Showing papers on "Electron backscatter diffraction published in 2007"

TEM Sample Preparation and FIB-Induced Damage

Abstract: One of the most important applications of a focused ion beam (FIB) workstation is preparing samples for transmission electron microscope (TEM) investigation. Samples must be uniformly thin to enable the analyzing beam of electrons to penetrate. The FIB enables not only the preparation of large, uniformly thick, sitespecific samples, but also the fabrication of lamellae used for TEM samples from composite samples consisting of inorganic and organic materials with very different properties. This article gives an overview of the variety of techniques that have been developed to prepare the final TEM specimen. The strengths of these methods as well as the problems, such as FIB-induced damage and Ga contamination, are illustrated with examples. Most recently, FIB-thinned lamellae were used to improve the spatial resolution of electron backscatter diffraction and energy-dispersive x-ray mapping. Examples are presented to illustrate the capabilities, difficulties, and future potential of FIB.

Influence of {10-12} extension twinning on the flow behavior of AZ31 Mg alloy

Abstract: Uniaxial compression tests were performed on samples cut along the extrusion direction from AZ31 Mg alloy tubes. A stage of increasing work hardening rate was observed on representative true sigma-epsilon curves. Specimens compressed to various strain levels were examined by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the widespread formation of intersecting {10-12} extension twins is responsible for the increased strain hardening rate. (c) 2006 Elsevier B.V. All rights reserved.

ARPGE: a computer program to automatically reconstruct the parent grains from electron backscatter diffraction data

Abstract: A computer program called ARPGE written in Python uses the theoretical results generated by the computer program GenOVa to automatically reconstruct the parent grains from electron backscatter diffraction data obtained on phase transition materials with or without residual parent phase. The misorientations between daughter grains are identified with operators, the daughter grains are identified with indexed variants, the orientations of the parent grains are determined, and some statistics on the variants and operators are established. Some examples with martensitic transformations in iron and titanium alloys were treated. Variant selection phenomena were revealed.

Microstructure and texture of a lightly deformed TRIP-assisted steel characterized by means of the EBSD technique

Abstract: The microstructural and textural changes after a tensile strain of 10% were observed by orientation contrast measurements in a TRIP-assisted steel. On the undeformed samples it was shown that the electron back scatter diffraction (EBSD) technique could be used successfully for determining the volume fraction of the microstructural constituents bainite, ferrite and austenite, whereas after deformation only the BCC and FCC phases could be separated. The results show that the tensile strain of 10% gave rise to a drop in residual austenite content from 10 to 4%, which was also confirmed by magnetic measurements. The texture data showed only minor orientation rotations after 10% tensile strain for the BCC ferrite and bainite grains, whereas the residual austenite did show a significant texture change. By meticulously monitoring the local intra-granular misorientations it was concluded that the BCC phases (ferrite and bainite) took up the larger part of the nominal strain whereas the residual austenite primarily responded to the mechanical load by a partial (stress-induced) martensite transformation. Hence, the texture change observed in the residual austenite could be attributed to the orientation selective character of the phase transformation.

Thermal stability of ECAP processed pure copper

Abstract: Microstructure and texture evolution of pure copper (99.95%) after equal-channel angular pressing (ECAP) with route Bc up to 12 passes and subsequent heat treatment were investigated. After deformation the samples were annealed at different temperatures. The deformed and annealed states were characterized by X-ray pole figures, electron back scatter diffraction (EBSD), TEM and microhardness tests. It was shown that the observed texture and microstructure changes during subsequent annealing have to be associated with discontinuous recrystallization. The study revealed a very low thermal stability of this ECAP processed pure copper. The observed decrease of the apparent activation energy for recrystallization is most likely due to improved nucleation conditions of ECAP deformed material.

Correlation of microstructure and charpy impact properties in API X70 and X80 line-pipe steels

Abstract: This study aims at correlating microstructure and Charpy impact properties in high-toughness API X70 and X80 line-pipe steels. Three kinds of steels were fabricated by varying alloying elements and hot rolling conditions, and their microstructures and Charpy impact properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstructures were composed of acicular ferrites. In the X80 steel rolled in the single phase region, the decreased energy transition temperature (ETT) could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. Thus, it had excellent mechanical properties in yield and tensile strengths, absorbed energy, and transition temperature, except in ductility.

Three-Dimensional Characterization of Microstructure by Electron Back-Scatter Diffraction

Abstract: The characterization of microstructures in three dimensions is reviewed, with an emphasis on the use of automated electron back-scatter diffraction techniques. Both statistical reconstruction of polycrystalline structures from multiple cross sections and reconstruction from parallel, serial sections are discussed. In addition, statistical reconstruction of second-phase particle microstructures from multiple cross sections is reviewed.

