Nanoscale Compositional Homogenization by Severe Plastic Deformation-Induced Twinning for Achieving Desensitization in Stainless Steel

Sensitization of stainless steels is well known to substantially degrade their corrosion resistance through localized intergranular attack. We report the use of ultrasonic nanocrystalline surface modification (UNSM), a high strain-rate surface peening technology to rapidly desensitize a sensitized AISI 304H austenitic stainless steel microstructure. High-resolution transmission electron microscopy and selected area electron diffraction analysis confirmed that the high amount of localized strain and strain rate imparted by the UNSM treatment triggered nanoscale deformation twinning in the austenite matrix. Atom probe tomography (APT) measurements in the vicinity of the sensitization-induced grain boundary chromium carbides revealed that this deformation twinning facilitated a nanoscale homogenization of the Cr-content distribution in the austenite matrix. As a result, the lowest Cr content in the austenitic matrix was raised from 7 at. pct in the as-sensitized condition to similar to 12 at. pct in the UNSM-treated sample, meeting the threshold Cr content of 11-12 at. pct required for passivation. Detailed crystallographic analysis of the spatial distribution of the Cr-concentration and Cr-carbide precipitate morphology suggests that atomic transport across twin planes during twin thickening (i.e., during glide of the a/6 <(11) over bar2 > Shockley partials) is responsible for the rapid composition homogenization in the austenitic matrix at room temperature.