Elements of a Method for Multiscale Characterization of Recrystallization in Deformed Metals

Sonja Rosenlund Ahl

Research output: Book/ReportPh.D. thesisResearch

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Recrystallization of deformed metals is a complex multiscale process which involves the migration of the boundary of a recrystallizing grain into the surrounding deformed structure. The lattice of a recrystallizing grain is almost defect-free whereas the deformed structure consists of subgrains separated by low angle boundaries made up of dislocations. The migrating grain boundary is a high angle boundary and its shape is related to the characteristics of the neighbouring subgrains.
To study the structures and dynamics involved in recrystallization an appropriate experimental technique is required. Such a technique should be able to produce three-dimensional grain maps of the whole mm-sized sample and at the same time be able to zoom in on individual grains, grain boundary segments or subgrains. The technique should be fast enough to study dynamics and have high enough resolution to resolve submicron-sized structures, millidegree
angular differences and strain on the order of 10-4. Penetrative power to reach embedded bulk structures is necessary and preferably the technique should allow for in situ annealing.
X-ray imaging techniques such as three dimensional x-ray diffraction (3DXRD) and diffraction contrast tomography (DCT), that have been developed over recent years, utilize hard x-rays from a synchrotron source to produce grain maps. Zooms on individual grains are facilitated by the implementation of topo-tomography. With the introduction of a new technique, dark field x-ray microscopy (DFXRM), a spatial resolution better than 100 nm is reached together with millidegree angular resolution.
In this thesis a first application of DFXRM to recrystallization is presented together with a framework for analysing DFXRM data in terms of distortion fields and dislocation densities. Three-dimensional maps of the internal structure of recrystallizing grains in cold-rolled aluminium AA1050 were obtained by means of DFXRM in a few hours. Mosaicity and strain maps revealed that the investigated recrystallizing grains possessed a well-ordered internal network of ultra low angle boundaries. The misorientation across these boundaries was less than 0.1° and some boundaries were sharp to within the instrumental resolution whereas others were several μm wide. A clear correlation between the intragranular network and the shape of the grain boundary was observed, with retrusions at points where the ultra low angle boundaries met the external grain boundary. Furthermore, there were indications that the ultra low angle boundary network was related to the subgrain structure that it grew from, and during annealing the intragranular structure became more well-ordered.
A new method for monitoring the structural evolution of several thousand individual subgrains during recovery is also demonstrated. At constant heat rate for temperatures well below the onset of recrystallization the size, orientation and internal strain of individual subgrains was observed to change while the overall size distribution remained unaltered. Each subgrain exhibited its own growth kinetics and in contradiction with prevalent models for recovery this study found no correlation between subgrain size and growth rate. A simple model for thermally activated recovery was applied to extract individual critical temperatures and a region of increased dynamics was observed around 160°C. Furthermore, promising first results from a preliminary test of a triangulation method for mapping subgrain structures is presented.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages142
Publication statusPublished - 2018


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