Formation and strengthening mechanism of kink bands in an ultra-coarse-grained Fe-Cr-Al alloy

Abstract

Fe-Cr-Al alloys, owing to their absence of allotropic transformation, require multiple cycles of rolling and recrystallization annealing processes to achieve substantial grain refinement, ultimately leading to the attainment of outstanding mechanical properties. However, the corresponding manufacturing costs will also increase greatly. In this work, we have proposed a new microstructural preparation process. Simply using warm rolling for an ultra-coarse-grained Fe-Cr-Al alloy to introduce lamellar kink bands (KBs) into the matrix, the mechanical properties can be significantly improved. By using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), and combined with Schmid factor (SF) calculation, the formation mechanism of KBs has been revealed. When the slip plane and direction are nearly perpendicular to the loading force direction (LFD) during the continuous grain rotation, the dislocation wall will evolve into the KBs boundaries. Simultaneously, a huge orientation separation between the matrix and KBs will be produced. As strain continues to rise, KBs undergo a transformation, transitioning from low-angle-grain boundaries (LAGBs) to high-angle-grain boundaries (HAGBs), occasionally adopting a configuration as coincident site lattice (CSL) boundaries with reduced interface energy. Results of the tensile test, cyclic loading-unloading-reloading tensile test, and the strengthening calculation show that KBs can pronouncedly enhance the strength by their heterogeneous refinement on the original grains and hetero-deformation induced (HDI) strengthening effect from the dislocation density discrepancy between the matrix and internal KBs, the grains containing KBs (KBGs) and the grains without KBs (or KBs-free-grains, KFGs). The theoretical calculation value of the strengthening contribution from KBs on yield strength can be up to 225.5 MPa, with a minimum value exceeding 153 MPa. On the other hand, the ductility can be retained to some extent through stimulating the KBs boundary delamination mechanism. The present study provides a low-cost and feasible processing method for fabricating Fe-Cr-Al alloy with high strength and good ductility.