**4. Conclusions**

In this study, we investigated the mechanical response of single crystal and bi-crystal FeCrAl alloys using in situ micropillar compression and tension testing. According to the EBSD data, we selected grains with specific orientations which favor the type and number of activated slip systems. In order to explicitly envision the influence of the orientation and microstructures on the stress strain responses of micropillars, Figure 8 shows the mechanical responses of single crystal micropillars with one activated slip system, two activated slip systems, and multiple activated slip systems and bi-crystal micropillars. Firstly, it is expected that these micropillars will show different stress strain responses, corresponding to plastic anisotropy associated with crystal plasticity. However, we obtained a very close glide resistance of about 225 MPa for slips on {110} and {112} planes when the 0.2% offset yield strength and the largest Schmid factor were used to estimate the glide resistance. This conclusion is consistent with our systematic study in single crystal micropillars [52]. Secondly, the strain hardening rates derived from these tests clearly indicated that the weakest hardening is associated with pillars with one activated slip system, the largest hardening is observed in pillars with multiple activated slip systems. With increasing the number of activated slip systems, the strain hardening rate increases. Thirdly, the critical strain for reaching a stable strain hardening rate increases with the number of activated slip systems in micropillars and pillars' diameter. Lastly, for the bi-crystal pillars, the strain hardening rate is also related to the geometrical compatibility factor, the higher the factor is, the lower the strain hardening rate and the weaker the strengthening effect are. These results can be used to develop the mechanisms-based meso/micro/macro-scale constitutive laws for FeCrAl polycrystalline aggregates in order to accelerate designing and predicting mechanical response of structural components [61,62].

**Figure 8.** Comparison of stress-strain responses and corresponding strain hardening rates of micropillars with different orientations.

*Crystals* **2020**, *10*, 943

**Author Contributions:** Conceptualization and methodology, J.W.; software, D.X. and B.W.; formal analysis, D.X., B.W. and W.W.; investigation, D.X. and B.W.; resources, J.W.; data curation, D.X., B.W. and W.W.; writing—original draft preparation, D.X.; writing—review and editing, J.W.; supervision, J.W.; project administration, J.W.; funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was fully supported by the Department of Energy (DOE) Office of Nuclear Energy and Nuclear Energy University Program through Award No. DE-NEUP-18-15703.

**Acknowledgments:** The research was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience (and/or NERCF), which are supported by the National Science Foundation under Award ECCS: 2025298, and the Nebraska Research Initiative. Valuable discussions with Kaisheng Ming and Shun Xu is really appreciated.

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