Emerging order in dislocation structures during metal loading
W. Pantleon 1, B. Jakobsen 1, H.F. Poulsen 1, J. Almer 2, U. Lienert 2
1 Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risoe National Laboratory, Roskilde, Denmark
2 XOR, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, USA
The wide spread use of metals is based on their superior mechanical properties. Metals cannot only endure a certain elastic deformation, but are also able to yield plastically. Plastic deformation occurs by motion of line defects (called dislocations) through the crystalline lattice. In the course of plastic deformation more and more dislocations are accumulated in the crystal and an enormous line length of dislocations is stored, typically 10 million km in a centimeter cube.
Dislocations have been observed directly by transmission electron microscopy (TEM): In cell-forming metals already after a small amount of deformation, the initially disordered structure with randomly arranged dislocations is replaced by an ordered structure with dislocation boundaries separating dislocation free volumes. Rather different dislocation structures emerge depending on deformation mode and orientation of the crystalline lattice with respect to the main deformation axes, but in general, hierarchical arrangements of dislocation boundaries of different types evolve.
The emerging order manifested in dislocation boundaries has been a long lasting challenge. For understanding of work-hardening and modeling plastic deformation, it is a central issue, how dislocations organize into boundaries and whether the observed dislocation boundaries are just “graveyards” where dislocations gather after deformation or the dislocation structures are indeed present during ongoing deformation? Despite abundant post mortem observations of ordered dislocation arrangements, the dynamic existence of dislocation boundaries in-situ during plastic deformation has not been proven yet.
In the present work, this problem is approached by a newly developed experimental technique allowing the observation of X-ray diffraction peaks of single grains while simultaneously deforming. The suggested approach is advantageous over previous attempts in a twofold manner because the investigated peaks are stemming from single grains and averaging over different orientations is avoided and the analysis will be based on models for dislocation boundaries reflecting the information gathered by electron microscopy.
For proofing the concept, a dedicated set-up has been established at the 1-ID-XOR beam line of the Advanced Photon Source (APS) at the Argonne National Laboratory. It extends the 3‑Dimensional X-ray Diffraction (3DXRD) method developed by the Risoe group to generation of reciprocal space maps of a set of reflections all arising from the same embedded grain. Each of these 3D reciprocal space maps comprises information about the intensity distribution as function of the Bragg angle 2 q , i.e. the radial peak profile, as well as a complete description of the mosaic spread of the peak, i.e. the azimuthal spreading.
The feasibility of the 3DXRD-based peak shape analysis has been demonstrated on a polycrystalline Al specimen subjected to tensile strain. A 1 mm thick tensile specimen was mounted in a displacement-controlled stress-rig, which in turn was installed on a 3-circle goniometer. The experiment was performed with a collimated beam of 30 keV X-rays of narrow bandwidth ( D E/E < 0.5 × 10 -4). For characterisation of individual peak shapes with respect to the reciprocal space parameters, a CCD was positioned at a large distance of about 3 m from the specimen. The instrumental resolution in radial (2 q ) direction was estimated to be D d/d » 10 -4. A second CCD positioned closer records all low-index diffraction spots arising from all illuminated grains allowing the multi-grain indexing program GRAINDEX to sort the diffraction spots according to which grain they originate from and to determine the orientation of these grains. 20 reflections of a grain with its [4 11 3] direction along the tensile axis were characterised in-situ under tensile loading up to a strain of 4.5%. This is illustrated by three 2D projections of the 3D peak shape of the ` 113 reflection at a strain of 2.5%. The azimuthal broadening in j (rotation around tensile axis) and h (along the diffraction ring) is rather smooth, with only a slight distortion due to a weak secondary maximum.
Peak shapes, in particular their asymmetry and spikiness, are sensitive to changes in the dislocation arrangements. The in-situ determination of peak shapes from x-ray diffraction during loading and unloading will result in unique information on the formation of dislocation structures and lead to novel insights in the governing processes. With the proposed technique there will be for the first time a possibility for testing the existence of dislocation boundaries during deformation.
Acknowledgements
WP, BJ and HFP acknowledge the Danish National Research Foundation for supporting the Center of Fundamental Research: Metal Structures in Four Dimensions, within which part of this work was performed, as well as the Danish Research Council SNF for financial support via Dansync. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38.
Reference
W. Pantleon, H.F. Poulsen, J. Almer, U. Lienert. In situ X-ray peak shape analysis of embedded individual grains during plastic deformation of metals. Mater. Sci. Eng. A (2004) in press.
