Extended plasticity mechanisms (EPM) allow a metal to undergo extended plastic deformation without failure. These mechanisms are responsible for the extended plastic deformation characteristic of hot working processes. In this thesis it is shown that electron backscatter diffraction (EBSD) is capable of detecting EPM artifacts in the final microstructure of AISI 304L stainless steel (304L). Results also indicate that dislocation cells form in hot worked AISI 304L stainless steel. Additionally, in this study EBSD data collection and analysis is used with a design of experiments approach to model the presence of EPM artifacts in the final microstructure of friction stir welded 304L. Texture analysis of the welded material reveals a dominant shear deformation texture and a lack of the rotated cube texture. The shear deformation texture is characteristic of dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX), while the rotated cube texture is characteristic of discontinuous dynamic recrystallization (DDRX). The texture analysis results indicate that dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX) play a role in the final microstructure of the welded material, while DDRX does not. Design of experiments was used to find the relationships between the fraction of cell boundaries and spindle speed, travel speed, location in the stir zone, and tool temperature. The regression analyses reported that predicted fraction of cell boundaries were relatively high (approximately 0.70 or more) and changed by less that 20% in the stir zone and 10% in the TMAZ. The relatively high predictions indicate that in FSW 304L DRV dominates and limited CDRX occurs. The small changes in predictions across the experimental space indicate that the effects, while statistically significant, are not practically significant. Finally, an alternate tool temperature basis was developed, which provides a valid method for selecting welds which should have constant tool temperature.

The current study focus on investigating a relationship between processing variables and microstructure evolution mechanism in 304L stainless steel subjected to hot torsion. The Gleeble 3800 with Mobile Torsion Unit (MTU) is utilized in the current study to conduct hot torsion test of 304L stainless steel. Samples are rotated at 1100℃ in the shear strain rate range of 0.02s-1 to 4.70s-1 and the shear strain range of 0.5 to 4. Orientation imaging microscopy (OIM) technique is used to collect and analyze the microstructure. At low strains (≤1) and strain rate (0.02s-1), average grain size remains relatively constant, but the lengths of DSs and LABs increase within grains. These are characteristics of the dynamic recovery (DRV). With increasing strain and strain rate, the lengths of DSs, LABs and HABs increase, accompanied by the decrease of average grain size. Subgrains with HAB segments are observed. These are characteristics of continuous dynamic recrystallization (CDRX). At strain rates greater than or equal to 0.94s-1, the fraction of deformation texture is about 3 times higher than that of rotated cube texture. The average grain size increases relative to that at a strain rate of 0.20s-1, accompanied by the increase of twin length per area. This indicates that grain growth take place after CDRX. Sigma phase is not observed in the current study due to the lack of static recrystallization (SRX) and the higher cooling rate.