Insight into dislocation activity during ECAP processing of AISI 304 stainless steel studied by X-ray diffraction profile analysis

Despite many advantages of austenitic stainless steels, such as excellent formability, good corrosion resistance, and acceptable weldability, they suffer from low yield strength (YS). Grain refinement using equal channel angular pressing (ECAP) has been preferred as a promising method to strengthen these alloys. Due to the important role of dislocations both in the stage of microstructure refinement during deformation process and their subsequent work hardening effect in the final processed material, it is necessary to obtain information on the dislocation content of the material as well as the character and fraction of dislocations. The topic has been studied extensively on the stainless steels after deformation by conventional processes such cold rolling. However, a systematic study presenting sufficient detail after processing by severe plastic deformation methods such as ECAP is missing. For the first time, X-ray diffraction peak profiles have been analyzed for a severely deformed (by ECAP at 350 °C up to eight passes) AISI 304 austenitic stainless steel by this research. The modified Williamson–Hall and Warren–Averbach approaches were used to this end. Crystallite sizes obtained using these two methods are in a good agreement indicating that processing by eight passes ECAP refines the coherently scattering domain size down to nanometer range (average size D = 68 nm). Dislocation density ( ρ ) was found to increase during the initial stages of deformation and then to decrease after reaching a maximum. After that, it shows an upward trend again up to eight passes and approaches value of 4.71 × 10 15 m −2 . The population of screw dislocations decreases gradually by applying ECAP deformation. A relatively equal fractions of both types of dislocations are present in the ECAP-ed material between equivalent strain of 2.8 and 4.8. With more deformation, screw dislocations in the structure increases again and reaches 74% after eight passes. At the same time, the effective outer cutoff radius of dislocations (Re) after a decrease during initial passes levels off to a saturated value around 80 nm, with continued deformation. Deformation-induced martensite (DIM) transformation does not occur during ECAP. This behavior was explained by a combination of high pressing temperature and greater stability of austenite phase in the studied material. Scanning transmission electron microscopy (STEM) observations of the microstructure showed a good correspondence with XRD data.