Structure of nanometer-sized polycrystalline iron investigated by positron lifetime spectroscopy

Abstract

Nanometer-sized polycrystalline materials are polycrystals prepared by compacting very small crystallites (5--10 nm in diameter) under high pressures. The initial studies of Gleiter and co-workers indicate a wide distribution of interatomic distances within the disordered intercrystalline phase, which can be investigated accurately due to its high relative volume fraction in these materials. In the present paper the investigation of nanometer-sized Fe polycrystals by positron lifetime spectroscopy is reported. The influence of the compacting pressure and thermal annealing was studied. The positron lifetimes ${\ensuremath{\tau}}_{1}$=180\ifmmode\pm\else\textpm\fi{}15 ps, ${\ensuremath{\tau}}_{2}$=360\ifmmode\pm\else\textpm\fi{}30 ps, and long-lived components between 1 and 5 ns, have been observed with saturation trapping of positrons. These values are different from the positron lifetimes in well-annealed bulk iron, in amorphous iron alloys, or in the uncompacted fine nanometer-sized iron crystals (\ensuremath{\tau}=443 ps). Based on the present results the lifetime ${\ensuremath{\tau}}_{1}$ in nanometer-sized Fe polycrystals is attributed to positron trapping in vacancy-size free volumes in the crystallite interfaces. This is in agreement with the hypothesis of an interfacial structure with a wide distribution of interatomic distances. The lifetime ${\ensuremath{\tau}}_{2}$ is ascribed to positron annihilation in microvoids at the intersections of interfaces. Hence, positron lifetime spectroscopy on nanometer-sized polycrystalline materials appears to supply a specific tool for studying the interfacial structure of solids. The long-lived components indicate ortho-positronium (o-Ps) formation in larger voids.