Triple line diffusion in nanocrystalline Fe/Cr and its impact on thermal stability

The thermal reaction of iron–chromium multilayers is analyzed by atom-probe tomography. Samples were prepared using ion-beam sputter deposition and cutting by focused ion beams. Isothermal and isochronal annealing sequences were carried out in a vacuum furnace. Effects of atomic transport are observed at temperatures above 773 K. Segregation along line-shaped zones is noticed to very high concentrations. These zones, with a diameter of 1.5 nm, are identified as triple lines of the grain structure. While these defects could not be resolved by TEM, the outstanding potential of a 3D analysis provided by APT allowed their detailed investigation. Evaluating the dependence of the segregation amplitude on time and temperature, the segregation enthalpy and diffusivity of the triple lines are quantified. The segregation enthalpy is determined to be 0.076 eV, which indicates the considerable excess volume at the triple line.

Research Highlights
► Triple lines (TL) are topologically required linear defects of grain boundary structure and may control the properties of nanocrystalline material.
► The thermal stability of a nanocrystalline Fe/Cr multilayer system is investigated by atom-probe tomography.
► A clear segregation tendency of TLs is observed, exceeding grain boundary segregation.
► Segregation enthalpy of 0.076 eV is determined, 40% less than experimentally determined surface segregation energy.
► The segregation enthalpy indicates a considerable excess free volume at the TL.