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Physical metallurgical (PM) and data-driven approaches can be independently applied to alloy design. Steel technology is a field of physical metallurgy around which some of the most comprehensive understanding has been developed, with vast models on the relationship between composition, processing, microstructure and properties. They have been applied to the design of new steel alloys in the pursuit of grades of improved properties. With the advent of rapid computing and low-cost data storage, a wealth of data has become available to a suite of modelling techniques referred to as machine learning (ML). ML is being emergingly applied in materials discovery while it requires data mining with its adoption being limited by insufficient high-quality datasets, often leading to unrealistic materials design predictions outside the boundaries of the intended properties. It is therefore required to appraise the strength and weaknesses of PM and ML approach, to assess the real design power of each towards designing novel steel grades. This work incorporates models and datasets from well-established literature on marageing steels. Combining genetic algorithm (GA) with PM models to optimise the parameters adopted for each dataset to maximise the prediction accuracy of PM models, and the results were compared with ML models. The results indicate that PM approaches provide a clearer picture of the overall composition-microstructure-properties relationship but are highly sensitive to the alloy system and hence lack on exploration ability of new domains. ML conversely provides little explicit physical insight whilst yielding a stronger prediction accuracy for large-scale data. Hybrid PM/ML approaches provide solutions maximising accuracy, while leading to a clearer physical picture and the desired properties.

We tackle the role of austenite in multiphase steels on hydrogen diffusion systematically for the first time, considering a range of factors such as morphology, interface kinetics and the additional effect of point traps using both experiments and modelling. This follows the findings from part I where we showed that austenite cannot be parametrised and modelled as point traps under the assumption of local equilibrium, unlike grain boundaries and dislocations. To solve this, we introduce a 2D hydrogen diffusion model accounting for the difference in diffusivities and solubilities between the phases. We first revisit the as-quenched martensite permeation results from part I and show that the extremely low H diffusivity there can be partly explained with the new description of austenite but is partly likely due to quench vacancies. We then also look at the H absorption and desorption rates in a duplex steel as a case study using a combination of simulations and experiments. The rates are shown to depend heavily on austenite morphology and the kinetics of H transition from ferrite to austenite and that an energy barrier is likely associated to this transition. We show that H diffusion through the ferrite matrix and austenite islands proceeds at similar rates and the assumption of negligible concentration gradients in ferrite occasionally applied in the literature is a poor approximation. This approach is also applicable to other austenite-containing steels as well as other multiphase alloys.

We quantified systematically the H trap density in martensite resulting from the presence of dislocations, grain boundaries and retained austenite through a combination of detailed microstructural characterisation, H permeation, thermal desorption and diffusion modelling. This thorough analysis allowed for the first time to deconvolve key microstructural constituents affecting H diffusion in multi-trap martensite. Three microstructures were investigated – as-quenched, tempered at 300 °C and tempered at 450 °C. The first two microstructures had identical dislocation densities and grain size, while the as-quenched one also contained 3.5 vol.% of retained austenite. The two tempered microstructures showed a large difference in dislocation density with few other microstructural differences. The as-quenched microstructure exhibited over an order of magnitude lower H diffusivity and increased H trapping due to retained austenite, while the tempered samples exhibited very similar diffusivities, indicating that dislocations have a limited effect on H trapping. Trap density scaling laws were successfully identified and validated through diffusion simulations and experiments. It was also shown that in martensite and heavily deformed ferrite, where the average grain size is small, grain boundaries are more effective trapping sites than dislocations. Our results also show that retained austenite cannot be effectively modelled as a point trap under the local equilibrium assumption, which is frequently used to model its effect on H diffusion, and that bulk trapping must be considered at least in two dimensions, which is addressed in part II of this series.

Nanotwin formation is observed during laser powder bed fusion (LPBF) of austenitic stainless steels fabricated with various process parameters. Transmission electron microscopy was employed to reveal the nature of such twins, which are formed due to the high strain rapid solidification inherent to LPBF. Dynamic recrystallization (DRX) was also activated during LPBF, and induced by the deformation nanotwins. A thermostatistical model is proposed to determine the critical conditions for twinning-induced DRX; the model is validated with the reported experimental results. This modelling approach offers a method to microstructurally engineer austenitic stainless steels for potential applications needing high strength and ductility., This work was supported by the Royal Academy of Engineering (RCSRF1718/5/32), and by EPSRC via DARE grant (EP/L025213/1). HF acknowledges the support by National Natural Science Foundation of China (51971011). CZ acknowledges the financial support by Tsinghua University research grant (20197050027). DSM is grateful to the Regional Government of Madrid (Spain) for the support via Mat4.0 project (S2018/NMT-4381).

