3D Mapping of Intragranular Residual Strain and Microstructure in Recrystallized Iron Using Dark-Field X-ray Microscopy

Abstract

Grain growth is a key process in the thermomechanical treatment of metals. Recently, the presence of local residual stresses within fully recrystallized grains has attracted increasing interest in connection with shear-coupled grain boundary migration mechanisms. In this work, we provide the first direct experimental measurements of residual elastic strain variations in fully recrystallized commercial-purity iron, on the order of $10^{-4}$. Using dark-field X-ray microscopy (DFXM), we performed non-destructive three-dimensional measurements of strain and orientation variations within individual grains. Our results reveal heterogeneous strain distributions across all measured grains. In one case, we observed several isolated dislocations accommodating two second-phase particles, exhibiting a localized strain signature with no detectable long-range effect. The formation mechanisms of intragranular residual strains and their potential influence on grain boundary migration during subsequent grain growth are discussed. This work highlights the importance of accounting for such residual elastic strains in future grain growth models.

Figures (4)

• Figure 1: Schematics of the DFXM setup showing the incident beam illuminating the sample and the diffracted beam passing through the X-ray objective to form a dark field image with diffraction contrast sensitive to lattice spacing and misorientation along the 110 vector of iron. Sample rotations $\phi$ and $\chi$ are indicated. Two illumination modes were used: (i) a box beam (blue) providing full-volume projection of the diffracting grain and (ii) a line-focused beam (red) provides information through the height of the grains, as the sample is translated across the horizontal beam in the $z$ direction to obtain three-dimensional information, enabling height-resolved imaging with a spatial sensitivity of about $500\,\mathrm{nm}$ in this direction.
• Figure 2: (a) Weak-beam condition (left and right), highlighting dislocations, and strong-beam condition, showing two regions of reduced contrast that likely correspond to second-phase particles (highlighted in pink). Following the red arrows, a superimposed image of the three conditions shows the positions of the particles (pink) and the dislocation contributions from the left (blue) and right (green) weak-beam conditions. (b) maximum value of the rocking curve (peak of intensity during the mosaicity scan) of the layers 3, 4, 5, and 6, respectively. The arrows indicate the position of the particles shown in Fig.\ref{['fig:strain_mosa']}.
• Figure 3: DFXM results on eight layers ($0-7$) of a single grain of interest (G2). The layers are separated by $1\micro$m in height. The different rows represent (a) mosaicity, (b) kernel average misorientation, (c) local orientation maps of $\phi$. (d) strain scans. Next to each scan, there is the corresponding colormap. The grain is approximately $\sim40\micro$m large and contains some particles, represented by circles, one red and one yellow with a black outline and other two in black and grey, and several dislocations surrounding them.
• Figure 4: (a) Dislocation density distributions of the same layers of grain G2 as shown in Fig. \ref{['fig:strain_mosa']}. (b) Area-weighted strain distributions of the individual layers of grain G2. (c) Volume-weighted strain distributions of all investigated grains. In (c), solid lines correspond to grains for which multiple layers were measured using the line beam, while dashed lines correspond to strain distributions obtained from projection measurements using the box beam; for these grains, a spherical grain shape was assumed to estimate the contributing volume.