Search

The effect of Luders banding on hydrogen embrittlement (HE) susceptibility of medium Mn steels was investigated by tuning the degree of yield point elongation (YPE). It was found that the steel with a larger YPE was more susceptible to HE. A larger YPE led to a higher local flow stress and a higher fraction of strain-induced martensite, which in turns resulted in more H-induced cracks in the Luders banding area and the consequent catastrophic failure. It thus suggests that Luders banding and the associated localized deformation play a key role in influencing the overall HE susceptibility of medium Mn steels.

Autonomous healing of creep-induced grain boundary cavities by Au-rich and W-rich precipitates was studied in a Fe-3Au-4W (wt pct) alloy at a fixed temperature of 823 K (550 °C) with different applied stresses. The ternary alloy, with two supersaturated healing solutes, serves as a model system to study the interplay between two separate healing agents. The creep properties are evaluated and compared with those of the previously studied Fe-Au and Fe-W binary systems. The microstructures of the creep-failed samples are studied by electron microscopy to investigate the cavity filling behavior and the mass transfer of supersaturated solute to the defect sites. Compared to the Fe-Au and Fe-W alloys, the new Fe-Au-W alloy has the lowest steady-state strain rate and the longest lifetime. The site-selective filling of the creep-induced cavities is attributed to two different categories of precipitates: micron-sized Au-rich precipitates and nano-sized W-rich precipitates. The Au-rich precipitates are found capable to fully heal the cavities, while the W-rich precipitates show only a limited degree of healing. The two types of precipitates show a reluctance to coexistence, and the formation of W-rich precipitates is suppressed strongly. A model is proposed to describe the competitive healing behavior of the Au-rich and W-rich precipitates.

The current study reports the detailed analysis of an observation of the local pinning of a slowly moving austenite-ferrite interface by a single nanosized oxidic particle. The observations were made during an in situ cyclic partial phase transformation experiment on a Fe-0.1C-1.0Mn alloy close to the inversion stage at which the interface migrates at a rather low velocity. The low velocity allowed capturing the interface pinning effect over a period of no less than 16 seconds. From our observations, it was possible to follow the progression of the pinning effect from the initial stages all the way through to the release of the interface. The pinning force exerted by the individual particle having a diameter of 140 nm on the austenite-ferrite interface was estimated as 175 nJ m−1, while the maximum pinning length was approximately 750 nm to either side of the particle, leading to an interface line tension of 170 nJ m−1. The observed pinning behavior is compared with the most relevant models in the literature.

The degradation of creep resistance in Nickel-based single crystal superalloys is essentially ascribed to their microstructure evolution. Yet there is a lack of work that manages to simulate the effect of alloying element concentrations on microstructure degradation. In this research, a computational model is developed to connect the rafting kinetics of Ni superalloys with their chemical composition, by combining thermodynamics calculation and an energy-based microstructure model. The isotropic coarsening rate and γ/γ misfit stresses have been selected as composition related parameter, and the effect of service temperature, time and applied stress are also taken into consideration to simulate the evolutions of microstructure parameters during creep process. The different generations of commercial Ni superalloys are selected and their chemical compositions are calculated based on this model. The simulated microstructure parameters are validated by the results from experimental results and the existing analytical model. The capability of the model in predicting the microstructure characteristics may provide instructional thought in developing a novel computational guided design approach in Ni superalloys.

Chemical boundary engineering expands the dimensionality of alloy design., For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys.

Herein, the effect of Nb content on the phase transformation kinetics, microstructure, and mechanical properties of hot-rolled quenching and partitioning (Q&P) steel is investigated. The characteristics of three C–Mn–Si–Ti steels (0.18C, 2.0Si, 2.6Mn, and 0.015Ti) containing 0, 0.027, or 0.061 wt% Nb are compared. Results reveal that grain boundary pinning by precipitates and Nb solute drag effects refine the austenite grain size during the hot-rolling process; the microstructural refinement is carried over to the final microstructure subjected to the Q&P treatment. The remaining supersaturated Nb suppresses the bainite formation and decreases the final bainite fraction formed in the Q&P process. The microstructural evolution leads to an increase in the ultimate tensile strength (UTS) of the steel containing 0.027 wt% Nb from 1169 to 1228 MPa, while keeping the total elongation at 18%. When the Nb content is increased to 0.061 wt%, the UTS of the steel increases to 1313 MPa, but the elongation at break drops to 16%. The effect is due to the carbon consumption by the Nb precipitates, which causes a decrease in the stability of the retained austenite and reduces the strain hardening at high strain levels.

The degradation of creep resistance in Ni-based single-crystal superalloys is essentially ascribed to their microstructural evolution. Yet there is a lack of work that manages to predict (even qualitatively) the effect of alloying element concentrations on the rate of microstructural degradation. In this research, a computational model is presented to connect the rafting kinetics of Ni superalloys to their chemical composition by combining thermodynamics calculation and a modified microstructural model. To simulate the evolution of key microstructural parameters during creep, the isotropic coarsening rate and γ/γ′ misfit stress are defined as composition-related parameters, and the effect of service temperature, time, and applied stress are taken into consideration. Two commercial superalloys, for which the kinetics of the rafting process are selected as the reference alloys, and the corresponding microstructural parameters are simulated and compared with experimental observations reported in the literature. The results confirm that our physical model not requiring any fitting parameters manages to predict (semiquantitatively) the microstructural parameters for different service conditions, as well as the effects of alloying element concentrations. The model can contribute to the computational design of new Ni-based superalloys.

