Microstructural Evolutions Under Ultrasonic Treatment In 304 And 316 Austenitic Stainless Steels: Impact Of Stacking Fault Energy

Herein, the influence of ultrasonic treatment (UST) on the microstructure of AISI-304 and AISI-316 stainless steels as two common commercial grades with similar properties but different levels of stacking fault energy (SFE) is compared. The softening effect of the ultrasonic wave on the predeformed structure is demonstrated by microhardness measurements, and the relaxation of tensile residual stresses is affirmed through the X-ray diffraction (XRD) peak shifting. Electron and optical microscopy reveals a significant impact of ultrasound on the reduction of the deformation twins' fraction. A new mechanism for detwinning under the action of ultrasonic vibration is proposed using electron backscatter diffraction (EBSD) analysis. Reductions of 25% and 34% are detected in dislocation density of the 10% predeformed tensile sample after 300 W UST for the 304SS and 316SS alloys, respectively. The effect of the SFE is discussed, and it turns out that cross-slip is the main mechanism of dislocation annihilation as a result of UST. Observation of the low-deformation regions close to the grain boundaries indicates the occurrence of the recrystallization phenomenon during UST. Dislocation annihilation, detwinning, dislocation absorption to grain boundaries, and recrystallization are regarded as the softening and relaxation mechanisms of UST for austenitic stainless steels.