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Lecture

Nano- micro mechanical behavior of Ni alloys

Friday (28.09.2018)
11:15 - 11:30 S1/01 - A2
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Most high strength and high-performance alloys when exposed to hydrogen experience a premature rupture. This phenomenon is called hydrogen embrittlement. Among the experimental approaches to study the hydrogen-induced degradation, small-scale testing has the capability to resolve the hydrogen interaction with microstructure and the crystal defects in the same length scale. Since hydrogen has a strong tendency to segregate in structural defects, the importance of grain boundary (GB) becomes even more dominant in the hydrogen embrittlement phenomenon. GBs are considered as one of the potential sites for initiation of this catastrophic phenomenon in the polycrystalline materials.

Ni super-alloys with the different manufacturing processes, composition, and heat treatment backgrounds shows a different failure mode in the presence of hydrogen. Precipitates have a significant influence on the embrittlement phenomenon especially when they segregate in the GBs.

In this study, we used an in-situ electrochemical micro-cantilever bending (ECCB) method for evaluating the effect of different alloying elements and heat treatment processes on the hydrogen induced cracking. Charging the Ni sample cathodically during ECCB testing of micro-sized bi-crystal beams, assured a uniform concentration of hydrogen in the micro-beams during the bending test.

Single and bi-crystal cantilevers of Ni-alloys were compared in two different H- charged and H-free conditions. The effect of sulfur segregation on inter-granular failure was also investigated and compared with the non-segregated condition. Hydrogen-charged, sulfur segregated bi-crystals revealed complete inter-granular brittle failure. The load-displacement curves of these bi-crystal beams showed earlier initiation of the continuous decrease in the flow stress. Precipitation effect on the failure mechanism was evaluated in this study in the presence of hydrogen and compared with the H-free condition.

Speaker:
Tarlan Hajilou
Norwegian University of Science and Technology NTNU
Additional Authors:
  • Prof. Afrooz Barnoush
    Norwegian University of Science and Technology NTNU