Mesoscale quantification of grain boundary character and local plasticity in hydrogen-assisted intergranular and intergranular-like cracking paths in tempered martensitic steel

Abstract

The effects of the misorientation of prior austenite grain boundary (PAGB) segments on the local plasticity evolution in intergranular (IG) and IG-like fractures were investigated using a tempered martensitic steel. When a diffusible hydrogen content of 6.3 mass ppm was introduced, IG fractures with smooth fracture surfaces occurred in the elastic regime of the tensile test. According to the grain orientation spread (GOS) distribution, local plasticity did not contribute to the mesoscopic (block size) scale, irrespective of the PAGB segment misorientation. With a relatively low hydrogen content (4.0 mass ppm), an IG-like feature appeared on the fracture surface, which included plasticity-related traces such as tear ridges or nanovoids. The GOS values near the IG-like cracks were higher than those near the IG cracks and exhibited significant misorientation dependence. Specifically, at the low-angle PAGB segments, the GOS values of the grains neighboring the IG-like cracks increased with a decreasing misorientation. On the other hand, most of the cracked high-angle PAGB segments exhibited nearly constant GOS values. Remarkably, a Σ3 PAGB segment exhibited an abnormally high GOS value. These facts and corresponding microstructural observation results indicate that the low angle and Σ3 PAGB segments allow crack-tip blunting before crack growth.