Extended x-ray absorption fine structure of metastable bcc and fcc phases in mechanically alloyed Fe–Cu

The extended x-ray absorption fine structure (EXAFS) technique has been employed to monitor the formation of thermodynamically unstable solid solutions induced by mechanical alloying in the positive heat of mixing Fe–Cu system. Local structural changes are clearly observed, with Cu taking on bcc coordination in the bcc (Fe80Cu20) solid solution, and Fe taking on fcc coordination in the fcc (Fe50Cu50) solid solution. Atomic level alloying is demonstrated through curve fitting to ab initio simulations. A difference is observed in the behavior of bcc and fcc ball milled samples, with a large reduction in first shell coordination number observed in bcc nanocrystalline structures (pure Fe and Fe80Cu20), but not in the fcc nanocrystalline structures (Fe50Cu50). This is believed to be due to the inability of the fcc structure to retain a high defect density.