Metastable structures of immiscible FexCu100-x system induced by mechanical alloying

The immiscible FexCu100-x alloys have been investigated by x-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM). The TEM results show that the smaller size nanocrystalline particles are obtained by milling for 160 h for Fe-Cu alloys with higher Cu concentration (Fe60Cu40). The XAFS results indicate that the local structure of Fe atoms in Fe60Cu40 gradually changes from a bce structure to a fee one while the Cu atoms maintain the original coordination geometry. In contrast, the local structures around both Fe and Cu atoms in Fe80Cu20 alloys which have a larger average grain size indicated a bcc structure after 160 h milling. This suggests that in a mechanical alloying process the grain size strongly affects the alloying limit in the immiscible region. Possible mechanisms of bcc-to-fcc change in Fe60Cu40 are discussed in relation to the interdiffusion and the internal pressure effect as a result of small grain size.