Characterization of nanoscaled heterogeneities in mechanically alloyed and compacted CuFe

Elemental distribution, solute solubility and defect free volume in nanocrystalline restricted-equilibrium Cu-Ag alloys

In this article we study the elemental distribution and solute solubility in nanocrystalline alloys of immiscible components near restricted equilibrium for the case of the binary Cu-Ag system. As predicted from thermodynamic considerations, a grain boundary segregated monophase alloy is observed in the annealed mechanically alloyed state for low Ag content by using atom probe tomography. From the detected Ag solute grain boundary enrichment the segregation free enthalpy is estimated to range between -25 and -49 kJ mol(-1) following the McLean equation, in agreement with values reported for coarse-grained Cu-Ag. The extension of the alloying range is described by a two-domain thermodynamic model that considers the excess free volume in the grain boundaries and the strain in the grain interior on the basis of the universal equation of state at negative pressure. To access the grain boundary volumetric strain experimentally, a method based on a combination of density measurements and microscopical quantification of closed pore areas is presented. Moreover, we apply x-ray diffraction line broadening analysis to determine the local strain amplitude, which yields a root-mean-square microstrain of ~0.3% for a grain size of ~30 nm. It is shown that the grain boundary free volume represents the major origin for the global solubility enhancement in nanocrystalline Cu-Ag at 503 K.

Superferromagnetism in mechanically alloyed fcc Fe(23)Cu(77) with bimodal cluster size distribution

Magnetic measurements, x-ray diffraction and Mössbauer spectroscopy were used to characterize a nanostructured fcc Fe(23)Cu(77) at.% alloy prepared by high-energy ball-milling, addressing in particular the effect of clustering on the nature of the interacting magnetic entities. The interpretation of magnetization measurements leads to the conclusion that grains, whose mean size is ∼16 nm, contain two populations of magnetic Fe-rich nanoclusters with a bimodal size distribution. These two sets of clusters contain about 14 and 400 Fe atoms and have magnetic moments of 30 µ(B) and 860 µ(B), respectively. The inter-cluster ferromagnetic interactions that lead to superferromagnetism with a Curie temperature T(C)∼220 K can be described by a mean field determined by the smaller clusters only, which account for 90% of the magnetization.