EBSD Research Papers - Academia.edu (original) (raw)

The exoskeleton of the lobster Homarus americanus is a multiphase biological composite material which consists of an organic matrix (crystalline α-chitin fibers and various types of non-crystalline proteins) and minerals (mainly calcite). In this study we discuss experimental data about the mesoscopic structure and the crystallographic texture (orientation distribution) of the α-chitin–protein fiber network in this material. The synchrotron measurements reveal very strong crystallographic textures of the α-chitin. According to these data, a large fraction of the α-chitin lattice cells is arranged with their longest axis parallel to the normal of the surface of the exoskeleton. Additionally, a smaller fraction of the α-chitin cells is oriented with their longest axis perpendicular to the cuticle surface. These structural investigations reveal the pronounced role of crystallographic orientation distributions in mineralized biological composite materials which may be of relevance for an improved understanding of biological and bio-inspired nano-composites.

- by
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- Materials Engineering, Mechanical Engineering, Materials Science, Crystallography
- by Milan Dopita
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- Engineering, Microstructure, Materials, Transmission Electron Microscopy

Titanium alloys have been of great interest in recent years because of their very attractive combination of high strength, low density and corrosion resistance. Application of these alloys in areas where wear resistance is also of importance calls for thorough investigations of their tribological properties. In this work, Ti–6Al–4V and Ti–24Al–11Nb alloys were subjected to dry sliding wear against hardened-steel counter bodies and their tribological response was investigated. A pin-on-disc type apparatus was used with a normal load of 15–45N and sliding speed of 1.88 ms−1. In the steady state, it was demonstrated that Ti–24Al–11Nb had a substantially higher wear resistance (about 48 times) than that of the Ti–6Al–4V alloy tested under a normal load of 45 N. Severe delamination is found to be responsible for the low wear resistance of Ti-6Al-4V. In the case of Ti–24Al–11Nb, two wear mechanisms have been suggested: delamination with a lower degree of severity and oxidative wear. It is thought that the ability of Ti–24Al–11Nb to form a protective oxide layer during wear results in a much lower wear rate in this alloy.

- by Haseeb ASMA
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- Microstructure, Advanced Oxidation Processes, Tribology, EBSD
- by Robert Keller
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- Materials Engineering, Condensed Matter Physics, Crystallography, Microscopy

Ferritic–martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (lDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within ‘‘hot zones’’ with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution lDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels.

- by Calixto Garcia
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- Engineering, Microstructure, Image Analysis, Electron Diffraction

PTCLab is an abbreviation of phase transformation crystallography lab. The purpose of this program is to calculate the phase transformation crystallography after a phase transformation and represent the results in graphical way such as in stereo graphic projection. The lattice matching near the interface, the superimposed diffraction patterns, EBSD pattern and so on can be readily simulated and indexed with present program. The crystallographic model involves calculate orientation relationship according to different criteria, such as O-line method based on O-lattice theory, edge-to-edge method, Near coincidence site lattices method, martensite theory (including Frank's model, Phenomenal theory for martensite transformation, double shear model, prism matching model).This program is free, opensource and runnable on multiple platform. Available from http://sourceforge.net/projects/tclab/

- by Xinfu Gu
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- STEM, EBSD, Transformation Crystallography, 3DAP

A laser beam welding process via heat conduction was applied to join DC01 steel with aluminum (Al)in overlap conﬁguration without ﬁller wire. The effect of the applied laser power (1.7, 1.8, 2.1, and2.4kW) on the formation and evolution of the interfaces between steel and Al was analyzed. Twointermetallic compounds were found at the interface, namely, one adjacent to the steel layer (Al
5
Fe
2
)and one close to the solidiﬁed Al (Al
13
Fe
4
). The thickness of the intermetallic reaction layer increaseswith laser power, while the morphology of its individual components evolves due to differences inaccumulated thermal cycles. Correlations between simulations and measurements show that the peaktemperature has signiﬁcantly stronger inﬂuence on the thickness of the intermetallic reaction layerthan cooling time and the integral of temperature over the time. Shear/tensile strength tests reveal thatall the specimens fail in the Al heat affected zone

- by Dierk Raabe
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- Electron Backscatter Diffraction (EBSD), Laser Welding, Welding Metallurgy, EBSD

Depending on the strain and temperature regime examined recovery of bcc metals during hot rolling or annealingsubsequent to cold deformation often leads to the preservation of certain deformation texture components. A broad variety of mesoscopic and macroscopic texture datafrom both hot rolled and cold rolled and annealed bcc metals (Fe, Ta, Mo, Nb) and alloys (low-carbon steels, ferritic stainless steels, transformer steels) is re-examined with respect to such phenomena.