Hydrogen embrittlement is increasingly important in advanced high strength steels (AHHS) as strength levels increase well above 1000MPa. This work developed a detailed understanding of the embrittling mechanism in model AHHS steels based on Fe-Ti-Mo and Fe-V-Mo, both strengthened through interphase precipitation. Hydrogen charging led to an increase in the dislocation density and an enlarged strain field around precipitates, resulting in an increase in residual stress. This was much greater for the Ti-Mo steel compared to the V-Mo. Important differences in the hydrogen trapping behaviour was seen between the two steels, with hydrogen believed to be trapped at the matrix/precipitate interface for the Ti-Mo steel, but within the precipitate for the V-Mo steel. The effects of hydrogen were investigated in detail for slow strain rate tensile tests and double notched tensile samples. Hydrogen charging resulted in a loss in strength and ductility, with the Ti-Mo steel failing at yield, while the V-Mo steel exhibited a ∼13% loss in strength and a ∼ 35% loss of ductility. Crack initiation in tensile samples occurred at high strain gradient dislocation boundaries. However, crack propagation rapidly became quasi-cleavage, along the {100} plane in ferrite, and also along the martensite/ferrite grain boundaries on the {110} plane in the martensite. Minimal plasticity was observed associated with the crack tip, which was believed to be a result of the suppression of dislocation emission at the crack tip by the hydrogen.

An integral modelling approach for understanding the strengthening mechanisms in Ti alloys is presented and applied to alloys undergoing deformation via dislocation slip. The model incorporates contributions from solid solution, grain boundary, dislocation forest and strain hardening. The metal forming and thermomechanical processing factors influence both grain size and the stored strain energy. The strain hardening of Ti-Fe-Sn-Nb alloys was modelled by considering dislocation accumulation and annihilation terms. By tailoring the contribution of each strengthening effect, the yield stress and plasticity of advanced Ti alloys can be optimised.

An explanation is proposed to martensite inhibition beyond characteristic concentration thresholds in titanium binary alloys. The method combines the phenomenological theory of martensite crystallography (PTMC) and thermodynamic calculations (TCs) to describe the conditions under which martensite formation is favourable. It is shown that martensite can be crystallographically prevented while being thermodynamically favourable. The PTMC is implemented by taking into account the influence of composition. After a comprehensive comparison to experiments, two twinning systems and two glide systems are inferred to be able to produce the lattice invariant shear. The critical concentrations above which martensite cannot form are computed and compared to experimental results on binary and ternary systems, showing good agreement. The proposed method may be used as a guide to design titanium alloys for controlled martensitic behaviour.

A new approach to modelling the microstructure evolution and yield strength in laser powder bed fusion components is introduced. Restoration mechanisms such as discontinuous dynamic recrystallization, continuous dynamic recrystallization, and dynamic recovery were found to be activated during laser powder bed fusion of austenitic stainless steels; these are modelled both via classical Zener-Hollomon and thermostatistical approaches. A mechanism is suggested for the formation of dislocation cells from solidification cells and dendrites, and their further transformation to low-angle grain boundaries to form subgrains. This occurs due to dynamic recovery during laser powder bed fusion. The yield strength is successfully modelled via a Hall–Petch-type relationship in terms of the subgrain size, instead of the actual grain size or the dislocation cell size. The validated Hall–Petch-type equation for austenitic stainless steels provides a guideline for the strengthening of laser powder bed fusion alloys with subgrain refinement, via increasing the low-angle grain boundary fraction (grain boundary engineering). To obtain higher strength, dynamic recovery should be promoted as the main mechanism to induce low-angle grain boundaries. The dependency of yield stress on process parameters and alloy composition is quantitatively described., This work was supported by the Royal Academy of Engineering for chair funding (RCSRF1718/5/32), and by EPSRC via DARE grant (EP/L025213/1). HF acknowledges the support by National Natural Science Foundation of China (51971011) and Beihang Top Young Talent Support Programme (KG12079901).

