Numerous studies have demonstrated the viability of lightweight Fe-Mn-Al-C steels for exhibiting an improved balance of high strength and high ductility in automotive applications. However, their high-cycle fatigue behaviour has been scarcely studied. This work examines the effect of κ-carbides formed during the aging treatment on the high-cycle fatigue performance of an austenitic Fe-29Mn-8.7Al-1C (wt. %) steel. The material is studied in solution-treated, under-aged, and peak-aged conditions. High-cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high-cycle fatigue performance in the aged material, somewhat better than for high Mn steels. Fatigue crack formation and growth occur predominantly via a quasi-cleavage mechanism along the [1 1 1] crystallographic planes, which is also a plane for planar glide and the formation of persistent slip bands during plastic deformation. The nanoscale intragranular κ-carbides in the aged samples interact with the gliding dislocations, resulting in the shearing of nanoscale κ-carbides in a weakly coupled regime. The resistance of particles to shearing is determined by their size, volume fraction, and antiphase boundary energy (γ_APB), which vary during the aging process. The aged Fe-29Mn-8.7Al-1C steel significantly improves the fatigue strength as the formation of persistent slip bands is delayed due to an additional energy barrier related to the shearing of the κ-carbides. This improvement peaks in the under-aged condition and decreases with further aging time.

This study focuses on the microstructure's evolution upon different aging conditions of a high-strength low-density steel with a composition of Fe–28Mn–9Al–1C. The steel is hot rolled, subsequently quenched without any solution treatment, and then aged under different conditions. The microstructure of the samples was studied by means of Scanning Electron Microscopy, Electron Backscatter Diffraction, and Transmission Electron Microscopy. The aging treatment leads to the formation of an ordered face-centered cubic L1₂ phase named κ-carbide. This study aims to characterize the formation and growth of these κ-carbides qualitatively and quantitatively under different aging conditions. Then, an effort is made to relate the fraction and size of this phase with the tensile properties of the steel to determine the optimal aging conditions that will lead to a good combination of strength and ductility. It has been found that the κ-carbides start to form intragranularly through concentration fluctuations of aluminum and manganese inside the austenite grain. Then, with the process of spinodal decomposition, they grow in size coherently with the matrix. During this process, the strength and hardness of the steel increase while maintaining a relatively high elongation. The best combination of high strength and ductility was achieved at the aging condition of 8 h at 550 °C with an ultimate tensile strength up to 1157 MPa and total elongation of 51%. Increasing the aging temperature and time, κ-carbides start to form intergranularly, lose their coherency with the matrix and severely compromise the hardness and strength. The shearing of the carbides during deformation is also studied.

The strengthening via precipitation of nano-sized κ-carbides leads to exceptional strength-ductility balance in low-density steels. During aging, such nanocarbides form through spinodal decomposition by fluctuations in the aluminum and carbon content in the austenite, followed by short-range ordering. At lower aging temperatures and short aging times, the κ-carbides are very fine, coherent with the matrix, and homogeneously distributed. When the aging temperature increases, heterogeneous nucleation initiates on the grain boundaries, and the κ-carbides become coarse and lose coherency with the austenite matrix, leading to the deterioration of the mechanical properties. This work studies a fully austenitic hot rolled Fe-29Mn-8.7Al-0.9C alloy after different aging treatments. Two aging treatments were selected for a detailed study of the microstructure based on the exceptional strength-ductility balance demonstrated by these conditions. The samples aged at 550 °C for 8 h exhibited an ultimate tensile strength of 1141 MPa and strain at failure around 49%. The second aging treatment selected was aging at 600 °C for 1 h, and these samples exhibited an ultimate tensile strength of 1084 MPa, with strain at failure 62%. The size and morphology of the austenite grains and the annealing twins were studied through EBSD. Additionally, the size, morphology, and volume fraction of the nano-sized κ-carbides were studied using TEM. Both aging conditions led to microstructures consisting of a matrix formed by equiaxed austenite grains with homogeneously distributed intragranular κ-carbides. The κ-carbides were coherent with the matrix and showed globular morphology with a diameter between 3 and 6 nm and coherent with the austenite matrix. The interaction between gliding dislocations and κ-carbides was analyzed. It was shown that the mechanical behavior of the studied material is characterized by very high sensitivity to the size of κ-carbides and, therefore, can be tailored by appropriate aging treatment.