Precession Electron Diffraction Multi-tilt Approach for Reliable Carbide Identification in Q&P Steels

Quenching and partitioning steels belong to the third generation of advanced high-strength steels and play a crucial role in the automotive industry. During its production, carbide precipitation occurs during martensite tempering. The carbide phase depends on steel composition and annealing temperature. Identifying the carbide phases is essential for understanding partitioning kinetics during quenching and partitioning and for tailoring material design to achieve the desired macroscopic properties of the final product. This study investigated single carbide precipitates occurring in Fe–0.2C–3Mn–1.1Si and Fe–0.2C–3Mn–1.2Al steel alloys annealed at 320 °C and 420 °C. The carbide phases were identified through a crystallographic analysis via transmission electron microscopy. Following a multi-tilt approach, at least three diffraction patterns were recorded for each carbide at different orientations using zero-loss energy-filtered precession electron diffraction. The comparison of measured lattice spacings and sample tilting angles with simulations enabled the unambiguous identification of θ-Fe 3 C, ε-Fe 3 C and η-Fe 2 C carbides. The multi-tilt approach allowed reliable distinction between the hexagonal ε-Fe 3 C and orthorhombic (quasi-hexagonal) η-Fe 2 C transition carbides. Simultaneous acquisition of martensite diffraction patterns yielded dependable orientation relations between the carbides and the surrounding martensite matrix. The proposed method can be extended to other material systems that contain spatially confined phases that are difficult to distinguish.