Elon Musk stated that SpaceX will use modern versions of Inconel superalloys for Raptor rocket engines. They will use the SX 300 and soon SX 500 superalloys. They will have high strength at high temperatures and provide extreme oxidation resistance. The super allows are needed for ~800 atmosphere, hot, oxygen-rich turbopump on Raptor rocket engines.
SpaceX has been justified in using more expensive materials and parts to enable higher and higher levels of reusability. They went to more expensive titanium fins to control the descent of their Falcon 9 rockets. They will be justified in using expensive alloys to enable the Raptor engines and the SpaceX BFR to be reused hundreds of times.