Yeah, that's definitely a stress fracture at the back of that pin. I'll talk to the metallurgist I work with for his suggestion on the best steel to use. Other than that I'd figure 4140 heat treated to RC45. The firing pin on a 1911 is specified as medium high to high carbon steel between 40 and 46 Rockwell. If it doesn't hold up at least we tried.
I have CNC produced 416 stainless triggers to replace the plastic triggers on Colt Mustangs, Mustang Plus II's, MK IV Government .380's and Sig P-238's. Video on installation in Sig;http://www.youtube.com/watch?v=Jh2v_-87DRM
Plus Llama .32 and .380 recoil spring buttons, checkered nicely and blued.