In the present work, three different scratch tests are compared on their ability to rank the abrasion resistance of low alloy steels for industrial applications where the abrasion play a key role, e.g., in earthmoving, agricultural and mining equipment. The first test involves single pass scratching of pristine surfaces with a relatively large rigid indenter. The second test involves multi-pass scratching along a fixed track using the same large indenter. The third test involves the creation of a multi-pass scratch track using the same large indenter followed by final scratching of the abrasion track with a sharp indenter, i.e., Multi-Pass Dual Indenter (MPDI) scratch test. The three test protocols activate different abrasion mechanisms. Five low alloy construction steel grades with different strain hardening capabilities, i.e., Interstitial-free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel), are used. The results show that for both single (large) indenter scratch test protocols, the scratch depths always increase with indenter load and the failure mechanism is pure ploughing except for the IF steel due to the nature of softness. While for the dual-indenter scratch test, the scratch depth is a more complex function of the load applied during creation of the work hardened surface layer. The conventional scratch test protocols cannot well reveal micro-cracks and defect in (sub) surface and cannot well reflect the real response of strain hardening of steels. However, the MPDI test can distinguish between different steels with different initial hardness and different strain hardening behaviour and reveal the damage and, equally important, can indicate how average loading conditions may affect the relative abrasion resistance of construction steels during steady state conditions.

The thermal stability of the microstructure, in particular that of the precipitates, has been known to be a critical factor for the creep strength of 9–12Cr martensitic heat-resistant steels. In the present study, four martensitic heat-resistant steels were designed to have different precipitate types and morphologies. Steel NS1 (nitride strengthened, NS) has a low density of fine nitrides and Steel NS2 is strengthened by a high density of fine nitrides. Steel CNS (carbide and nitride strengthened, CNS) contains large size carbides and fine carbonitrides and Steel CS (carbide strengthened, CS) mainly possesses large size carbides. Both aging and creep tests at 650 °C were employed to evaluate the stability of the microstructure and properties of the steels during high-temperature exposure as a function of the precipitate morphology. It was found that Steel NS1 and Steel NS2 displayed a good creep performance, but they already recrystallized after a relatively short-term aging treatment. On the other hand, Steel CNS and Steel CS gave poor creep strength, but they could still maintain the tempered martensitic microstructure during long-term aging. The relation between microstructural stability and precipitate morphology/density was linked to the dislocation-precipitate interaction.

In the literature many researchers have tried to correlate the (dry) abrasion resistance of steels to their standard mechanical properties as determined in simple tensile and hardness testing. However, many unclear or even conflicting correlations have been reported, in part due to small data sets being used, different testing methods and relatively simple data processing techniques. In the present work 40 different low alloy steels were tested using the same ASTM G65 abrasion test and standard mechanical testing. The data set consists of 20 samples having the same chemical composition yet processed to different microstructures and different mechanical properties, as well as 20 steel grades with various compositions but having mechanical properties comparable to the first data set. The results show that for steels of the same composition significantly higher correlations between the abrasion resistance and certain mechanical properties (in particular strength coefficient K , hardness and UTS) are observed than those for steels having different compositions. In case both data sets were combined and a new regression analysis was applied, the quality of the correlation dropped significantly and the correlations became barely statistically relevant. Furthermore, when taking into account the measurement errors in both independent and dependent variables, the correlation coefficient became even worse and no statistically relevant correlation was observed at all. The current study clearly casts doubts on the validity of reported correlations between material (mechanical) properties and abrasion resistance.

Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating. This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

Over the past decades, the mechanism of interface migration during the austenite-ferrite transformation in steels has attracted significant attention from physical metallurgists. There are two challenging research questions in this field: (i) What is the effect of (substitutional) alloying elements on migrating interfaces? and (ii) How to accurately determine the value of interface mobility?. Recently, a cyclic partial phase transformation approach has been proposed to study interface migration, and new insights into the above two questions have been provided. An overview of the cyclic partial phase transformation concept is given, and pathways for future research are suggested.

A novel probe has been developed to characterize particles in boilers of waste incineration plants, which are under suspicion to essentially drive the corrosion processes. The probe collects larger particles on impaction plates and smaller particles on membranes, whose deposition characteristics have been studied by computational fluid dynamics. Artifacts by condensation of salt vapors are largely suppressed. The size distribution, morphology and chemical properties of the particles can be investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The probe has been employed for measurements in a waste incineration plant to characterize the aerosol on its way through the boiler.

