Three generations of advanced high strength steels (AHSS) have attracted considerable attention due to their excellent mechanical properties and relatively low cost. While there has been extensive research on the basic mechanical properties of AHSS, the impact energy absorption capacity, a critical property for ensuring passenger safety, has not been systematically investigated. In addition, the absence of standardized impact testing protocols for materials or structures hinders the comparison of results across different studies. The present review aims to provide a comparative analysis of the impact performance of thin-walled structures and sheet specimens made from the three generations of AHSS. First, an introduction to the background of AHSS is presented. Widely used experimental techniques and specimen geometries are then reviewed. This is followed by a detailed review of recent relevant studies on the first, second, and third generations of AHSS. Emphasis is placed on investigating the influence of microstructure on impact performance and the underlying mechanisms governing high-strain-rate plastic deformation under impact loading. Various strategies to improve the impact performance of AHSS are also discussed.

Increasing train speeds and the reduction of maintenance slots places high demands on the railway rails. To meet the challenging demands, producers regularly introduce new steel types. In this experimental investigation is the mechanical behavior of an air-cooled vanadium-alloyed hypereutectoid rail steel presented. The rail is produced applying conventional hot rolling of a reheated bloom and is then cooled on a cooling bed. The mechanical behavior is determined by performing standardized linear elastic fracture mechanics tests. The necessary specimens are extracted from new rails that are made in series production. Monotonic tensile test results have shown that the strain-hardenability of the steel is comparable to standard grade eutectoid rail steel and is higher than that of an accelerated-cooled eutectoid rail grade. The fracture toughness test results showed, statistically, no difference when compared with the fracture toughness values of the accelerated-cooled eutectoid rail grade. The tests were performed at room temperature. The fatigue crack growth rates are, in the linear Paris-regime, slightly higher than in the previously mentioned steels. The results are explained considering the distinct microstructural characteristics of the air-cooled vanadium-alloyed hypereutectoid steel and the fractured surface of the specimens. This experimental investigation contributes to selecting railway steels and predicting the actual in-service behavior.

In sparsely populated areas single-track railway lines are still common. Despite the low-density traffic and low axle loads, rail damage is observed to initiate in the rails of these lines. Not only large cracks requiring repair are found, but also newly initiated, post-grinding damage is observed. Rail specimens from the track containing representative damages are extracted to identify the reasons for the damage initiation in the R260Mn steel rail. At the rail surface, three years after the preventive grinding maintenance, the characteristic grinding facets and roughness patterns are still present. White etching layers are observed to surround the residual grinding grooves, maintaining the roughness as a result of the high wear resistance of these layers. The strain orientation at the gauge side of the rail is uni-directional due to lateral creep in the wheel–rail contact while in the center of the contact surface the strain patterns evidence shear stress reversal associated with the bi-directional traffic. The damage initiation mechanism after preventive grinding is associated with low-wear conditions and stresses concentrated around long-lasting grinding grooves. The findings show that preventive grinding maintenance specifications for regional single-stock railway lines must be improved. Specific points of interest are stringent requirements on the number of facets and surface roughness, as well as directions for the removal of corrugation.

Preventive grinding of rails is a recurring maintenance routine to remove damage initiated in the wheel–rail contact. The grinding routine increases the service life of rails and reduces operational costs. Despite these benefits, grinding-related defects are observed. In this work a field test is performed to investigate the contact-surface formation and to better understand its durability. Surfaces after grinding are studied at different stages of the test to characterize wear mechanisms and deformation. The freshly ground surface exhibits a higher roughness and is composed of facets. It is determined that roughness asperities are extruded and fill grinding grooves in the process. High contact stresses at the facet transitions accelerate the extrusion of roughness asperities and the fast formation of the contact surface. The analysis further shows that deeper grinding grooves prevent homogeneous deformation. Strain concentrations arise due to the inhomogeneous deformation leading to damage initiation sites. These grooves are still present in the rail surface after the test. The evolution of the ground surface is captured in a schematic wear model.

