All sources that I can find indicate that they are isogrid (or possibly orthogrid?) structural tanks manufactured of milled aluminum. However, I cannot find further detail about of which alloy specifically they are composed.
Are they Al-Li 2198, as is Falcon 9, or are they more similar to the Apollo-era Al 2014 used on upper stages of the Saturn family?
(Despite one being kerolox and one being hydrolox, I assume ULA makes their two booster families out of the same material to simplify logistics. I may be very wrong about that!)