On the Determination of Operative Slip Systems of η-Fe₂Al₅ via Room-Temperature Single Crystal Micropillar Compression Investigation

Hot-dip aluminized steels have extensively been used for sheet materials for automotive applications such as exhaust systems and heat shields and have recently received increasing attention as a possible candidate for the replacement of Zn-coated steels used in large quantities in the automotive industries. Some Fe-Al intermetallic phases are reported to form in the reaction layer on the substrate steel during the subsequent heat treatment process. Among them, η-Fe₂Al₅ is revealed to be the major phase to occupy a large volume fraction in the reaction layer and is thus expected to play a decisive role in determining the mechanical properties of the hot-dip aluminized steels. However, almost nothing is known about the mechanical properties of η-Fe₂Al₅, in spite of the importance of getting some useful strategy to mitigate the extent of cracking and peeling of the coating layer from the substrate steel in press forming. In the present study, we, for the first time, identified room-temperature plastic flow of η-Fe₂Al₅ occurring by the operation of six different slip systems at very high critical resolved shear stresses (CRSSs) above 1 GPa through micropillar compression. The operation of slip systems depends on crystal orientation; the (001)<010>, (001)<110>, (001)<130> and {23}[110] slip systems operate in the middle of the stereographic triangle with the similar CRSS values in the range of 1.1~1.23 GPa, while in orientations close to [001], the {311}<03> slip system as well as the {301}<03> slip system operate with a much higher CRSS values around 1.5 GPa. On the other hand, premature failure occurs without the operation of any slip systems in orientations close to the [100]-[110]-[010] symmetry line. We also make a discussion on the selection of slip systems, their CRSS values, and the possible dislocation dissociation modes with the use of the overlapped atom volume that occurs during the shear deformation by taking the partial occupancies of Al atoms in the c-axis chain into account. A smaller overlapped atom volume is proved to play a decisive role in determining the operative slip system as it leads to the lower CRSS. These new findings on the operative slip systems and their CRSS values in η-Fe₂Al₅definitely contribute to the understanding of the mechanical properties of this intermetallic phase, which can successfully be applied to get useful strategy to mitigate the extent of cracking and peeling of the coating layer from the substrate steel in press forming.