Composite wires obtained from aluminium shell, micron sized aluminium powder and nano - sized TiN, SiC and Al2O3 powders were fabricated by a two stage process. The stages included ultrasonic treatment and hot extrusion. TEM and SEM observations were performed to investigate the wires’ microstructure. Individual nano powder particles as well as particle agglomerations were observed by two microscopic methods, as well as nanoparticles distribution and localization in the matrix. It was found that the observed nanoparticles and clusters were in the volume of micron-sized Al particles and along the grain boundaries of the matrix. The provided EDX analysis confirms the presence of nanoparticles in the aluminium matrix. From the obtained results, it can be concluded that the two stage fabrication process of nanocomposite wires leads to better wetting of the nano powders from the aluminium melt. Nanocomposites obtained by mixing and extruding TiN, SiC and Al2O3 nano powders and Al micro powders can be used as modifiers of aluminium alloys, as the nanoparticles will contribute to grain refinement and will participate as additional crystallization centers.

The combination of mechanical properties (strength, toughness, and crack arrestability) and weldability that is required of steel pipelines has led to the pipeline steel grades that are used today. The only way to control the mechanical properties of these grades is by controlling microstructure and crystallographic texture of the steel plates. In this chapter, descriptions are provided of the microstructure and texture in the most commonly used pipeline steel grades. The composition ranges of these grades are summarized, together with a brief discussion of segregation effects. After a short historical review, the general principles of microstructure and texture formation during the thermomechanically controlled processing of plate and sheet are discussed. These are illustrated with examples taken from the literature and the authors' research. Special attention is paid to texture formation, methods of texture control, and the influence of texture on mechanical properties, anisotropy, and fracture behavior of these microalloyed steels.

This study compares the fracture toughness of high-speed steel produced by powder metallurgy and subjected to different heat treatments to obtain either martensitic or bainitic/martensitic microstructures. The heat-treatment process involved austenitization at 1150 °C, followed by either martempering or austempering at 235 °C, and final tempering. Microstructural analysis was performed using electron backscatter diffraction (EBSD), field-emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). Fracture toughness was evaluated using circumferential notched tensile (CNT) specimens. The results showed that austempered CNT samples exhibited significantly higher fracture toughness compared to martempered ones, indicating improved resistance to crack propagation. Microstructural characterization revealed distinct differences: the austempered samples featured bainitic laths, retained austenite blocks, and martensite plates, whereas the martempered sample contained martensite plates and austenite islands. However, small differences in prior austenite grain size, lath thickness, and dislocation density were insufficient to fully account for the enhanced toughness in the austempered sample. Further analysis indicated that the increased fraction of high-angle grain boundaries and higher kernel average misorientation (KAM) in the austempered sample acted as effective barriers to crack propagation. Additionally, a greater volume fraction of nano-sized carbides contributed to a more pronounced strengthening effect, further enhancing fracture toughness.

Wire arc additive manufacturing (WAAM) is a significant area of interest within the field of additive manufacturing (AM). In the present research, WAAM technology was employed to deposit a Ni-based alloy on a ductile cast iron substrate to fabricate a bimetallic structure of Ni-45 %Fe alloy and ductile cast iron. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) were used to study phase transformations, microstructure and crystallographic texture development in interfacial regions as well as deposited material. The mechanical properties were also studied using micro-hardness and profilometry-based indentation plastometry (PIP) measurements. The results showed that a wide variety of phases are generated within the heat-affected zone (HAZ) and partially melted zone (PMZ). These phases form complex microstructures with single and double shell morphology. The deposited alloy has a face-centred cubic (FCC) structure, with some carbides and graphite that are formed during the solidification of the first deposited layer. The compositional changes were also observed across the interface. The texture of the deposited alloy showed around 30° deviation from 〈100〉 II building direction due to the shape and overlap of the melt pools. The present results provide a better understanding of interface development mechanisms during WAAM of bimetallic structures. The peak of the hardness across the interface was observed in PMZ because of the formation of a martensitic matrix. The PIP measurements showed that the σ_y and the UTS of deposited alloy are lower than the cast iron base metal.

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.

