Local structures of mechanically alloyed Fe_100−xCu_x solid solutions studied by X-ray absorption fine structure

The local structures of the immiscible Fe_100-xCu_x alloys (x= 0, 10, 20, 40, 60, 80 and 100) produced by mechanical alloying have been investigated by XAFS. For the Fe_100-xCu_x (x ≥ 40) solid solutions, the local structures around Fe atoms change from bcc structure to fcc one and the Cu atoms maintain the original coordination geometry after milling for 160 hours. On the contrary, the local structures around Cu atoms in both of Fe₈₀Cu₂₀ and Fe₉₀Cu₁₀ alloys appear a transition from fcc to bcc structure. We found that the Debye-waller factor σ of fcc Fe-Cu phase is larger than that of bcc Fe-Cu phase, and the σ (0.099 Å) around Fe atoms is larger than that (0.089 Å) of Cu in the Fe_100-xCu_x (x ≥ 40) solid solutions. This suggests that the mechanically alloyed Fe_100-xCu_x supersaturated solid solution is not a homogeneous alloy, and consists of Fe-rich and Cu-rich regions for various compositions. A possible mechanism for bcc-to-fcc and fcc-to-bcc changes in Fe_100-xCu_x solid solutions is discussed in relation to the interdiffusion and transition induced by the ball milling.