**5. Discussion**

The global and local deformation and damage behavior of modified 16MnCrS5 steel (composition in Table 1) is analyzed. Before the in situ testing, the specimens were prepared using a special grinding and polishing technique (presented in detail in Appendix A), and detailed EBSD data were collected to understand the morphology of the matrix and the second phase inclusions. The outcome of the statistical analysis of the collected microstructural data is presented in Figures 3 and 4. It is evident that, generally, the non-metallic inclusions are quite small, and the MnS inclusions are slightly larger and more visible at the set magnification. Although the MnS inclusions were the focus of this work, other inclusions are also expected to behave similarly.

The parameters of the crystal plasticity model were calibrated and then incorporated in the full phase model employing the recorded and cleaned EBSD data. It is observed that the global results of the simulation match well with the experimental observations. Therefore, researchers can successfully use the identified parameters to model this material using DAMASK in the future. However, one can argue that this match of global results is because a similar data set was used to calibrate these parameters. To answer that critique, the damage pattern around a comparable inclusion in experiments and simulations was made at different strains, and a remarkable similarity in the trends was observed.

Apart from the global results, the local simulation results provide grea<sup>t</sup> insight into how the inclusion size, position within the matrix, and their distribution affect the local stress, strain, damage, and triaxiality. The trends of local strain distribution match well with the previously published data [28,54]. A qualitative and quantitative comparison of the three different zones was presented in Section 4.2. All three zones are intrinsically different based on the composition, inclusion size, and distribution. In addition, there is a clear difference in the damage behavior in these zones, and local stress distribution is observed. These local attributes account for the different local material behavior observed in the experimental part of this work.

Since the previous works have shown that the second phase inclusions are major players in defining the formability of the material [15,41,55], a special focus was given to them in this work. Previous work [51–56] shows that the morphology and distribution of the second phase inclusions dictate the formability limit and damage degradation in a material. In this work, in situ tests were carried out focusing on the strain evolution and damage evolution around and within these second phase inclusions. The second phase inclusion size distribution is observed to be close to 1 μm with a few larger inclusions in the specimen. The local strain around and within one of these exceptionally large MnS inclusions is studied in detail. MnS inclusions, due to their larger size, are easily identifiable at the magnification range selected in this study, and therefore they appear predominantly in this work. However, it is assumed that other inclusions behave similarly. This is also verified by the simulation results presented in Section 4.2, where all the inclusions behave almost indifferently under applied external load.

The results of the local strain measurement from the in situ tests are presented in Section 4.3. It is observed that the strain in the matrix is higher at some points and lower at some points, depending on the orientation of the corresponding ferrite grain. The inclusion is observed to have a low strain distribution until a brittle fracture appears, which damages the whole microstructure, and it becomes hard to track the local strain in the material further. Small MnS inclusions were also tracked in this work at increasing external loads. For these smaller inclusions, it is observed that matrix/inclusion interface decohesion starts to take place and grow at high strain regimes of >550 MPa. These findings match well with the previously published work of other researchers [4,6,19,44,56].

Although similar research with in situ tests and crystal plasticity simulations has been previously carried out by some researchers before [21,27,28,57,58], it was either pure simulation or pure experimental work. No correlation using both methods has been presented before concerning how the inclusion distribution, size, and morphology affect the material deformation and degradation. However, in this work, the findings of the developed crystal plasticity-based numerical simulation model have been validated. Researchers and industry can now use this tool to analyze and optimize the non-metallic inclusion size, distribution, and morphology to attain the desired material formability.
