Showing papers on "Electron backscatter diffraction published in 2008"

Dynamic recrystallization during high temperature deformation of magnesium

Abstract: As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in the deformation of magnesium, particularly at a deformation temperature of 200 °C, where a transition from brittle to ductile behavior is observed. Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperatures and strain rates to examine the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains (ɛ ∼ −1.4) seemed to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain growth at elevated deformation temperatures, i.e. 400 °C, even at a low strain rate of 10−4 s−1. Certain deformation conditions gave rise to a desired fully recrystallized microstructure with an average grain size of ∼18 μm and an almost random crystallographic texture. For samples deformed at 200 °C/10−2 s−1, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.

Three-Dimensional Orientation Microscopy in a Focused Ion Beam-Scanning Electron Microscope: A New Dimension of Microstructure Characterization

Abstract: In the present work, we report our recent progress in the development, optimization, and application of a technique for the three-dimensional (3-D) high-resolution characterization of crystalline microstructures. The technique is based on automated serial sectioning using a focused ion beam (FIB) and characterization of the sections by orientation microscopy based on electron backscatter diffraction (EBSD) in a combined FIB–scanning electron microscope (SEM). On our system, consisting of a Zeiss–Crossbeam FIB-SEM and an EDAX-TSL EBSD system, the technique currently reaches a spatial resolution of 100 · 100 · 100 nm 3 as a standard, but a resolution of 50 · 50 · 50 nm 3 seems to be a realistic optimum. The maximum observable volume is on the order of 50 · 50 · 50 lm 3 . The technique extends all the powerful features of two-dimensional (2-D) EBSD-based orientation microscopy into the third dimension of space. This allows new parameters of the microstructure to be obtained—for example, the full crystallographic characterization of all kinds of interfaces, including the morphology and the crystallographic indices of the interface planes. The technique is illustrated by four examples, including the characterization of pearlite colonies in a carbon steel, of twins in pseudonanocrystalline NiCo thin films, the description of deformation patterns formed under nanoindents in copper single crystals, and the characterization of fatigue cracks in an aluminum alloy. In view of these examples, we discuss the possibilities and limits of the technique. Furthermore, we give an extensive overview of parallel developments of 3-D orientation microscopy (with a focus on the EBSD-based techniques) in other groups.

Dynamic recrystallization of Mg and Mg–Y alloys: Crystallographic texture development

Abstract: Pure magnesium and three binary Mg–Y alloys (023, 084 and 271 wt%Y) have been deformed in plane strain compression under conditions where dynamic recrystallization (DRX) is observed The development of crystallographic texture during deformation has been determined for both the parent and DRX grains In all but the highest Y alloy, the texture of the DRX grains was found to follow that of the parent grains closely in all alloys, implying that the DRX texture is dominated by the deformation conditions, rather than preferred nucleation or growth In the highest Y alloy, the DRX texture is randomized, which also suggests that preferred nucleation or growth is not responsible for texture formation in this alloy A transition in macrotexture development was observed in going from pure Mg deformed at 250 °C, to Mg–271 wt%Y deformed at 450 °C This can be attributed to activation of additional slip modes, and a concomitant decrease in the contribution of twinning to deformation

Nano-grained copper strip produced by accumulative roll bonding process

Abstract: Accumulative roll bonding (ARB) process is a severe plastic deformation (SPD) process that has been used for pure copper (99.9%). The ARB process up to 8 cycles was performed at ambient temperature under unlubricated conditions. Microstructural characterizations were done by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). It was found that continuous recrystallization resulted in microstructure covered with small recrystallized grains with an average diameter below 100 nm. The tensile strength and hardness of the ARB processed copper has become two times higher than initial value. On the other hand, the elongation dropped abruptly at the first cycle and then increased slightly. Strengthening in ARB processed copper may be attributed to strain hardening and grain refinement. In order to clarify the failure mode, fracture surfaces after tensile tests were observed by scanning electron microscopy (SEM). Observations revealed that failure mode in ARB processed copper is shear ductile rupture with elongated small dimples.

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Abstract: Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10−1 s−1. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.

Microstructure evolution and mechanical properties of AA1100 aluminum sheet processed by accumulative roll bonding

Abstract: In this study, accumulative roll bonding (ARB) process was carried out on an AA1100 aluminum sheet up to 10 cycles. Electron backscattering diffraction (EBSD) method was utilized to investigate the microstructural evolution during the ARB process. It was observed that the ARB is a promising process for fabricating ultra-fine grained structures in aluminum sheets. The results indicate that several mechanisms are responsible for the microstructural changes at different levels of strain during this process. Grain subdivision as well as the development of sub-grains are the major mechanisms at the early stages of ARB. Strain induced transition of low angle to high angle grain boundaries and the formation of thin lamellar structure occurs at the medium levels of strain. Finally, the progressive break up of this thin lamellar structure into more equiaxed grains is the dominant mechanism at relatively high strains. By reducing the grain size, the yield stress and the tensile strength of the ARBed sheets increased significantly and reached the maximum values of 282 and 333 MPa after the tenth cycle. The strength held Hall–Petch relationship and was in a good conformity with the microstructural changes.

Synthesis of flower-like CuO nanostructures as a sensitive sensor for catalysis

Abstract: The flower-like CuO nanostructures were hydrothermally synthesized without using any template. The influences of hydrothermal temperature and time on the growth of nanostructures were investigated. The samples were characterized by means of scanning electron microscope (SEM), X-ray powder diffraction (XRD), transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM), selected area electron diffraction (ED), and N2 adsorption isotherm. Interestingly, these architectures are made of three-order structures. The formation mechanism of the flower-like CuO was proposed and explained. Furthermore, the chemiluminescence (CL) and catalysis properties of the flower-like CuO were also investigated. The flower-like nanostructures showed the high-CL intensities and reactive activities for CO oxidation. The flower-like CuO can be used to fabricate a highly sensitive CL detector. This CL mode is a rapid and effective method for the selection of new catalysts from thousands of materials.

