Constitutive and transformation kinetics modeling of ε-, α′-Martensite and mechanical twinning in steels containing austenite

In the present study, a physically based constitutive and transformation kinetics model of stress assisted (SA) and strain induced (SI) ε-, α′-Martensite and twinning in steels containing austenite, is presented. The deformation behavior of γ-Austenite, α′-Martensite, α- and δ-Ferrite is modeled in terms of the dislocation density. The macroscopic elastoplastic response of the material in uniaxial tension is calculated with respect to the partitioned stress and strain in each phase using a homogenization method. Inelastic strain accommodation below the yield strength of austenite is considered due to the SA transformation. The kinetics of SA and SI ε-, α′-Martensite and twinning in austenite, are calculated in terms of the ε-, α′- and twin embryo nucleation rate and the subsequent growth of shear bands, promoted by the applied stress and strain, respectively. Heterogeneous nucleation of α′-Martensite only on ε and twin shear band intersections is considered, presenting a stress state dependence. Austenite stability against TRIP and TWIP is modeled as a function of the austenite stacking fault energy and the separation distance of Shockley partial dislocations. Fitting the model to experimental stress-strain data allows for the prediction of α′-, ε-Martensite and mechanical twin fractions, without a prior notion of the transformation kinetics. A MATLAB implementation and an Extended Methodology section are provided as Supplementary Material. The model could be used to aid in the design of novel alloys with exceptional properties, like medium Mn steels. © 2022 Acta Materialia Inc

Composition and processing design of medium-Mn steels based on CALPHAD, SFE modeling, and genetic optimization

Medium-Mn steels are being thoroughly investigated as potential candidates for the 3rd generation of advanced high strength steels. Despite the wide experimental work, limited attempts have been presented to systematically optimize the chemical composition and heat treatment process to obtain desired microstructural features. In the present work, CALPHAD-based thermodynamic and kinetic modeling, coupled with multi-objective genetic optimization was adopted for the development of δ-ferrite containing medium-Mn steels with optimized microstructure, meeting set design requirements associated with retained austenite fraction and stability. A new sub-regular solution model for the prediction of the austenite stacking fault energy (SFE) was developed and compared to experimental literature data. A MATLAB implementation of the SFE model is provided as supplementary material. Pareto optimal compositions and associated process windows were identified via thermodynamic modeling coupled with the NSGA-II algorithm. A single optimized steel was selected for further consideration through kinetic simulation of the entire process chain including solidification, hot-rolling, accelerated cooling, quenching, and intercritical annealing, considering the effect of δ-ferrite on retained austenite stability and the martensite to austenite transformation kinetics. Temporal optimization resulted in the selection of an optimal intercritical annealing time. Model predictions were validated with metallographic observations on two different δ-ferrite containing medium-Mn steels, revealing excellent agreement between predicted and observed phase fractions. © 2020 Acta Materialia Inc

Alloy Design Based on Computational Thermodynamics and Multi-objective Optimization: The Case of Medium-Mn Steels

A new alloy design methodology is presented for the identification of alloy compositions, which exhibit process windows (PWs) satisfying specific design objectives and optimized for overall performance. The methodology is applied to the design of medium-Mn steels containing Al and/or Ni. By implementing computational alloy thermodynamics, a large composition space was investigated systematically to map the fraction and stability of retained austenite as a function of intercritical annealing temperature. Alloys exhibiting PWs, i.e., an intercritical annealing range, which when applied satisfies the given design objectives, were identified. A multi-objective optimization method, involving Pareto optimality, was then applied to identify a list of optimum alloy compositions, which maximized retained austenite amount and stability, as well as intercritical annealing temperature, while minimized overall alloy content. A heuristic approach was finally employed in order to rank the optimum alloys. The methodology provided a final short list of alloy compositions and associated PWs ranked according to their overall performance. The proposed methodology could be the first step in the process of computational alloy design of medium-Mn steels or other alloy systems. © 2017, The Minerals, Metals & Materials Society and ASM International

Fatigue and fracture behavior of pearlitic Grade 900A steel used in railway applications

