Showing papers on "Electron backscatter diffraction published in 2010"

Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD

Abstract: We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

The Origin of Microstructural Diversity, Texture, and Mechanical Properties in Electron Beam Melted Ti-6Al-4V

Abstract: An additive layer manufacture (ALM) technique, electron beam melting, has been used for the production of simple geometries, from prealloyed Ti-6Al-4V powder. Microstructure, texture, and mechanical properties achieved under standard operating conditions have been investigated. Three transitional regions are observed with a change in microstructural formation dependent on the thermal mass of deposited material. Prior β-phase reconstruction, from room temperature α-phase electron backscatter diffraction (EBSD) data, reveals a strong texture perpendicular to the build axis. Variation of build temperature within the processing window of 898 K to 973 K (625 °C to 700 °C) is seen to have a significant effect on the properties and microstructure of both as-deposited and hot isostatically pressed (HIP) samples.

The effect of grain size and grain orientation on deformation twinning in a Fe-22 wt.% Mn-0.6 wt.% C TWIP steel

Abstract: 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 3 μm and 50 μm 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 〈1 1 1〉//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.

Statistical analyses of deformation twinning in magnesium

Abstract: To extract quantitative and meaningful relationships between material microstructure and deformation twinning in magnesium, we conduct a statistical analysis on large data sets generated by electron backscattering diffraction (EBSD). The analyses show that not all grains of similar orientation and grain size form twins, and twinning does not occur exclusively in grains with high twin Schmid factors or in the relatively large grains of the sample. The number of twins per twinned grain increases with grain area, but twin thickness and the fraction of grains with at least one visible twin are independent of grain area. On the other hand, an analysis of twin pairs joined at a boundary indicates that grain boundary misorientation angle strongly influences twin nucleation and growth. These results question the use of deterministic rules for twin nucleation and Hall–Petch laws for size effects on twinning. Instead, they encourage an examination of the defect structures of grain boundaries and their role in twin ...

A study of ternary Cu2SnS3 and Cu3SnS4 thin films prepared by sulfurizing stacked metal precursors

Abstract: Thin films of Cu2SnS3 and Cu3SnS4 were grown by sulfurization of dc magnetron sputtered Sn–Cu metallic precursors in a S2 atmosphere. Different maximum sulfurization temperatures were tested which allowed the study of the Cu2SnS3 phase changes. For a temperature of 350 °C the films were composed of tetragonal (I-42m) Cu2SnS3. The films sulfurized at a maximum temperature of 400 °C presented a cubic (F-43m) Cu2SnS3 phase. On increasing the temperature up to 520 °C, the Sn content of the layer decreased and orthorhombic (Pmn21) Cu3SnS4 was formed. The phase identification and structural analysis were performed using x-ray diffraction (XRD) and electron backscattered diffraction (EBSD) analysis. Raman scattering analysis was also performed and a comparison with XRD and EBSD data allowed the assignment of peaks at 336 and 351 cm−1 for tetragonal Cu2SnS3, 303 and 355 cm−1 for cubic Cu2SnS3, and 318, 348 and 295 cm−1 for the Cu3SnS4 phase. Compositional analysis was done using energy dispersive spectroscopy and induced coupled plasma analysis. Scanning electron microscopy was used to study the morphology of the layers. Transmittance and reflectance measurements permitted the estimation of absorbance and band gap. These ternary compounds present a high absorbance value close to 104 cm−1. The estimated band gap energy was 1.35 eV for tetragonal (I-42m) Cu2SnS3, 0.96 eV for cubic (F-43m) Cu2SnS3 and 1.60 eV for orthorhombic (Pmn21) Cu3SnS4. A hot point probe was used for the determination of semiconductor conductivity type. The results show that all the samples are p-type semiconductors. A four-point probe was used to obtain the resistivity of these samples. The resistivities for tetragonal Cu2SnS3, cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4 are 4.59 × 10−2 Ω cm, 1.26 × 10−2 Ω cm, 7.40 × 10−4 Ω cm, respectively.

