Recrystallisation is a thermally activated process during which the energy stored in a plastically deformed metal or alloy is reduced [1]. Recrystallisation is generally separated into two distinct processes: nucleation and growth, which may occur consecutively or simultaneously. During nucleation, almost defect-free nuclei form in the deformed microstructure. During growth, these nuclei grow by grain boundary migration until they impinge upon each other. Following impingement the grains may continue to grow at much reduced velocities by curvature driven growth (secondary recrystallisation).

A plethora of models exists to simulate recrystallisation ranging from purely geometrical numerical models to fundamental analytical models. A common characteristic of the models is that they focus is on the average behaviour of the nuclei. Hence, one or more of the following assumptions are generally applied: randomly distributed nucleation sites, instantaneous or constant nucleation rates, and a growth rate identical for all the nuclei. The main reason for these assumptions is the lack of experimental data for the behaviour of the individual nuclei.

Here we present the first result of an X-ray diffraction technique for in situ studies of the nucleation and growth of individual grains during recrystallisation of metals (Figure 94). The experiment was performed using the 3DXRD microscope at beamline ID11 [2,3]. Using high energy X-rays (50-100 keV) from a synchrotron source ensures the observation of true bulk behaviour. A suitable intrinsic gauge volume is defined by focusing the incoming beam and by performing ray tracing on the diffracted beams, using a two-dimensional detector. By this procedure, the orientation and volume of several hundred nuclei can be determined simultaneously during annealing. The first results relate to 90% cold-rolled 99.5% pure aluminium of type AA1050. As seen in Figure 95 the threshold for observation of the nuclei is in this case a radius of 1 micrometre. In contrast to model assumptions the variations in nucleation time and growth velocity are substantial. Note also the slow increase in integrated intensity indicative of an incubation process taking place prior to the onset of the grain growth (for example, grain #4 and #6 in Figure 95a).

In short, a general-purpose technique for bulk studies of recrystallisation is presented. It allows in situ studies of the nucleation and growth of individual nuclei. The setup used here for aluminium applies to all other metals and alloys, as penetration depths of 1 mm are universally obtainable with hard X-rays. The specific results obtained for AA1050 type Al include qualitative new information. Contrary to what is generally believed, incubation periods are observed and the curvature driven growth can be sufficiently fast for small grains to disappear abruptly (for example, grain #4 in Figure 95b).

[1] Proc. of ReX'99: The Fourth International Conference on Recrystallisation and Related Phenomena, Ed: T. Sakai, H.G. Suzuki.
[2] H.F. Poulsen, S. Garbe, T. Lorentzen, D. Juul Jensen, F.W. Poulsen, N.H. Andersen, T. Frello, R. Feidenhans'l, H. Graafsma. J. Synchrotron Rad., 4, 147-154 (1997).
[3] U.Lienert, H.F. Poulsen, Å. Kvick. Proceedings of the 40th conference of the American Institute of Aeronautics and Astronautics on Structures, Structural Dynamics and Materials, St. Louis (USA), April 1999.

Principal Publication and Authors
E.M. Lauridsen (a), D.J. Jensen (a), U.Lienert (b), H.F. Poulsen (a), Scripta Mater., accepted for publication.

(a) Materials Research Department, Risø National Laboratory, Roskilde (Denmark)
(b) ESRF