Observations on interphase boundary structure

A review is presented of experimental observations reported on the structure of interphase boundaries since 1968. Nearly all observed boundaries are of the partially or fully coherent type, and hence the principal structural features examined were misfit dislocations and ledges. Observations were drawn from studies of precipitation, spinodal decomposition, oriented overgrowths and eutectics. Interphase boundary structures were found to be little affected in the main by the type of reaction through which they were developed and were consistent with Van der Merwe theory and a theory of precipitate morphology. Observations of these structures have been considerably facilitated by the efforts of Weatherly and co-workers to define the visibility and the optimum TEM viewing conditions of misfit dislocations and ledges. Important new observations on misfit dislocations made since 1968 include: misfit dislocation spacing in the Cr(Mo)/NiAl eutectic in good agreement with theoretical expectation because misfit dislocations are also glide dislocations and the interface plane is also the glide plane; misfit dislocations on θ′ Al-Cu and η Al-Au plates with a Burgers vector (a/2<100>) stable only at an interphase boundary; reduction of the interfacial energy of partially coherent boundaries on the same precipitates by dislocation interactions and rearrangements; reduction of interfacial energy of interfaces developed during spinodal decomposition through rotation of the interface itself from {100} to {110}; and misfit dislocation boundaries between f.c.c. (Cu-rich) and b.c.c. (Cr) crystals made possible with the assistance of ‘structural ledges’. Growth ledges have now been observed in a number of alloy systems which have undergone precipitation from solid solution or eutectic solidification; they appear to be confirmed as a customary feature of migrating partially or fully coherent interphase boundaries. Much new experimental information has been obtained on the sources (exceedingly diverse), heights (usually appreciably higher than monatomic), spacings (irregular) and migration kinetics (diffusion-controlled if their edges are disordered) of ledges. New observations continue to confirm the complexity of the processes through which misfit dislocations are acquired. The Ashby-Johnson and the Brown-Woolhouse theories of misfit dislocation nucleation or acquisition by spherical precipitates are in encouraging agreement with experiment.     

EBSD characterization of carbide–carbide boundaries in WC–Co composites

A sample of WC‐6wt%Co was investigated for grain boundary character distribution and occurrence of coincidence site lattice (CSL) boundaries on a statistical basis. For this purpose orientation measurements of the grains were carried out using electron back‐scattered diffraction (EBSD). The dominant misorientation relationships were determined by complementary EBSD data representation tools such as orientation maps, misorientation angle distribution histograms and the sectioned three‐dimensional misorientation space. It was found that the grain boundary character distribution of the material is nearly random and the CSL boundaries are not present in statistically significant amounts. It was also found that the amount of binder phase does not play a role in the formation of special boundaries. The paper focuses on the methodology of characterizing grain boundaries in a hexagonal material using EBSD.     

EBSD as a tool to identify and quantify bainite and ferrite in low-alloyed Al-TRIP steels

Bainite is thought to play an important role for the chemical and mechanical stabilization of metastable austenite in low‐alloyed TRIP steels. Therefore, in order to understand and improve the material properties, it is important to locate and quantify the bainitic phase. To this aim, electron backscatter diffraction–based orientation microscopy has been employed. The main difficulty herewith is to distinguish bainitic ferrite from ferrite because both have bcc crystal structure. The most important difference between them is the occurrence of transformation induced geometrically necessary dislocations in the bainitic phase. To determine the areas with larger geometrically necessary dislocation density, the following orientation microscopy maps were explored: pattern quality maps, grain reference orientation deviation maps and kernel average misorientation maps. We show that only the latter allow a reliable separation of the bainitic and ferritic phase. The kernel average misorientation threshold value that separates both constituents is determined by an algorithm that searches for the smoothness of the boundaries between them.     

Effect of nitriding on wear resistance of iron alloys with various lattice types

The paper considers the characteristics of the formation of the wear-resistant structure of nitrided model alloys with bcc and fcc lattices: Fe-Mo, Fe-Cr, Fe-Al, Fe-Ni-Cr, Fe-Ni-Al, Fe-Ni-Ti, and Fe-Ni-Cr-Al-Ti. The relationships between the structure characteristics of the nitrided layer and the wear resistance of the alloys are found. The maximal wear resistance of the alloys with the bcc lattice of the matrix is shown to be attained when incoherent particles of nitrides of the alloying elements appear; the maximal hardness corresponds to the alloy that contains coherent nitrides. In the alloys with the fcc lattice, the maximal hardening under nitriding and the maximal wear resistance are achieved at the stage that precedes the rupture of the coherent bond between the nitride and the matrix. The relationships between the structure characteristics of the nitrided layer and the wear resistance are discussed from the viewpoint of the formation of a wear-resistant structure state of the alloys.     

