Papers by Keyword: Electron Backscatter Diffraction (EBSD)

Abstract: Dislocations play a critical role in metal forming processes, and accurate values of dislocation density are important in modelling these processes. However, direct determination of the dislocation density is challenging. In this study, electron backscatter diffraction is used to estimate the evolution of geometrically necessary dislocation density as a function of plastic strain, strain rate and temperature in hot compression of AA7050 alloy. The geometrically necessary dislocation density was found to increase at a higher strain rate and lower temperature; the higher dislocation density in these samples promoted continuous dynamic recrystallisation leading to grain refinement. At lower strain rates and higher temperatures, the dislocation densities were lower and dislocations formed into walls, channels and cells. These observations agree with accepted theories of dislocation evolution and demonstrate the capability of electron backscatter diffraction to provide representative dislocation density values as well as comprehensive information linking plastic flow with microstructural evolution.Keywords: electron backscatter diffraction (EBSD), geometrically necessary dislocations (GNDs), hot deformation, AA7050

Abstract: Equal-channel angular pressing (ECAP) is often used as effective tool for grain refinement for many different metallic materials. It is well known that grain size is an important microstructural feature influencing superplastic properties of fcc materials like aluminum alloys. The magnitude of introduced shear strain depends on geometrical parameters of the ECAP channel. In this contribution, the impact of different geometrical parameters of the ECAP channel on the resulting magnitude of introduced shear strain is analyzed. ECAP on AA5083 aluminum sheets with the dimensions of 200x200x1.8 mm3 is performed. Microhardness measurements reveal a considerable increase of hardness after ECAP and microstructural investigations by electron backscatter diffraction (EBSD) show the beginning formation of a deformation-induced substructure which is known to be a preliminary stage of the grain refinement process. It is assumed that this fine-grained microstructure results in an enhanced superplastic forming capability. Furthermore, a numerical model of the process based on the experimental results is established. The bending of the ECAP processed sheet metal as well as its microhardness are used for the validation of the model. The friction coefficient between the channel and the aluminum sheet significantly influences the results of the simulation. With the applied model different channel angles and inner corner radii are varied in order to determine a maximum magnitude of deformation resulting in sufficient grain refinement of the investigated material. With the help of the results gained in this study, suitable ECAP parameters for sheet metals can be derived that enable creating ultrafine-grained materials for superplastic forming operations.

Abstract: Evaluation of surface damage layers formed by mechanical grinding processes is indispensable in epi-ready SiC wafer preparation. As well as microstructure, the analysis of local strain distribution in the damage layers gives a clue on control of the wafer quality. Advanced electron backscatter diffraction (EBSD) technique is applied to evaluate the strain distribution of the damage layers. It is revealed that the elastic strain distribution can be classified into a hierarchy of three regions with respect to depth from the surface. Combining EBSD analysis with TEM observation, large compressive elastic strain and misorientation are introduced in the highly-defective region underneath the ground wafer surface. In addition, the gradient distribution of the strain is observed clearly below the highly-defective region. The knowledge of correlating between strain distribution and microstructure is promising to control the damage layer for the wafer preparation.

Abstract: The present study aims to study the texture heterogeneity of an AA5754/AA6061 composite processed by cross accumulative roll-bonding at room temperature. Both Al alloys were first roll-bonded with a 50% reduction and the product was cut in half. Then, both parts were stacked and finally another roll-bonding was carried out but the initial rolling direction was turned by 90°. As a result, a strong gradient appears in the thickness of the composite because of shearing but also due to the alloys composition dissimilarity. As a consequence, it appears each of the four resulted layers has its own texture. In the bulk, AA5754 layer promotes the ND-rotated Brass {011}<755> and S {123}<634> components while AA6061 develops the ND-rotated Brass and Dillamore {4 4 11}<11 11 8> components. On the surface, AA5754 favors the Dillamore component whereas AA6061 shows the rotated-Cube {001}<110> component.

Abstract: In this paper, tensile tests were performed at elevated temperature and strain rate in order to investigate the plastic flow behavior, anisotropic characteristics and microstructural evolution of Ti6Al4V sheets under testing conditions similar to the ones experienced during hot stamping operations. It is shown that the Ti6Al4V anisotropic characteristics under the investigated forming conditions, different from the ones of the superplastic regime, are influenced by the variation of the material texture as a function of the testing temperature. The Ti6Al4V flow stress behavior was analyzed as a function of the deformation temperature and strain rate. Afterwards, the Arrhenius constitutive model was proposed to predict the flow behavior of Ti6Al4V sheets at elevated temperature and strain rate. The statistical analysis of its predictive capabilities suggests that the Arrhenius model guarantees a good accuracy in reproducing the flow behavior of Ti6Al4V sheets.

