**4. Conclusions**

An in situ topotomographic experiment has been carried out at a synchrotron facility to collect a very detailed 4D dataset to study the onset of plastic slip under tension in an aluminum lithium alloy. Initial DCT imaging allowed measuring the initial microstructure and selecting a set of three grains in the bulk of the specimen for further investigations. Upon loading, incipient plasticity was observed non-destructively in the form of band contrast in X-ray topographs, which were further related geometrically to active slip planes in the bulk.

Information collected from rocking curves was presented in the form of polar plots illustrating the anisotropy of mean lattice curvature. The polar plots have a characteristic dumbbell shape, which can be attributed to lattice bending with respect to some specific axis, which could be determined in the experiment for the three grains during deformation.

Crystal plasticity simulations were carried out to compare the prediction obtained with a classical continuum model with our experimental observations. Essential features, such as slip system activation and average misorientation per grain, are well captured by the model, although some discrepancies remain for one grain. A major contribution of the work is the comparison of experimental and simulated misorientation polar plots showing characteristic dumbbell shapes quantifying the anisotropy of lattice curvature.

A striking feature of the presented experiment is the direct observation of inelastic deformation mechanisms in the bulk (plastic slip, but also twinning or phase transformation are possible). The detected slip system activity could be used in the material parameter identification procedure instead of macroscopic tests only. This would require a specific treatment (for instance using grain averaged quantities) since the full field calculation is costly and not well suited for an optimization routine.

One of the limitations of the present work is that only one specimen could be tested in the given beam-time. Building on this experiment and using the developed automation algorithms, future work will target more specimens and more grains per specimen to obtain more statistics and study slip transmission in more detail.

The adaptation of advanced reconstruction techniques [13] to this type of combined DCT and topotomography acquisitions might allow one to retrieve finer details of the orientation fields and their evolution at increasing levels of applied strain. This could be used to study more complex inelastic mechanisms in metals or to extract more dependable constitutive parameters minimizing the discrepancies between experimental observations and numerical simulations on the digital twins of the tested specimen.

**Supplementary Materials:** The following are available online at www.mdpi.com/1996-1944/11/10/2018/s1: Video S1: Full set of integrated topographs over 360 degrees (every 4◦) for Grain 4 in the undeformed state; Video S2: Full set of integrated topographs over 360 degrees (every 4◦) for Grain 4 in the deformed state.

**Author Contributions:** H.P. and W.L. conceived of and designed the experiments. N.G., W.L. and H.P. performed the experiments. N.G. and H.P. analyzed the data. S.F. contributed the crystal plasticity model. N.G. ran the simulations. H.P. and N.G. wrote the paper. W.L. and S.F. provided comments on the manuscript.

**Funding:** MINES Paristech is acknowledged for funding the PhD of N.G.; this research received an additional funding by Association Instituts Carnot gran<sup>t</sup> number MIN ANR CA 6263.

**Acknowledgments:** The authors thank the ESRF for providing the beam time for this experiment under proposal MA2285.

**Conflicts of Interest:** The authors declare no conflict of interest.