Depending on the alloy composition, intercritical annealing may provide different phases in the microstructure. For low-alloyed dual-phase (DP) steels it is usually ferrite and martensite, while for medium-Mn steels retained austenite is also formed. In a present study, a wide intercritical temperature range was applied to a 5% Mn steel to investigate possible microstructure combinations: ranging from fully ferritic, through ferritic-austenitic, multiphase, to fully martensitic, which were next investigated in terms of mechanical properties to clarify the behavior of this type of material. The obtained results together with technological issues and economic indicators were next compared to mechanical properties of typical DP steels in order to assess the possibility of replacing this material in car production. The mechanical properties were evaluated using static tensile and hardness tests. The phase composition was determined qualitatively and quantitatively using dilatometry, X-ray diffraction measurements, and electron backscatter diffraction analysis. The results suggest that both initial austenite and martensite fractions have a decisive influence on the yielding and elongation of steel; however, the tensile strength depends mainly on the sum of martensite initially present in the microstructure and the strain-induced martensite formed from the plastically deformed austenite regardless of the initial retained austenite—martensite ratio. The results indicate superior total elongation of medium-Mn steels reaching 30% compared to DP steels with a similar strength level in the range between 900 and 1400 MPa. However, medium-Mn steels could be a significant competitor to dual phase steels only if some technological problems like discontinuous yielding and serrations are significantly reduced.

Additively manufactured Nitinol (NiTi) architectured materials, designed with unit cell architectures, hold promise for customisable applications. However, the common assumption of homogeneity in modeling and additive manufacturing of these architectured materials needs further investigation because geometric-dependent melt pool behaviour results in inhomogeneous microstructure and thermomechanical properties. This study shows that property inhomogeneity at the mesoscale is one reason for pseudo-linear response and partial superelasticity of the fabricated NiTi body-centered cubic (BCC) architectured materials. We modeled using a phenomenological constitutive relation and additively manufactured NiTi architectured materials with varying relative densities. These fabricated samples showed distinct microstructural textures and compositions that affected their local recoverability. The edge effects and laser turn regions were identified as the causes underlying the observed microstructural inhomogeneity. The dimensionless Fourier number is used to describe the transition of printing modes. This study provides valuable information on rigorous experimental/computational consistency in future work.

Rolling and annealing is a crucial technology to produce electrical steel sheets. This technology is not just aimed to control the geometry of steel sheets but more importantly to enhance the magnetic properties of the final products via appropriate microstructure and crystallographic texture. In this study, the evolution of microstructures and textures of an Fe-1.2 wt.% Si alloy through the entire processing route (from reheating, warm rolling to annealing) is monitored by electron back-scatter diffraction. Plastic flows of the material during conventional and asymmetric rolling are analyzed in detail based on geometric parameters of the rolling gaps. Deformation textures are accurately predicted by the full-constraint Taylor and advanced Lamel (ALAMEL) crystal plasticity models. The development of recrystallization textures is accounted for by the plastically stored energy in deformed crystals, which in turn is approximated by the plastically dissipated power (i.e., the Taylor factor) as predicted by the full constraint Taylor model. Although asymmetric warm rolling does not produce an improved texture or microstructure for electrical steels, the present study provides useful information on the evolution of the recrystallization microstructure and texture in steels with a complex strain history after asymmetric warm rolling.

Austenitic stainless steels similar to type AISI 316L are widely used structural materials in current and future nuclear reactors. Careful development and characterization of these materials and their welds is needed to verify the structural integrity of large-scale multicomponent structures. Understanding the local deformation behavior in heterogeneous materials and the mechanisms involved is key to further improve the performance and reliability of the materials at the global scale and can help in developing more accurate models and design rules. The full-field 3D digital image correlation (3D-DIC) technique was used to characterize two 316L multi-pass welds, based on cylindrical uniaxial tensile tests at room temperature, 350 °C, and 450 °C. The results were compared to solution annealed 316L material. The inhomogeneous character and dynamic behavior of the 316L base and weld materials were successfully characterized using 3D-DIC data, yielding high-quality and accurate local strain calculations for geometrically challenging conditions. The difference in character of the dynamic strain aging (DSA) effect present in base and weld materials was identified, where local inhomogeneous straining in weld material resulted in discontinuous type A Portevin–Le Châtelier (PLC) bands. This technique characterized the difference between local and global material behavior, whereas standard mechanical tests could not.