A general computational alloy design approach, based on thermodynamics and thermokinetics and coupled with a genetic algorithm optimization routine, was applied to the design of novel creep martensitic resistant steels. The optimal alloy suggested by the model has a high density of barely coarsening MX carbonitride precipitates. The model yielded precise values for the concentrations of the 10 alloying elements considered. The model alloy was produced on a 10 kg lab scale. Samples of the new alloy of one of the best commercial martensitic steels on the market P92 were subjected to a high aging temperature of 923 K (650 °C) for times up to 1000 hours. The microstructure of the new alloy in the as-produced state as well as after 1000 hours exposure has all the intended features as predicted by the model. The coarsening rate of the MX rate carbonitrides was substantially lower than that of the precipitates in the P92 steel. The very low coarsening rate explains the superior hardness at very long exposure times.

An experimental investigation of the scratch and abrasive wear behaviour of a lean C–Mn construction steel in its tempered fully martensitic (TM) state is presented. The scratch resistance and the corresponding failure mechanisms as a function of the tempering temperature (200–500 °C) were evaluated using a multi-pass dual-indenter (MPDI) scratch test applying different loading conditions. Results show that the scratch resistance depends not only on the tempering temperature, but also on the load applied during scratching. The optimal tempering temperature depends on the applied load. For both low and high loading conditions, the dual phase (ferrite–martensite) variant with an optimised martensite volume fraction and morphology yields an even better combination of scratch/abrasion resistance and hardness. The scratch resistance at different loading conditions is linked to the strength coefficient K in the Hollomon equation ( σ = K e n ). The scratch behaviour in the MPDI scratch test at a low load correlates quite well with the standard ASTM G65 multi-particle abrasion test.

The formation of bainitic ferrite and that of grain boundary ferrite in low alloy steels have been two of the most important and interesting research topics in the field of solid state ferrous phase transformation for several decades, and various aspects of these two transformations have been discussed extensively in the literature. Recently, a so-called Gibbs energy balance (GEB) model was proposed by the authors to evaluate alloying element effects on the growth of bainitic ferrite and grain boundary ferrite. The model predicts a growth mode transition from paraequilibrium, negligible partitioning to partitioning during the isothermal formation of bainitic ferrite and grain boundary ferrite. Transformation stasis and bay phenomenon are well explained by the GEB model and both of them are found to be due to alloying element diffusion at the interface. This overview gives a summary of the authors’ recent progress in the understanding of the growth of bainitic ferrite and grain boundary ferrite, with particular focus on the growth mode transition, the transformation stasis phenomenon and the bay phenomenon.

Reduced-activation ferritic/martensitic steels are prime candidate materials for structural applications in nuclear power reactors. However, their creep strength is much lower than that of creep-resistant steel developed for conventional fossil-fired power plants as alloying elements with a high neutron activation cannot be used. To improve the creep strength and to maintain a low activation, a high-throughput computational alloy design model coupling thermodynamics, precipitate-coarsening kinetics and an optimization genetic algorithm, is developed. Twelve relevant alloying elements with either low or high activation are considered simultaneously. The activity levels at 0–10 year after the end of irradiation are taken as optimization parameter. The creep-strength values (after exposure for 10 years at 650 °C) are estimated on the basis of the solid-solution strengthening and the precipitation hardening (taking into account precipitate coarsening). Potential alloy compositions leading to a high austenite fraction or a high percentage of undesirable second phase particles are rejected automatically in the optimization cycle. The newly identified alloys have a much higher precipitation hardening and solid-solution strengthening at the same activity level as existing reduced-activation ferritic/martensitic steels.

The transition between grain boundary ferrite (GBF) and bainitic ferrite (BF) formation in Fe–C–Mo and Fe–C–Mn alloys has been investigated from a diffusional point of view by a so called Gibbs energy balance approach based on solute drag theory, in which the chemical driving force is assumed to be equal to energy dissipation due to alloying elements diffusion inside the interface. The GEB approach predicted that with decreasing temperature the transformation is dominated by GBF and BF formation, respectively, which provided a rationale for the transition between GBF and BF formation. The presence of transformation bay phenomenon in Fe–C–Mo alloys was well captured by the GEB approach, and the bay phenomenon was found to be due to strong solute drag effect of Mo. No bay phenomenon was predicted in Fe–C–Mn alloys. The effect of Mo and Mn on bainite start temperature has also been well predicted by the GEB approach.

A systematic experimental investigation concerning the effect of ferrite–martensite morphology on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels is reported. A hot rolled 22MnB5 steel was subjected to different heat treatments to generate dual phase microstructures with different ferrite–martensite morphologies. The effects of morphology on the scratch resistance and the corresponding failure mechanisms were unravelled using a multi-pass dual-indenter (MPDI) scratch test applying different load combinations. Results show that the ferrite–martensite morphology has a significant influence on scratch resistance and that the effect is contact load dependent. The scratch behaviour is linked to the strength coefficient K in the Hollomon model ( σ = K e n ) as well as the initial indentation hardness. Results suggest that the strength coefficient K corresponds well with the scratch resistance. The optimal microstructure to yield the best combination of abrasion resistance and hardness depends on the working conditions. At a low loading condition the relative ranking of the scratch resistance of the various microstructures created is in good agreement with that of the ASTM G65 abrasion test.