Developing lighter, stronger and more ductile aerospace metallic materials is in demand for energy efficiency strategies. Alloys with twinning-induced plasticity (TWIP) and/or transformation-induced plasticity (TRIP) effects have been exploited to defeat the conflict of strength versus ductility, yet very few if any physically informed methods exist to address the complex interactions between transitions. Here we report a facile route to deploy transformation-mediated strengthening in lightweight Ti alloys, which particularly focuses on the controlled activation of TRIP and TWIP via a mechanism-driven modelling approach. New alloys were comparatively developed and presented notable resistances to strain localisation, but interestingly through distinct mechanical characteristics. Specifically, extraordinary strain-hardening rate with a peak value of 2.4 GPa was achieved in Ti-10Mo-5Nb (wt.%), resulting from the synergetic activation of hierarchical transformations. A model integrating TRIP and TWIP was applied to understand the interplays between the transition mechanisms that individually contribute to strength and uniform elongation.

Embrittlement originates from dislocation nanocellular structures stabilized by the presence of hydrogen., Hydrogen embrittlement is shown to proceed through a previously unidentified mechanism. Upon ingress to the microstructure, hydrogen promotes the formation of low-energy dislocation nanostructures. These are characterized by cell patterns whose misorientation increases with strain, which concomitantly attracts further hydrogen up to a critical amount inducing failure. The appearance of the failure zone resembles the “fish eye” associated to inclusions as stress concentrators, a commonly accepted cause for failure. It is shown that the actual crack initiation is the dislocation nanostructure and its associated strain partitioning.

During laser powder bed fusion (LPBF) the powder bed undergoes several thermal cycles incorporating complex thermo-mechanical processing. Different restoration mechanisms such as dynamic recovery, dynamic recrystallization and grain growth can be activated at different thermal cycles, leading to a very fine average grain size. This is modelled via classical and thermostatistical approaches for an austenitic stainless steel. Four subsequent thermal cycles in each layer induce various microstructural transitions for each individual grain. The high cooling rate solidification in the first two thermal cycles leads to the formation of a highly deformed cellular microstructure. Discontinuous and continuous dynamic recrystallization are activated in the third thermal cycle to induce grain refinement. The fourth thermal cycle undergoes dynamic recovery and grain growth. The as-built alloys exhibit an excellent combination of high yield and ultimate tensile strength. The high strength is attributed to the activation of the various dynamic recrystallization mechanisms, as well as to the development of the cellular structures resulting from a high cooling rate upon solidification. A methodology to design alloys with tailored microstructures is presented.

Understanding the factors influencing yield strengthening in alloys processed by laser powder bed fusion (LPBF) is critical in designing new formulations, and for predicting the optimum parameters for their processing. In this work, a relationship between the heat input and strengthening and softening mechanisms is proposed for a titanium, nickel and stainless steel alloy (Ti-6Al-4V, IN718 and 316L, respectively). Maximum strength is obtained with increasing heat input in 316L stainless steel; whereas IN718 and Ti-6Al-4V require low heat inputs. The results demonstrate that yield strength can be described in terms of the normalised enthalpy. The variation in the yield strength of LPBFed alloys depends prominently on dislocation multiplication/annihilation at certain processing temperatures and thermal straining, which are alloy dependent; as well as on dislocation strengthening and heat dissipation during cooling, which are process dependent. These dependencies are modelled via well-known metallurgical approaches. The relative contribution of various strengthening mechanisms is revealed. The findings of this work can be used as a metric for the prediction and further improvement of yield strength based on the choice of LPBF process parameters and chemical composition.

A ball-on-rod RCF tester was employed to investigate the failure mechanisms of hydrogen-rich rolling components. The formation of defects, voids and surface cracks, is significantly facilitated in hydrogen-rich bearing steels. In samples with RCF cycles of 1.6 × 107, the void density in hydrogen-rich samples is about three times that of hydrogen-free samples, whilst their crack length density four times that of hydrogen-free samples. This is due to a higher stress intensity factor around inclusions which is altered by hydrogen. Further characterisation confirms that grain boundaries are preferential sites for void formation and crack propagation.

