Fabrication of gradient structured AISI 304 stainless steels with aid of ultrasound vibration and electropulsing

Abstract

An AISI 304 stainless steel was processed by 8 mm diameter tool tip pressing aided with ultrasound (20 kHZ) and/or electropulsing (pulse frequency, f=500 Hz, effective current density, J=10 A/mm²). Systematical characterizations on the microstructure and hardening along depth reveal that the tool tip pressing alone induces no apparent surface deformation and hardening. However, aiding with ultrasound and/or electropulsing achieves a thick deformation layer (>700 μm), significant grain refinement below 7 nm and hardening above 6 GPa. Ultrasound adds extra stress (∼70 MPa) and electropulsing decreases the critical resolved shear stress for generating dislocations and twins, benefiting plastic deformation and grain refinement. Ultrasound and electropulsing allows significant grain refinement without sacrificing surface quality. Additionally, ultrasound and electropulsing promote deformation-induced martensitic transformation. The wide tuning of the deformation and the grain refinement in terms of stress open a window to fabrication of high performance fine-grained materials and surfaces, showing potential scientific and technological importance.