A systematic experimental investigation was conducted to study the effect of the martensite volume fraction on the scratch and abrasion resistance of ferrite–martensite dual phase (DP) steels. Dual phase microstructures with relatively high martensite fractions were generated by tailored intercritical annealing treatments. The scratch resistance and corresponding failure mechanisms were evaluated using a multi-pass dual-indenter (MPDI) scratch test for different loading conditions. Results show that the scratch resistance depends on not only the volume fractions of each phase, but also the loading conditions employed. Under low load conditions, the scratch depth decreases with increasing martensite fraction, in line with the increasing hardness evolution. However, under high load conditions, the scratch depth initially decreases with increasing martensite fraction up to a critical value (optimal fraction), beyond which the scratch depth starts to rise. This optimal fraction displays the best scratch resistance despite of a lower hardness compared to that of the full martensite. The scratch behaviour was correlated to the tensile behaviour and the corresponding work hardening characteristics. A two-stage strain hardening model was applied to interpret the scratch behaviour at different loads. The scratch behaviour at low load was found to correlate well with the standard ASTM G65 abrasion test.

Upon austenite decomposition, either of two ferritic morphologies may result depending on the steel composition and the transformation temperature, i.e. equiaxed (i.e. quasi spherical) or plate-like ferrite. A 2-D model based on the Gibbs Energy Balance (GEB) model and the Zener–Hillert equation is presented to explain the conditions leading to either morphology. To this aim the kinetics of isotropic growth and plate-lengthening, plate-thickening are modeled with appropriate assumptions regarding the interface conditions respectively. The competition between the two morphologies is determined by the fastest minimization of the total Gibbs energy of the system during transformation. The model is first applied to a ternary alloy containing an austenite forming element (Fe-0.23C-1.86Mn) and the predicted morphology competition is in good agreement with experimental observations. In the refined model, the effect of nucleation on the growth of a plate is rationalized. For the plate-like bainitic transformation, a new concept of ‘single plate-thickening stasis’ is proposed, in addition to the commonly discussed ‘global stasis of the bainitic transformation’, i.e. the incomplete bainitic transformation. A general model of the global transformation kinetics bridging the single plate stasis and the global stasis is depicted. Finally, without changing any of the model assumptions or transition conditions the model is applied to a ternary alloy containing a ferrite forming element (Fe-0.375C-1.48Si) The predicted morphology competition is fully in line with the experimental observations for samples subjected to isothermal decomposition at different temperatures.

Predicting the effect of alloying elements on the degree of incomplete austenite to bainite transformation in low carbon steels is of great industrial importance. This study introduces an extended Gibbs energy balance model which makes use of an additive approach to calculate the coupled effect of Mn, Si and Mo on the fraction of bainitic ferrite after the incomplete transformation in multicomponent steels. The model predicts significant effects of Mn and Mo and the negligible effect of Si levels on the fraction of bainitic ferrite. This is attributed to the high value of dissipation of Gibbs energy caused by interfacial diffusion of Mn and Mo and low values caused by Si diffusion. The model predictions for quaternary Fe–C–Mn–Si system are comparable with the experimentally measured values of bainite fraction. For the Fe–C–Mn–Mo system, the agreement is less accurate and the accuracy decreases with increasing Mo content, which is attributed a substantial carbide formation but interaction effects between Mn and Mo or a temperature dependent binding energy cannot be ruled out.

Formation of the microstructural ferrite/pearlite bands in medium Mn steels is an undesirable phenomenon commonly addressed through fast cooling treatments. In this study, a novel approach using the cyclic partial phase transformation concept is applied successfully to prevent microstructural band formation in a micro-chemically banded Fe-C-Mn-Si steel. The effectiveness of the new approach is assessed using the ASTM E1268-01 standard. The cyclic intercritical treatments lead to formation of isotropic microstructures even for cooling rates far below the critical one determined in conventional continuous cooling. In contrast, isothermal intercritical experiments have no effect on the critical cooling rate to suppress microstructural band formation. The origin of the suppression of band formation either by means of fast cooling or a cyclic partial phase transformation is investigated in detail. Theoretical modeling and microstructural observations confirm that band formation is suppressed only if the intercritical annealing treatment leads to partial reversion of the austenite-ferrite interfaces. The resulting interfacial Mn enrichment is responsible for suppression of the band formation upon final cooling at low cooling rates.

A 3D model has been developed to predict the average ferrite grain size and grain size distribution for an austenite-to-ferrite phase transformation during continuous cooling of an Fe-C-Mn steel. Using a Voronoi construction to represent the austenite grains, the ferrite is assumed to nucleate at the grain corners and to grow as spheres. Classical nucleation theory is used to estimate the density of ferrite nuclei. By assuming a negligible partition of manganese, the moving ferrite–austenite interface is treated with a mixed-mode model in which the soft impingement of the carbon diffusion fields is considered. The ferrite volume fraction, the average ferrite grain size, and the ferrite grain size distribution are derived as a function of temperature. The results of the present model are compared with those of a published phase-field model simulating the ferritic microstructure evolution during linear cooling of an Fe-0.10C-0.49Mn (wt pct) steel. It turns out that the present model can adequately reproduce the phase-field modeling results as well as the experimental dilatometry data. The model presented here provides a versatile tool to analyze the evolution of the ferrite grain size distribution at low computational costs.

