This study uses small-angle neutron scattering (SANS) to investigate vanadium carbide (VC) random precipitation (RP) kinetics in two nanosteels with varying vanadium and carbon contents during aging for up to 10 hours at 600 and 650 °C. Starting from a martensitic microstructure, the evolution of the VC precipitate size distributions is tracked over time. Atom probe tomography (APT) and scanning transmission electron microscopy (STEM) provide complementary characterization of precipitate shape, morphology, and composition. The precipitation process follows a sequence of burst nucleation, rapid growth, and coarsening, driven by enhanced diffusion along dislocations and interfaces. VC precipitates form primarily at martensitic interfaces, adopting predominantly oblate ellipsoidal particles. Analysis of the time-dependent precipitate size indicates that pipe diffusion along dislocations is the dominant diffusion mechanism during coarsening. Using the Ashby-Orowan model, we estimate the strength enhancement from VC precipitation. Maximum VC strengthening occurs at different aging times depending on steel composition. For steels with a high carbon and vanadium content, peak strengthening occurs after 120 minutes of aging at 650 °C. Reducing the carbon and vanadium content prolongs the time required to reach peak strengthening, extending it to 300 minutes at 650 °C. The nuclear-to-magnetic scattering ratio shows a time-dependent evolution indicating a temporal compositional change in the VC precipitates during aging. ... Read more

This paper reports on the two-scale fractal structure of chromatin organization in the nucleus of the HeLa cell. Two neutron scattering methods, small-angle neutron scattering (SANS) and spin-echo SANS, are used to unambiguously identify the large-scale structure as being a logarithmic fractal with the correlation function (r) - ln(r/E). The smaller-scale structural level is shown to be a volume fractal with dimension DF = 2.41. By definition, the volume fractal is self-similar at different scales, while the logarithmic fractal is hierarchically changed upon scaling. As a result, the logarithmic fractal is more compact than the volume fractal but still has a rather high surface area, which provides accessibility at all length scales. Apparently such bi-fractal chromatin organization is the result of an evolutionary process of optimizing the compactness and accessibility of gene packing. As they are in a water solution, the HeLa nuclei tend to agglomerate over time. The large-scale logarithmic fractal structure of chromatin provides the HeLa nucleus with the possibility of penetrating deeply into the adjacent nucleus during the agglomeration process. The interpenetration phenomenon of the HeLa nuclei shows that the chromatin-free space of one nucleus is not negligible but is as large as the volume occupied by chromatin itself. It is speculated that it is the logarithmic fractal architecture of chromatin that provides a comfortable compartment for this most important function of the cell. ... Read more