Abstract
This study focuses on the time-dependent intergranular cracking of a newly developed Ni-base superalloy ALLVAC 718PLUS. Grain-boundary (GB) cracking is due to the penetration of oxygen down grain boundaries and subsequent oxidation of GB particles, i.e., carbides and possibly delta phase present at the grain boundaries. The GB oxidation and subsequent brittle intergranular failure of this material is highly dependent upon moisture level in the testing atmosphere. With increasing moisture level in the test atmosphere, it is demonstrated that hydrogen may become dominant over oxygen as the embrittling species. The degree of susceptibility to time-dependent intergranular cracking is also highly dependent upon microstructure. The behavior exhibited by ALLVAC 718PLUS is common to a wide range of Ni- base superalloys when tested in air or other aggressive environments. A six-step model for the grain-boundary cracking mechanism is presented. The model is presented for hydrogen in high-moisture environments but is equally applicable to oxygen at lower-moisture levels, as well. Structures which provide increased resistance to time-dependent intergranular cracking are also discussed.
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