Effect of rare earth elements on the microstructure and texture development in magnesium-manganese alloys during extrusion

Abstract: Single additions of the rare earth (RE) elements cerium, yttrium or neodymium have been made to magnesium–manganese alloys in order to investigate their influence on the microstructure and texture formed during indirect extrusion and the resulting mechanical properties. Whereas the binary Mg–Mn alloy M1 exhibits a 〈10.0〉 or 〈10.0〉–〈11.0〉 fibre texture depending on the extrusion rate, the RE-containing alloys exhibit weaker recrystallisation textures and the formation of a new texture component. The preferential growth of grains having 〈11.0〉 parallel to the extrusion direction was hindered in these alloys. For the rare earth elements used in this work it appears that Nd is a much stronger texture modifier compared to Ce or Y in Mg–Mn alloys. The weaker texture leads to increased ductility, lower yield and ultimate stresses, but a decrease in the asymmetric yield behaviour of the extruded bars.

Determination of elastic strain fields and geometrically necessary dislocation distributions near nanoindents using electron back scatter diffraction

Abstract: The deformation around a 500-nm deep Berkovich indent in a large grained Fe sample has been studied using high resolution electron back scatter diffraction (EBSD). EBSD patterns were obtained in a two-dimensional map around the indent on the free surface. A cross-correlation-based analysis of small shifts in many sub-regions of the EBSD patterns was used to determine the variation of elastic strain and lattice rotations across the map at a sensitivity of ∼±10−4. Elastic strains were smaller than lattice rotations, with radial strains found to be compressive and hoop strains tensile as expected. Several analyses based on Nye's dislocation tensor were used to estimate the distribution of geometrically necessary dislocations (GNDs) around the indent. The results obtained using different assumed dislocation geometries, optimisation routines and different contributions from the measured lattice rotation and strain fields are compared. Our favoured approach is to seek a combination of GND types which support th...

Automated nanocrystal orientation and phase mapping in the transmission electron microscope on the basis of precession electron diffraction

Abstract: An automated technique for the mapping of nanocrystal phases and orientations in a transmission electron microscope is described. It is primarily based on the projected reciprocal lattice geometry that is extracted from electron diffraction spot patterns. Precession electron diffraction patterns are especially useful for this purpose. The required hardware allows for a scanning-precession movement of the primary electron beam on the crystalline sample and can be interfaced to any older or newer mid-voltage transmission electron microscope (TEM). Experimentally obtained crystal phase and orientation maps are shown for a variety of samples. Comprehensive commercial and open-access crystallographic databases may be used in support of the nanocrystal phase identification process and are briefly mentioned.

Development of a Software Suite on X-ray Diffraction Experiments

Abstract: Angle dispersive powder X-ray diffraction experiments using a flat imaging plate (IP) are one of the most popular methods in high-pressure material science. In order to support such experiments, we developed two software, IPAnalyzer and PDIndexer. IPAnalyzer can convert a two-dimensional Debye-ring pattern to one-dimensional (Bragg-Brentano) geometry. IPAnalyzer can also calibrate experimental parameters (wave length, camera length, and so on) automatically. PDIndexer can display the converted pattern(s) and diffraction peaks calculated for any crystals.

Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 °C for more than 100,000 h

Abstract: There are rather few quantitative data on the microstructure of the 9-12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 ◦ C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 ◦ C up to 113,431 h. Electron Backscatter Diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.

The effect of crystal orientation on the indentation response of commercially pure titanium: experiments and simulations

Abstract: This study combines nanoindentation, electron backscatter diffraction (EBSD) and crystal plasticity finite element analysis to examine the anisotropy in the indentation behaviour of individual grains within an α-Ti polycrystal. Nanoindentation is utilized to mechanically probe small volumes of material within grains for which orientations are known from prior EBSD mapping. Both indentation modulus and hardness decrease significantly as the indentation axis is inclined further from the c -axis; the plastic response showing the more marked anisotropy. Recently developed high angular resolution EBSD has been utilized to examine selected indents, providing maps of elastic strain variations and lattice rotations. From such maps lower bound solutions for the density of geometrically necessary dislocations (GNDs) have been established. Crystal plasticity modelling showed promise in capturing correctly the orientation dependence of load–displacement response and in lattice rotations local to the indenter, particularly for indentation into a basal plane which generated threefold rotational symmetry about an axis parallel with the indentation direction which was also observed in experiments.

