Hydrogen effects on fracture resistances of bulk cementite evaluated by in-situ microbending test during cathodic hydrogen charging

Abstract

Hydrogen absorption characteristics and mechanical properties in hydrogen environment of cementite were evaluated by low-temperature thermal desorption analysis and in-situ microbending tests during cathodic hydrogen charging using bulk cementite plates obtained through a vacuum carburizing process. In the low-temperature thermal desorption analysis, no hydrogen desorption was identified up to 1073 K. In the microbending test, notched microcantilevers experienced cleavage fracture in an elastic deformation range in air. The cathodic hydrogen charging increased fracture load (i.e., fracture toughness) with appearance of plasticity while it did not change the fracture surface morphology and Young's modulus. In the present hydrogen charging conditions, the hydrogen atoms are present only near the specimen surface because of high hydrogen migration energy in the cementite. It seems that no hydrogen desorption is detected because the hydrogen atoms are absent in most regions of the specimens. The invariance of the Young's modulus and the fracture surface morphology can be explained by the same reason. On the other hand, it is considered that the fracture toughness is improved because the hydrogen atoms charged near notch bottom of the microcantilever enhance dislocation nucleation and glide, and cause blunting of the notch during the bending.