A microstructural investigation of adiabatic shear bands in an interstitial free steel

Abstract: We report the microstructural characterization of induced adiabatic shear bands (ASB) in a hot-rolled interstitial free (IF) steel deformed at high strain rates (>2.8 × 104 s−1) in a split Hopkinson bar under controlled conditions at −50 and 25 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to reveal the degree of subdivision within ASB and neighboring grains. Deformation twins were found in adjacent grains suggesting that twinning occurs before the flow associated to shear banding. Progressive subgrain misorientation (PriSM) recrystallization is a plausible mechanism to explain the development of a new structure consisting of weakly textured ultrafine grains (0.1–0.5 μm) within the ASB. Recrystallization is proposed to occur by the formation and mechanical rotation of subgrains during deformation, coupled with boundary refinement via diffusion during shear band cooling. The presence of elongated subgrains and grains perfectly aligned within regions resembling a former lamellar structure within bands supports the occurrence of such a mechanism.

Characterization of Microstructure by Electron Back-Scatter Diffraction

Abstract: The characterization of microstructures in three dimensions is reviewed, with an emphasis on the use of automated electron backscatter diffraction techniques. Both statistical reconstruction of polycrystalline structures from multiple cross sections and reconstruction from parallel, serial sections are discussed. In addition, statistical reconstruction of second-phase particle microstructures from multiple cross sections is reviewed.

High strain gradient plasticity associated with wedge indentation into face-centered cubic single crystals: Geometrically necessary dislocation densities

Abstract: Experimental studies on indentation into face-centered cubic (FCC) single crystals such as copper and aluminum were performed to reveal the spatially resolved variation in crystal lattice rotation induced due to wedge indentation. The crystal lattice curvature tensors of the indented crystals were calculated from the in-plane lattice rotation results as measured by electron backscatter diffraction (EBSD). Nye's dislocation density tensors for plane strain deformation of both crystals were determined from the lattice curvature tensors. The least L 2 -norm solutions to the geometrically necessary dislocation densities for the case in which three effective in-plane slip systems were activated in the single crystals associated with the indentation were determined. Results show the formation of lattice rotation discontinuities along with a very high density of geometrically necessary dislocations.

Texture development in Ti-6Al-4V linear friction welds

Abstract: Microstructure and microtexture development in as-welded and post weld heat treated (PWHT) lab-scale (LS) and full-scale (FS) Ti–6Al–4V linear friction welds have been characterized using scanning electron microscopy and electron back-scatter diffraction (EBSD). The full-scale specimens exhibited a plastically affected zone (PAZ) about twice the size compared to lab-scale test welds. At the weld line a region of very fine martensitically formed microstructure was observed in both the LS and FS friction welds. EBSD texture scans across the weld line revealed a sharp texture variation in the PAZ. At the weld line a strong { 1 0 1 ¯ 0 } 〈 1 1 2 ¯ 0 〉 transverse texture for the as-welded and PWHTed condition was observed in both welds. Whereas the lab-scale linear friction welds displayed a dramatic change from almost random to strong transverse texture in the PAZ, the full-scale linear friction welds exhibited transverse texture at the weld line, followed by dispersed bands of transverse and { 1 1 2 ¯ 2 } 〈 1 1 2 ¯ 3 〉 type rolling texture in the PAZ region. The observed difference in LS and FS weld region textures is attributed to a difference in the variant selections during β → α transformation.

Deformation induced martensite in an AISI 301LN stainless steel: characterization and influence on pitting corrosion resistance

Abstract: In austenitic stainless steels, plastic deformation can induce martensite formation. The induced martensite is related to the austenite (g) instability at temperatures close or below room temperature. The metastability of austenite stainless steels increases with the decreasing of stacking fault energy (SFE). In this work, the deformation induced martensite was analyzed by X ray diffraction, electron back scatter diffraction (EBSD), magnetic methods and atomic force microscope (AFM) in samples of a low SFE austenitic stainless steel, AISI 301LN and compared with a medium SFE stainless steel, AISI 316L. Both techniques, X ray diffraction and EBSD, presented similar quantities for the a’-martensite. Texture results indicate that the crystallographic orientation of the formed a’-martensite is {001} and {103} . The morphology of a’-martensite was analyzed by AFM. Corrosion tests showed that deformation reduces pitting corrosion and generalized corrosion resistance in both steels.

The Study of Nanovolumes of Amorphous Materials Using Electron Scattering

Abstract: Many of the structural elements of importance in materials applications (e.g., thin films, barrier layers, intergranular films in ceramics) are small in volume and amorphous. Although the characterization of the structure of amorphous materials by X-ray and neutron diffraction methods is well established, these techniques are not suitable for studies of nanovolumes of materials because of the relatively small scattering cross sections. This chapter reviews recent developments in electron techniques, and particularly electron diffraction, for overcoming this problem.

Application of electron backscatter diffraction to the study of phase transformations

Abstract: The application of the electron backscatter diffraction technique to the investigation of phase transformations is reviewed. The wide variety of results obtained using this technique is illustrated and discussed, focusing on thermodynamics and kinetics of phase transformations, solidification, solid state phase transformations, environmentally assisted reactions and thin film deposition. Emphasis is also placed on two rapidly growing developments: coupling electron backscatter diffraction with advanced experimental techniques and with more and more complex modelling of phase transformations and of resulting material properties.