High-entropy alloys (HEAs), originally introduced to the literature due to their ability to stabilize a single phase across large temperature ranges, have recently demonstrated to display multiphase systems undergoing a variety of strengthening mechanisms. Previous reports have focused on solid solution strengthening and precipitation hardening; however, other hardening mechanisms such as twinning and martensite formation have been reported to play a key role in controlling their mechanical behavior. Such deformation mechanisms display significant variations with temperature and strain rate. The present contribution provides an outline of the various hardening mechanisms reported in the literature for HEAs. For each mechanism, a modeling strategy is proposed to describe the associated mechanical behavior. The mechanisms are combined into a single framework to discover new HEAs of improved mechanical behavior. A strategy for HEA design is presented, and the advantages of adopting additive layer manufacturing to improve mechanical behavior are discussed.

Three major types of microstructural alterations occurring under rolling contact fatigue, white etching areas (WEAs), dark etching regions (DERs) and white etching bands (WEBs), are modelled under a unified approach: dislocation-assisted carbon migration theory. Following our previous work on DERs and WEBs, a novel model is proposed to describe dislocation cell formation in WEAs. The proposed model yields predictions of WEAs appearance, agreeing with experimental observations. Bearing life can be estimated by the WEA appearance model. The three microstructural alterations models are combined and, for the first time, it becomes possible to predict the occurrence and the formation progress of WEAs, DERs and WEBs with a unified theory. Microstructural alteration maps are plotted as a function of number of cycles, temperature, contact pressure and stress cycle frequency. The models are validated by the experimental results reported over the last 50 years.

A modelling suite for hydrogen transport during electrochemical permeation, degassing and thermal desorption spectroscopy is presented. The approach is based on Fick's diffusion laws, where the initial concentration and diffusion coefficients depend on microstructure and charging conditions. The evolution equations are shown to reduce to classical models for hydrogen diffusion and thermal desorption spectroscopy. The number density of trapping sites is found to be proportional to the mean spacing of each microstructural feature, including dislocations, grain boundaries and various precipitates. The model is validated with several steel grades and polycrystalline nickel for a wide range of processing conditions and microstructures. A systematic study of the factors affecting hydrogen mobility in martensitic steels showed that dislocations control the effective diffusion coefficient of hydrogen. However, they also release hydrogen into the lattice more rapidly than other kind of traps. It is suggested that these effects contribute to the increased susceptibility to hydrogen embrittlement in martensitic and other high–strength steels. These results show that the methodology can be employed as a tool for alloy and process design, and that dislocation kinematics play a crucial role in such design.

The present work develops a numerical approach combining thermodynamic and kinetic simulations to investigate the austenitisation process on spheroidised bearing steel. The approach incorporates the dissolution of spheroidised cementite present prior to austenitisation and the influence of austenitisation temperature. It allows predictions including the chemical driving force of austenite formation, the evolution of phase constituents and their chemical compositions during austenitisation, as well as an assessment on the austenite stability upon quenching. The calculated results further allow to predict the hardness of the produced martensitic steels. The model predictions are validated against experimental data in two commercial bearing steels with six austenitisation processes. Good agreement between the experimental results and numerical predictions is obtained on the steel microstructure, austenite stability and material hardness. In addition, comparison of the two steels show that 100Cr6 requires to be austenitised at temperatures 10 °C higher than 100CrMnSi6-4, to achieve the same driving force for austenite formation, and 20 °C higher to achieve identical austenite stability upon quenching. The method can be adopted beyond bearing steels to design austenitisation processing schedules., This research is supported by SKF Engineering & Research Centre and financed by SKF AB. Mr. Javier Vara from the Phase Transformations Laboratory in CENIM-CSIC is greatly acknowledged for the experimental support with high resolution dilatometry experiments.

The microstructure of most hard steels used in tribological applications is inhomogeneous at a micro-scale. This results in local variations in chemical composition and mechanical properties. On a similar scale, tribofilms formed by ZDDP and other anti-wear additives are commonly observed to exhibit a patch-like morphology. ZDDP tribofilms formed under controlled contact conditions on four different steel grades were carefully studied with a new AFM technique to analyse the relationship between the steel microstructure and the tribofilm morphology. Tribofilms were found to be thinner on residual carbides than on the martensitic matrix in all grades containing residual carbides. In most cases, the difference in tribofilm thickness is larger than the carbide protrusion.

