Homogeneity of mechanically alloyed nano-crystalline Fe-Cu-powders

Abstract

In this contribution Atom Probe Tomography has been applied systematically to characterise the homogeneity of mechanically alloyed Fe-Cu powders for two different compositions (Fe-10 at.% Cu, and Cu-2.5 at.% Fe). Fe-Cu is among the most attractive systems for technical application since it exhibits the Invar effect. Additionally it serves as a binary model system and hence is representative for those immiscible systems characterised by a large positive heat of mixing. Results on the extension of the solubility limits up to 10 at.% and on the homogeneity of the alloy - depending on composition and milling time - are presented, discussed and compared to earlier works. Chemical homogeneity of the sample as measured by the atom probe was never fully reached on the nano-scale, even for the longest milling time and for those samples which appear to be homogeneous via X-ray analysis. Furthermore, impurities such as oxygen, stemming from the preparation procedure, were frequently detected and analysed for the different samples. In addition, structural characterisation was performed using transmission electron microscopy and diffraction measurements to confirm the transition of Fe from bcc to fcc. Thus, nano-scale information on the local concentration, segregation effects and the distribution of impurities for the different nominal compositions and processing parameters could be gained.