In the last 20 years, the Crystal Plasticity Finite Element Method (CPFEM) has developed into an extremely versatile tool for describing the mechanical response of crystalline materials on all length scales from single crystals to engineering parts. While this is clearly reflected by an ever increasing number of publications in scientific journals, to date there is no comprehensive monograph on the topic. To change this situation the authors have brought together their experience with CPFEM into the current book.

- by Dierk Raabe
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- Microstructure, Finite Element Methods, Mechanical properties, Crystal Plasticity

Dual-phase (DP) steel is the flagship of advanced high-strength steels, which were the first among various candidate alloy systems to find application in weight-reduced automotive components. On the one hand, this is a metallurgical success story: Lean alloying and simple thermomechanical treatment
enable use of less material to accomplish more performance while complying with demanding environmental and economic constraints. On the other hand, the enormous literature onDP steels demonstrates the immense complexity ofmicrostructure physics inmultiphase alloys: Roughly 50 years after the first reports on ferrite-martensite steels, there are still various open scientific questions. Fortunately, the last decades witnessed enormous advances in the development of enabling experimental and simulation techniques, significantly improving the understanding of DP steels. This review provides
a detailed account of these improvements, focusing specifically on (a) microstructure evolution during processing, (b) experimental characterization of micromechanical behavior, and (c) the simulation of mechanical behavior, to highlight the critical unresolved issues and to guide future research efforts.

This is a study on grain-scale micromechanics of polycrystal surfaces during plastic straining. We use Al–Mg–Si sheets (alloy AA6022) as model material. The work aims at understanding the relationship between microstrain heterogeneity and surface roughness in plastically strained polycrystals in terms of the surface and through-thickness microstructure. Experiments were conducted on polycrystals with identical composition but different processing and microstructures. We performed tensile and bending tests on sheet samples cut in transverse and rolling directions. We investigated the plastic surface microstrains (photogrametry), surface topography (confocal microscopy), particle distribution (metallography, SEM), microtexture (EBSD), and grain size distribution (EBSD) in the same sample regions.
We also conducted in-situ straining experiments where the microtexture, surface topography, and stress–strain behavior
were simultaneously determined. The results reveal a relationship between the heterogeneity of plastic surface microstrains, roughness, and microstructure. In particular a correlation could be established between microstrains and banded microtexture components (Cube, Goss, {111}[uvw]).

Here we present cellular automaton models in materials science. It gives an introduction to the fundamentals of cellular automata and reviews applications, particularly for those that predict recrystallization phenomena. Cellular automata for recrystallization are typically discrete in time, physical space, and orientation space and often use quantities such as dislocation density and crystal orientation as state variables. Cellular automata can be defined on a regular or nonregular two- or three-dimensional lattice considering the first, second, and third neighbor shell for the calculation of the local driving forces. The kinetic transformation rules are usually formulated to map a linearized symmetric rate equation for sharp grain boundary segment motion. While deterministic cellular automata directly perform cell switches by sweeping the corresponding set of neighbor cells in accord with the underlying rate equation, probabilistic cellular automata calculate the switching probability of each lattice point and make the actual decision about a switching event by evaluating the local switching probability using a Monte Carlo step. Switches are in a cellular automaton algorithm generally performed as a function of the previous state of a lattice point and the state of the neighboring lattice points. The transformation rules can be scaled in terms of time and space using, for instance, the ratio of the local and the maximum possible grain boundary mobility, the local crystallographic texture, the ratio of the local and the maximum-occurring driving forces, or appropriate scaling measures derived from a real initial specimen. The cell state update in a cellular automaton is made in synchrony for all cells. The review deals, in particular, with the prediction of the kinetics, microstructure, and texture of recrystallization. Couplings between cellular automata and crystal plasticity finite element models are also discussed.