High entropy alloys (HEAs) is a fascinating field of research, with an increasing number of new alloys discovered. This would hardly be conceivable without the aid of materials modeling and computational alloy design to investigate the immense compositional space. The simplicity of the microstructure achieved contrasts with the enormous complexity of its composition, which, in turn, increases the variety of property behavior observed. Simulation and modeling techniques are of paramount importance in the understanding of such material performance. There are numerous examples of how different models have explained the observed experimental results; yet, there are theories and approaches developed for conventional alloys, where the presence of one element is predominant, that need to be adapted or re-developed. In this paper, we review of the current state of the art of the modeling techniques applied to explain HEAs properties, identifying the potential new areas of research to improve the predictability of these techniques., I.T.C. is grateful for financial support of the fellowship 2016-T2/IND-1693, from the Programme Atracción de talento investigador (Consejería de Educación, Juventud y Deporte, Comunidad de Madrid). J.S.W. acknowledges the financial support from the Foundation of Polish Science Grant HOMING (No. Homing/2016-1/12). The HOMING programme is co-financed by the European Union under the European Regional Development Fund. University of Warsaw, under Grant No. GA69-30. The work at CCFE has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant Agreement No. 633053 and funding from the RCUK Energy Programme [Grant No. EP/P012450/1]. P.E.J.R.D.C.’s work was supported by Grant EP/L025213/1 from the UK Engineering and Physical Sciences Research Council (EPSRC).

The nucleation and growth of lenticular carbides (LCs) in bearing steels occur near to deformed ferrite bands after exposure to prolonged rolling contact fatigue (RCF). Since the first observations in 1947, a large number of attempts have been made to explain the formation mechanisms of such stress-induced microstructural alterations, but a reliable model was still not available. In this research, a novel theory is proposed to describe the carbon redistribution process during LC formation. The theory suggests a dislocation assisted LC growth mechanism on the basis of the classic Cottrell atmosphere formation theory. The mechanism considers (1) JLC=Jd, the carbon flux equilibrium between LC thickening (JLC) and dislocation-assisted carbon migration (Jd), and (2) M0=MLC+Mb, the carbon mass conservation of the system, where M0 denotes the total amount of carbon within the system, MLC denotes the amount of carbon within a LC, and Mb denotes the amount of carbon left within the ferrite band, respectively. The solution to these two equations, which addresses the problem that has been puzzling researchers for several decades, makes good predictions on LC thickening rate under various testing conditions. The stress-induced carbide precipitation was examined using high resolution characterisation techniques such as scanning and transmission electron microscopy, obtaining significant evidence to support the postulated theory. The successful description of LC growth implies a potential extension of the theory to other types of stress induced microstructural changes in bearing steels where carbon redistribution occurs. The model presented here provides a more comprehensive understanding of RCF from a microstructural point of view, and thus can enhance the accuracy of traditional bearing life prediction approaches.

A unified description for the evolution of e– and α′– martensite, and twinning in austenitic steels is presented. The formation of micron—scale e and twin bands is obtained by considering the evolution of hierarchically arranged nano–sized e and twins (embryos). The critical size and applied stress when these structures form is obtained by minimising their free energy of formation. The difference between forming an e plate or a twin lies in the number of overlapping stacking faults in their structure. A nucleation rate criterion is proposed in terms of the critical embryo size, resolved shear stress and embryo number density. Based on Olson and Cohen's classical α′–martensite transformation model, the nucleation rate of α′ is considered proportional to that for e. These results, combined with dislocation–based approximations, are employed to prescribe the microstructure and flow stress response in steels where transformation–induced–plasticity (TRIP) and/or twinning–induced–plasticity (TWIP) effects operate; these include austenitic stainless and high–Mn steels. Maps showing the operation range of e, α′ and twinning in terms of the stacking fault energy at different strain levels are defined. Effects of chemical composition in the microstructure and mechanical response in stainless steels are also explored. These results allow identifying potential compositional scenarios when the TRIP and/or TWIP effects are promoted in austenitic steels.

A model to predict the conditions for printability is presented. The model focuses on crack prevention, as well as on avoiding the formation of defects such as keyholes, balls and lack of fusion. Crack prevention is ensured by controlling the solidification temperature range and path, as well as via quantifying its ability to resist thermal stresses upon solidification. Defect formation prevention is ensured by controlling the melt pool geometry and by taking into consideration the melting properties. The model&rsquo, s core relies on thermodynamics and physical analysis to ensure optimal printability, and in turn offers key information for alloy design and selective laser melting process control. The model is shown to describe accurately defect formation of 316L austenitic stainless steels reported in the literature.

