Papers by Keyword: Electron Backscatter Diffraction (EBSD)

Abstract: Sample preparation of metastable austenitic-ferritic steels can have a significant effect on the apparent microstructure due to the transformation of austenite to martensite (γ - α'). As a result, these steels often have a complex microstructure with ferrite and martensite, which have relatively similar crystal structures, making it very difficult to analyse. However, the quantitative analysis of such microstructures and the effect of the sample preparation are very important for the further study of the steel. In this research, the effect of sample preparation in metastable austenitic-ferritic stainless steel was studied by using three different sample preparation methods. In addition to conventional mechanical etching with colloidical silica and electropolishing, focused ion beam (FIB) milling was used to create an optimal sample surface to be further analysed with electron backscatter diffraction (EBSD). Micrographs were obtained from each sample before and after sample preparation using field emission scanning electron microscopy (FESEM) and laser scanning confocal microscopy (LSCM), and the microstructure was analysed using EBSD. The surface flatness required for good EBSD analysis was significantly better using FIB milling than mechanical polishing, while electropolishing results in the greatest topography and an arched sample surface. The amount of martensite was found to be dependent on the sample preparation: least martensite was formed during electropolishing, while surprisingly mechanical polishing and FIB milling resulted in equal amounts of martensite.

Abstract: An ECAP (equal channel angular pressing) processed UFG Al-5Cu alloy was characterized by electron backscatter diffraction (EBSD). It is revealed that a bimodal grain structure, i.e. ultrafine grains accompanied by micron-sized grains was developed after 4 passes. A high strength (~501 MPa) and a relatively large elongation to failure (~28%) with ~5% uniform elongation were achieved simultaneously after 4 passes of ECAP. The high strength is due to a combination of strengthening by solute, high density of dislocations and ultrafine grains. The enhancement of uniform elongation is primarily due to the enhanced work hardening resulted from the solute Cu content and the bimodal grain structure. The large post-uniform elongation is attributed to the high strain rate sensitivity of the UFG Al-5Cu alloy. More importantly, the present work revealed that during ECAP high solid solution content of Cu and coarse secondary phase particles can introduce inhomogeneous deformation resulting in a desirable bimodal grain structure, which can be utilized as a strategy to gain both high strength and relatively good ductility.

Abstract: The duplex stainless steels (UNS S32304) after solid solution annealing at two different temperature (1323K and 1573K) were subjected to the same cold rolling with ε =3 and subsequent annealing for 230 min at 1323 K . The corresponding interface character distribution (ICD) were determined by electron backscatter diffraction (EBSD). The results show that a larger population of phase boundaries (PB) having K-S orientation relationship (OR) between the neighboring δ and γ grains was introduced and therefore higher intergrannular corrosion resistance (ICR) were resulted in the specimen initially solid-solution annealed at 1573K.

Abstract: Typical microstructures of dual-phase (DP) steels consist of hard martensite particles dispersed within a ductile ferritic matrix. These microstructures possess a complex network of grain and interphase boundaries, which, together with the mechanical contrast of their phase composition, control micro-damage initiation mechanisms, induced by deformation. Accordingly, in this study we analyze the influence of individual microstructural features and interfaces on damage nucleation and progression in DP steels with respect to applied tensile strain. Prominent micro-damage mechanisms include cracking of martensite and damage initiation at interphase boundaries. Influence of martensite morphology is discussed based on a statistical analysis of the damage features as observed by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) maps. Prior austenite grain boundaries (PAGbs) in martensite show a brittle behavior and are highly susceptible to crack propagation.

Abstract: Negligible factors in bulk materials, such as grain-size effects, have proven inappropriate to be neglected for micro-forming processes. Studies had shown that material behavior varies greatly with the increasing of the scale in the micro-forming world. Therefore, in every micro-forming-related process, especially in micro-stamping, studies and analyses of each material used for the process have to be considered as indispensable in order to be able to understand their behavior and to be able to correlate their behavior with the process. Uniaxial tensile-testing experiments have been carried out to determine the strip’s properties, behavior and its correlation with the feeding process in micro-stamping/micro-sheet-forming application. Based on the results of the uniaxial tensile-test experiments conducted, the flow stress was found to decrease with the decrease of the strip thickness and vice versa, due to the size/scale effect. A surface model was used to explain the findings.

