Showing papers on "Electron backscatter diffraction published in 2011"

A review of strain analysis using electron backscatter diffraction.

Abstract: Since the automation of the electron backscatter diffraction (EBSD) technique, EBSD systems have become commonplace in microscopy facilities within materials science and geology research laboratories around the world. The acceptance of the technique is primarily due to the capability of EBSD to aid the research scientist in understanding the crystallographic aspects of microstructure. There has been considerable interest in using EBSD to quantify strain at the submicron scale. To apply EBSD to the characterization of strain, it is important to understand what is practically possible and the underlying assumptions and limitations. This work reviews the current state of technology in terms of strain analysis using EBSD. First, the effects of both elastic and plastic strain on individual EBSD patterns will be considered. Second, the use of EBSD maps for characterizing plastic strain will be explored. Both the potential of the technique and its limitations will be discussed along with the sensitivity of various calculation and mapping parameters.

Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic‐preferred orientation

Abstract: [1] We performed triaxial compressive creep experiments on aggregates of San Carlos olivine to develop a flow law and to examine microstructural development in the dislocation-accommodated grain boundary sliding regime (GBS). Each experiment included load and temperature steps to determine both the stress exponent and the activation energy. Grain boundary maps, created with electron backscatter diffraction data, were used to quantify grain size distributions for each sample. Inversion of the resulting data produced the following flow law for GBS: GBS = 104.8 ± 0.8 (σ2.9 ± 0.3/d0.7 ± 0.1) exp[(−445 ± 20 kJ mol−1)/RT], with σ, d, and GBS in units of MPa, μm, and s−1, respectively. Although relatively weak, crystallographic-preferred orientations (CPOs) have [010] maxima parallel to the compression direction along with [100] and [001] girdles perpendicular to the compression direction. CPOs and subgrain boundary misorientation axes suggest that the (010)[100] slip system contributes significantly to deformation. We propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations. Extrapolation of our flow laws to mantle conditions suggests that GBS is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow, and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep.

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Abstract: The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

In situ characterization of the deformation and failure behavior of non-stochastic porous structures processed by selective laser melting

Abstract: Cellular materials are promising candidates for load adapted light-weight structures Direct manufacturing (DM) tools are effective methods to produce non-stochastic structures Many DM studies currently focus on optimization of the geometric nature of the structures obtained The literature available so far reports on the mechanical properties but local deformation mechanisms are not taken into account In order to fill this gap, the current study addresses the deformation behavior of a lattice structure produced by selective laser melting (SLM) on the local scale by means of a comprehensive experimental in situ approach, including electron backscatter diffraction, scanning electron microscopy and digital image correlation SLM-processed as well as heat treated lattice structures made from TiAl6V4 alloy were employed for mechanical testing It is demonstrated that the current approach provides means to understand the microstructure-mechanical property–local deformation relationship to allow for optimization of load adapted lattice structures

Automated electron diffraction tomography – a new tool for nano crystal structure analysis

Abstract: Automated electron Diffraction Tomography (ADT) comprises an upcoming method for “ab intio” structure analysis of nano crystals. ADT allows fine sampling of the reciprocal space by sequential collection of electron diffraction patterns while tilting a nano crystal in fixed tilt steps around an arbitrary axis. Electron diffraction is collected in nano diffraction mode (NED) with a semi-parallel beam with a diameter down to 50 nm. For crystal tracking micro-probe STEM imaging is used. Full automation of the acquisition procedure allowed optimisation of the electron dose distribution and therefore analysis of highly beam sensitive samples. Cell parameters, space group and reflection intensities can be determined directly within a reconstructed 3d diffraction volume using a dedicated software package (ADT3D). Intensity data sets extracted from such a volume usually show a high coverage and significantly reduced dynamical effects due to “off-zone” acquisition. The use of this data for “ab initio” structure solution by direct methods implemented in standard programs for X-ray crystallography is demonstrated. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hot deformation behavior and microstructural evolution of homogenized 7050 aluminum alloy during compression at elevated temperature

