AerMet100超高强度钢以其优异的力学性能广泛应用于飞机起落架。其超高强度主要源于合金回火过程中产生的大量M₂C型针状析出相的二次硬化作用。由于AerMet100高强钢中的纳米析出相与马氏体基体共格且尺寸细小，无法进行萃取，很难采用传统成像手段对其尺寸和成分进行准确表征。本工作结合TEM和XRD的观测结果，采用小角中子散射(SANS)技术分别对454、482、486和498℃回火5 h的AerMet100合金钢中M₂C型针状析出相的尺寸、分布和含量进行了定量表征。通过在SANS实验时施加1.1 T横向磁场并对数据沿平行和垂直磁场方向按角度规约，实现了对数据的核、磁散射分离。对核散射曲线的模型拟合结果显示，合金中针状析出相平均长度为6~16 nm，平均直径为1~2 nm，针状碳化物相的体积分数随退火温度升高由0.54%逐渐提高至5.19%。磁散射数据得到的碳化物结构参数均大于核散射结果，说明在碳化物与马氏体基体相界面存在自旋错排的纳米磁畴区。此外，通过对碳化物和基体相中子核磁散射长度密度的对比计算，推断出针状碳化物的可能的化学组成和密度分别为：(Cr_0.4Mo_1.6)C和8.55 g/cm³。

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 AerMet100 steel tempered for 5 h at 454, 482, 486, and 498^oC were quantitatively 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.