To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations with respect to the lamellar interface were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load monotonically increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The load reduction with respect to the displacement was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.

To investigate hydrogen embrittlement susceptibility of an individual grain boundary, microcantilevers were fabricated on Ni-Cr bialloy surfaces by focused ion beam, and microbending tests were conducted during hydrogen charging by electrochemical nanoindentation. For microcantilevers fabricated across a twin boundary and inside a grain, no crack was formed by the bending both in air and during hydrogen charging. On the other hand, for microcantilevers fabricated across a random grain boundary, a crack was formed by the bending during hydrogen charging, whereas no crack was formed in air. Thus, it was identified that, for Ni-Cr bialloy, random grain boundaries are more susceptible to hydrogen than twin boundaries and crystalline planes. To validate these experimental results, slow strain-rate tensile tests were also performed, and subcracks of the fractured specimens were analyzed by electron beam back-scattering diffraction. In accordance with the microbending test results, subcracks were predominantly formed along random grain boundaries, but never along twin boundaries and inside grains. Higher hydrogen susceptibility of the random grain boundaries with lower cohesive energy (i.e., work for separation), smooth fracture surfaces without dimples or tear ridges, and no correlation between strain accumulation and subcrack formation indicate that hydrogen embrittlement of Ni-Cr bialloy is caused by decohesion mechanism, where hydrogen atoms lower cohesive energy at grain boundaries.

To investigate causes of superior hydrogen embrittlement resistance of drawn pearlitic steel, notched microcantilevers with different notch orientations were fabricated by focused ion beam, and microbending tests were conducted in air and during cathodic hydrogen charging by electrochemical nanoindentation. In air, indentation load increased with increase in indentation displacement, and no crack appeared for any notch orientations. During hydrogen charging, indentation load declined, and a crack appeared. The degree in the load reduction was larger, and the crack was deeper for the notch parallel to the lamellar interface than that normal to the lamellar interface. Furthermore, stationary cracks in the microcantilevers were observed by scanning electron microscopy and scanning transmission electron microscopy. For the notch parallel to the lamellar interface, a sharp long crack was identified along the lamellar interface. The crack stopped at the position where the cementite lamellae are disconnected. In lattice images, cementite was identified in one side of the crack, and ferrite in another side of the same crack. On the other hand, for the notch normal to the lamellar interface, a blunt short crack was identified. Thus, it was concluded that the ferrite-cementite interface is a preferential crack path, and hydrogen embrittlement resistance in the direction parallel to the lamellar interface is superior to that normal to the lamellar interface. The present results also indicate that directional lamellar alignment of the drawn pearlitic steel suppresses crack propagation in the radial direction of the drawn wire, improving the hydrogen embrittlement resistance in the drawing direction.

To improve the life performance of Polymer Electrolyte Fuel Cell (PEFC), the degradation behavior of the cell is diagnosed at the early stage, and an appropriate treatment should be applied to the cell. Our proposal diagnostics evaluates the transient response that includes the resistance polarization, the activation polarization and the diffusion polarization. The obtained diagnostic parameters from this transient response include the degradation factors. This paper aims the establishment of diagnostics by clarifying the correlation of diagnostic parameter and each polarization. Because the anode polarization is small extremely compared with the cathode polarization, the performance of most fuel cells are controlled by the cathode reaction. This paper focused on the cathode polarization and resistance polarization of the membrane as the diagnostic parameter. As a result, our diagnostics can diagnose each degradation factor that originates on cathode side, and the degradation factor distribution in the same electrode can be measured by applying this diagnostics to the quadrisection separator.