Microscopic observation of the mechanism for brittle striation formation in low carbon steel fatigued in hydrogen gas TEM and EBSD observations corresponding to fractography

In order to investigate the brittle-striation formation mechanism of low carbon steel JIS S10C in hydrogen gas environment, TEM/EBSD observations corresponding to fractography were conducted. Main results are as follows. (1) Quasi-cleavage (QC) facets plane with brittle striations does not coincides with (100) cleavage plane. (2) Slip deformation distributions reflecting the brittle striation formation processes are observed with TEM. One of the conceivable brittle-striation formation mechanisms which can explain these results is as follows. A striation line is formed during the loading part of the cycle as a trace of blunting by slip. Then, stable ductile crack growth starts. These processes were similar to those in the normal ductile fracture from a crack, that is, ductile tearing process in tension.