[Bruce Lancaster]

Ductile iron indeed fits its nomenclature, and has the ability to bend and deform under impact without cracking. I believe that some modern OEM spindles are made of ductile, with a forged pin inserted for the actual spindle pin, so I would concede that it is probably an adequate material for an axle...but I will stick to forged and heat-treated Ford EE steel that I know is virtually unbreakable under really severe bending or impact. And absolutely no thanks on aftermarket cast spindles.
Ford introduced cast-iron cranks in 1934, a revolutionary step that also required special alloy that was able to withstand torsion...I think this was the ancestor of ductile. In the crank, it had some advantages, besides price, though its ultimate strength was less than a forging. It allowed much better counterweighting and was stiffer, a big deal since the '32-6 cranks were so thin. The stiffer cranks apparently made more power at high speeds...the forgings were twisting enough to run things out of time. Anecdotal evidence from 1930's stock car racers is that deuces picked up several MPH in their circa 100MPH racing range when fitted with '34 cranks.