An investigation into the reverse transformation of ferrite to austenite during friction stir processing of a duplex low-density steel

The reverse transformation of ferrite-to-austenite during friction stir processing of a duplex lightweight steel resulted in an abrupt decrease of the ferrite volume fraction within the stirred zone. This was explained in terms of diffusion-governed phenomena from the thermodynamic points of view. However, owing to the recorded thermal history of the process and the low exposure time at the maximum temperature, the probability of shear assisted diffusion was considered. The fraction of high angle and low angle boundaries within the stirred zone was extremely higher than those observed in thermomechanically affected zone or heat affected zone. These high/low angle boundaries acted as preferred diffusion paths and provided a proper condition to increase the lattice diffusion. The results of electron probe micro-analysis indicated that the alloying elements remained relatively un-partitioned in long range order. In addition, the grains were arbitrarily sheared towards the shear surface/ direction and volume fraction of shear texture component was substantially higher than that of recrystallization texture component. The above observations suggest that the abrupt drop in the ferrite content at the vicinity of the rotating tool in the shear layer may be also induced by shear assisted massive transformation.