Deformation Fields in Alloys by Microtomography
X-ray absorption tomography is capable of mapping internal structure of solids consisting of components with different X-ray absorption. In medical science, for example, tomography is used to map bones and organs. The method has also been used to map the distribution of chemically distinct phases in metallic materials, providing e.g. 3D images of shape, size and spatial distribution of particles in aluminium alloys. However, the previous work has been limited to "snapshots" of the particle distribution in different samples at several stages in a thermomechanical process.
During 2002, a technique to identify and track particles in a sample at different stages in the deformation process has been developed in the Center. Using this technique, quantitative information on the trajectories of several thousand individual tungsten particles during compression of an aluminium-tungsten alloy has been extracted. The sample was compressed in steps and microtomography images were recorded after each step using hard X-ray radiation from the synchrotron at HASYLAB in Hamburg, Germany. Particle trajectories were determined by tracking particle positions in successive images. Subsequent analysis yields maps of the local plastic state inside the sample. Even in a test sample with a small grain size, inhomogeneous flow has been found.
Trajectories of 2544 particles within a 0.4 mm3 cube in the bulk of the sample. The numbers on the axes are in units of pixels of 1.5 micrometer.
This novel technique has great potential for studies of deformation fields around cracks, material flow in metal forming operations, and a wide variety of related phenomena. Since the method does not depend on diffraction, it is applicable to all types of materials containing marker particles, including glasses, polymers and biological samples.
3D Grain Maps
Non-destructive 3D spatial mapping of a grain structure is essential to in-situ studies of processes involving, e.g., migration of grain boundaries or nucleation of new grains. Such maps may be derived from intensity distributions of diffracted X-rays measured by area detectors as done by the 3DXRD microscope. The 3D maps are constructed from such distributions from different layers in the sample. Previous work has demonstrated a spatial resolution of 25 mm. During 2002, new algorithms which aim at a resolution of the order of 5 mm have been constructed and tested.
Mathematically, the problem resembles the reconstruction problems underlying medical imaging devices, such as CAT- and PET-scanners. However, it is substantially more complex. In addition to the three spatial co-ordinates, the relevant space for grain maps encompasses the three parameters giving the crystallographic orientation.
During 2002, a mathematical formalism has been derived which is capable of dealing with this complexity. A map of 4 neighbouring grains in a randomly chosen layer of an undeformed aluminium polycrystal was generated from 3DXRD data. From the amount of overlap and voids between grains we infer a spatial resolution of £ 5 mm. 3D maps are presently being generated.
Independent reconstruction of 4 neighboring grains marked by different colors. Void regions as well as regions of overlap between the grains are marked by red.
Based on these first results, we plan to establish a general-purpose reconstruction program. The implementation will be performed in collaboration with the group of applied mathematicians headed by G. Herman, CUNY, New York. This group has over the last 30 years developed the methodology behind medical scanners and produced application software.
Rotation of Individual Grains
The new 3 dimensional X-ray technique is the only existing technique with sufficient penetration depth and spatial resolution to follow structural changes in metals during deformation on the scale of individual grains. During deformation, the individual grains change their shape and must rotate, i.e. change the orientation of their crystallographic lattice, to stay in contact with the neighboring grains.
For the first time, this rotation has been monitored for about 100 grains deeply embedded in an aluminum sample subjected to tensile deformation. Initially, the grains were randomly distributed in orientation space. During deformation, the grains rotate towards the stable tensile texture components. There is a clear correlation between rotation direction and the initial orientation of the grains, indicating that interaction with their different neighbors does not dominate the rotation behavior.
Raw images from the 3DXRD microscope of diffraction spots from aluminium. The inserts show a spot from the same grain before and after 6% elongation. It is seen that the spot spreads out and moves as the lattice rotates.
From the point of fundamental research, this is very important new experimental information. Until now the experimental basis for this understanding has been limited to studies of similar but not identical samples before and after deformation which does not give any information on the rotation of individual grains. This information is essential to identify the factors controlling the rotations. The research also has technological importance because many properties, e.g. mechanical and magnetic, depend on the ensemble of crystallographic orientations present in a material.
Finer Structures by Deformation
The smaller the scale of the microstructure the stronger a metal is. This general experience lies behind extensive research to produce and optimize metals and alloys with microstructures on scale ranging from micrometers to nanometers. Such metals are difficult to produce but cold deformation is a promising route. Large plastic deformation is required since even a 100 times thickness reduction in plate rolling of aluminum, nickel and copper can only bring the scale of the microstructure down to about 100-200 nanometer. A way to obtain extreme degrees of deformation is to press metal parts together and slide them against each other. This technique has been used in a collaborative research with Dr. D. A. Hughes Sandia National Laboratories, USA. In this study copper block samples were pressed against a sliding steel platen in such a way that the copper surface is heavily deformed by ridgelike asperities on the steel, which during sliding plow the surface of the copper.
Transmission electron microscopy revealed that the scale of the resulting structure gradually increases with the distance from the surface, i.e. with decreasing degree of deformation. In the surface region structures as small as 10-20 nm were found. Surprisingly, the structure in the surface region looked like a miniature of the structure in regions far below the surface which were on a much larger scale. This shows that the pattern of deformation holds down to a very small scale. Metals with finer and finer structures may, therefore, behave according to the principles known from conventionally processed metals. The results provide guidelines for processing of strong metals to be used industrially in, for example miniature parts of computers or cars. The results also open up for new research pushing the scale of the microstructure even further downwards to explore if and where the typical behavior will break down. |