The structural properties of the metastable fcc Cu-Fe alloys obtained by high-energy ball milling of elemental powders have been studied by transmission electron microscopy. The same technique has been used to study the effect of thermal annealing on the alloyed samples. Experimental observations show that in some Cu-rich grains, even for short processing times, there is an Fe concentration up to about 30 at. % with an essentially flat concentration profile. In the same grains one observes also a relatively low density of dislocations, several small aggregates of point defects and small particles of oxide precipitates all having the same orientation with respect to the matrix. In a few cases it is also possible to observe a lamellar structure with a periodicity of a few nm which has been interpreted as arising from spinodal decomposition. The same origin has been attributed to structures modulated on a similar length scale, observed quite frequently in the annealed samples. All these observations suggest that during plastic deformation there is a transient supersaturation of point defects which increases considerably the atomic mobility and the kinetics of the solid-state reactions in the deformed grains, and that the driving force for alloying arises from the mechanical energy supplied to the sample by the ball impact. It is also shown that the observation of (partial) spinodal decomposition, prior to and after annealing, is compatible with the magnetic properties of these samples if one accepts that Fe atoms in fcc Fe-Cu alloys are present in two electronic states, as observed in other fcc iron-late transition metal alloys. \textcopyright{} 1996 The American Physical Society.