The protoype Junkers all-metal aircraft reportedly used frames constructed from steel angle channel and I-beams. While I've seen nothing on the alloy designation, I suspect it was similar to what Fokker was using. The production model that served in the Great War used primarily aluminum tubing for the frame and a corrugated aluminum skin, though they did use a 5mm "chrome-nickel" steel sheet on the forward fusealge for engine and crew protection. I've seen the sole surving sample which resides in the National Aviation Museum in Ottawa.