Deformation microstructures of bulk ultrafine-grained austenitic AISI 304L stainless steels were analyzed by electron backscatter
diffraction. Samples with grain sizes below 500 nm showed transition from grain-scale deformation to the collective phenomenon of shear banding. This was assisted by strain-induced grain rotation and coalescence. This phenomenon was suppressed in samples with a bimodal grain size distribution (maxima at 650 and 1400 nm) due to deformation-induced martensite formation, yielding high tensile strength and ductility (1348 MPa ultimate tensile strength at 0.36 max. true strain).

- by Dierk Raabe
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- Microstructure, EBSD, Texture, Recrystallization
- by Dierk Raabe
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- Materials Engineering, Mechanical Engineering, Materials Science, Crystallography

In the modern practice of stamping simulation of complex industrial parts the prediction of springback still lacks accuracy. In commercial software packages various empirical constitutive laws for stamping are available. Limited to simple empirical models for material anisotropy they do not take into account in a full manner the effects of microstructure and its evolution during the deformation process. The crystal plasticity finite element method bridges the gap between the polycrystalline texture and macroscopic mechanical properties that opens the way for more profound consideration of metal anisotropy in the stamping process simulation. In this paper the application of crystal plasticity FEM within the concept of virtual material testing with a representative volume element (RVE) is demonstrated. Using virtual tests it becomes possible, for example, to determine the actual shape of the yield locus and Lankford parameters and to use this information to calibrate empirical constitutive models. Along with standard uniaxial tensile tests other strain paths can be investigated like biaxial tensile, compressive or shear tests.
The application of the crystal plasticity FEM for the virtual testing is demonstrated for DC04 and H320LA steel grades. The parameters of the Vegter yield locus are calibrated and the use case demonstration is completed by simulation of a typical industrial part in PAMSTAMP 2G.

We present a study about the dependence of nanoindentation pile-up patterns and of microtextures on the crystallographic
orientation using high purity copper single crystals. Experiments were conducted on a Hysitron nanoindentation setup using a
conical indenter in order to avoid symmetries others than those of the crystal structure. Orientation measurements were conducted
using a high resolution electron back-scatter diffraction technique for the automated acquisition of texture mappings around the
indents. Simulations were carried out by means of a 3D elastic–viscoplastic crystal plasticity finite element method which takes full
account of crystallographic slip and orientation changes during indentation. The experiments as well as the simulations show that
the pile-up patterns on the surfaces of (0 0 1)-, (0 1 1)- and (1 1 1)-oriented single crystals have four-, two-, and sixfold symmetry,
respectively. The different pile-up patterns can be explained in terms of the strong crystallographic anisotropy of the out-of-plane
displacements around the indents. Pronounced accumulation of material entailing characteristic pile-up patterns occurs along the
intersection vectors between the primary crystallographic slip planes and the indented surface planes.

- by Dierk Raabe
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- Microstructure, Digital Image Correlation, EBSD, Nanoindentation, Simulation

Two plain carbon steels with varying manganese content (0.87 wt pct and 1.63 wt pct) were refined to approximately 1 μm by large strain warm deformation and subsequently subjected to intercritical annealing to produce an ultrafine grained ferrite/martensite dual-phase steel. The influence of the Mn content on microstructure evolution is studied by scanning electron microscopy (SEM).