The classical Johnson–Mehl–Avrami–Kolmogorov equation was modified to take into account the normal local strain distribution in deformed samples. This new approach is not only able to describe the influence of the local heterogeneity of recrystallization but also to produce an average apparent Avrami exponent to characterize the entire recrystallization process. In particular, it predicts that the apparent Avrami exponent should be within a narrow range of 1 to 2 and converges to 1 when the local strain varies greatly. Moreover, the apparent Avrami exponent is predicted to be insensitive to temperature and deformation conditions. These predictions are in excellent agreement with the experimental observations on static recrystallization after hot deformation in different steels and other metallic alloys.

Multi-pass dual-indenter (MPDI) scratch tests were carried out on a wide range of steels with different microstructures to investigate the relation between the formation of subsurface deformation layers under repetitive local sliding contact and their abrasion resistance. Five steel grades with different work hardening capability, i.e. Interstitial-Free steel, Fully Martensitic steel, Dual Phase steel, Quench Partitioning steel and TWining Induced Plasticity steel were selected. The results of this well-defined local contact damage test method and those of the standard ASTM G65 dry sand rubber wheel abrasion test were correlated. Results suggest that the abrasion resistance of steels is controlled by the properties of the work hardening layer formed beneath the abraded surface and depends on the thickness of the work hardening layer. When applying the correct evaluation parameters the new scratch test MPDI methodology reproduces the material response to the ASTM G65 abrasion test rather well and provides a reproducible and quantitative method to screen the abrasion resistance of new construction steels. Moreover, the MPDI test allows to quantitatively evaluate the abrasion resistance of steels for working conditions beyond those of the standardized G65 test.

Recently, the Laves phase precipitate family has been identified as a promising alternative for Precipitation Hardening (PH) in ferritic creep resistant steels owing to its significant contribution to creep strength. A computational alloy design approach to create novel ferritic creep resistant steels strengthened by optimized Laves phase is presented. This novel approach aims at a simultaneous optimization of the steel’s chemical composition involving 9 chemical elements to obtain a ferritic steel optimally strengthened by Laves phases with an ideal combination of a high volume fraction and a reduced coarsening rate at the intended service temperature. Using this approach, three alloys possessing ideal PH contributions at their intended service temperatures, i.e. 650, 700 and 750 C, respectively, are designed. This approach is validated by analyzing PH contributions in existing 15Cr ferritic steels at different service temperatures. Good agreement between reported experimental performance and model predictions is found. The newly designed alloys are predicted to have a substantially higher PH contribution at corresponding service temperature than those of the existing alloy. While Laves phases generally precipitate slowly during service and have a more significant strengthening effect at the late stage, the precipitation of the alternative M23C6 precipitates can be used to increase the initial strength of ferritic steel. Hence, our computational approach was extended to aim for a high strengthening contribution at both initial and late stages at the intended service temperature.

Alumina encapsulated molybdenum silicide (MoSi2) intermetallic particles were synthesized using a simple precipitation method followed by calcining at temperatures of 800°C–1000°C, to prevent the premature oxidation of MoSi2 at high temperatures. The shell composition and the influence of the calcining temperature on microcapsule integrity were investigated by means of X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results demonstrate that the composition and the mechanical stability of the alumina shell can be tuned by the annealing temperature. After calcining at 800°C and 850°C the alumina shell remains intact. Calcining at higher temperature promotes the formation of mullite, which leads to cracking of the shell. However, when annealed at 1000°C for 24 h these cracks were filled with mullite and preserved the molybdenum silicide particles. Furthermore, the mechanical stability of the shell was improved by applying an intermediate calcining treatment at 450°C prior to the annealing process at 1000°C.

The synthesis of a high purity Cr 2 AlC metallo-ceramic involving pressure-less sintering (PLS) of elemental powders followed by spark plasma sintering (SPS) sintering of the crushed reaction product of the PLS product is described. High purity Cr 2 AlC was obtained for PLS 1400 °C for 1 h in argon. Depending on the ball milling conditions either intermediate coarse equiaxed Cr 2 AlC particles or intermediate small flake like Cr 2 AlC particles were obtained. SPS of the coarse powder resulted in high density bulk samples with a random texture. SPS of the fine powder resulted in high density bulk samples with a well-developed texture such that the (0 0 0 1) planes are perpendicular to the compression direction. The hardness of fine-grained Cr 2 AlC bulk ceramic along different directions is both higher than that of coarse-grained Cr 2 AlC bulk ceramic. Due to its texture the fine-grained Cr 2 AlC bulk ceramic shows an anisotropic fracture behavior.

**By Shasha Zhang,* Cees Kwakernaak, Willem Sloof, Ekkes Bruck, Sybrand van der Zwaag€and Niels van DijkIn the present study we demonstrate that autonomousrepairof high-temperature creep damagecan beachieved in iron by alloying with a small amount of gold. Adopting a heat treatment that providesmobilegoldwithintheironmatrixisfoundtosignificantlyextendthecreeplifetimeatatemperatureof550°C. Combined electron-microscopy techniques demonstrate that the improved creep propertiesoriginate from selective precipitation of gold atoms within early-stage creep cavities, before they canmergeintomicro-cracksalongthegrainboundaries.Theselectiveprecipitationofgoldatomsatthefreesurface of a creep cavity results in pore filling, and as a result autonomous repairof the creep damage.Grain boundaries and dislocations act as fast routes for solute gold transport from the matrix to thecreep damage. Pore filling preferentially takes place at the highest loaded grain boundaries orientedperpendiculartotheappliedload.Theefficiencytohealcreepdamageisfoundtodependstronglyontheapplied stress. For lower stress levels filling fractions of up to 80% of the open-volume creep damagehave been observed.

