On the two-state microstructure of nanocrystalline chromium

High-angle neutron powder diffraction was used to investigate the grain size dependence of the Debye-Waller parameter (DWP) of nanocrystalline and coarse-grained chromium samples. The DWP measured at 20 K depends linearly on inverse grain size and is consistent with a two-state model in which defects with short-ranged displacement fields are present in differing concentrations in two distinct microstructural regions within individual grains. One possible model microstructure consists of grain boundary and free surface regions with significantly larger concentrations of point defects than in grain interiors. Evidence is also seen for an enhancement of the temperature-dependent component of the DWP of chromium with decreasing grain size, indicating different behavior than seen previously for nanocrystalline palladium.