- by Dierk Raabe
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- Electron Backscatter Diffraction (EBSD), 2D EBSD, EBSD, Grain size

We report on the microstructure, texture and deformation mechanisms of a novel ductile lean duplex stainless steel (Fe–19.9Cr–0.42Ni–0.16N–4.79Mn–0.11C–0.46Cu–0.35Si, wt.%). The austenite is stabilized by Mn, C, and N (instead of Ni). The microstructure is characterized by electron channeling contrast imaging (ECCI) for dislocation mapping and electron backscattering diffraction (EBSD) for texture and phase mapping. The material has 1 GPa ultimate tensile strength and an elongation to fracture of above 60%. The mechanical behavior is interpreted in terms of the strength of both the starting phases, austenite and ferrite, and the amount, dispersion, and transformation kinetics of the mechanically induced martensite (TRIP effect). Transformation proceeds from austenite to hexagonal martensite to near cubic martensite (c !e ! a0). The e-martensite forms in the austenite with an orientation relationship close to Shoji–Nishiyama. The a0-martensite nucleates at the intersections of deformation bands, especially e-bands, with Kurdjumov–Sachs and Nishiyama–Wassermann relationships. The ferrite deforms by dislocation slip and contains cell substructures.

- by Dierk Raabe
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- Thermodynamics, Microstructure, Ferrite, High temperature corrosion

- by Thomas Bieler and +1
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- Microstructure, Finite Element Methods, Mechanical properties, Crystal Plasticity
- by Andrey Belyakov
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- Engineering, SEM, TEM, EBSD

We discuss methods to map crystallographic textures in crystal plasticity finite element simulations. Fourier-type series expansion methods which use spherical harmonic library functions as well as the direct pole figure inversion methods are not well suited to reproduce texture information in a sufficiently localized spherical form onto finite element grids. Mathematically compact Gauss-shaped spherical texture components represent a better approach for including textures in finite element models since they represent an excellent compromise between discreteness (spherical localization), compactness (simple functions), mathematical precision (very good approximation also of complex orientation distribution functions already with small sets of texture components), scalability (the number of used texture components can be systematically varied according to the desired exactness of the texture fit), conceptual simplicity (simple mathematical handling), and physical significance (texture components can be directly linked to characteristic metallurgical mechanisms). The use of texture component functions has also advantages over the use of large sets of discrete single orientations with equal scatter and height since they are more compact, practical, and provide better physical insight into microstructural mechanisms and composition sensitive effects. The article presents a new approach for the mathematical reproduction of such crystallographic texture components in crystal plasticity finite element simulations. It explains in some detail why they are particularly suited for this purpose and how they can be used to map and recover textures in/from plasticity simulations.

- by Dierk Raabe
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- Microstructure, Finite Element Methods, Metal Forming, Sheet Metal Forming

The effect of Mn content on the microstructure and mechanical properties of two ultraﬁne grained0.2%C–Mn steels has been investigated. The ultraﬁne grained microstructure was produced by use of largestrain warm deformation and subsequent annealing. The ﬁnal microstructure consists of ﬁne cementite par-ticles within an ultraﬁne grained ferrite matrix. The increase in the Mn content leads to a decrease in the average ferrite grain size (from 1.3 to 0.8 um for an increase in the Mn content from 0.74 to 1.52mass%). This can be attributed to the enrichment of Mn in the cementite particles, which becomes ﬁner in the steelwith a higher Mn content. The increase in the Mn content results in an increase in strength at equal ductility and toughness.

- by Dierk Raabe
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- Microstructure, Electron Backscatter Diffraction (EBSD), Grain Boundaries, Steel
- by alok bhadauria and +4
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- Mechanical Engineering, Materials Science, Microstructure, Stainless Steel