A proper understanding of abrasion resistance and associated damage mechanisms is of vital importance in the design of improved abrasion resistant construction steels. The conventional scratch test, sliding a rigid indenter under a controlled load and speed against a smooth surface, mimics the nature of the abrasion process and can be used to evaluate the abrasion resistance of various microstructures. However, scratch tests are mostly done on the initial surface, which can be very different from that formed during the abrasion process and hence do not truly reflect its abrasion resistant response. In the present work, a new scratch test methodology is developed to approach the real abrasion condition by carrying out a multi-pass dual indenter scratch tests, in which a small indenter scratches a pre-scratched surface produced by a large indenter. Five steel grades with different work hardening capacities, i.e. Interstitial-Free Ferritic steel (IF steel), Fully Martensitic steel (FM steel), Dual Phase steel (DP steel), Quench Partitioning steel (Q&P steel) and TWining Induced Plasticity steel (TWIP steel) were selected. Systematic scratch resistance experiments were performed to investigate the damage mechanisms, the work hardening behavior and the development of subsurface deformation layers. Results suggest that the work hardening layer formed beneath the abraded surface plays the dominant role in determining the abrasion resistance. Steels grades of DP, Q&P and TWIP display superior scratch and abrasion resistances, notwithstanding their relative low hardness compared to that of a corresponding steel with a fully martensitic microstructure.

The precipitation of Au-rich precipitates on the external surfaces of Fe-Au alloys has been studied by scanning and transmission electron microscopy. The surface precipitates formed at elevated temperatures are found to self-organize in regular patterns and their growth rate is determined quantitatively. The observed surface precipitation at a free surface is compared to the precipitation at the internal surface of grain boundary cavities induced by creep loading. A close agreement between both processes is observed.

The stability of individual metastable austenite grains in low-alloyed TRIP steels has been studied during tensile loading using high-energy X-ray diffraction. The carbon concentration, grain volume and grain orientation with respect to the loading direction was monitored for a large number of individual grains in the bulk microstructure. Most austenite grains transform into martensite in a single transformation step once a critical load is reached. The orientation-dependent stability of austenite grains was found to depend on their Schmid factor with respect to the loading direction. Under the applied tensile stress the average Schmid factor decreased from an initial value of 0.44 to 0.41 at 243 MPa. The present study reveals the complex interplay of microstructural parameters on the mechanical stability of individual austenite grains, where the largest grains with the lowest carbon content tend to transform first. Under the applied tensile stress the average carbon concentration of the austenite grains increased from an initial value of 0.90 to 1.00 wt% C at 243 MPa, while the average grain volume of the austenite grains decreased from an initial value of 19 to 15 µm3 at 243 MPa.

Generally, Laves phase and M23C6 are regarded as undesirable phases in creep-resistant steels due to their very high-coarsening rates and the resulting depletion of beneficial alloying elements from the matrix. In this study, a computational alloy design approach is presented to develop martensitic steels strengthened by Laves phase and/or M23C6, for which the coarsening rates are tailored such that they are at least one order of magnitude lower than those in existing alloys. Their volume fractions are optimized by tuning the chemical composition in parallel. The composition domain covering 10 alloying elements at realistic levels is searched by a genetic algorithm to explore the full potential of simultaneous maximization of the volume fraction and minimization of the precipitates coarsening rate. The calculations show that Co and W can drastically reduce the coarsening rate of Laves and M23C6 and yield high-volume fractions of precipitates. Mo on the other hand was shown to have a minimal effect on coarsening. The strengthening effects of Laves phase and M23C6 in the newly designed alloys are compared to existing counterparts, showing substantially higher precipitation-strengthening contributions especially after a long service time. New alloys were designed in which both Laves phase and M23C6 precipitates act as strengthening precipitates. Successfully combining MX and M23C6 was found to be impossible.

The effect of different heat treatment on the microstructure and mechanical properties with compression tests for alloy Ti – 10% V – 2% Fe – 3% Al with a metastable β-structure is studied.

