A. B. Pandey, Pratt & Whitney Rocketdyne, West Palm Beach, FL

Summary:

There has been increasing interest to develop high strength and high temperature aluminum alloys to reduce the weight of propulsion systems. Typically, hydrogen turbopumps are made out of nickel base superalloys and steels, which have almost three times higher density than aluminum alloys resulting in an increased weight for turbopumps. The use of an aluminum alloy can reduce the weight of turbpumps significantly providing a substantial contribution to the system’s thrust-to-weight ratio. New aluminum alloys containing nanocrystalline particles of L1₂ strengthening phase are being developed to provide a significant weight reduction benefit for the hydrogen turbomachinery.

The proposed aluminum alloy is based on Al-Sc-X system. Aluminum-scandium alloys form Al3Sc based precipitate that has an L1₂ structure and is thermally stable up to high temperatures (750°F). The thermal stability and strength of Al₃Sc precipitate can further be improved by addition of gadolinium and zirconium. These dispersoids provide strengthening in the alloy through Orowan strengthening, antiphase boundary energy strengthening and grain size strengthening. This alloy is made by a powder metallurgy technique that includes compaction of powder and extrusion. The proposed alloy has demonstrated very high strength for a range of temperatures up to 600°F. This alloy also has an excellent compatibility with hydrogen. The purpose of this paper is to provide an overview of Al-Sc-X alloys and discuss the relevant strengthening mechanisms in this class of alloy systems.