What material are GE engine High Pressure Compressor (spools) made of ? i.e. aluminum, titanium ?
Various alloys. In general , I believe even the same stage may have different alloys in various engines. As the air progresses through the compressor it heats and alloys are chosen accordingly. Over the years new alloy modifications are developed ; some more subtle changes are longitudinal grain orientation and single crystal blades. I have done failure analysis on gas turbines, steam turbines and gas compressors and don't recall which blade went where. However, I never saw an aluminum blade ( or vane). GE , and others, have various 13% Cr steel blades they often used. As far as I know Ti and graphite are only used in the first stage fan. I only worked on older design engines, I expect Ti is used in earlier compression stages. The final stages of compression are so hot that nickel alloys are used. I mean you need a very specific question to get a specific alloy.