Analysis of the diffraction-line broadening on nanostructured Fe: size-strain effects induced by milling and heating.

The microstructure of ball milled Fe powder as a function of the milling time has been investigated using room temperature x-ray powder diffraction and transmission electron microscopy. The powder microstructure changes when the milling time increases in a twofold way: (i) a reduction of the crystalline grain size to around 20 nm after 80 h of milling time and (ii) a significant amount of microstrain is induced (up to similar to 0.75%), together with a slight increase of the crystalline lattice parameter. Moreover, the temperature dependence of the microstructure has been studied by means of in situ neutron powder thermo-diffraction in the range between 300 and 1220 K for the sample milled for 80 h. The heating of the nanostructured powder produces a progressive grain growth starting at around 450 K, and the disappearance of the microstrain above 850 K due to relaxation processes induced by thermally activated atomic diffusion. The kinetics of both processes at two different heating rates of 1 and 10 K min(-1) has been compared. A detailed analysis of the diffraction patterns has been performed using the Rietveld method. All this microstructural information can be correlated with the temperature dependence of the magnetization of nanostructured Fe and the differences found with regard to the case of bulk Fe.