Microstructures and mechanical properties of FSWed Fe-xAl and Fe-0.1C-xAl alloys

The microstructural evolution and mechanical properties of SZs of FSWed Fe-(0.05, 2, 5)Al and Fe-0.1C-(0.05, 5)Al steels were investigated. The hot rolled bands were heat-treated at 700 °C for 30 min prior to FSW. Phase diagram calculations, along with EDS, TEM, and Kikuchi pattern analyses by SEM-EBSD, confirmed the presence of cementite (Fe₃C, Orthorhombic) in Fe-0.1C-0.05Al and κ-carbides (Fe₃AlC_x, L1₂ perovskite-type structure) in Fe-0.1C-5Al. FSW significantly refined grain sizes of 8.6 μm–4.6 μm and altered carbide morphologies. During the process, coarse cementites in BM were fragmented into smaller particles, while κ-carbides in the BM changed to smaller κ-carbides together with very small cementites at grain boundaries. Higher Al and C contents synergistically strengthened SZs mainly through grain refinement, solid solution, and precipitation strengthening. Specifically, the elongation of the SZs decreased with increasing Al content in Fe-xAl alloys, whereas it increased with the addition of 5 wt% Al in Fe-0.1C-xAl alloys. Furthermore, both the ductile-brittle transition temperature (DBTT) and upper shelf energy increased with Al content. The DBTT increment with 0.1 wt% C addition was much larger in the 0.05 wt% Al alloys compared to the 5 wt% Al alloys. These features of ductility and toughness of the SZs are attributed to the changes in carbides with the 5 wt% Al addition. Below the DBTT, cracks in the Fe-0.1C-0.05Al alloy initiated within cementite and propagated into ferrite, while in the Fe-0.1C-5Al alloy, cracks formed and propagated into the ferrite matrix despite the presence of κ-carbides and fine cementite.