Proceedings of the Third International Conference on Metals & Hydrogen P45

Effect of Nickel-based Alloy 718 Microstructure on Hydrogen Embrittlement Fracture Mode

Brian J. Kagay (1)1 , Kip O. Findley (1)1 , Stephen P. Coryell (2)2

  • (1) 1

    Advanced Steel Processing and Products Research Center, Colorado School of Mines, USA

  • (2) 2

    Special Metals Corporation, USA


Hydrogen embrittlement fracture modes of several alloy 718 microstructures were evaluated with electron backscatter diffraction (EBSD) of secondary cracks and scanning electron microscopy of fracture surfaces.  Aging of alloy 718 was performed to produce an under-aged microstructure with no grain boundary δ phase, peak-aged and over-aged microstructures with minimal δ phase, and a peak-aged microstructure with extensive δ phase precipitation. EBSD scans were performed on secondary cracks formed in the alloy 718 microstructures during slow strain rate testing with cathodic hydrogen charging. The grain boundary misorientation distributions for cracked boundaries and for the bulk microstructure, the percentage of transgranular cracking, and the percentage of intergranular cracking along Σ3 twin boundaries are compared between microstructures. The quantitative results from the EBSD scans are also correlated to scanning electron micrographs of the fracture surfaces. A significant increase in intergranular cracking on Σ3 twin boundaries was observed for the under-aged microstructure. The microstructures with grain boundary δ phase exhibited mostly intergranular cracking along high-angle grain boundaries. A decreased amount of δ phase resulted in an increase in percentage of transgranular cracking, close to 50% of the total crack length for the under-aged microstructure, which indicates that δ phase increases the susceptibility of alloy 718 to intergranular cracking but is not required for intergranular cracking to occur. The relationships between microstructures of alloy 718 and hydrogen embrittlement fracture mechanisms are discussed.


  • Precipitation hardened nickel-base superalloys
  • Hydrogen embrittlement
  • Electron backscatter diffraction
  • Grain boundaries
  • Fractography