It's a steel frame, and I believe the dropouts are cast from aluminum.

I didn't buy the frame where I did because it was cheaper, I bought it there because they were the only place I found that had them in stock. I tried the LBS first, and they told me the frames were sold out until 2015. It could have been a returned frame - one of the stickers had been partially peeled off when I received it.

I built this bike with a Rohloff Speedhub, and spent many hours machining a part to allow the Speedhub axle plate to secure to the frame. Replacing the frame with a different one would undo all this work.