【正文】 cent to elongated austenite islands in the TMAZ. Phase maps of regions, CEN and in the weld are shown in All regions consist of a ferrite matrix with the austenite islands similar to that of the base material. Distribution of the austenite islands in regions and CEN is similar to that in the base material, but the austenite islands contain more grain boundaries than the base material. The grain size profile ( Fig. 8) showed that the austenite and ferrite grains in the stir zone were smaller than those in the base material. Additionally, the phase maps showed that both the austenite and ferrite phases in the stir zone did not exhibit a heavily deformed microstructure, . many low angle grain boundaries. Both the grain size profile and phase maps suggest that dynamic recrystallisation occurred both in the austenite and ferrite phases during FSW. It is generally known that dynamic recrystallisation easily occurs in the austenitic stainless steels, while ferritic steels hardly experience dynamic recrystallisation because the ferrite phase has a high stacking fault energy. In the case of the duplex stainless steels, however, deformation is localized in the ferrite matrix at high temperatures, because the ferrite phase is relatively weaker than the austenite. Consequently, the recrystallised grains are often formed in ferrite phase more easily than in austenite phase. Some research] suggests that the recrystallised grains in the ferrite phase are formed By continuous dynamic recrystallisation, which is characterized by straininduced progressive rotation of subgrains with little boundary migration. In the present study, the duplex stainless steel experienced plastic deformation by therotating tool at relatively high temperatures during FSW. As such, it is likely that the ferrite matrix in the stir zone undergoes continuous dynamic recrystallisation through the samescenario. On the other hand, the morphology of austenite islands in the stir zone was much different from that of the base material, as shown in Figs. 5 and 6. This suggests that the austenite islands also experienced intense plastic strain during FSW, leading to dynamic recrystallisation in the austenite phase. After dynamic recrystallisation, the recrystallised grains grow during the oncooling thermal cycle . As mentioned above, since the ferrite phase is more likely to undergo dynamic recrystallisation than the austenite phase, as a result of the high temperature deformation of FSW, it is likely that the recrystallised ferrite grains would grow early than those of the austenite phase. This is likely the reason why the ferrite phase exhibits a larger grain size than the austenite phase in the stir zone. Region , located just outside the stir zone, had the similar morphology of austenite islands to the asreceived base material. Grain size and texture of the austenite phase in region were roughly the same as those in the base material, but the ferrite phase had the different texture ponents than that of the base material. This result suggests that only the ferrite matrix underwent deformation and recrystallisation during FSW because the ferrite phase has a lower flow stress at elevated temperature than the austenite phase in the duplex stainless steel, as mentioned above. It is generally known in Al alloys that dislocations introduced into the TMAZ rearranged or migrate at temperature producing a recovered microstructure [42–45].However, the ferrite phase in region did not exhibit a deFig. 11. Hardness profile across the stir zone in the weld. formed microstructure. This result suggests that the ferrite phase in this region may undergo sufficient plastic strains to induce continuous dynamic recrystallisation as a result of the significant localisation of deformation in the ferrite phase. . Mechanical properties and effect of microstructure on mechanical properties A typical transverse hardness profile of a FSW in 2507 super duplex stainless steel is indicated in Fig. 11 . Given the fact that the ferrite content in the stir zone is roughly uniform, the increase of hardness in the stir zone suggests that the hardness profile is related to the grain sizes of ferrite and austenite phases in the weld Transverse tensile properties of the weld are shown in Fig. 12 (a). The asFSW 2507 super duplex exhibits roughly the same % offset yield and ultimate tensile strengths as the base material, with the exception of the elongation. Total elongation to failure based on the standard 51 mm gauge length was roughly 50% of the base material. However, given the amount of reduction in area as observed in Fig. 12 (b),the actual percentage of ductility of the FSW specimens is likely much higher than reported. All tensile failures occurred roughly 7 mm from the weld centre at the retreating side, . near the border of the stir zone and TMAZ, as shown in Fig. 12 (b). This is consistent with the data presente