Correlation between the Microstructure, Growth Mechanism, and Growth Kinetics of Alumina Scales on a FeCrAlY Alloy

Abstract: The microstructural development of an alumina scale formed on a model FeCrAlY alloy during oxidation at 1200 °C was characterized for up to 2000 hours of growth. Quantitative scanning electron microscopy (SEM) studies revealed that the scale had a columnar microstructure, with the grain size being a linear function of the distance from the scale/gas interface. For a given fixed distance from the scale/gas interface, there was found to be no change in the oxide grain size for exposure times ranging from 24 to 2000 hours at 1200 °C, up to 100 hours at 1250 °C. Thus, there was no significant coarsening of existing grains in the scale. Through oxygen tracer experiments, the scale-growth mechanism was shown to be predominated by inward oxygen diffusion along the oxide grain boundaries. Electron backscatter diffraction (EBSD) analysis further revealed that a competitive oxide-grain growth mechanism operates at the scale/alloy interface, which is manifested by a preferential crystallographic grain orientation. The scale-thickening kinetics were modeled using the experimentally-derived, microstructural parameters and were found to be in excellent agreement with converted thermogravimetric (TG) measurements. The model predicted a subparabolic oxidation rate, with the time exponent decreasing with increasing exposure time. The values of the time exponent were shown to be approximately 0.35 to 0.37, at oxidation times commonly reached in the TG experiments, i.e., a few tens of hours. At longer oxidation times of a few thousand hours and with a constant rate of average oxide-grain size increase, the time exponent was predicted to approach 0.33, corresponding to an ideal cubic oxidation rate.

Crystallographic texture and microstructure of terebratulide brachiopod shell calcite: An optimized materials design with hierarchical architecture

Abstract: We analyzed the microstructure, microchemistry, and microhardness variations across the architectural elements of the shells of the brachiopod species Megerlia truncata and Terebratalia transversa with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), and Vickers microhardness indentation (VMHI). The brachiopod valves consist of two principal layers of distinct calcite biomineralization: a thin, nanocrystalline, outer, hard protective layer with VMHI values exceeding 200 HV and a much thicker, inner, secondary layer of a hybrid organic-inorganic fiber composite material. The secondary layer is further structured into two sublayers, an outer part with VMHI values varying between 110 and 140 HV, and a softer inner part (70 < HV < 110). Whereas the size of the calcite crystals within the primary layer varies between a few tens of nanometers and 2 μm, calcite crystals within the secondary layer are fibrous, commonly reaching lengths exceeding 150 μm. Cross sections of these fibrous crystals are spade shaped, their dimensions being about 5 × 20 μm. The fibers are aligned parallel to each other. They are single crystals with their morphological fiber axes pointing almost parallel to the shell vault. The crystallographic orientation of the morphological fiber axes, however, is arbitrary within the a – b plane of the calcite lattice, whereas the c -axis (hexagonal unit-cell setting) is perpendicular to the morphological fiber axes and thus parallel to the radial vector of the valve vault. This morphology strongly indicates that fibrous growth is controlled by confinement within a cell in an organic matrix and not by attachment of biomolecules to specific crystallographic faces. We observe inhomogeneous Sr2+ and Mg2+ concentrations in the shell calcite within the 0.1–0.9 wt% range. Design of the shell appears to be highly optimized for mechanical performance. Crystal morphology and orientation as well as incorporated organic matter are structured hierarchially at different length levels forming a hybrid organic-inorganic fiber composite architecture.

Recrystallization mechanisms in 5251 H14 and 5251 O aluminum friction stir welds

Abstract: The combination of deformation and heat in the nugget of friction stir welding (FSW) tends to transform by dynamic recrystallization the large grains of the base metal into a microstructure where the grains are completely equiaxed and strongly disorientated. To follow the different stages of dynamic recrystallization in the nugget, the evolution of the microstructure of two aluminum samples, cold rolled or annealed, is observed from the parent metal to the nugget using electron back-scattered diffraction (EBSD). The dynamic recrystallization mechanisms in each region of the weld strongly depend on the initial microstructural state of the aluminum sheet, in particular on the deformation energy stored. So, a static recrystallization prior to a continuous dynamic recrystallization was evidenced for the initially cold rolled Al alloy, while a geometric dynamic recrystallization occurred for the initially annealed microstructure. Fractions of low angle boundaries, crystallographic texture and grain size were compared in both weld nuggets.

Modeling and experiments on the indentation deformation and recrystallization of a single-crystal nickel-base superalloy

Abstract: This study presents crystal-plasticity finite-element calculations of room temperature deformation of a single-crystal nickel-base superalloy and simulation results on the microstructural development during subsequent recrystallization. The predictions are compared to corresponding experiments. Single-crystalline material is deformed by Brinell-type indentation using a spherical indenter of 1 mm in diameter. A succeeding annealing under inert atmosphere leads to the formation of recrystallized grains around the indents. The crystal plasticity finite element method is used to predict the distribution of crystallographic slip around the indents. The amount of accumulated slip is used to estimate the stored deformation energy in the indented volume. A 2D probabilistic cellular automaton simulation is then applied to the predicted distribution of the stored energy for the simulation of the formation and growth of new grains around the indents. The cellular automaton predicts the kinetics, microstructures, and crystallographic texture evolving during recrystallization.