Elucidating deformation behavior of cementite and κ-carbide during friction stir welding of Fe-0.1C-xAl (x = 0.05 and 5 mass%) steels below A

The deformation behavior of cementite and κ-carbide of Fe-0.1C-0.05Al and Fe-0.1C-5Al (mass%) steels at below A 1 friction stir welding (FSW) was investigated. Quantitative nano-hardness measurements revealed that cementite exhibited high hardness (13.0 GPa) and strong resistance to deformation (0.22 μN/nm 2 ), whereas κ-carbide had a lower hardness of 6.9 GPa and resistance force of 0.083 μN/nm 2 for the first time. During FSW under an axial load of 40 kN at a rotation rate of 80 rpm (peak temperatures ~700 °C, below A 1 ), few dislocations were introduced into the structure of cementite, which is hard and brittle and is prone to fragmentation owing to the accumulation of dislocations at the ferrite/cementite interface. In contrast, κ-carbides, being softer, exhibited considerable plastic deformation and were presumed to be sheared by the high-density dislocations introduced under the same conditions. The shear processing plausibly triggers dynamic redistribution of the C atoms, where dislocations penetrate through the κ-carbides, disrupting Fe C bonding. These phenomena enable the transfer of C atoms from the κ-carbides to the matrix and subsequent precipitation of fine cementite along part of the grain boundaries during the cooling stage after FSW. Furthermore, the introduced dislocations remain within the shearable κ-carbides and may serve as dislocation sources in subsequent deformation processes, thereby enhancing the ductility of the steels. The present study is the first to clearly reveal the internal dislocation features of carbides and analyze the distinct deformation behavior of different carbides during FSW below A 1 . • Focus on deformation of cementite and κ-carbide during FSW in Fe-C-Al steels. • Quantified the hardness and deformation resistance of cementite and κ-carbide. • Cementite had high hardness but low plasticity, causing fragmentation during FSW. • κ-carbide had clear plastic deformation and was sheared by dislocations during FSW. • Dislocation interaction with C promoted fine cementite formed after FSW cooling.