Precipitation hardening and solid-solution strengthening (SSS) are the most effective strengthening mechanisms in creep-resistant, corrosion-resistant ferritic/martensitic steels. However, it is difficult to achieve a simultaneous maximization of both mechanisms as alloying elements may affect both mechanisms in a different way and upset considerations based on one strengthening mechanism only. To address this challenge, a generic alloy-by-design model was developed. The model uses quantitative descriptions for both the time-dependent precipitate population and its resulting strengthening contribution and the time-independent SSS due to chemical elements remaining in solid solution. Furthermore, the model involves full thermodynamic calculations at appropriate temperatures. The formulations were found to capture the contributions of both strengthening mechanisms in existing Cr containing martensitic steel grades rather precisely. Subsequently, the model was used to probe new steel compositions in the compositional domain of 9–12% Cr martensitic creep-resistant steels containing nine other alloying elements, each at different concentration levels. A genetic algorithm was used to probe the compositional search domain. The calculations were performed assuming a continuous operating temperature of 650 °C and a lifetime of at least 10 5 h. Alloy compositions and heat treatment conditions leading to too large amounts of undesirable phases and other detrimental effects were automatically rejected. A Pareto front linking the strength contributions according to precipitation hardening or SSS was constructed by combining results of two separate optimization routes. From the Pareto front analysis, steel compositions with optimized combinations of strengthening mechanisms were identified. The robustness of the new solutions was tested by enforcing concentration variations as encountered in industrial practice and to test for abrupt changes in predicted creep strength values. The newly designed steel is predicted to significantly outperform existing commercial creep-resistant martensitic steel grades and has a target composition such that the usual deviations in elemental concentration levels do not significantly affect the final creep strength.

In the present study, X-ray computer tomography is used to determine damage evolution in extrusion weld seams loaded close to failure for two comparable AlSiMg alloys with distinctively different grain structures. In the dispersoid free alloy, recrystallisation occurs upon extrusion and the weld seam has no effect on mechanical properties. No preferential void formation in the weld seam region was detected. In the non-recrystallising dispersoid rich alloy, the weld seam leads to a reduced ductility. There are no signs of internal void formation, and damage initiation was found to be close to the surface. It is argued that for the non-recrystallising alloy, the effect of the weld seam on the ductility is related to a sharp texture gradient across the weld seam.

The martensitic transformation behaviour of the metastable austenite phase in low-alloyed transformation-induced plasticity (TRIP) steels has been studiedin situusing high-energy X-ray diffraction during deformation. The austenite stability during tensile deformation has been evaluated at different length scales. A powder diffraction analysis has been performed to correlate the macroscopic behaviour of the material to the observed changes in the volume phase fraction. Moreover, the austenite deformation response has been studied at the length scale of individual grains, where an in-depth characterization of four selected grains has been performed, including grain volume, local carbon concentration and grain orientation. For the first time, a high-resolution far-field detector was used to study the initial and evolving structure of individual austenite grains during uniaxial tensile deformation. It was found that the austenite subgrain size does not change significantly during tensile deformation. Most austenite grains show a complete martensitic transformation in a single loading step.

This work presents an artificial intelligence based design of a series of novel advanced high performance steels for ambient and high temperature applications, following the principle of the materials genome initiative, using an integrated thermodynamics/kinetics based model in combination with a genetic algorithm optimization routine. Novel steel compositions and associated key heat treatment parameters are designed both for applications at the room temperature (ultra-high strength maraging stainless steel) and at high temperatures (ferritic, martensitic and austenitic creep resistant steels). The strength of existing high end alloys of aforementioned four types are calculated according to the corresponding design criteria. The model validation studies suggest that the newly designed alloys have great potential in outperforming existing grades.

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation in a series of Fe-C-X (X = Mo, Ni, Si ) alloys has been analyzed. Both the Gibbs energy balance (GEB) approach and the \(T_0\) model have been applied to model the “transformation stasis” phenomenon, and their predictions are compared with experimental results. The \(T_0\) model failed in predicting the transformation stasis (TS) phenomenon for the alloys investigated here, while the GEB predictions are in very good agreement with experimental data. It is found that Mo has a very strong effect on the TS phenomenon, while the effect of Si is found to be negligible. Ni has an intermediate effect.

Solid solution strengthening can play a very important role in the performance of heat resistant steels, not only because of its substantial strengthening effect as such, but also because of the fact that the effect that solid solution strengthening is, by definition, time independent. In the present study a computational alloy design approach is presented to explore the chemical composition of high strength ferritic heat resistant steel designed to have an optimal solid solution strengthening, while minimising the presence of undesirable microstructural components. Alloy compositions covering eleven elements are considered in the calculations. The chemical composition and the associating optimal annealing temperature are optimised simultaneously via a genetic algorithm. To validate the approach, the solid solution strengthening contribution in existing commercial ferritic stainless steel was calculated using the same computational procedures, and it is shown that the documented relative experimental performance of the reference alloys agrees very well with the model predictions. According to the predictions results, the newly designed alloys have a substantially higher solid solution strengthening than found in existing alloys.

This work reports on a study into the design of creep-resistant precipitation-hardened austenitic steels using an integrated thermodynamics-based model in combination with a genetic algorithm optimization routine. The key optimization parameter is the secondary stage creep strain at the intended service temperature and time, taking into account the coarsening rate of MX carbonitrides and its effect on the threshold stress for secondary creep. The creep stress to reach a maximal allowed creep strain (taken as 1%) at a given combination of service temperature and time is formulated and maximized. The model was found to predict the behaviour of commercial austenitic creep-resistant steels accurately. By means of the alloy optimization scheme, three new steel compositions are presented, yielding optimal creep strength for various intended service times (10, 10 3 , 10 5 h). According to the evaluation parameter employed, the newly defined compositions will outperform existing precipitate-strengthened austenitic creep-resistant steels.

