AFRL researchers teamed with scientists from Universal Energy Systems (UES), Inc., on a Small Business Innovation Research (SBIR)-funded effort designed to meet requirements of the IHPRPT program. Their goal was to improve the thrust-to-weight ratio in rocket engines by identifying an aluminum alloy with (1) specific strength values equal to or exceeding those of the high-strength titanium alloy Ti-5Al-2.5Sn ELI (extra low interstitial), (2) significantly reduced weight, and (3) ductility of no less than 7%. Currently, metallurgists produce highstrength aluminum alloys through an expensive nanophase aluminum process that includes production of the alloy powder; mechanical milling of the powder in liquid nitrogen to produce a nanophase structure; and powder compaction by hot isostatic pressing, extrusion, and forging. Unfortunately, the resulting alloys do not possess the combination of strength and ductility required for use in cryogenic rocket engine applications. During this SBIR effort, AFRL and UES scientists, collaborating with scientists from Rocketdyne, selected an alternative processing approach: they produced alloys using conventional casting technology, which is less expensive than powder metallurgy processes. The properties of new alloys are determined by alloy composition and thermomechanical treatment; the combined effect in the new alloys produces very fine, nonsoluble dispersoids and grain refinement.

During Phase I of the SBIR effort, the research team investigated the microstructure, hardness, and tensile properties of nine alloys. Two different foundries cast 3 in. diameter billets, and the researchers examined them in as-cast and hot-extruded (see Figure 2) conditions using AFRL’s Materials Processing Laboratory. They heat-treated the extruded samples to maximum hardness and then determined the tensile properties of the heat-treated samples at standard and cryogenic temperatures. The researchers also analyzed the microstructures of both heattreated and deformed samples to determine fracture modes.

The research team subsequently selected three of the original nine alloys for further investigation during Phase II of the SBIR effort. The scientists optimized the heat treatment of as-cast alloys to achieve maximum strength and reasonable ductility. They determined the transverse tensile properties of the hot-extruded alloys to verify that their properties would meet design requirements. To ensure that the processing parameters would be acceptable for forging, they also performed compression tests. The team produced several forging pancakes from the alloys with different processing histories and determined tensile properties in different cross sections after heat treatment. This testing allowed the scientists to optimize their forging parameters and determine each alloy’s microstructure at different processing steps.