Abstract: Hot compression tests of homogenized 7050 aluminum alloy were carried out on the Gleeble-1500 thermal simulation machine, and the associated microstructure was studied using electron back scattered diffraction technique and transmission electron microscopy. The results showed that the peak stress levels decreased with the increase of deformation temperatures or the decrease of strain rates, which can be represented by the Zener–Hollomon parameter in the exponent-type equation with the hot deformation activation energy of 160.3 kJ/mol. With the decrease of Z values, results showed a continuous decrease in very low angle boundaries, exhibiting a misorientation between 2° and 5°, associated to substructure, and a steady increase in the other higher angle boundaries, especially with the misorientation angles between 30° and 60°. And such an evolution is due to the increase of subgrain size with the decrease of Z values. At lower Z , the dislocations collected into more widely spaced and less dense tangles. As strain rose, the tangles reorganized into subgrains with walls that were more widely spaced and straighter; they had fewer, more regularly arranged dislocations. The main softening mechanism of homogenized 7050 aluminum alloy is dynamic recovery.

Development of Microstructure and Crystallographic Texture during Stationary Shoulder Friction Stir Welding of Ti-6Al-4V

Abstract: Electron backscattering diffraction (EBSD) was used to study a stationary shoulder friction stir weld in Ti-6Al-4V. Weld temperatures exceeded the β-transus, resulting in a supertransus zone (STZ) that encompassed all of the thermomechanically affected zone (TMAZ) and a portion of the heat-affected zone (HAZ). Standard EBSD provided limited information on the material behavior at high temperature in the β phase field, so in-house software was used to reconstruct the crystallographic orientations of the high-temperature β phase. The portion of the HAZ that lay within the STZ exhibited the same β texture at high temperature as the retained β phase in the unaffected parent material. In the TMAZ, material was deformed in the high-temperature β phase field and, on cooling, transformed to a fully lamellar microstructure. The β textures at high temperature were dominated by the D 2 $$ \left( {\bar{1}\bar{1}2} \right)\left[ {111} \right] $$ simple shear texture component. The α phase textures in the fully lamellar microstructure that formed on cooling were inherited from the shear textures of the β phase, but significant variant selection occurred.

High spatial resolution semi-automatic crystallite orientation and phase mapping of nanocrystals in transmission electron microscopes

Abstract: A semi-automatic technique for the mapping of nanocrystal phases and orientations in a transmission electron microscope (TEM) is described. It is based primarily on the projected reciprocal lattice geometry, but also utilizes the intensity of reflections that are extracted from precession-enhanced electron diffraction spot patterns of polycrystalline materials and multi-material composites. At the core of the method, experimental (precession-enhanced) electron diffraction spot patterns are cross correlated with pre-calculated templates for a set of model structures. The required hardware facilitates a scanning-precession movement of the primary electron beam on the polycrystalline and/or multi-material sample and can be interfaced to any newer or older mid-voltage TEM. The software that goes with this hardware is so flexible in its intake of experimental data that it can also create crystallite orientation and phase maps of nanocrystals from the amplitude part of Fourier transforms of high resolution TEM images. Experimentally obtained crystallite orientation and phase maps are shown for a clausthalite nanocrystal powder sample, polycrystalline aluminum and copper films, fine-grained palladium films, as well as MnAs crystallites that are partly embedded in a GaAs wafer. Comprehensive open-access and commercial crystallographic databases that may provide reference data in support of the nanocrystal phase identification process of the software are briefly mentioned. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic microstructural changes during hot extrusion and mechanical properties of a Mg–5.0 Zn–0.9 Y–0.16 Zr (wt.%) alloy

Abstract: In this study, firstly, dynamic microstructural changes of an as-cast Mg–5.0 Zn–0.9 Y–0.16 Zr (wt.%) alloy (designated ZWK510) during hot extrusion at 350 °C and a ram speed of 3.33 mm s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. The dynamic recrystallization (DRX) mechanism during hot extrusion was discussed. Then, the effect of microstructure and texture on the mechanical properties of the as-extruded alloy specimens at room temperature was discussed. The as-cast ZWK510 alloy consists of a-Mg and quasicrystalline I-phase. During hot extrusion at 350 °C, the main DRX mechanism is the continuous DRX near the original grain boundaries. The I-phase can accelerate the DRX behavior near these areas by obstructing the slip of dislocations. The deformation twins and massive blocky substructures formed in original grains can coordinate the DRX process near the original grain boundaries, however the DRX seldom occurs inside of these area. After further deformation, these deformation twins and massive blocky substructures are elongated along the material flow and become so-called unDRXed area, then a bimodal “necklace structure” composed of fine DRXed grains of about 2.1 μm and unrecrystallized coarse area is formed. The extruded ZWK510 alloy shows a DRX ratio of about 58% and a typical basal fiber texture of ( 0 0 0 1 ) 1 0 1 ¯ 0 matrix / / extrusion direction (ED). In the DRXed area around the crushed eutectic I-phase a large number of fine I-phase precipitates are observed pinning at the newly formed DRXed grain boundaries. The 0.2% proof strength and the ultimate tensile strength of the extruded ZWK510 alloy specimen are 317 and 363 MPa, respectively, with an elongation to failure of 12%, which have been attributed to strong basal fiber texture, refined grain size as well as the existence of fine precipitates formed during the hot extrusion.