In overloaded railway networks, rail surface defects developed under rolling contact fatigue (RCF) if left untreated, may propagate to the bulk of the rail and become critical for its structural integrity. To avoid critical situations, the fatigue and damage tolerance behavior of railway steels is of high importance. In this experimental study, the fatigue and fracture performance of a pearlitic Grade 900A rail steel, which is used in the railway network of Attiko Metro in Athens has been investigated. The rail material is an annealed steel with pearlitic microstructure, combining high strength with a high strain hardening rate and excellent high cycle fatigue behavior. On the other hand, the material exhibits brittle fracture characteristics with inferior fracture toughness and fatigue crack growth resistance compared to other pearlitic rail steels. The mechanical performance suggests that the material resists crack initiation, but for application purposes maintenance schedules should be able to identify fatigue cracks at early stages to avoid propagation of cracks to critical lengths compromising structural integrity. © 2016 Elsevier Ltd

Grain Size Evolution during Multipass Hot-Rolling of C-Mn Steels: Comparison of Phase Field and Extended JMAK Modeling

The design of multipass hot-rolling schedules is of great importance to steel industry and an effort is made to predict the microstructure evolution. In the present study, a multi-phase field model is employed for the simulation of the grain size evolution due to static recrystallization and grain growth during multipass hot-rolling of C-Mn steels. A variable stored energy per rolling pass, a temperature dependent interface mobility, and nucleation site density are considered. The model is compared with an extended JMAK model and validated with experimental data obtained from two hot-rolling schedules. The results indicate that both models describe the experimental data well, however the phase field model avoids certain discontinuities between static recrystallization and grain growth. A statistical analysis is conducted to investigate the effect of the microstructure domain size on the phase field results and grain size distributions. The effect of key process parameters on the kinetics of static recrystallization and grain growth are determined by the temporal evolution of equivalent circular grain diameter distributions. Both approaches have the potential to be used for the computational design of multipass hot-rolling processes in steels. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel. © 2022 by the authors

Rolling contact fatigue cracking in rails subjected to in-service loading

Rolling contact fatigue (RCF) is one of the most important failure mechanisms in rails with significant cost- and safety-related implications on the operation of railway systems. In this work, a metallurgical analysis of RCF crack initiation and propagation, including geometrical characteristics of RCF cracks – length, depth from surface, angle of propagation and spacing between cracks, is presented. The role of proeutectoid ferrite in crack initiation has been studied. Analysis of the fracture surface of an RCF crack revealed a ductile initiation zone followed by a quasi-cleavage crack propagation. Iron oxide formed in the interior of all cracks in rails exposed to stagnant water with implications to crack propagation rate because of crack closure effects. Sequential sectioning parallel to the rolling surface revealed that RCF cracks possess convoluted surfaces. The crack trace expands with depth from the rolling surface. Subsurface crack initiation has also been documented. © 2016 Wiley Publishing Ltd

Human bone ingrowth into a porous tantalum acetabular cup

Porous Tantalum is increasingly used as a structural scaffold in orthopaedic applications. Information on the mechanisms of human bone ingrowth into trabecular metal implants is rather limited. In this work we have studied, qualitatively, human bone ingrowth into a retrieved porous tantalum monoblock acetabular cup using optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis. According to the results and taking into account the short operational life (4 years) of the implant, bone ingrowth on the acetabular cup took place in the first two-rows of porous tantalum cells to an estimated depth of 1.5 to 2 mm. The bone material, grown inside the first raw of cells, had almost identical composition with the attached bone on the cup surface, as verified by the same Ca:P ratio. Bone ingrowth has been a gradual process starting with Ca deposition on the tantalum struts, followed by bone formation into the tantalum cells, with gradual densification of the bone tissue into hydroxyapatite. A critical step in this process has been the attachment of bone material to the tantalum struts following the topology of the porous tantalum scaffold. These results provide insight to the human bone ingrowth process into porous tantalum implants. © 2017 Gregory N. Haidemenopoulos, et al

High entropy oxides-exploring a paradigm of promising catalysts: A review

This review presents the structural elements and discusses the great potential of high entropy oxides (HEO) as promising catalysts. A critical comparison is provided with the medium and low entropy metal oxides in terms of the important functionality of HEO to exhibit higher oxygen mobility and withhold high population of oxygen vacancies as well as leading to high metal dispersion. This review study critically compares the performance of the thermal, electro- and photo- high entropy oxides catalysts with the conventional metal oxides and demonstrates their superiority over them while discussing the governing characteristics of HEO. The HEO complex structure is highlighted using ab initio calculations on understanding and tuning their electronic/structural properties. Catalysts’ design criteria and direction in the studies of the HEO as catalysts for energy and sustainability are proposed. © 2021 The Author