The formation of white etching bands (WEBs) occurs at the subsurface of rolling contact-fatigued bearing inner rings, exhibiting microstructural decay detrimental to bearing life. Despite the fact that WEBs have been observed in bearing steels for nearly 70 years, the understanding of WEB formation is still limited and mostly qualitative. Therefore, a systematic investigation is carried out in this research to reveal the evolution of WEBs with respect to the number of contact cycles. WEBs formed at different stages are reproduced by full-scale bearing RCF tests with predetermined numbers of cycles. Multi-scale characterisation techniques such as optical microscopy, micro-indentation, scanning and transmission electron microscopy and atomic force microscopy are conducted on the microstructural alterations to study the development and microstructure of WEBs. WEBs are found in the absence of dark etching regions which is attributed to the heat treatment. With an increasing number of cycles, WEBs grow in number density and in all three dimensions, and their formation is found to be controlled by the maximum shear stress component. Ferrite bands within WEBs that contain dislocation cells manifest accumulated plastic strain in the material. Based on the characterisation results, the evolution of plastic strain under RCF is quantified.

The microstructural influence on hydrogen permeation and trapping in pure iron and two ferritic-pearlitic steels, AISI 1018 and AISI 4340 is quantified. To this end, hydrogen is introduced into specimens of these materials through electrochemical charging and the total hydrogen content of the specimens are quantified following gas fusion analysis principle. Furthermore, a modeling framework based on Fickian diffusion equations including the relevant microstructural features, electrochemical charging conditions and three-dimensional geometry of the specimen affecting the overall diffusion behavior is adopted to describe the time-dependence of hydrogen content in the three materials. The approach quantitatively describes the hydrogen ingress into the three materials, as well as its distribution across various defects and microstructural features. Traps of two potencies are identified, dislocations and grain boundaries (trap 1), and ferrite/cementite interfaces (trap 2). The former are shown to be responsible for the trapped hydrogen at early stages of its ingress, whereas trap 2 is shown to gather trapped hydrogen at later stages. The ability to design microstructures to control hydrogen ingress and diffusion is discussed, showing how the framework presented here can be adopted for controlling hydrogen in commercial components, and how this can delay hydrogen-related embrittlement. Keywords: Hydrogen diffusion, Hydrogen quantification, Hydrogen embrittlement, Steels, Microstructure, Modeling

The mechanical characteristics and the operative deformation mechanisms of a metallic alloy can be optimised by explicitly controlling phase stability. Here an integrated thermoelastic and pseudoelastic model is presented to evaluate the β stability in Ti alloys. The energy landscape of β → α ′ / α ″ martensitic transformation was expressed in terms of the dilatational and transformational strain energy, the Gibbs free energy change, the external mechanical work as well as the internal frictional resistance. To test the model, new alloys were developed by tailoring two base alloys, Ti-6Al-4V and Ti-6Al-7Nb, with the addition of β-stabilising element Mo. The alloys exhibited versatile mechanical behaviours with enhanced plasticity. Martensitic nucleation and growth was fundamentally dominated by the competition between elastic strain energy and chemical driving force, where the latter term tends to lower the transformational energy barrier. The model incorporates thermodynamics and micromechanics to quantitatively investigate the threshold energy for operating transformation-induced plasticity and further guides alloy design.

The presence of stress raisers in bearing steels is known to be highly detrimental to rolling contact fatigue performance. In VIM–VAR (vacuum induction melted–vacuum arc remelted) M50, primary carbides inherited from the solidification stage can develop damage in the form of butterflies, possibly leading to failure by spalling. A ball-on-rod rolling contact fatigue test rig was used to induce damage under different loading conditions. The butterfly distribution as well as the spatial variation of number, size, and orientation relative to the racetrack surface was investigated. The effect of the load on the butterfly formation is also detailed. Finally, the nucleation and growth of butterflies was observed throughout a series of tests suspended after different number of stress cycles. The trends unveiled in this paper could highly benefit the development and validation of models aiming at predicting the effect of stress raisers on the performance of materials subjected to rolling contact fatigue.

