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A deeper look into recrystallisation in heavily deformed metal alloys

Dark-field X-ray microscopy at beamline ID06-HXM was utilised to investigate microscopic changes in the structure of an industrial alloy during thermal annealing. A deeper understanding of this phenomenon is relevant because the structure determines useful properties such as hardness and ductility.

The industrial processing of metals typically includes techniques such as hot or cold rolling, which puts the metal under stress, causing plastic deformation (permanent changes to the material s crystal lattice structure), followed by thermal annealing. The density and distribution of dislocations in the crystal structure, and the growth of recrystallised grains can determine the brittleness and ductility (and therefore the workability) of a metal, but can also affect properties such as electrical conductivity, magnetic response, resistance to corrosion, etc. Understanding the nucleation and growth of recrystallised grains in deformed materials is therefore of great industrial interest.

Dark-field X-ray microscopy (DFXM) is a novel X-ray imaging technique enabling the non-destructive mapping of strain and orientation in bulk materials with high spatial and angular resolution [1]. By placing an X-ray objective lens in the diffracted beam path, a diffraction volume can be imaged (Figure 108a). Furthermore, the objective filters out unwanted overlap and stray diffracted

signals, thereby focusing on a grain of interest (GOI). The technique can be coupled with coarser grain methods to obtain grain-averaged strain and texture information.

In this work, DFXM was used at beamline ID06- HXM to investigate recrystallisation of a cold-rolled, heavily deformed (85% reduction in size) ferrous alloy (Fe-3%Si-0.1%Sn) during thermal annealing. The technique revealed the 3D microstructure of the 200µm-thick sample, demonstrating, for the first time, a direct observation of the relation between the deformed matrix and recrystallised grains within high- stored- energy regions. The results have unveiled the specific preferred orientation patterns of recrystallisation and grain growth paths within these regions, indicating that higher misorientation zones, such as grain boundaries or deformation bands, are preferential grain nucleation regions.

Figure 108d shows the first ever DFXM image of a highly deformed grain, revealing fine details of the deformed matrix, such as orientation gradients and deformation bands with distinct alignment with respect to the rolling direction. Compressive strains larger than 0.001 are accumulated around these deformations. These high deformation and stress concentration points act as nucleation sites upon annealing.

Figure 109 shows the early stages of recrystallisation. While the orientation gradient observed in Figure 108d is still visible, new recrystallised grains have nucleated within and around the deformed parent grain, with

Fig. 108: a) Schematics of DFXM experiment. b) Electron Backscatter Diffraction Normal Direction-Inverse Pole Figure map of the initial microstructure of the as-deformed state through thickness. c) DFXM local {110} pole figure colour key of the two sample tilts. d) DFXM mosaicity map of a selected layer of the as-deformed state. The mosaicity scans at the constant angle reveals the spatial

variation of the orientation of {110} plane.