The mechanical properties and deformation behavior of each constituent layer of multilayered steel composites were examined using microtensile testing. Three-layered integrated steels consisting of SUS420 and SPCC (cold-reduced carbon steel sheets) were fabricated by a cold-rolling process. Different heat treatment processes were used to prepare three types of specimens (as-rolled, 823K-2 min heat-treated, and 823K-500 min heat-treated), and the effect of heat treatment on their mechanical properties was investigated. In the as-rolled specimens, the average tensile strengths in the SUS420 and SPCC layers were 1063 and 606 MPa, respectively, while in the specimens heat-treated for 500 min, they were 680 and 451 MPa, respectively. The tensile strength decreased with the increase in the heat treatment time. The tensile strength of the specimens was also calculated by using the rule of mixture. For the as-rolled specimens and the 823K-2 min heat-treated specimens, the calculated value was consistent with the measured value; however, for the 823K-500 min heat-treated specimens, the calculated value was lower than the measured value. This result suggests that the necking of this layered structure was effectively obstructed by the outer ductile layer. The micromechanical characterization technique used in this study is useful not only for investigating deformation behavior but also for designing multilayered steel composites with superior mechanical properties.

New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles.

Structures of nanoparticles and their role in dual-ion irradiated Fe-16Cr-4.5Al-0.3Ti-2W-0.37Y2O3 (K3) ODS ferritic steel produced by mechanical alloying (MA) were studied using high-resolution transmission electron microscopy (HRTEM) techniques. The observation of Y4Al2O9 complex-oxide nanoparticles in the ODS steel imply that decomposition of Y2O3 in association with internal oxidation of Al occurred during mechanical alloying. HRTEM observations of crystalline and partially crystalline nanoparticles larger than ~2 nm and amorphous cluster-domains smaller than ~2 nm provide an insight into the formation mechanism of nanoparticles/clusters in MA/ODS steels, which we believe involves solid-state amorphization and re-crystallization. The role of nanoparticles/clusters in suppressing radiation-induced swelling is revealed through TEM examinations of cavity distributions in (Fe + He) dual-ion irradiated K3-ODS steel. HRTEM observations of helium-filled cavities (helium bubbles) preferably trapped at nanoparticle/clusters in dual-ion irradiated K3-ODS are presented.

A systematic first-principles study was conducted on the stability of binary iron carbides. The calculations showed that all the binary iron carbides are unstable relative to the elemental solids (α-Fe and graphite). Apart from a cubic Fe23C6 phase, the energetically most favorable carbides exhibit hexagonal close-packed (hcp) Fesublattices. Structural relaxation of the hcp iron carbides was analyzed and discussed together with their relative thermodynamically stability. Finite-temperature analysis showed that contributions from lattice vibration and anomalous magnetic ordering (Curie-Weiss behavior), rather than from the conventional lattice mismatch with the matrix, are the origin of the high stability and predominance of cementite among the iron carbides in steels.

Cluster Variation Method (CVM) has been recognized as one of the most reliable theoretical tools to incorporate wide range of atomic correlations into a free energy formula. By combining CVM with electronic structure total energy calculations, one can perform first-principles calculations of alloy phase equilibria. The author attempted such CVM-based first-principles calculations for various alloy systems including noble metal alloys, transition-noble alloys, III-V semiconductor alloys and Fe-based alloy systems. Furthermore, CVM can be extended to two kinds of kinetics calculations. One is Path Probability Method (PPM) which is the natural extension of the CVM to time domain and is quite powerful to investigate atomistic kinetic phenomena. The other one is Phase Field Method (PFM) with the CVM free energy as a homogeneous free energy density term in the PFM. The author’s group applied the latter procedure to study time evolution process of ordered domains associated with disorder-L10 transition in Fe-Pd and Fe-Pt systems. CVM has, therefore, a potential applicability for the systematic studies covering atomistic to microstructural scales. It has been, however, pointed out that the conventional CVM is not able to include local lattice relaxation effects and that the resulting order-disorder transition temperatures are overestimated. In order to circumvent such inconveniences, Continuous Displacement Cluster Variation Method (CDCVM) has been developed. Since first-principles CDCVM calculations are still beyond the scope at the present stage, preliminary results on the two dimensional square lattice and an fcc lattice with primitive Lennard-Jones type potentials are demonstrated in the last section.

In this study, we use THz spectroscopy in the energy range between 45 GHz to 3000 GHz (1.5 cm-1 and 100 cm-1) as a non-destructive diagnostic tool to characterize the corrosion by-products (rust) on aged steel structural components that are usually embedded in concrete. The THz radiation has been shown to penetrate concrete with extinction coefficients between (1.0 to 2.3) x 10-2 GHz-1 cm-1, depending on the composition and the moisture content of the concrete. The previously reported antiferromagnetic resonance (AFR) near 140 GHz in iron oxide composites was found to be less than our current detection sensitivity (fractional absorbance sensitivity of >10%). However, a strong transition centered near 725 GHz (24 cm-1) has been observed for the first time. This feature has appeared in reflection from several different samples of mildly corroded steel plates and has been tentatively attributed to a broad phonon density of states commonly referred to as the “Boson peak” found in disordered materials. By taking advantage of this strong transition and a powerful excitation sources in the AFR region, we expect THz spectroscopy and imaging to be an effective diagnostic tool with broad applications in corrosion diagnostics and inspection.

Tensile-shear overload tests of spot welded cold-rolled 301LN plates deforming with slow and fast rates were carried out. The deformation behaviors of spot welds can be divided into two stages. Loading force with fast deformation rate increased more along with displacement in the first stage, while the force with slow rate increased more in the second stage. 21and 32(v.%) α´-martensite were introduced by failure deformation with fast and slow rates respectively. The hardness under the fracture surfaces was higher than HV400, which was more than two times of their original. The ultimate strengths and fracture energy absorptions of spot welds deforming with slow rate were higher than that with fast rate, especially for spot-welded thicker plates.