Hydrogen embrittlement remains of critical concern in the design of strong and reliable microstructures in steels. The role of microstructure in susceptibility to hydrogen trapping is evaluated using a numerical thermokinetic simulation approach. The simulation scheme is applied to evaluate variations in dislocation density and grain size in pure ferritic iron, ferritic and martensitic low alloy steels during cooling and ferritic steels under deformation. Additionally, variations in NbC nanoprecipitates in low alloy tempered martensitic steel, and coherent and incoherent TiC precipitates in low alloy steels were evaluated. These simulations were conducted to quantify the influence of such features on the trapping efficiency of interstitial hydrogen. To simulate the diffusion process in a complex microstructure, a mean field approach is applied. Modelling approaches adopting physically based formulations for the calculation of the trapping-affected concentration of hydrogen in the lattice are suggested, adopted in the present calculations and validated for a wide range of experimental and microstructural conditions. The combination of thermokinetic simulations with hydrogen trapping behaviours is the first of its kind and presents a means to incorporate the effects of various microstructural features, with respect to hydrogen migration and trapping, in the design of hydrogen embrittlement resistant steels. Keywords: Hydrogen, Steel, Microstructure, Precipitation, Multiscale simulation

A new AFM method for relating microstructure to tribofilm formation is presented. Three aspects of this method distinguish it. Firstly, the new method maps the same region employing atomic force microscopy (AFM) in three sequential steps: preEDTA and postEDTA, before and after the tribofilm removal; and as-etched, after revealing the steel microstructure. Secondly, a new software was written for correcting the displacement between the sequential AFM topography images, and for analysing the data. Thirdly, the method allows to correlate the tribofilm with the steel microstructure, and relating it with protrusion of residual carbides and tribofilm roughness for regions as large as 50 μm×50 μm. This allows to performing statistical analyses to correlate tribofilm thickness with different microstructural constituents.

Lattice distortion in high entropy alloys (HEAs) is one of their main crystallographic features. Its description is possible by means of unit cell parameter and bulk modulus variations of their constituent elements. The balance of forces acting on the lattice atoms under such distortion is related to the formation of a solid solution of a given crystal structure. This leads to the definition of a novel criterion for selecting HEA compositions. The main existing families of HEAs have been classified under this approach, in addition to an extensive list of multicomponent alloys including intermetallics and bulk metallic glasses. Criteria reported in the literature have been revised with the multicomponent alloy database used in this work which, together with the proposed approach, can be used to improve our understanding of HEAs formation.

Hydrogen embrittlement is, and has been for over a century, a prominent issue within many sectors of industry. Despite this, the mechanisms by which hydrogen embrittlement occur and the suitable means for its prevention are yet to be fully established. Hydrogen embrittlement is becoming an ever more pertinent issue. This has led to a considerable demand for novel hydrogen embrittlement-resistant alloys, notably within the bearings industry. This paper provides an overview of the literature surrounding hydrogen embrittlement in bearing steels, and the means by which manufacturers may optimise alloys and accompanying processes to prevent embrittlement. Notably, novel steels combining both high strength and hydrogen embrittlement resistance are reviewed with respect to their design, evaluation methods and required future work.This paper is part of a Themed Issue on Recent developments in bearing steels.

Rolling contact fatigue (RCF) of a variety of bearing steels was studied with a ball-on-rod configuration rig. Systematic sectioning of the specimens across the spalls revealed the presence of oxide near the cracks. Further characterisation of this phase using electron microscopy revealed that it was formed during testing, after crack initiation. It is highlighted that the discovered oxide could, in some cases, be mistaken for a non-metallic inclusion. The authors suggest that attributing RCF failure initiation to non-metallic inclusions should not only rely on optical microscopy but also on EDX measurements, as this reveals a composition inconsistent with non-metallic inclusions. A mechanism for the in situ formation of the oxide is suggested and its possible role on failure is discussed.