Determining grain resolved stresses in polycrystalline materials using three-dimensional X-ray diffraction

Abstract: An algorithm is presented for characterization of the grain resolved (type II) stress states in a polycrystalline sample based on monochromatic X-ray diffraction data. The algorithm is a robust 12-parameter-per-grain fit of the centre-of-mass grain positions, orientations and stress tensors including error estimation and outlier rejection. The algorithm is validated by simulations and by two experiments on interstitial free steel. In the first experiment, using only a far-field detector and a rotation range of 2 × 110°, 96 grains in one layer were monitored during elastic loading and unloading. Very consistent results were obtained, with mean resolutions for each grain of approximately 10 µm in position, 0.05° in orientation, and 8, 20 and 13 × 10−5 in the axial, normal and shear components of the strain, respectively. The corresponding mean deviations in stress are 30, 50 and 15 MPa in the axial, normal and shear components, respectively, though some grains may have larger errors. In the second experiment, where a near-field detector was added, ∼2000 grains were characterized with a positional accuracy of 3 µm.

Fracture toughness of polycrystalline tungsten alloys

Abstract: Tungsten and tungsten alloys show the typical change in fracture behavior from brittle at low temperatures to ductile at high temperatures. In order to improve the understanding of the effect of microstructure the fracture toughness of pure tungsten, potassium doped tungsten, tungsten with 1 wt.% La 2 O 3 and tungsten rhenium alloys were investigated by means of 3-point bending, double cantilever beam and compact tension specimens. All these materials show the expected increase in fracture toughness with increasing temperature. The experiments demonstrate that grain size, texture, chemical composition, grain boundary segregation and dislocation density seem to have a large effect on fracture toughness below the DBTT. These influences can be seen in the fracture behavior and morphology, where two kinds of fracture occur: on the one hand transgranular and on the other hand intergranular fracture. Therefore, techniques like electron backscatter diffraction (EBSD), Auger electron spectroscopy (AES) and X-ray line profile analysis were used to improve the understanding of the parameters influencing fracture toughness.

Rietveld texture analysis from TOF neutron diffraction data

Abstract: One of the advantages of a multidetector neutron time-of-flight diffractometer such as the high pressure preferred orientation diffractometer (HIPPO) at the Los Alamos Neutron Science Center is the capability to measure efficiently preferred orientation of bulk materials. A routine experimental method for measurements, both at ambient conditions, as well as high or low temperatures, has been established. However, only recently has the complex data analysis been streamlined to make it straightforward for a noninitiated user. Here, we describe the Rietveld texture analysis of HIPPO data with the computer code Materials Analysis Using Diffraction (MAUD) as a step-by-step procedure and illustrate it with a metamorphic quartz rock. Postprocessing of the results is described and neutron diffraction results are compared with electron backscatter diffraction measurements on the same sample.

Grain structure and texture evolution during friction stir welding of thin 6016 aluminum alloy sheets

Abstract: A “stop-action” technique was employed to study the grain structure and texture development during friction stir welding of thin 6016 aluminum sheets. The microstructural evolution ahead of the tool pin was found to be a complex process involving geometrical effects of strain as well as simultaneous development of continuous and discontinuous recrystallizations. At the circumference of the pin, the material flow was shown to be governed by the simple shear deformation induced by the rotating pin, which led to the formation of a pronounced {1 1 2} simple shear texture. Beyond the pin, the material experienced additional hot deformation, possibly caused by the influence of the tool shoulder, and limited static annealing during the weld cooling cycle. As a result, a well-recrystallized grain structure having a characteristic {1 0 0} cube texture finally developed in the stir zone.

Synthesis Mechanisms of Organized Gold Nanoparticles: Influence of Annealing Temperature and Atmosphere

Abstract: We report on the synthesis of organized arrays of gold (Au) nanoparticles on thermally oxidized Si wafers using sputtering as a metal deposition method in combination with sphere lithography. This simple process leads to the formation of a honeycomb mask of Au at room temperature (RT). We study the transformation mechanism of this honeycomb mask to a hexagonal array of Au nanoparticles by annealing at different temperatures and in different atmospheres. The underlying mechanisms of pattern formation during annealing are coalescence of particles and Ostwald ripening and depend on temperature and atmosphere. The crystallinity and orientation of the nanoparticles with respect to the underlying substrate is analyzed by electron backscatter diffraction (EBSD), and the control of the morphology, size, shape, and orientation in different atmospheres (argon (Ar), nitrogen (N2), air, hydrogen (H2), and vacuum) is discussed.

