利用小角中子核磁散射分离研究 AerMet100 钢中纳米析出相的形貌和成分

AerMet100 ultrahigh-strength steel is widely used in aircraft landing gears because of its excellent mechanical properties. Its high strength mainly results from the secondary hardening effect of a large amount of needle-shaped M₂C-type precipitates generated during the alloy tempering process. Given the nanoscale size and coherency with the martensitic matrix, the size and composition of the precipitates in the AerMet100 steel are difficult to extract and accurately characterize by using traditional imaging techniques. In this study, the size, distribution, and composition of M₂C-type needle-like precipitates in Aer-Met100 steel tempered for 5 h at 454, 482, 486, and 498^oC werequantitatively characterized by combining TEM, XRD, and small-angle neutron scattering (SANS) techniques. By applying a transverse magnetic field of 1.1 T during the SANS experiment and reducing the data along the parallel and perpendicular magnetic field directions, the separation of nuclear and magnetic scattering data was achieved. Model fitting of the nuclear scattering curves revealed an average length of 6-16 nm and an average diameter of 1-2 nm for the needle-like precipitates in the alloy. The volume fraction of the needle-like phase increases with the tempering temperature ranging from 0.54% to 5.19%. Given the nanodomains between the precipitates and matrix phase with spin misalignment, the structural parameters obtained from magnetic scattering data are much bigger than the nuclear ones. Furthermore, by comparing the neutron scattering length densities of the carbides and matrix phases, the possible chemical composition and physical density of the needle-like precipitates within are (Cr_0.4Mo_1.6)C and 8.55 g/cm³, respectively. This study demonstrates nondestructive quantitative neutron scattering analysis of the nanoscale morphology and chemical composition of nanophases in ferromagnetic alloys.