Abstract: Third-Generation advanced high strength steels are being developed with the goal of reducing the body-in-white weight while simultaneously increasing passenger safety. This requires not only the expected increase in strength and elongation, but also improved local formability. Optimizing elongation and formability were often contradictory goals in dual-phase steel developments. Recent results have shown that so-called "quench and partitioning" (Q&P) concepts can satisfy both requirements [1]. Many Q&P-concepts have been studied at thyssenkrupp Steel Europe. Thorough investigation of the microstructure has revealed relationships between features such as the amount, morphology and chemical stability of the retained austenite and the obtained mechanical properties. An evaluation of the lattice strain by means of electron-back-scattering-diffraction has also yielded a correlation to the obtained formability. The aim of this work is to present the interconnection between these microstructural features and propose hypotheses for the explanation of how these features influence the macroscopically observed properties.

Abstract: The ferritic stainless steel type 430 stabilized with Nb, with and without annealing after hot rolling, was cold rolled and subjected to isothermal annealing at temperatures 650, 700 and 750°C for times ranging between 10 to 86400 s. The recrystallization kinetics was evaluated by JMAK model through microhardness measurements and KAM and GOS parameters. The Avrami exponent data indicate the occurrence of an unidimensional grain growth due only to high angle boundaries migration, with values ranging between 0.9 and 1.2. The nucleation rate and grain growth decreased continuously with time. The evolution of the texture was analyzed via EBSD analysis by ODF maps. The steel recrystallization is based on combination of ON and SG theories, due to presence of {111}<121>, {554}<225> and {111}<112> related to γ fiber. The rotated cube component, feature of the hot rolled steel, decreased with annealing time.

Abstract: Dual-phase (DP) steel sheets composed of both soft ferritic and hard martensitic phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of the steel sheet is one of the important factors that influence press formability. However, the texture of the martensite itself in DP steels has not been discussed since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the effects of intercritical and γ single-phase annealing on the texture evolution in DP steels by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD) to obtain the texture of each phase separately. The chemical composition of the steel used was 0.1%C-1.2%Si-2.3%Mn-0.1%Ti (mass%). The 1st-annealing was carried out at 948K, which is below the Ac₁ temperature, in order to finish recrystallization after hot and cold rolling so as to focus on the transformation texture evolution itself. The steels were subsequently annealed both at 1123K in the intercritical region and at 1223K in the γ single-phase region to obtain DP microstructures with approximately 40% volume fraction of martensite. The overall texture including martensite in the case of intercritical annealing was similar to the initial texture before annealing, while the texture became randomized in the case of γ single-phase annealing. Moreover, our unique EBSD analysis method clearly showed that the textures of the martensite themselves were close to those of ferrite under the two annealing conditions.

Abstract: The recrystallization behavior of hot-deformed austenite of 0.55% C low alloy steels at 900, 850 and 800°C was investigated by a conventional double-hit compression test and a new method which reconstructs the parent austenite orientation map from an EBSD (electron backscattering diffraction) orientation map of daughter lath martensite. The new method can clearly reconstruct the parent austenite structure at high temperature from the daughter lath martensite structure and we can obtain the information on crystal orientation of the work-hardened austenite. It was revealed that recrystallization of austenite at 800 °C is significantly retarded by the addition of 0.1% V. The strong texture of <110> parallel to the compression direction develops just after the hot-deformation, but this texture becomes weaker as the recrystallization progresses. By applying the reconstruction method, it becomes possible to evaluate various phenomena related to the hot-deformation of austenite

Abstract: Axial suspension plasma spraying (ASPS) is a relatively new, innovative spraying technique which has produced thermal barrier coatings (TBCs) with attractive properties such as high durability and low thermal conductivity. Using a suspension, it is possible to spray with finer powder particles resulting in coatings that have a columnar microstructure and contain a wide range of pore sizes, both nm-and μm-sized pores. To optimize the thermal properties and to maintain them during service of the components, it will be important to design TBCs with optimal porosity. Hence, an important part in the assessment of ASPS coatings is therefore the characterization of the microstructure and how it is build up, and the determination of porosity. Both aspects are addressed by performing measurement on splats and ASPS-coating using electron backscatter diffraction (EBSD) technique and by measuring porosity by Mercury Intrusion Porosimetry (MIP).