In dual-phase (DP) steels, inherited microstructures and elemental distributions affect the kinetics and morphology of phase transformation phenomena and the mechanical properties of the final material. In order to study the inheritance process, we selected two model materials with the same average DP steel composition but with different initial microstructures, created by coiling at different temperatures
after hot rolling. These samples were submitted to a DP-steel heat treatment consisting of a short isothermal annealing in the pure austenite region and a quenching process. The evolution of microstructure, chemical composition and mechanical properties (hardness) during this treatment was investigated.
The initial samples had a bainitic–martensitic (B + M) microstructure for the material coiled at lower temperature and a ferritic–pearlitic (P + F) microstructure for that coiled at higher temperature. The P + F microstructure had a much more inhomogeneous distribution of substitutional elements (in particular of Mn) and of carbon. After complete heat treatment, both materials showed a typical DP microstructure (martensite islands embedded in ferrite) but the P + F material showed lower hardness compared to the B + M material. It was found that the inhomogeneous elemental distribution prevailed in the P + F material.
The inheritance process was studied by combining measurements of the elemental distribution by Wavelength-Dispersive X-ray spectroscopy (WDX), simulations of the evolution of the elemental composition via the DICTRA (diffusion-controlled reactions) computer programme, dilatometry to observe the kinetics of phase transformation, and observation and quantification of the microstructures by
Electron Backscatter Diffraction (EBSD) measurements. For the P + F material it was found that the a–c transformation during annealing is slowed down in regions of lower Mn content and is therefore not completed. During the subsequent cooling the incompletely autenitized material does not require ferrite
nucleation and the c–a transformation starts at relative high temperatures. For B + M, in contrast, nucleation of ferrite is needed and the transformation starts at lower temperatures. As a result the B + M material develops a higher martensite content as well as a higher density of geometrically necessary
dislocations (GNDs). It is speculated that for the B + M material the c–a transformation occurs through a bainitic (i.e. partly displacive) process while the transformation at higher temperatures in the P + F material proceeds exclusively in a diffusive way.

We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe–30.5Mn–2.1Al–1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales
inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening.

- by Dierk Raabe
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- Microstructure, Advanced high strength steels, Steel, Crystallographic Texture
- by Kim Verbeken
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- Materials Engineering, Steel, Manufacturing Engineering, EBSD

We investigate the effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3um and 50um were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall–Petch-type relation for the twinning stress using the same Hall–Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid’s law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 111//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid’s law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid’s law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains.

- by Dierk Raabe
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- Thermodynamics, Microstructure, Electron Backscatter Diffraction (EBSD), Ferrite

The role of grain boundaries during the early stages of oxidation in austenitic stainless steels containing alloyed Cu was investigated using APT, TEM, EBSD, EPMA, and XRD. The oxidation experiments were performed at 700 C in air with 20% water vapor. Within 4 um from the grain boundaries, the oxide layer exhibits a dual-layer structure consisting of a thin Fe-rich spinel oxide on a protective Cr2O3 oxide. Away from the grain boundaries, non-protective spinel oxide layers are formed as the outer and inner oxide layers. A critical grain size that prevents the formation of fast-growing spinel oxides is discussed.

- by Dierk Raabe and +1
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- Electron Backscatter Diffraction (EBSD), Corrosion Science, Transmission Electron Microscopy, Corrosion Engineering

This report aims at providing an improved fundamental under-
standing on the micro-mechanical response of lath martensitic
microstructures. Lath martensite is of immense importance for
structural alloys, since it is among the major strength-providing
microstructural constituents in martensitic or multi-phase steels (e.g. dual phase steel, transformation-induced plasticity steel, complex phase steels, quench-partition steels, etc.). Despite its long history and use, efforts to better understand the microstructure development and the mechanical behavior of lath martensite are still ongoing. Here, we are specifically interested in martensitic constituent size variation effects which have been rarely investigated so far [1–3], but drastically influence e.g. the autotempering behavior [4] and toughness properties [5]. Effectively any analysis associated with lath martensitic microstructures is hindered due to the complexities arising from (i) crystallographic and (ii) compositional aspects of the underlying microstructure. In order to motivate the novel analysis strategy developed here, we first discuss these two challenges in the following two paragraphs.
Regarding martensite crystallography most pioneering works
were based on transmission electron microscopy analyses [6,7].
TEM provides sufficient spatial resolution to resolve fine martensitic features (e.g. laths [6]), however, it provides only limited statistics of larger martensitic constituents (e.g. prior austenite grains) due to its limited field of view arising from the specimen and beam geometries. It is the development of the electron backscatter diffraction (EBSD) technique that enabled the systematic characterization of the hierarchical martensitic microstructure spanning multiple scales, i.e. ranging from prior austenite grains of hundreds of microns down to laths of tens of nanometers [8–11].
Yet, it is also clear that the standard 2D EBSD-based analysis provides a rather simplified representation of the lath martensite crystallography. For example, 3D EBSD and 3D FIB [12–15] analyses, as well as TEM observations [1,16,17] reveal significant heterogeneities in the size and morphology of martensite sub-units even within a single alloy, which cannot be fully captured by stand-alone 2D investigations. Also, even in optimized conditions, EBSD cannot resolve the fine details of the martensitic sub-structure.
To improve the fundamental understanding of the multi-scale characteristics of martensitic microstruc-
tures and their micro-mechanical properties, a multi-probe methodology is developed and applied to
low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron
channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels.