Abstract: Martensite (α’) formed in Fe-Ni alloy is mainly separated into four morphologies (thin plate, lenticular, butterfly and lath). The morphology of the α’ depends on its formation temperature because accommodation process for transformation strain in the α’ is changed with temperature. Also, crystal orientation relationship between α’ and austenite (γ) is depended on the morphology of the α’. In this study, temperature dependence of the microstructural evolution of the butterfly-type α’ in Fe-30mass%Ni alloy by tensile deformation is investigated. When the tensile test is performed at room temperature (RT), morphology of the butterfly-type gets close to lath-type α’. On the other hand, the morphology of the butterfly-type α’ is kept under the tensile test at lower temperature. These microstructural changes of the butterfly-type α’ depending on the tensile temperature are discussed in term of the change of the accommodation process for the transformation strain.

Abstract: One kind of high strength interstitial free steel sheets are annealed in a salt bath at 810 C for different times (1-30 s). The microstructure and recrystallized texture evolution during annealing are investigated using the optical microscopy and the Electron Back Scattered Diffraction technique. The results show that beginning and finishing of recrystallization are observed in the sample annealed at 810 C for 8 and for 20 s, respectively. The recrystallized grains nucleate in the order of ‹111›//ND, ‹110›//ND and ‹100›//ND. Recrystallized grains with ‹111›//ND orientation nucleate firstly in the ‹111›//ND deformed grains as well as at their boundaries and grow up by consuming the ‹111›//ND deformed grains at the initial stage of recrystallization. The ‹111›//ND recrystallized grains grow up by depleting the remained formed ‹100›//ND grains after the full consumption of the ‹111›//ND deformed grains. The overall recrystallization texture is mainly the ‹111›//ND component in the steel.

Abstract: To improve our understanding of the mechanisms of gold deposition, a comparison was made of the microstructures of a natural gold sample with a synthetic gold foil of similar alloy composition (approximately Au 90%, Ag 10%). The aim was to identify any similarities between the samples that could help increase our knowledge of how the natural gold microstructures formed and were modified post-mineralisation. The samples were analysed using electron backscatter diffraction to map their microstructure, with the synthetic gold foil then heated to and mapped at 400°C, 500°C, 600°C and 700°C. Both the natural and synthetic sample exhibited a dominance of ∑3 twin boundaries, but these were much less abundant in the synthetic sample prior to heating. The natural sample is dominated by coarse grains exhibiting lattice distortion and low angle grain boundaries, which more closely resemble the synthetic gold foil microstructure after recrystallisation has taken place, than the initial microstructure, implying that the grains have had time to grow. Performing experiments such as these allows direct comparison of gold microstructures where the formation conditions are known and the controlling mechanisms can be determined. This will improve our understanding of the important mechanisms behind gold deposition.

Abstract: Low cycle fatigue failure tests of the powder metallurgical nickel based superalloy FGH96 at 550°C and 720°C were carried out under total strain-controlled mode (R=-1). The fatigue failure behaviors were investigated by analyzing cyclic stress response and observing microstructure after fatigue through scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that FGH96 superalloy exhibits cyclic stability at 550°C, and cyclic softening afterwards at 720°C with Δε/2=0.4% , and of cyclic softening at 720°C with Δε/2=0.7%. After high temperature low cycle fatigue, the γ′ precipitates are cuboidal for all samples. No coarsening of γ′ precipitates was detected at 550°C and at 720°C with Δε/2=0.4%, but small γ′ precipitates get together to be larger precipitates at 720°C with Δε/2=0.7%. EBSD shows that the continuous small angle grain boundaries are produced during the fatigue. The small angle grain boundaries have a significant increase at 720°C, especially that case at Δε/2=0.7%.

Abstract: This overview highlights very recent application of electron backscatter diffraction (EBSD) to shape memory alloys, as main investigation technique but also as ancillary technique for other characterization methods. Over the last two decades EBSD in the scanning electron microscope has become a powerful tool for the characterization of many materials and transformation. In the mean time, shape memory alloys (SMA) are continuously studied: from a theoretical point of view, in order to clarify unsolved fundamentals of their phase transformations and characterize or develop new SMA systems, and from an engineering point of view, to solve design and processing problems related to the continuously growing examples of applications. Application of EBSD to SMA, even if hindered by limitations generally found also in other metallic system when phase transformation and martensitic phases are involved, provided useful information for both research areas.