Observations of Facet Formation in Near-α Titanium and Comments on the Role of Hydrogen

Abstract: Faceted features are frequently observed on the fracture surfaces of titanium alloys that have failed by static loading, continuous cycling, dwell fatigue loading, and stress corrosion cracking (SCC). Although the facets formed under different loading conditions seem qualitatively similar, there are significant differences in the spatial and crystallographic orientations of the facets as well as subtle differences in facet surface topography. The current study compares and contrasts facets for various loading conditions (cyclic, creep, SCC, and dwell) in the Ti-8Al-1Mo-1V alloy with the primary motivation being to understand the mechanisms of crack initiation and faceted growth during dwell fatigue. The spatial and crystallographic orientations of the facets were determined using quantitative tilt fractography and electron backscatter diffraction, whereas facet topography was examined using ultra-high-resolution scanning electron microscopy. Collectively, the experimental observations suggest that hydrogen may play an important role in facet formation and accelerating small crack growth rates during dwell fatigue loading.

Resistivity dominated by surface scattering in sub-50 nm Cu wires

Abstract: Electron scattering mechanisms in copper lines were investigated to understand the extendibility of copper interconnects when linewidth or thickness is less than the mean free path. Electron-beam lithography and a dual hard mask were used to produce interconnects with linewidths between 25 and 45 nm. Electron backscatter diffraction characterized grain structure. Temperature dependence of the line resistance determined resistivity, which was consistent with existing models for completely diffused surface scattering and line-edge roughness, with little contribution from grain boundary scattering. A simple analytical model was developed that describes resistivity from diffuse surface scattering and line-edge roughness.

Inferential statistics of electron backscatter diffraction data from within individual crystalline grains

Abstract: Highly concentrated distributed crystallographic orientation measurements within individual crystalline grains are analysed by means of ordinary statistics neglecting their spatial reference. Since crystallographic orientations are modelled as left cosets of a given subgroup of SO(3), the non-spatial statistical analysis adapts ideas borrowed from the Bingham quaternion distribution on {\bb S}^3. Special emphasis is put on the mathematical definition and the numerical determination of a `mean orientation' characterizing the crystallographic grain as well as on distinguishing several types of symmetry of the orientation distribution with respect to the mean orientation, like spherical, prolate or oblate symmetry. Applications to simulated as well as to experimental data are presented. All computations have been done with the free and open-source texture toolbox MTEX.

Influence of fraction of high angle boundaries on the mechanical behavior of an ultrafine grained Al–Mg alloy

Abstract: The mechanical behavior of ultrafine grained AA5052 processed through different techniques—rolled, annealed, friction stir processed (FSP) and equal channel angular pressed (ECAP)—were compared and correlated with microstructure. The microstructure was characterized using electron back scattered diffraction to obtain the boundary spacing, the fraction of high angle boundaries and to estimate the dislocation density from local misorientations. Both FSP and ECAP conditions had ultrafine boundary spacing, but the fraction of high angle boundaries was larger for the FSP condition. Tensile deformation carried out at 297 K and 10−3 s−1 showed a lower work-hardening rate and recovery rate for FSP as compared to the ECAP condition. It was inferred that low angle boundaries are more effective sinks for dislocations. When comparing differently processed materials, the strength, ductility and work-hardening behavior correlate better with the fraction of high angle boundaries than the boundary spacing.

Microstructure and texture evolution in dual-phase steels: Competition between recovery, recrystallization, and phase transformation

Abstract: 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. For low intercritical temperatures, recrystallization occurred before phase transformation. The sheets showed a similar through-thickness texture inhomogeneity with a plane-strain texture with strong -fiber and weak fiber as cold rolled sheets and a ferritic–martensitic band structure in the sheet center layers. An inverse correlation between the volume fractions of recrystallized ferrite and martensite was observed. This interdependence is attributed to a different phase transformation kinetics for recyrstallized and deformed ferrite and is discussed in terms of deformation strain energy, diffusion, and number of nucleation sites. © 2010 Elsevier B.V. All rights reserved.