Texture evolution of cold rolled and annealed Fe–24Mn–3Al–2Si–1Ni–0.06C TWIP steel

Abstract: The microstructure and texture evolution of 42% cold-rolled Fe–24Mn–3Al–2Si–1Ni–0.06C TWinning Induced Plasticity (TWIP) steel is investigated during isochronal annealing at temperatures between 600 and 850 °C. In the cold rolled condition, bulk texture returned the distinctive α-fibre for low stacking fault energy materials, with higher intensities for Goss ({1 1 2}〈 0 1 1 〉) compared to Brass ({1 1 1}〈 1 1 2 〉). A comparison between bulk and micro-textures, showed a significant slip contribution to the development of the Brass orientation, along with a possible role for micro-shear banding. Annealing twins contribute to recrystallisation from the early stages of nucleation and participate in generating new orientations thereafter. Unlike texture studies on other austenitic steels, the F ({1 1 1}〈 0 1 1 〉) and Rotated Copper ({1 1 2}〈 0 1 1 〉) orientations were detected in this work. The former is due to a more homogeneous distribution of nucleation sites, while the latter can be ascribed to second order twinning and the preferred-growth 30° 〈1 1 1〉 relation with the Brass rolling component. Based on the microstructural parameters from Electron Back-Scattering Diffraction (EBSD), the modified Hall–Petch (H–P) relation was successfully applied to the 0.2% proof stress.

Processing, microstructure and mechanical properties of nickel particles embedded aluminium matrix composite

Abstract: Nickel particles were embedded into an Al matrix by friction stir processing (FSP) to produce metal particle reinforced composite. FSP resulted in uniform dispersion of nickel particles with excellent interfacial bonding with the Al matrix and also lead to significant grain refinement of the matrix. The novelty of the process is that the composite was processed in one step without any pretreatment being given to the constituents and no harmful intermetallic formed. The novel feature of the composite is that it shows a three fold increase in the yield strength while appreciable amount of ductility is retained. The hardness also improved significantly. The fracture surface showed a ductile failure mode and also revealed the superior bonding between the particles and the matrix. Electron backscattered diffraction (EBSD) and transmission electron microscopy analysis revealed a dynamically recrystallized equiaxed microstructure. A gradual increase in misorientation from sub-grain to high-angle boundaries is observed from EBSD analysis pointing towards a continuous type dynamic recrystallization mechanism.

Three-Dimensional Orientation Mapping in the Transmission Electron Microscope

Abstract: Over the past decade, efforts have been made to develop nondestructive techniques for three-dimensional (3D) grain-orientation mapping in crystalline materials. 3D x-ray diffraction microscopy and differential-aperture x-ray microscopy can now be used to generate 3D orientation maps with a spatial resolution of 200 nanometers (nm). We describe here a nondestructive technique that enables 3D orientation mapping in the transmission electron microscope of mono- and multiphase nanocrystalline materials with a spatial resolution reaching 1 nm. We demonstrate the technique by an experimental study of a nanocrystalline aluminum sample and use simulations to validate the principles involved.

Preparation of ZnO Nanostructures by Chemical Precipitation Method

Abstract: ZnO nanostructures have been successfully prepared by dissolution–reprecipitation of micro-sized ZnO powder from aqueous ammonium hydrogencarbonate solution in the presence of thiourea, followed by calcination at 400°C for 1 h. The products were characterized by simultaneous thermal analysis, x-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction, and Fourier-transform infrared spectroscopy analysis. The results reveal that the morphology and particle size of the as-prepared ZnO samples are susceptible to the amount of added thiourea. The developed method has potential for application in industry due to simple processing and inexpensive reagents.