We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters.
Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by
which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe–22Mn–0.6C TWIP steel.

- by David Field
- •
- EBSD, Grain size distribution, Plastic deformation

Austenite formation, which originated from a fined-grained ferrite plus carbide microstructure, was observed during tensile testing at
973 K (60 K below Ae1, the equilibrium austenite–pearlite transformation temperature). Scanning electron microscopy, electron
backscatter diffraction and atom probe tomography results reveal the mechanism of austenitic transformation below Ae1. The initial
fine-grained microstructure, in combination with the warm deformation process, determines the occurrence of strain-induced austenite formation below Ae1. The initial fine-grained microstructure essentially contains a higher dislocation density to facilitate the formation of Cottrell atmospheres and a larger area fraction of ferrite/carbide interfaces which serve as austenite nucleation sites. The warm deformation promotes the Ostwald ripening process and the increase in dislocation density, and hence promotes the accumulation of local high carbon concentrations in the form of Cottrell atmospheres to reach a sufficiently high thermodynamic driving force for austenite nucleation. The critical carbon concentration required for the nucleation of austenite was calculated using classical nucleation theory, which correlated well with the experimental observations.

"We report the results of the microstructural characterization of a Ti-bearing IF-steel deformed at high strain rates in a split Hopkinson bar. The shock-loading tests were performed in hat-shaped specimens to induce the formation of adiabatic shear bands (ASB). The samples were deformed at 223 K and 298 K. High-resolution electron backscatter diffraction (EBSD) reveals the development of an ultrafine-grained structure within the ASB. A close inspection reveals the presence of deformation twins in grains adjacent to the shear band. These twins bend towards the ASB suggesting that mechanical twinning occurs before the flow associated to shear banding. The results of microtexture have indicated the presence of a sharp <111> gamma -fiber texture in the ASB for both temperatures"

A set of dynamically simulated electron backscatter patterns (EBSPs) for α-Ti crystals progressively rotated by 1° steps were analysed using cross-correlation to determine image shifts from which strains and rotations were calculated. At larger rotations the cross-correlation fails in certain regions of the EBSP where large shifts are generated. These incorrect shifts prevent standard least square error procedures from obtaining a valid solution for the strain and rotation, where the applied rotation exceeds ∼8°. Using a robust iterative fitting routine reliable strains and rotations can be obtained for applied rotations of up to and including ∼11° even though some image shifts are measured incorrectly. Finally, high resolution electron backscatter diffraction has been used to analyse the residual elastic strain, lattice rotations and density of stored geometrically necessary dislocations in a sample of copper deformed to 10% total strain. The robust iterative fitting analysis allows reliable analysis of a larger proportion of the map, the difference being most obviously beneficial in regions where significant lattice rotations have been generated.

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (l phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations,

The technique of electron backscatter Kikuchi diffraction patterns (BKDPs) in the scanning
electron microscope is reviewed. The paper focuses mainly on the crystallographic
applications of the technique, including discussions on point group and space group
determination and strain analysis. Orientation microscopy is discussed but not reviewed.
The geometrical configurations of BKDPs are reviewed in detail and the relationship
between BKDPs and the technique of electron channelling patterns (ECPs) is explored
briefly. Essential crystallography is discussed and methods of analysis of BKDPs to extract
crystallographic information are analyzed in detail. Some important characteristics of
diffraction contrast in BKDPs are analyzed with respect to the geometry of the technique,
the dynamical theory of electron diffraction and crystallographic applications. Examples of
the use of theoretical contrast in pattern interpretation are provided. Anomalous effects in
BKDPs are analyzed in detail and ways of identifying anomalous contrast in practice are
discussed. BKDPs included in the paper are zincblende (ZnS), silicon, germanium, GaAs,
chalcopyrite (CuFeS2), TaTe4 and Er2Ge2O7.

