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.

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

Quenching and partitioning (Q&P) treatment has been proven effective in manufacturing advanced high strength steels with high content of retained austenite, showing the improved balance of high strength and sufficient ductility. This method has been very well elaborated for carbon steel processing over the last two decades. Though it can also be potentially applied for processing other steel families, this has been scarcely studied. This article focuses on the effect of chemistry and heat treatment parameters on the microstructure and properties of Q&P treated martensitic stainless steels. Three different martensitic stainless steels with different contents of alloying elements are subjected to Q&P processing with varying quenching temperature or partitioning temperature and partitioning time. The tensile behavior of the Q&P treated steels is studied. The effect of chemistry and Q&P treatment parameters on the microstructure and tensile properties is analyzed. The effect of plastic deformation on the microstructure of the Q&P treated steels is also investigated. It is demonstrated that the Q&P treated martensitic stainless steels can show a good combination of enhanced strength and sufficient tensile ductility. Their uniform elongation increases with the increasing volume fraction of retained austenite due to the transformation induced plasticity (TRIP) effect. The ability of the martensitic matrix to accumulate plastic deformation also plays an important role. The Q&P process - microstructure - property relationship is discussed.

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.

Recent studies have demonstrated the viability of quenching and partitioning (Q&P) treatment for processing martensitic stainless steels showing an improved balance of high strength and sufficient ductility. However, to date, the fatigue behaviour of these materials has not been explored. This study examines the effect of their complex hierarchic microstructure on high cycle fatigue performance. Three steels with different alloying element contents underwent Q&P processing, resulting in multiphase microstructures rich in retained austenite. 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 Q&P treated martensitic stainless steels, surpassing traditional counterparts. Fatigue cracks predominantly form and propagate along martensite packet and block boundaries, while prior austenite grain boundaries and MnS inclusions have minimal influence on fatigue crack formation and growth. Microplastic deformation at the fatigue crack tip enhances local KAM values and triggers localized transformation of retained austenite grains. It is hypothesized that the developed Q&P treated martensitic stainless steels exhibit improved resistance to low cycle fatigue.

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.

The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients through the sheet thickness resulting in a fast transformation of retained austenite, as well as the ability of the tempered martensitic matrix to accumulate plastic strain. Surprisingly, a Q&P steel with the highest volume fraction of retained austenite and highest tensile ductility shows the lowest formability among studied grades. The latter observation is related to the highest sum of fractions of initial fresh martensite and stress/strain induced martensite promoting formation of microcracks. Their role and ability of tempered martensitic matrix to accumulate plastic deformation during forming of Q&P steels is discussed.