News and highlights
Advanced texture measurements on WOMBAT- A round robin with iMateria / J-PARC and JAEA
Crystallographic texture is preferred orientation of the crystallites due to anisotropic properties of how solid material is processed. During plastic deformation slip systems are activated, leading to grain rotation into preferred orientation. These may compete with other processes, such as twinning and recrystallization, forming peculiar fingerprints of texture.
As the mechanical properties of such material are highly anisotropic, the knowledge, modeling, and engineering of texture is of utmost interest for industrial and practical applications. Furthermore, the thermo-mechanic history of an artifact can be determined through the help of texture analysis, which also applies to rocks in geological research and ice for the understanding of glaciers and climate.
Because neutrons have a high penetration depth, textures from the bulk of a sample can be obtained. The capability of texture measurements at the OPAL facility has been initiated within the ThermoMec.Pro project. We have now set up the WOMBAT high-intensity diffractometer for the advanced measurement of texture.
Smart orientation sampling in unconventional steps is employed making full use of the large, two-dimensional detector. Scripting within the dataRring package exists to extract pole figures for the mtex texture package, while input scripts for the MAUD Rietveld program can be made, taking track of all orientation angles.
The Materials Oscilloscope: Thermo-mechanical processing in a synchrotron beam
Figure: Authentic picture composition of a Materials Oscilloscope - load-frame with glowing specimen (front) and diffractogram (behind).
We have developed a new technique and named it the Materials Oscilloscope, termed for a time-resolved synchrotron high-energy X-ray technique to study rapid phase composition and microstructural related changes in a polycrystalline sample. This device has been developed for in-situ studies of specimens undergoing physical thermo-mechanical simulation.
Two-dimensional diffraction images of a fine synchrotron beam interacting with the specimen are recorded in time frames, such that reflections stemming from individual crystallites of the polycrystalline material can be distinguished. Data treatment is undertaken in a way that diffraction rings are straightened and presented line by line, streaked in time.
The traces, effectively timelines in azimuthal-angle/time plots, give insight in the processes happening in the material, while undergoing plastic deformation, or heating, or both. These timelines allow to distinguish grain growth or refinement, subgrain formation, slip deformation systems, crystallographic twinning, dynamic recovery, dynamic recrystallization, simultaneously in multiple phases.
Pioneering experiments on a variety of materials have been undertaken, such as room-temperature compression of copper, dynamic recovery and recrystallization of zirconium alloy, deformation of twinning-induced plasticity steel, cold and hot deformation of magnesium, and hot deformation of a two-phase titanium aluminide based intermetallics.
Figure: Liss-plot of magnesium undergoing two compression steps at higher and lower temperature. Horizontal axis - azimuthal angle (with longitudinal L and transverse T directions); vertical axis - time.
Directional atomic rearrangements during transformations between the alpha and gamma phases in titanium aluminides
Phase diagrams and microstructures of titanium aluminides are rather complex and, so far, little data were observed in-situ at elevated temperatures. We report on two-dimensional high energy X-ray diffraction and complementary laser scanning confocal microscopy to characterize the appearing phases and to follow the phase evolution in-situ and in real time (figure 1).
As an example, the microstructure evolution of a quenched γ-TiAl alloy, consisting of α2-Ti3Al grains at room temperature, has been followed in both reciprocal and direct space as a function of temperature up to 1400 °C. At 700 - 800 °C extremely fine γ-laths are formed in α2-grains occurring through an oriented rearrangement of atoms.
Streaks linking reflections of both phases testify to the coherent lattice and orientation gradients in the transforming crystallite. At temperatures around the eutectoid temperature recrystallization effects and the γ → α phase transition take place leading to grain refinement.
Figure 1: Making movies in-situ at glowing temperatures up to 1300°C through a microscope (false colour image) and from two-dimensional X-ray diffraction (movie frames) reveal the lattice correlations, gradients and intermediate structures during phase transformations in titanium aluminide. A quenched, α2-rich γ-based TiAl first approaches its equilibrium by α2 → γ on a heating ramp, disorders α2 → α and then evolves reversely γ → α, which are morphologically different processes. Cover story of Adv. Eng. Mater. 4/2008.
Synchrotron 3D Computed Tomography on Cast Magnesium
The microscopic morphology of magnesium alloy has been investigated by computed micro-tomography and compared to light microscopic studies. Precipitates of denser material are found but the information of microscopic studies alone are insufficient. In contrast, three-dimensional computed tomography tomography reveals the full topology in 3D which may be relevant for the mechanical properties of the material. See figure 2.
In-situ Quantitative Phase Analysis of Titanium-Aluminides
Synchrotron high-energy X-ray diffraction has been used to record 2D powder diffraction patterns in the bulk of Titanium Aluminides (figure 3). A quantitative Rietveld analysis shows the evolution in phase fractions, lattice parameters and order / disorder as a function of temperature and gives detailed insight into the kinetics and the phase transitions in this system (figure 4).
Collaborators: LaReine A. Yeoh, Klaus-Dieter Liss, Arno Bartels, Harald Chladil, Maxim Avdeev, Helmut Clemens, Rainer Gerling, Thomas Buslaps
In-situ studies of light metals at the ESRF
The recently established collaboration, between ANSTO and the University of Melbourne (funded for the year 2006/2007), brings together the know-how of mechanical engineering and analysis with diffraction methods. We successfully collected about 100 GB of data on four titanium aluminium samples of different composition and microstructures measuring in-situ on high-temperature ramps using high-energy X-Ray diffraction at ID15B at the European Synchrotron Radiation Facility (ESRF) .
Further room-temperature data of different thermo-mechanically processed samples, such as ball milling or ECAP, have been collected. At present, the data are being analysed.
Collaborators: LaReine A. Yeoh, Klaus-Dieter Liss, Kenong Xia, Wei Xu, Thomas Buslaps