) is described that can be used for the fast collection of high‐resolution spatial c‐axis orientation data from a set of digital polarized light images. At the onset of an analysis the user is presented with an axial‐distribution diagram (AVA –‘Achsenverteilungsanalyse’) of a thin section. It is then a simple matter to build‐up c‐axis pole figures from selected areas of interest. The c‐axis inclination and colatitudes at any pixel site is immediately available to create bulk fabric diagrams or to select measurements in individual areas. The system supports both the interactive selection of c‐axis measurement sites and grid array selection. A verification process allows the operator to exclude dubious measurements due to impurities, grain boundaries or bubbles. We present a comparison of bulk and individual c‐axis MiFA measurements to pole figures measured with an X‐ray texture goniometer and to data collected from a scanning electron microscope furnished with electron backscatter diffraction (EBSD) facility. A second sample, an experimentally deformed quartzite, illustrates that crystal orientations can be precisely linked to any location within an individual grain.     

Morphological analyses of minute crystals by using stereo-photogrammetric scanning electron microscopy and electron back-scattered diffraction

We present a new method for the morphological analyses of minute faceted crystals by combining stereo-photogrammetric analysis of scanning electron microscope images and electron back-scattered diffraction. Two scanning electron microscope images of the same crystal, recorded at different tilt angles of the specimen stage, are used to determine the orientations of crystal edges in a specimen-fixed coordinate system. The edge orientations are converted to the indices [ uvw ] in the crystal system using the crystal orientation determined by electron back-scattered diffraction analysis. The Miller indices of crystal facets are derived from the indices of the edges surrounding the facets. The method is applicable to very small crystal facets. The angular error, as derived from tests using a calcite crystal of known morphology, is a few degrees.
To demonstrate the applicability of the method, the morphology of boehmite (γ-AlOOH) precipitated from solution during the dissolution of anorthite was analyzed. The micrometre-sized boehmite crystals are surrounded by two {010} basal facets and eight equivalent side facets that can be indexed equally well as {323}, {434} or {545}. We suggest that these side facets are in fact {111}, the morphology having been modified slightly (by a few degrees) by a small extension associated with opening along (010) microcleavage planes. Tiny {140} facets are also commonly observed.     

球墨铸铁疲劳裂纹萌生及扩展机制的探讨

用电子显微断口分析和裂纹扩展途径的金相观察,探讨了不同珠光体和铁素体相对量的球墨铸铁的疲劳断裂机制,特别是疲劳裂纹萌生及早期扩展机制。结果表明：疲劳裂纹容易在石墨球和基体之间的界面萌生和扩展,石墨球边及基体中的次生裂纹对da/dN的影响有两重性。在疲劳裂纹萌生及早期扩展阶段,铁素体基本以周期解理和细条纹为主,珠光体基体为渗碳体解理和铁素体片上的疲劳条纹,80%珠光体球铁为上两种机制的混合。疲劳裂纹早期扩展速度为△K (th)所控制,疲劳裂纹孕育期N 0为σ s和△K (th)所控制。光棒或钝缺口下的N 0主要取决于σ s,尖锐缺口试样的N 0主要取决于△K (th)。在疲劳裂纹稳定扩展阶段,铁素体为疲劳条纹,珠光体为条纹与解理,80%珠光体中牛眼铁素体为条纹和晶间断。疲劳条纹的宽度能反映da/dN的变化,但两者往往不一致。疲劳裂纹快速扩展为条纹与静断特征混合,快速扩展速率受K (1c)的大小所控制。     

Precession electron diffraction‐assisted crystal phase mapping of metastable c‐GaN films grown on (001) GaAs

The control growth of the cubic meta‐stable nitride phase is a challenge because of the crystalline nature of the nitrides to grow in the hexagonal phase, and accurately identifying the phases and crystal orientations in local areas of the nitride semiconductor films is important for device applications. In this study, we obtained phase and orientation maps of a metastable cubic GaN thin film using precession electron diffraction (PED) under scanning mode with a point‐to‐point 1 nm probe size beam. The phase maps revealed a cubic GaN thin film with hexagonal GaN inclusions of columnar shape. The orientation maps showed that the inclusions have nucleation sites at the cubic GaN {111} facets. Different growth orientations of the inclusions were observed due to the possibility of the hexagonal {0001} plane to grow on any different {111} cubic facet. However, the generation of the hexagonal GaN inclusions is not always due to a 60° rotation of a {111} plane. These findings show the advantage of using PED along with phase and orientation mapping, and the analysis can be extended to differently composed semiconductor thin films. Microsc. Res. Tech. 77:980–985, 2014. © 2014 Wiley Periodicals, Inc.