The phase transformations, microstructure and properties of two Medium-Mn processed via quenching and partitioning steels were compared in this contribution and a new strategy for controlling mechanical properties by introducing and controlling cold-worked ferrite prior to heat treatment is proposed. It was found that during heating, the recovery and recrystallization of cold-worked ferrite compete with austenitization, thereby inhibiting the coarsening of austenite. The cold-worked ferrite interface will significantly delay the austenitization kinetics during the partitioning local equilibrium stage compared to martensite. These results lead to a diverse parent austenite, as well as a refined martensite substructure. As a result, the randomly distributed variants increase the number of effective grain boundaries, thus enhancing yield strength. The intercritical annealing process at a temperature of 860 °C resulted in the formation of fresh martensite-retained austenite (M/RA) constituents exhibiting a remarkably fine (<2 μm) and uniform grain morphology. Such microstructure yielded substantial improvement in both the strength and ductility of the steel. The proposed treatment led to excellent elongation (24%) at fracture, combined with very high ultimate tensile strength and yield strength of 1345 MPa and 1163 MPa, respectively, of the steel, resulting in a product of strength and elongation that exceed 32 GPa%.

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

Damage in bearings is closely associated with the presence of microstructural alterations, known as white etching areas (WEAs) and white etching cracks (WECs). One of the main reasons for the creation of these microstructural alterations is the presence of defects in the material, such as non-metallic inclusions. Manganese sulfides and aluminum oxides are widely reported in the literature as the most common types of non-metallic inclusions found in bearing steels. This study classifies 280 non-metallic inclusions in an investigated bearing steel according to several criteria: bonded/debonded with the matrix, size, shape, orientation angle, depth below the raceway surface, and chemical composition. Contrary to the findings in the literature, this investigation reports that the chemical composition of the inclusion (MnS + Al₂O₃) is of secondary importance when considering factors for damage initiation. The orientation of the microstructural alterations is observed to coincide with the high-stress regions, indicating a relation between the formation of butterfly wings and the white etching crack. In our investigation, butterfly wings typically exhibit a 45-degree pattern originating from the non-metallic inclusions. Conversely, the white etching crack starts from the non-metallic inclusion at a shallower angle in correspondence to the raceway. This can be attributed to the stress state, which corresponds to a region where extensive white etching cracks are formed. In conclusion, the microstructural observations demonstrate that the state of non-metallic inclusion—i.e., whether they are bonded or not to the steel matrix—plays an essential role in initiating rolling contact fatigue damage.

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 influence of carbon concentration variations on pearlite formation (20 h at 600 °C) in a case-carburized steel is investigated. The resultant microstructure shows three distinct regions: carburized case, a transition region, and the original core. The microstructural transition from the case to the core regions is observed to be relatively sharp. The investigated region of the carburized case (0.9 wt.% C) contains two types of pearlite: ferrite + cementite and ferrite + M₂₃C₆, where the pearlitic aggregate with M₂₃C₆ shows faster formation kinetics. The kinetics of pearlite formation in the transition region (0.3 wt.% C) is very slow and is observed with only M₂₃C₆ carbide. Only around 40% austenite decomposes into pearlite in the transition region, which, in comparison to the carburized case region of 0.9 wt.% C is a fraction that is lower by a factor of two. Pearlite is absent in the investigated core region (0.16 wt.% C). The microstructure in this region is predominantly martensite and pro-eutectoid ferrite, with a fraction of ferrite well below the equilibrium fraction. Ferrite formation in this region is limited by the redistribution of mainly Ni, Mn, and Cr, and their resulting solute drag effect on the austenite/ferrite interface. A thermodynamic and kinetic argumentation of these observations is provided with the help of thermodynamic data, precipitation simulations, and a general mixed-mode Gibbs energy balance model.

Bearing failure is a cause of concern in a variety of machinery such as turbines, transmissions, drills, engines, etc. It is often associated with rolling contact fatigue (RCF) triggered from damage initiation at non-metallic inclusions (NMI's). Experimental evidence shows that damage initiation lifetime is highly sensitive to the NMI characteristics and its bonding with the steel matrix. This study numerically investigates the role of NMI features and its bonding with the steel matrix on damage initiation lifetime. NMI characteristics modelled in this study are derived from an experimental investigation of a failed bearing. Simulation results highlight a near to instantaneous debonding at the matrix-inclusion interface followed by accelerated crack initiation. The critical depth for damage initiation shifts towards the surface with the increase in friction coefficient between roller and raceway. The simulations also reveal that larger inclusions show earlier damage initiation, indicating a size effect. The damage hotspots from the simulation results were compared with experimental findings and a hypothesis for crack initiation from a NMI is put forward.

The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.