A computational method is presented to design alloys of lower susceptibility to solidification cracking, while preventing the formation of porosity and defects during laser powder bed fusion (LPBF). The method is developed for austenitic stainless steels, on which a wealth of data are available as various conditions for crack and pore/defect formation have been reported. The model is based on an alloy design approach combining thermodynamic calculations with a genetic algorithm to discover novel austenitic stainless steel compositions; the new alloys are expected to be crack-free whilst showing improved strength. A new crack prevention factor is proposed to relate composition to solidification crack formation. The factor incorporates quantitative criteria for the solidification temperature range, the performance index (ratio between yield stress and coefficient of thermal expansion) and the solidification path. Overall, the design methodology is validated by literature data on 316L austenitic stainless steel. Although cracking is not an issue during LPBF of 316L stainless steel, this material is a good choice to show under which conditions the cracks form. As for porosity and defect prevention, it is shown how this can be achieved by providing a sufficient amount of energy to melt the powder bed, and by controlling the melt pool geometry; such criteria are dissimilar to those reported in the literature. Process maps have been developed to show the effects of process parameters on the formation of pores and defects based on the proposed criteria. The model is applied to optimise such parameters to produce 316L austenitic stainless steel, and it is shown that a defect-free LPBFed stainless steel can be achieved, performing better under tensile testing compared to its wrought counterpart. The conditions for the application of such model to other alloy families displaying cracking, such as marageing steels and nickel alloys, are discussed.

International audience; New theory is presented to describe the occurrence of plasticity-induced transitions in titanium alloys. The approach is able to predict the composition dependence of transformation induced plasticity (TRIP), superelasticity, as well as martensite formation upon quenching. Martensite formation in the absence of stress is considered as the result of a competition between elastic strain energy and chemical driving force. Assuming that the formation of martensite is the result of a thermally activated nucleation process followed by athermal growth, a nucle-ation parameter is postulated to describe the conditions under which martensite is formed upon quenching; the parameter accounts for the ratio between the available thermal energy and an energy barrier for nucleation, suggesting that phase is not the main factor controlling martensite inhibition. This nucleation parameter is able to describe, for the first time, martensite occurrence in 130 alloys from the literature, quantifying the martensite start temperature (M s) reported for 49 alloys with great precision. An empirical parameter ([Fe] eq) is proposed and, when combined with the M s prediction, it allows to define regions within which TRIP and superelasticity occur. By defining threshold values for the M s , the [Fe] eq and the nucleation parameter, candidate alloys likely to display TRIP, superelasticity or martensitic transformation upon quenching can be identified. As a result, this method can be adopted to design alloys with tailored plasticity behaviour.

A multiscale dislocation-based model was built to describe, for the first time, the microstructural evolution and strain-hardening of {332}⟨113⟩ TWIP (twinning-induced plasticity) Ti alloys. This model not only incorporates the reduced dislocation mean free path by emerging twin obstacles, but also quantifies the internal stress fields present at β-matrix/twin interfaces. The model was validated with the novel Ti-11Mo-5Sn-5Nb alloy (wt.%), as well as an extensive series of alloys undergoing {332}⟨113⟩ twinning at various deformation conditions. The quantitative model revealed that solid solution hardening is the main contributor to the yield stress, where multicomponent alloys or alloys containing eutectoid β-stabilisers exhibited higher yield strength. The evolution of twinning volume fraction, intertwin spacing, dislocation density and flow stress were successfully described. Particular attention was devoted to investigate the effect of strain rate on the twinning kinetics and dislocation annihilation. The modelling results clarified the role of each strengthening mechanism and established the influence of phase stability on twinning enhanced strain-hardening. Strain-hardening stems from the formation of twin obstacles in early stages, whereas the internal stress fields provide a long-lasting strengthening effect throughout the plastic deformation. A tool for alloy design by controlling TWIP is presented.

The ductility of materials can be reduced by hydrogen. Such reduction can result in a form of premature failure often termed as hydrogen embrittlement. In super duplex stainless steels, although both austenite and ferrite are susceptible to hydrogen embrittlement, there is a lack of understanding into the hydrogen embrittlement contribution of each phase. In this study, in situ neutron diffraction tests were applied on H-charged samples to investigate the hydrogen embrittlement behaviour in super duplex stainless steels. The result reveals that austenite maintains good plasticity during tensile testing, whilst a loss of it is realised in ferrite. Fractography analysis reveals that there is a brittle-to-ductile transition from the sample surface towards the centre; hydrogen embrittlement vanishes as the specimen's centre is approached, while it is demonstrated to disappear first in austenite but not in ferrite. This transition can be predicted by applying a physics-based hydrogen embrittlement model which incorporates the effects of hydrogen concentration, hydrogen diffusivity, residual stress, loading state and temperature. The present work demonstrates the dissimilar susceptibility of austenite and ferrite to hydrogen embrittlement, providing a tool to describe it.