MECHANICALLY DRIVEN ALLOYING AND STRUCTURAL EVOLUTION OF NANOCRYSTALLINE FE60CU40 POWDER

Highly supersaturated nanocrystalline fee Fe60Cu40 alloy has been prep ared by mechanical alloying of elemental powders. The phase transforma tion is monitored by X-ray diffraction (XRD), Mossbauer spectroscopy a nd extended X-ray absorption fine structure (EXAFS). The powder obtain ed after milling is of single fee structure with grain size of nanomet er order. The Mossbauer spectra of the milled powder can be fitted by two subspectra whose hyperfine magnetic fields are 16 MA/m and 20 MA/m while that of pure Fe disappeared. EXAFS results show that the radial structure function (RSF) of Fe K-edge changed drastically and finally became similar to that of reference Cu K-edge, while that of Cu K-edg e nearly keeps unchanged in the process of milling. These,imply that b cc Fe really transforms to fee structure and alloying between Fe and C u occurs truly on an atomic scale. EXAFS results indicate that iron at oms tend to segregate at the boundaries and Cu atoms are rich in the f ee lattice. Annealing experiments show that the Fe atoms at the interf aces are easy to cluster to alpha-Fe at a lower temperature, whereas t he iron atoms in the lattice will form gamma-Fe first at temperature a bove 350 degrees C, and then transform to bcc Fe.