Analysis of the mechanical properties and deformation behavior of nanostructured commercially pure Al processed by equal channel angular pressing (ECAP)

Abstract: In the present paper commercially pure Al was processed by equal channel angular pressing (ECAP) up to 8 passes using route B C . For ECAP processing a proper die set was designed and constructed. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) were used to evaluate the microstructure of the pressed materials. Mechanical properties and the deformation behavior of the ECAP processed material were investigated by the hardness and compression tests. The significant increase in hardness and yield stress after ECAP was discussed by two strengthening mechanisms. Based on these mechanisms, variations of the hardness and yield stress as a function of the pass numbers were related to the calculated dislocation densities and the average boundary spacing. Also it was suggested that the absorption of the dislocations into grain boundaries would be an effective recovery process for the absence of the strain hardening.

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

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

Electron Diffraction Based Analysis of Phase Fractions and Texture in Nanocrystalline Thin Films, Part I: Principles

Abstract: A method for phase analysis, similar to the Rietveld method in X-ray diffraction, was not developed for electron diffraction (ED) in the transmission electron microscope (TEM), mainly due to the dynamic nature of ED. Nowadays, TEM laboratories encounter many thin samples with grain size in the 1–30 nm range, not too far from the kinematic ED conditions. This article describes a method that performs (semi)quantitative phase analysis for nanocrystalline samples from selected area electron diffraction (SAED) patterns. Fractions of the different nanocrystalline components are determined from rotationally symmetric ring patters. Both randomly oriented nanopowders and textured nanopowders, observed from the direction of the texture axis produce such SAED patterns. The textured fraction is determined as a separate component by fitting the spectral components, calculated for the previously identified phases with a priori known structures, to the measured distribution. The Blackman correction is applied to the set of kinematic diffraction lines to take into account dynamic effects for medium grain size. Parameters of the peak shapes and the other experimental parameters are refined by exploring the parameter space with the help of the Downhill-SIMPLEX. Part I presents the principles, while future publication of Parts II and III will elaborate on current implementation and will demonstrate its usage by examples, respectively.

Variant Selection in Grain Boundary Nucleation of Upper Bainite

Abstract: The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling.

Mechanical properties and microstructure evolution of 1050 aluminum severely deformed by ECAP to 16 passes

Abstract: Variation of mechanical properties and microstructure evolution in 1050 aluminum processed by equal channel angle pressing are investigated using Route BC and up to 16 passes. Micro hardness and compression testing were used to evaluate mechanical properties, whereas electron back scattered diffraction (EBSD) was used to document the variation of cell size and misorientation angle with number of passes. The hardness and yield strength exhibited an instant increase by a factor of about 2.75 and 2.96, respectively, compared to the annealed state, after only the first pass. It was found that the cell size gradually decreased with number of passes and attained an average value of about 0.6 μm in the face plane (normal to the extrusion direction), and 0.85 μm in the flow plane (parallel to the extrusion direction) after the sixteenth pass. The average misorientation angle evolved in both the face and flow planes ended up to about 27° and 26°, respectively.

Orientation dependence of dislocation structure evolution during cold rolling of aluminum

Abstract: A well-organized dislocation structure forms in many polycrystalline metals during plastic deformation. This structure is described qualitatively with no explanation of the quantitative characterization. In this work, the evolution of dislocation structure in commercial purity aluminum is described by means of the excess dislocation density and by quantitative characterization of the cell structure as seen on a plane surface. The measurements were performed on a pseudo-internal surface of a split specimen deformed by channel die deformation. The results show a clear dependence of cell structure formation on orientation of the crystallite with respect to the imposed deformation gradient with the largest excess dislocation density occurring in grains of { 011 }[1 2 2] orientation for plane strain deformation. Neighboring grain and non-local effects are shown to be of importance in the type of dislocation structures that evolve. © 2007 Elsevier B.V. All rights reserved.

Crystal rotation in Cu single crystal micropillars: In situ Laue and electron backscatter diffraction

Abstract: In situ microdiffraction experiments were conducted on focused ion beam machined single crystal Cu pillars oriented for double slip. During deformation, the crystal undergoes lattice rotation on both the primary and critical slip system. In spite of the initial homogeneous microstructure of the Cu pillar, rotation sets in already at yield and is more important at the top of the pillar than at the bottom, demonstrating the inhomogeneous stress state during a microcompression experiment. The rotation results are confirmed by electron backscatter diffraction measurements.

Effect of rolling temperature on microstructure and mechanical properties of 6063 Al alloy

Abstract: Aluminium alloy (6063) was severely rolled upto 92% thickness reduction at liquid nitrogen temperature and room temperature to study the effect of rolling temperature on its mechanical properties and microstructural characteristics by using tensile tests and SEM/electron back scattered diffraction (EBSD), transmission electron microscope (TEM), DSC, X-ray diffraction (XRD) as compared to room temperature rolled (RTR) material with the same deformation strain. An improved strength (257 MPa) of cryorolled 6063 Al alloy was observed as compared to the room temperature rolled alloy (232 MPa). The improved strength of cryorolled alloy is due to the accumulation of higher dislocation density than the room temperature rolled material. The tensile properties of cryorolled alloy and the alloy subjected to different annealing treatments were measured. The cryorolled alloy subjected to annealing treatment at 300 °C for 5 min exhibits an ultrafine-grained (UFG) microstructure with improved tensile strength and ductility.