Nickel-Based Superalloy Resists Embrittlement by Hydrogen
- Thursday, 29 May 2008
This alloy also exhibits high strength and ductility.
A nickel-based superalloy that resists embrittlement by hydrogen more strongly than does nickel alloy 718 has been developed. Nickel alloy 718 is the most widely used superalloy. It has excellent strength and resistance to corrosion as well as acceptably high ductility, and is recognized as the best alloy for many high- temperature applications. However, nickel alloy 718 is susceptible to embrittlement by hydrogen and to delayed failure and reduced tensile properties in gaseous hydrogen. The greater resistance of the present nickel- based superalloy to adverse effects of hydrogen makes this alloy a superior alternative to nickel alloy 718 for applications that involve production, transfer, and storage of hydrogen, thereby potentially contributing to the commercial viability of hydrogen as a clean-burning fuel.
The table shows the composition of the present improved nickel-based superalloy in comparison with that of nickel alloy 718. This composition was chosen to obtain high resistance to embrittlement by hydrogen while maintaining high strength and exceptional resistance to oxidation and corrosion. The alloy-design approach followed to arrive at this composition was based on accounting for the simultaneous effects of several additions. The approach included systematic modification of γ- matrix compositions for increased resistance to embrittlement by hydrogen, increasing the volume fraction of the γ ′phase, adding γ-matrix-strengthening elements to increase strength, and obtaining precipitate-free grain boundaries. Substantial amounts of chromium and nickel were also included to obtain excellent resistance to oxidation and corrosion. Microstructural stability was maintained through improved solid solubility of the γ matrix along with the addition of alloying elements that retard η-phase precipitation. This alloy represents a material system that greatly extends ranges of composition beyond those of prior nickel-base superalloys that resist embrittlement by hydrogen.
This alloy is first processed by a combination of vacuum induction melting and vacuum arc remelting. Typically, the resulting alloy ingot is homogenized at a temperature of 2,100 °C for 24 hours and then hotrolled in the range of 927 to 1,093 °C into 1.6-cm-thick plates. The plates are subjected to a solution heat treatment at 1,050 °C for 1 hour, followed by aging at 718 °C for 8 hours, then 621 °C for 8 hours.
The most novel property of this alloy is that it resists embrittlement by hydrogen while retaining tensile strength >175 kpsi (>1.2 GPa). This alloy exhibits a tensile elongation of more than 20 percent in hydrogen at a pressure of 5 kpsi (≈34 MPa) without loss of ductility. This amount of elongation corresponds to 50 percent more ductility than that exhibited by nickel alloy 718 under the same test conditions.