On the formation of κ-carbides in low-density steels

This work aims to clarify the fundamental precipitation mechanisms in the Fe–Al–Mn–C system, spanning from atomic-level processes to the overall mechanical response. A multiscale characterization approach, including high-resolution transmission electron microscopy, atom probe tomography, and synchrotron X-ray diffraction, was combined to characterize the evolution of κ-carbides during aging, providing atomic-level insights into their formation. This enabled the analysis of carbide size, morphology, interparticle spacing, and volume fraction at different stages of aging. Tensile and hardness tests on samples aged at 550 ℃ for different times were performed to correlate macroscale mechanical properties with the nanoscale κ-carbides. The results show that spinodal decomposition occurs throughout the 8 h of aging, forming solute-rich regions. In contrast, the formation of κ-carbides begins in the solute-rich areas through short-range ordering in the early stages of aging, i.e., within 30 min at 550 ℃, and both the fraction and size increase with longer aging times. After 2.5 h of aging, the fraction and lattice parameter of the κ-carbides saturate and stop increasing. The yield strength and hardness follow the same trend as the κ-carbide fraction, with a steep increase in the initial stages of aging, up to 2.5 h, reaching 925 MPa and 360 HV, and then saturating after 2.5 h. This work provides a clear mechanistic understanding of the underlying strengthening mechanisms and the resulting mechanical behavior.