Advanced Structural Materials II

Abstract: Materials exhibit microstructures and textures that influence their use and properties. Xray and neutron diffraction allow characterization of the bulk texture, whereas Electron Backscattered Diffraction (EBSD) permits determination of the local texture. In many cases Transmission Electron Microscopy (TEM) remains necessary to characterize the substructure and the local texture for highly deformed materials. Depending on the scale considered, all these complementary techniques permit the coupling of texture and microstructure so that it becomes possible to control microstructure and its evolution during a processing route. Some examples in titanium aluminides, (α + β) titanium alloys and an Fe-Ni alloy will illustrate this challenge.

Abstract: Automated Electron Backscatter Diffraction (EBSD) or Orientation Imaging Microscopy (OIM) has proven to be a viable technique for investigating microtexture in polycrystalline materials. It is particularly useful for investigating orientation relationships between phases in multiphase materials. However, when phases do not significantly vary in crystallographic structure, OIM is limited in its capability to reliably differentiate between phases. Through simultaneous collection of EBSD data and chemical data via X-Ray Energy Dispersive Spectroscopy (EDS) it is possible to dramatically improve upon the phase differentiation capabilities of either technique individually. This presentation will introduce a methodology for combining the two techniques as well as show a few example applications.

Abstract: The effect of heating rate and annealing time on the microstructure and texture of a commercial 0.04 %C steel, cold rolled up to 80 %, is studied. Samples have been isothermally annealed at various heating rates (12 °C/h, 20 °C/h, 40 °C/h and 650 °C/min) and then soaked at 700 °C for 15 hours. The microstructural evolution of the samples during the heating process and hold period has been followed by optical microscopy, scanning and transmission electron microscopy. The electron back-scattered diffraction technique is used to reveal the texture of the samples. Tensile tests and hardness measurements are correlated with the microstructural features. Results show that (a) recrystallization occurs between 600 and 650 °C; (b) a “pancake” structure develops during recrystallization at low heating rates without appreciable grain growth; (c) samples heated at 650 °C/min exhibit an equiaxed grain structure and significant grain growth; (d) only at low heating rates the material develops a strong {111} recrystallization texture, in ccordance with the high plastic anisotropy found by mechanical testing.

Abstract: The present work is an attempt to understand the recrystallisation mechanisms in Fe-3% Si alloys used in transformer cores. After secondary recrystallisation silicon steels exhibit a Goss texture with a more or less important spread depending on the details of the processing route, namely, Conventional Grain Orientation CGO or High Permeability Hi-B. The mechanisms of Goss grain formation during hot rolling and primary recrystallisation, as well as those controlling the first steps of abnormal growth, are not yet well understood. The present work mainly deals with texture characterization of the hot rolled state. Surface, quarter and half thickness samples are prepared from hot-rolled sheet. Global and local textures are characterized by neutron diffraction and electron backscattered diffraction, respectively. The Orientation Distribution Functions and the volume fraction of the different texture components are calculated. The components from global texture measurements are (001)[1-10], (112)[1-10] (α fiber ), (011)[100] (Goss) and (111)[1-21] (111) [1-10](γ fiber). EBSD measurements have shown large variations of texture from the surface to the half thickness of the sheet. These local measurements are related to the global results by rotation about the transverse direction. Moreover, the grain size appears to be inhomogeneous.

Abstract: The microstructural and mechanical properties of an ultra-clean low carbon steel sheet with 0.035% Cr have been evaluated. The isothermal recrystallization kinetics at 800 oC is analyzed by using the Kolomogorov-Johnson-Mehl-Avrami (KJMA) equation. The obtained results indicate that Cr retards the recrystallization kinetics owing to the presence of fine particles of chromium carbides. The annealing process produces a fully recrystallized microstructure. The main texture components in the specimens are: {111}<112>, {112}<110> and {111}<110>. These are related with the Lankford value and the elongation. The obtained results suggest that the formability of this steel is excellent.

Abstract: This research work analyses the effect of cold working level produced by drawing, on the work hardening exponent of 0.18 and 0.43 % C ferrite-pearlite steels. Such analysis is carried out by means of true stress-true strain curves derived from uniaxial tension tests. The work hardening exponent behaviour was determined by using Hollomon and differential Crussard-Jaoul models. It is found that the work hardening exponent decreases as a function of the applied cold-drawing level, and negative values were obtained when differential analysis is used. The results indicate that the Hollomon analysis shows some deviations from the experimentally determined true stress - true strain curves while the differential Crussard-Jaoul analysis fits better when two work hardening exponents are considered. This analysis establishes two exponents for different stages of plastic deformation which are determined by the sharp slope change in the plot of ln (d σ/d ε) - ln ε.

Abstract: The fracture surfaces of SAE-1018 steel tension and impact test specimens with different grain sizes are analyzed in order to explore the possible relations between the microstructure and the self-affine fracture surface parameters such as the roughness exponent, ζ, and the correlation length, ξ. The topography of the fracture surfaces was observed and quantified by means of scanning electron microscopy, atomic force microscopy, optical microscopy and optical digitizer. It is confirmed that the fracture surfaces exhibit a self-affine behavior extending over six decades of length scale, from nanometers up to a few millimeters. The roughness exponent exhibits a value of ζ∼0.82 for all the cases regardless of the microstructural condition.

Abstract: Quasicrystals can deform Plastically at high temperatures by the movement of dislocations like in crystals. Little is known, however, on the mechanisms that control dislocation movements in these structures. In a first part, the crystallography of quasicrystals is introduced as the projection, in the physical space, of a periodic structure in a hyperspace with more than three dimensions (6 in the case of icosahedral structures). The main properties of dislocations are defined in the physical space and in the six-dimensional space. New results are then presented, showing that dislocation movements may have a large component of climb, in contrast with all existing models that are based on glide processes only. Dislocation movements in two-fold and five-fold planes are compared and discussed in connection with macroscopic mechanical properties.

Abstract: An in situ study of the plastic deformation of <001> single crystals of an industrial superalloy has been performed at 850 °C in a TEM to observe directly the micromechanisms which control the deformation under the actual temperature and stress conditions experienced by this material in aeroengines. A comparison between the creation and propagation modes of moving dislocations in the standard microstructure after annealing and the rafted microstructure after 20 h of creep at 1050°C evidence the important role of the width of the γ channels as well as the strength of the γ/γ’ interface in controlling the shearing events of γ’ channels.

Abstract: The mechanical behavior of the sintered nanostructured intermetallic alloys, Al67Ti25Mn8 and Al67Ti25Fe8 has been investigated by means of compression tests as a function of temperature. These intermetallic materials are produced by mechanical alloying and spark plasma sintering. The sintered alloys have been characterized by X-Ray Diffraction and Transmission Electron Microscopy. Their nanostructure consists of a single-phase with an L12 (cubic) structure and an average grain size in the nanoscale (lower than 30 nm). These nanostructured intermetallics show considerably high ductility in compression at high temperature but are brittle at temperatures below 400 °C. In such cases the compressive fracture strength can reach values as high as 1.8 GPa (Al67Ti25Fe8). At 500 °C, some ductility is found together with a relatively high flow stress (around 1 GPa), the corresponding deformation curve shows strain hardening and in some cases stress serrations (strain aging). At 600 °C, a low flow stress is measured (~300 MPa) with high ductility. At 700 and 800 °C, a quasi-superplastic behavior is found with a total deformation of around 45 and very low flow stresses. No evidence of dislocation motion is found at temperatures above 700 °C suggesting a deformation mechanism based on grain boundary sliding.