High Mn TWinning-Induced Plasticity (TWIP) steels have mechanical properties which make them suitable for effective vehicle mass containment and an enhanced passenger safety in automotive applications. High Mn TWIP steels with additions of C and Al are fully austenitic at room temperature and have a stacking fault energy (SFE) within the narrow range of 20-30 mJ/m2 required for mechanical twin formation. The present contribution reviews the state-of-the-science on TWIP steels, and highlights those areas where there is still a lack of fundamental understanding of their properties, such as the effect of the anti-ferromagnetic transition, the influence of interstitial C, the twinning mechanism, the effect of slip and twinning on the crystallographic texture evolution and the delayed fracture phenomenon.

Steels with transformation induced plasticity (TRIP) offer an excellent combination of high strength and ductility. The transformation of meta-stable austenite into martensite during straining leads to strong local hardening and prevents early localization of strain. Therefore, the mechanical properties of TRIP steels, including the damage resistance depend to a significant extent on the stability of retained austenite. The aim of this study was to evaluate the effect of texture on the stability of retained austenite. In order to compare the changes in both tension and compression the steel was deformed by a micro 3-point-bending device. The texture development upon bending was followed by electron backscatter diffraction (EBSD) technique. Based on a simple analysis using the relation between face centered cube (FCC) and body centered cube (BCC) shear geometries theoretically expected changes of texture components due to deformation are proposed. Using the results of this analysis the observed changes of the austenite texture due to deformation could be distinguished from those due to transformation, by comparing the experimental results with the theoretically expected behavior. From this comparison, austenite grains with “Brass (B) {011} <211>” and “Goss (G) {110} <100>” texture components were found to transform into martensite much easier than differently oriented grains.

A unified physically-based representation of the microstructure in martensitic steels is developed to investigate its effects on the initiation and evolution of failure modes at different physical scales that occur due to a myriad of factors, such as texture, grain size and shape, grain heterogeneous microstructures, and grain boundary (GB) misorientations and distributions. The microstructural formulation is based on a dislocation-density based multiple-slip crystal plasticity model that accounts for variant distributions, orientations, and morphologies. This formulation is coupled to specialized finite-element methods to predict the scale-dependent heterogeneous microstructure, and failure phenomena such as shearstrain localization, and void coalescence.

Plastic deformation in body centered cubic iron is dominated by glide of screw dislocations with non-planar dislocation cores. This causes a strong strain rate and temperature dependence of flow stress, the breakdown of Schmid’s law and a dependence of dislocation mobility on shear stress components that do not contribute to the mechanical driving force for dislocation glide. Based on the framework of crystal plasticity, we developed a constitutive plasticity model that takes all these phenomena into account. To parameterize this continuum plasticity model molecular statics simulations using a semi-empirical potential have been performed. These atomistic calculations yielded quantitative relationships for the influence of all components of the local stress tensor on dislocation mobility. Together with experimental data from the literature on the kinetics of screw dislocations in bcc iron the constitutive relation presented here has been developed. As application example of the model, we calculated the tension compression asymmetry and the strain rate dependence of the hardening behavior within a bcc iron crystal.

Stainless steels are among the most important engineering materials, finding their principal scope in industry, specifically in cutlery, food production, storage, architecture, medical equipment, etc. Austenitic stainless steels form the largest sub-category of stainless steels having as the main building blocks the paramagnetic substitutional disordered Fe-Cr-Ni-based alloys. Because of that, austenitic steels represent the primary choice for non-magnetic engineering materials. The presence of the chemical and magnetic disorder hindered any previous attempt to calculate the fundamental electronic, structural and mechanical properties of austenitic stainless steels from first-principles theories. Our ability to reach an ab initio atomistic level approach in this exciting field has become possible by the Exact Muffin-Tin Orbitals (EMTO) method. This method, in combination with the coherent potential approximation, has proved an accurate tool in the description of the concentrated random alloys. Using the EMTO method, we presented an insight to the electronic and magnetic structure, and micromechanical properties of austenitic stainless steel alloys. In the present contribution, we will discuss the role of magnetism on the stacking fault energies and elastic properties of paramagnetic Fe-based alloys.

The work presented gives an insight into using formation enthalpies determined from ab initio calculations for computing solubility products in steels. The role of enthalpy and entropy contributions to the solubility product is discussed. As an illustration of the method, we present solubility products for observed stoichiometric precipitate phases in ferrite from first-principles calculations and in austenite as obtained from the combined approach based on ab initio and experimental phase diagram analysis. The results are compared with experimental data where available.

A high strength ferritic steel with finely dispersive precipitates was investigated to reveal the fundamental strengthening mechanisms in this alloy. Using energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscope (TEM), fine carbides with an average diameter of 10 nm were observed in the ferrite matrix of the 0.08%Ti steel, and some cubic M23C6 precipitates were also observed at the grain boundaries and the interior of grains. The dual precipitate structure of finely dispersive TiC precipitates in the matrix and coarse M23C6 at grain boundaries provides combined matrix and grain boundary strengthening. The resulting yield stress is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. The effect of the particle size, particle distribution and intrinsic particle strength have been investigated through dislocation dynamics (DD) simulations and the relationship for resolved shear stress for single crystal under this condition has been presented using simulation data. The results show that the finely dispersive precipitates can strengthen the material by pinning the dislocations up to a certain shear stress and retarding the recovery as well as annihilation of dislocations. The DD results also show that strengthening is not only a function of the density of the nano-scale precipitates but also of their size. This size effect is explained using a mechanistic model developed based on dislocation-particle interaction.