Grain boundary character distributions in Ni-16Cr-9Fe using selected area channeling patterns: Methodology and results

Selected area channeling patterns imaged on an SEM are digitized and displayed on the screen of a Macintosh computer, on which the user selects channeling bands that are measured to determine orientation. Grain boundary misorientations are found using the orientation information for pairs of grains adjacent at grain boundaries, and the boundaries are classified as low angle boundaries (LABs), coincident site lattice boundaries (CSLBs), or general boundaries (GHABs) based on the misorientation information. The technique was implemented to analyze the grain boundary character distributions (GBCDs) in Ni-16Cr-9Fe. The GBCDs of solution annealed material were similar to those expected in an aggregate of randomly oriented polycrystals. However, sequential thermomechanical treatments (5% tensile strain + 945°C:75 min + 2% tensile strain + 890°C:15 h + 3% tensile strain + 890°C:20 h or 9% compressive strain + 890°C:20 h + 9% compressive strain + 890°C:20 h + 3% compressive strain + 890°C:15 h) applied after the solution anneal lowered the proportions of GHABs in the GBCDs from 76–79% to 47–64%. The CSL-enhanced GBCDs of both the tensile-deformed samples and the compression-deformed sample appear to have evolved mainly through impingement of twin and twin-related boundaries during recrystallization; the CSL-enhanced GBCD of a compression-deformed sample appears to have been influenced by grain rotation processes to a greater degree than were the tensile-deformed samples. The CSL boundaries in the CSL-enhanced GBCDs were, in general, closer to the exact CSL misorientations than were those in the near-random GBCDs of the solution annealed material. An analysis of the distribution of misorientation axes did not indicate any correlation between grain misorientation texture and GBCD evolution.     

Effect of composition and structure of nitrided iron alloys on wear resistance under friction and sliding

Characteristic features of the formation of the wear-resistant structure of the nitrided model alloys with body-centered cubic (bcc) and face-centered cubic (fcc) lattices are studied, including Fe-Mo, Fe-Cr, Fe-Al, Fe-Ni-Cr, Fe-Ni-Al, Fe-Ni-Ti, and Fe-Ni-Cr-Al-Ti. Interrelations are established between the parameters of the structure of the nitrided layer and the wear resistance of alloys. It is shown that, in alloys with the bcc lattice, maximum wear resistance is achieved by the formation of incoherent particles of nitrides of alloying elements, and the maximum hardness can be found in the alloy with coherent nitrides. In alloys with the fcc lattice, maximum hardening during nitriding and wear resistance are achieved at the stage preceding the rupture of coherent communication of the nitride with the lattice. Interconnections of characteristics of the structure of the nitrided layer and the wear resistance are discussed in the context of the formation of the wear-resistant structural state of alloys.     

Nondestructive three-dimensional characterization of grain boundaries by X-ray crystal microscopy

The results from an emerging method of nondestructive grain boundary characterization, with unprecedented sensitivity to neighbor-grain misorientation and grain boundary morphology are reported. The method utilizes differential aperture X-ray microscopy to determine the local crystallographic orientation of submicron volumes within polycrystalline materials. Initial measurements are described for a recrystallized Ni sample where the grain boundary type was identified at 85 grain boundaries within the framework of an ideal coincident site lattice (CSL) model. The remarkable resolution of this method is demonstrated by the <0.03 degrees deviations of misorientation measured for Sigma3 (twin) boundaries. Because of its high angular and spatial resolution, this new approach to grain boundary characterization can provide quantitative tests of grain boundary models with new insights for grain boundary engineering efforts.     

Experimental study of ELNES at grain boundaries in alumina: intergranular radiation damage effects on Al-L23 and O-K edges

The need for understanding the structural and chemical properties of interfaces and grain boundaries in materials is being paralleled by new developments in transmission electron microscopy methods. The extraction of data on grain boundaries has to be carefully evaluated, freed from artefacts and allowing fairly direct interpretation. Besides improvements in data processing, a primary requirement is an improved knowledge of the ELNES fine structures of the relevant absorption edge. For the case of alumina, we first present a compilation of Al-L(23) edges with an energy resolution of 0.5-0.6eV. This allows identification of the ELNES features which are more likely to vary as a function of the aluminium atomic environment, i.e. the excitonic peaks between 77 and 79eV and the near-edge features between 83 and 86eV, which both clearly depend on the Al site environment. Our second investigation concerns the likely occurrence of electron radiation damage at interfaces which may adversely interfere with the identification of new bonding types. The Al-L(23) and O-K ELNES changes associated with several cases of damage are detailed.     

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.     

