Hydrogen-Assisted Twin Boundary Fracture of Type 304 Austenitic Stainless Steel at Low Temperature Investigated by Scanning Probe Microscopy

Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and magnetic force microscopy (MFM) are used to characterize the morphology and strain-induced α′ martensite distribution on the twin fracture surfaces of hydrogen-precharged type 304 austenitic stainless steel tensile tested at 200–218 K. The STM images of the twin fracture surfaces show that the topographies of two conjugate fracture surfaces match well with each other at the micron scale but not at the nanoscale. Three sets of shallow grooves, intersecting each other at an angle of 120°, are formed on the twin fracture surfaces and the parallel nanoplate-like features are formed between the shallow grooves, resulting from the intersection of hydrogen-enhanced deformation bands with the twin boundary. The α′ martensite distribution observed by MFM from two conjugate fracture surfaces and longitudinal section of microcracks indicate that the fracture occurs along the phase boundary between the austenite and strain-induced α′ martensite near the twin boundary. The hydrogen-assisted twin boundary fracture processes are discussed in the theoretical framework of hydrogen-enhanced localized plasticity (HELP).Copyright © 2013 by ASME