- by Karim Z Baba-Kishi
- •
- EBSD

The proposal of configurational entropy maximization to produce massive solid-solution (SS)-strengthened, single-phase high-entropy alloy (HEA) systems has gained much scientific interest. Although most of this interest focuses on the basic role of configurational entropy in SS formability, setting future research directions also requires the overall property benefits of massive SS strengthening to be carefully investigated. To this end, taking the most promising CoCrFeMnNi HEA system as the starting point, we investigate SS formability, deformation mechanisms, and the achievable mechanical property ranges of different compositions and microstructural states. A comparative assessment of the results with respect to room temperature behavior of binary Fe-Mn alloys reveals only limited benefits of massive SS formation. Nevertheless, the results also clarify that the compositional requirements in this alloy system to stabilize the face-centered cubic (fcc) SS are sufficiently relaxed to allow considering nonequiatomic compositions and exploring improved strength–ductility combinations at reduced alloying costs.

Microstructures of multi-phase alloys undergo morphological and crystallographic changes upon deformation, corresponding to the associated microstructural strain fields. The multiple length and time scales involved therein create immense complexity, especially when microstructural damage mechanisms are also activated. An understanding of the relationship between microstructure and damage initiation can often not be achieved by post-mortem microstructural characterization alone. Here, we present a novel multi-probe analysis approach. It couples various scanning electron microscopy methods to microscopic-digital image correlation (l-DIC), to overcome various challenges associated with concurrent mapping of the deforming microstructure along with the associated microstrain fields. For this purpose a contrast- and resolution-optimized l-DIC patterning method and a selective pattern/microstructure imaging strategy were developed. They jointly enable imaging of (i) microstructure-independent pattern maps and (ii) pattern-independent microstructure maps. We apply this approach here to the study of damage nucleation in ferrite/martensite dual-phase (DP) steel. The analyses provide four specific design guidelines for developing damage-resistant DP steels.

The microstructure and texture evolution of dual-phase steel sheets with a cold reduction of about 50%, annealed at ferritic and intercritical temperatures, were analyzed by scanning electron microscopy and electron backscatter diffraction. The competition between recrystallization and phase transformation was of particular interest. The sheets were annealed in salt bath or were annealed in a MULTIPAS annealing simulator under variation of annealing temperature, annealing time, and heating rate.

SEM-based electron backscattered diffraction (EBSD) measurements characterize constituent-particle nucleated fatigue crack path relative to local grain orientation and crack wake defect distribution for Al-Zn-Mg-Cu alloy 7050-T7451 stressed in moist air. Crack propagation is primarily transgranular; consisting of facets parallel to 100, 110 and high-index planes with no evidence of 111 slip-based cracking; and is also inter-subgranular involving pre-existing or fatigue process zone generated subgrain boundaries. Dislocation substructure develops close to the fatigue crack surface due to dynamic recovery of crack tip cyclic plasticity. Crack growth through subgrain structure explains the broad occurrence of crack features without a low-index orientation and is justified based on trapped-hydrogen embrittlement. A failure criterion for environmental fatigue modeling must capture a failure mechanism based on: a) formation of localized defect structure from cumulative cyclic plasticity (perhaps H sensitive), and b) subsequent embrittlement due to interaction of H trapped at this defect structure with microstructure-sensitive local tensile stresses normal to this weakened interface. Crack interaction with subgrain (and grain) boundaries produces local deflections and branches that arrest over a short distance. Such features should cause a distribution of microstructure-sensitive growth rates.

- by Vipul Gupta
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- EBSD, Environmental Fatigue, Misorientation, Microstructurally Small Crack
- by Thomas Bieler
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- Microstructure, Finite Element Methods, Mechanical properties, Crystal Plasticity
- by Jack Telesman
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- Grain Boundaries, Creep, EBSD