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 bee n employed to monitor the formation of thermodynamically unstable soli d solutions induced by mechanical alloying in the positive heat of mix ing Fe-Cu system. Local structural changes are clearly observed, with Cu taking on bcc coordination in the bcc (Fe80Cu20) solid solution, an d Fe taking on fee coordination in the fee (Fe50Cu50) solid solution. Atomic level alloying is demonstrated through curve fitting to ab init io simulations. A difference is observed in the behavior of bcc and fe e ball milled samples, with a large reduction in first shell coordinat ion number observed in bcc nanocrystalline structures (pure Fe and Fe8 0Cu20), but not in the fee nanocrystalline structures (Fe50Cu50). This is believed to be due to the inability of the fee structure to retain a high defect density. (C) 1996 American Institute of Physics.