Self-organized nanotemplating on misfit dislocation networks investigated by scanning tunneling microscopy

Self-ordering growth of nanoarrays on strained metallic interfaces is an attractive option for preparing highly ordered nanotemplates. The great potential of this natural templating approach is that symmetry, feature sizes, and density are predicted to depend on the interfacial stress in these strained layers, which can be adjusted by changing the substrate-thin film composition, temperature, and adlayer coverage. This bottom-up approach of growing nanostructured two-dimensional ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films and surfaces requires a detailed understanding of the nucleation and film-adsorbate interaction processes. Here we show how high resolution, large scale, variable temperature scanning tunneling microscopy imaging can improve our understanding of these self-assembly processes.     

Unsupervised approval criteria for automated EBSP investigation of deformed metals

Unsupervised approval criteria have been investigated for orientations gathered from cold deformed samples (medium to high strain range) using the electron backscattering pattern technique. For such samples, the dislocation cell-size is on the order of the available step-size and pattern quality is generally low. Approval criteria for assessing the validity of measured orientations under these conditions were determined using, as a calibration, channel die cold deformed single crystals of stable orientations. In all cases, approval criteria based on an indexing confidence measure are found to be preferable. Different criteria are suggested, depending on whether the orientation data are subsequently to be used for texture analysis, or for a misorientation angle-based analysis. The latter is illustrated by an investigation of the number of deformation generated high angle boundaries introduced during a 90% cold reduction of a polycrystalline sample.     

Special grain boundaries in random polycrystalline aggregates

The observation of different types of order present in grain boundaries is discussed. It is shown that, while the majority of boundaries in a cubic random polycrystalline aggregate may possess order characteristics of a ‘plane matching’ or CAD situation, very few boundaries will possess order characteristic of a CSL. Frequent observations of CSL or CAD order in the electron microscope indicate a non random polycrystalline aggregate.     

An atomistic study of grain boundary stability and crystal rearrangement using molecular dynamics techniques

The stability of grain boundaries (GBs) and the dynamic behavior of atoms in the boundary region are investigated from an atomistic standpoint. Symmetric and non-symmetric GBs are constructed using an fcc configuration, and the GB energy is calculated as a function of the misorientation angles using a Lennard-Jones-type interatomic potential. Several specific angles are revealed to exhibit cusp-shaped low values. The effect of atomic relaxation at the GB is then simulated, showing a decrease in the GB energy. Changes in the morphology of a grain embedded in a bulk single crystal are also simulated. Using both a square-grain and a circular-grain model, the following results are obtained. In models with small misorientation angles, the grain changes orientation and the GB vanishes. When the orientations are initially stable, no change in the grain is observed. However, in models with non-stable orientations, local stabilization occurs by a rearrangement of the atoms around the GB, and the shape of the grain is transformed. Finally, a similar simulation is carried out at a high temperature, and this reveals that grain contraction occurs even in models that are stable at a low temperature, and that the grain eventually disappears.     

The influence of dislocation distribution density on curvature and interface stress in epitaxial thin films on a flexible substrate

We consider the problem of relaxation of coherency stresses by lattice misfit dislocations, which can be represented as edge dislocations uniformly distributed along the interface, and can be associated with the concept of dislocation density according to the mathematical theory of continuous distributions of dislocations. The orientation of dislocation line density is constrained within the plane of epitaxial layer and the Burgers vector density within the same plane is considered as a local variable of the problem. Under the external stress-free conditions the system is then enforced to satisfy the boundary conditions according to the geometry of the epitaxial film and the substrate together with the corresponding compatibility conditions. The dislocation density tensor is then connected with the deformation and rotation tensors and subsequently with the curvature of the system. The closed form solution of the problem is given for a generic one-dimensional case.     

铁素体球墨铸铁低温冲击断裂行为的影响因素

对QT400-18L牌号球墨铸铁件进行系列温度Charpy缺口冲击试验,采用扫描电镜对冲击断口形貌进行分析,基于X射线衍射方法测试在冲击过程中各个温度下冲击断口附近铁素体基体内位错密度的变化,分析表明QT400-18L铁素体球墨铸铁断口附近石墨球的损伤模式主要有以下几种:层状开裂或层状剥离、石墨核芯开裂以及石墨球基体界面解体;低温下位错在晶界处的塞积产生很高的内应力,微裂纹更容易在晶界处萌生,晶界处夹杂物的存在导致韧窝断口中往往存在一些细小的显微孔洞,这种由夹杂物引起的显微孔洞的连接导致了材料的最终断裂;随着温度的降低铁素体内位错密度有所下降,室温下QT400-18L铁素体内部位错密度约为1.32×10~(15)m~(-2),而-60℃下其位错密度约为0.39×10~(15)m~(-2)。     

