Structural transitions of mechanically alloyed Fe 100− xCu x systems studied by X-ray absorption fine structure

Abstract

The local structures of the immiscible Fe 100− x Cu x alloys ( x=0, 10, 20, 40, 60, 80 and 100) produced by mechanical alloying have been investigated by X-ray absorption fine structure technique. For the Fe 100− x Cu x solid solutions with x⩾40, the local environment around the Fe atoms changes from bcc to fcc structure and the Cu atoms maintain the original coordination geometry after milling for 160 h. In contrast, the local structures around the Cu atoms in both Fe 80Cu 20 and Fe 90Cu 10 alloys exhibit a transition from fcc to bcc structure. Furthermore, we found that the coordination numbers N in the first shell of the Fe and Cu atoms were largely deviated from the stoichiometric composition for the Fe 100− x Cu x solid solutions with x⩾40. The Debye–waller factor σ of the fcc Fe–Cu phase is larger than that of the bcc Fe–Cu phase, and the σ (0.099 Å) around Fe atoms is larger than that around Cu atoms (0.089 Å) in the Fe 100− x Cu x solid solutions with x⩾40. This indicates that the mechanically alloyed Fe 100− x Cu x supersaturated solid solutions with x⩾40 is not a homogeneous alloy, but consists of fcc Fe-rich and fcc Cu-rich regions. However, In Fe 100− x Cu x solid solutions with x⩽20, the Cu atoms were almost homogeneously solved into the bcc Fe–Cu phase. A possible mechanism for bcc-to-fcc and fcc-to-bcc changes in Fe 100− x Cu x solid solutions is discussed in relation to the interdiffusion and the transition induced by the ball milling.