The use of combined cathodoluminescence and EBSD analysis: a case study investigating grain boundary migration mechanisms in quartz

Grain boundary migration is an important mechanism of microstructural modification both in rocks and in metals. Combining detailed cathodoluminescence (CL) and electron backscatter diffraction (EBSD) analysis offers the opportunity to relate directly changes in crystallographic orientation to migrating boundaries. We observe the following features in naturally heated quartz grains from the thermal aureole of the Ballachulish Igneous Complex (Scotland, U.K.): (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area and (c) a gradual change in crystallographic orientation in the direction of growth. All these features are compatible with a growth mechanism in which single atoms are attached and detached both at random and at preferential sites, i.e. crystallographically controlled sites or kinks in boundary ledges. Additionally, strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. This study illustrates the usefulness of combined CL and EBSD in microprocess analysis. Further work in this direction may provide detailed insight into both the mechanism of static grain growth and the energies and mobilities of boundaries in terms of misorientation and grain boundary plane orientation.     

CBED study of grain misorientations in AlGaN epilayers

Large angle convergent beam electron diffraction (LACBED) has been used to examine AlGaN epilayers grown by facet-controlled epitaxial lateral overgrowth on GaN/(0001) sapphire substrates in prototype UV laser structures. The substrates, defined by masks with seed openings along a <10-10> stripe direction, had GaN seed columns with {11-22} surfaces. Studies were carried out on cross-sectional samples cut perpendicular to the stripe axis. An LACBED analysis of the orientation of (000 2) planes, and of the (11-20) planes parallel to the stripe axis, revealed that the AlGaN wings were both rotated by angles of 1-2 x 10(-2)radians about the 10-10 stripe axis with respect to the underlying GaN, and distorted due to misfit strains. It is shown that the results are consistent with the observed structure of the AlGaN/GaN and the wing/wing boundaries, and with a new model for the generation of a-type misfit dislocations at the AlGaN/GaN interface.     

An enhanced mean field material model incorporating dislocation strengthening for particle reinforced metal matrix composites

An analytic model of predicting the elastoplastic stress-strain curve of particle reinforced metal matrix composites is proposed. This model is enhanced so that Mori-Tanaka method can be applied to bimodal or particulate metal matrix composites that exhibit size effects due to the dislocation strengthening mechanisms. The thermal misfit and mechanical misfit strains between the inclusion and the matrix are accounted for by this model. Several aluminum-based metal matrix composite as well as a bimodal copper system are examined and their yield strengths and stress-strain curves are compared with published experimental data. The proposed model is simple, yet quite effective and reasonably accurate.     

Atom probe tomography of Ni-base superalloys Allvac 718Plus and Alloy 718

Atom probe tomography (APT) allows near atomic scale compositional- and morphological studies of, e.g. matrix, precipitates and interfaces in a wide range of materials. In this work two Ni-base superalloys with similar compositions, Alloy 718 and its derivative Allvac 718Plus, are subject for investigation with special emphasis on the latter alloy. The structural and chemical nuances of these alloys are important for their properties. Of special interest are grain boundaries as their structure and chemistry are important for the materials' ability to resist rapid environmentally induced crack propagation. APT has proved to be suitable for analyses of these types of alloys using voltage pulsed APT. However, for investigations of specimens containing grain boundaries and other interfaces the risk for early specimen fracture is high. Analyses using laser pulsing impose lower electrical field on the specimen thereby significantly increasing the success rate of investigations. Here, the effect of laser pulsing was studied and the derived appropriate acquisition parameters were then applied for microstructural studies, from which initial results are shown. Furthermore, the influence of the higher evaporation field experienced by the hardening γ′ Ni₃(Al,Nb) precipitates on the obtained results is discussed.     

Quantitative atom column position analysis at the incommensurate interfaces of a (PbS)(1.14)NbS(2) misfit layered compound with aberration-corrected HRTEM

Aberration-corrected HRTEM is applied to explore the potential of NCSI contrast imaging to quantitatively analyse the complex atomic structure of misfit layered compounds and their incommensurate interfaces. Using the (PbS)_1.14NbS₂ misfit layered compound as a model system it is shown that atom column position analyses at the incommensurate interfaces can be performed with precisions reaching a statistical accuracy of ±6 pm. The procedure adopted for these studies compares experimental images taken from compound regions free of defects and interface modulations with a structure model derived from XRD experiments and with multi-slice image simulations for the corresponding NCSI contrast conditions used. The high precision achievable in such experiments is confirmed by a detailed quantitative analysis of the atom column positions at the incommensurate interfaces, proving a tetragonal distortion of the monochalcogenide sublattice.