The final fractions of austenite after the isothermal austenite-to-ferrite transformation and its reverse transformations (including the martensite-to-austenite and the pearlite + ferrite-to-austenite transformations) in the intercritical annealing region have been studied using full equilibrium (FE), paraequilibrium (PE), and local equilibrium (LE) calculations and experiments. The LE model predictions are in a very good agreement with the experimental results, while the FE and PE model predictions deviate significantly. It is also found that the LEP/LENP transition for the austenite-to-ferrite transformation deviates from those of its reverse transformations. The magnitude of deviation increases with the increasing Mn concentration.

The influence of deformation-induced defects on the isothermal precipitation of Au was studied in high-purity Fe–Au and Fe–Au–B–N alloys. Preferential Au precipitation upon annealing at 550 °C is observed at local plastic indentations. In fractured Fe–Au–B–N, solute Au atoms were found to heterogeneously precipitate at grain boundaries and local micro-cracks. This is supported by in-situ creep tests that showed a strong tendency for Au precipitation at cracks and cavities also formed during creep loading at 550 °C. Complementary X-ray photoelectron spectroscopy experiments indicate a strong tendency of Au, B and N segregation onto free surface during aging. The observed site-specific precipitation of Au holds interesting opportunities for defect healing in steels subjected to creep deformation.

The microstructure evolution during shear loading of a low-alloyed TRIP steel with different amounts of the metastable austenite phase and its equivalent DP grade has been studied by in-situ high-energy X-ray diffraction. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing simultaneously the evolution of the austenite phase fraction and its carbon concentration, the load partitioning between the austenite and the ferritic matrix and the texture evolution of the constituent phases. Our results show that for shear deformation the TRIP effect extends over a significantly wider deformation range than for simple uniaxial loading. A clear increase in average carbon content during the mechanically-induced transformation indicates that austenite grains with a low carbon concentration are least stable during shear loading. The observed texture evolution indicates that under shear loading the orientation dependence of the austenite stability is relatively weak, while it has previously been found that under tensile load the {110}〈001〉 component transforms preferentially. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between the interstitial carbon concentration in the austenite, the grain orientation and the load partitioning.

Cyclic phase transformation experiments are performed in a series of Fe-C-xMn, Fe-C-Mn-xNi, and Fe-C-Mn-xCo alloys to study the effect of alloying elements on the length of the stagnant stage during a cyclic partial phase transformation in the austenite–ferrite two-phase region. The length of stagnant stage increases linearly with the increasing Mn or Ni concentration, while Co has no effect. It was experimentally proven that the effects of alloying elements on the length of stagnant stage are additive, and the experimental results matched the predictions of the local equilibrium model very well.

This work concentrates on assessment of the TEP change of AA6061 during isothermal aging at 177 °C and the following interrupted aging at 65 °C. The results show that the TEP is sensitive to follow the microstructural changes undergone during all aging stages. Multiple sub-ambient temperature dependences TEP of binary Al-X alloys as well as the AA6061 subjected to the above mentioned heat treatments were undertaken. The solute level of individual element of the alloy, particularly those contributing to the clustering/precipitation, can be extracted and evaluated.

A new genetic alloy design approach based on thermodynamic and kinetic principles is presented to calculate the optimal composition of MX carbonitrides precipitation strengthened austenitic heat-resistant steels. Taking the coarsening of the MX carbonitrides as the process controlling the life time for steels in high temperature use, the high temperature strength is calculated as a function of steel chemistry, service temperature and time. New steel compositions for different service conditions are found yielding optimal combinations of strength and stability of the strengthening precipitation for specific applications such as fire-resistant steels (short-time property guarantee) and creep-resistant steels (long-time property guarantee). Using the same modelling approach, the high temperature strength and lifetime of existing commercial austenitic creep-resistant steels were also calculated and a good qualitative agreement with reported experimental results was obtained. According to the evaluation parame...

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation has been investigated in a series of Fe–C–Mn and Fe–C–Mn–Si alloys. The Gibbs energy balance (GEB) approach is applied to model the transformation stasis phenomenon, and theoretical predictions are compared with experimental observations. The good agreement over several alloy systems demonstrates that the transformation stasis is caused by diffusion of alloying elements into the migrating austenite/bainitic ferrite interfaces. It is found that the occurrence of transformation stasis in the Fe–C–Mn and Fe–C–Mn–Si alloys depends on the concentration of Mn, while the addition of Si has a negligible effect on transformation stasis. The GEB model clearly outperforms the diffusionless T0 model.

International audience; New interrupted cooling experiments have been designed to study the kinetics of bainitic ferrite formation starting from a mixture of austenite and bainitic ferrite. It is found that the kinetics of bainitic ferrite formation during the cooling stage is determined by the isothermal holding time. The formation rate of bainitic ferrite at the beginning of the cooling decreases with increasing prior isothermal holding time. An unexpected stagnant stage during the cooling stage appears when the isothermal holding time increases to a critical point. There are two reasons for the occurrence of the stagnant stage: (i) a solute spike in front of the interface; and (ii) kinetic transition. A so-called Gibbs energy balance approach, in which the dissipation of Gibbs energy due to diffusion inside the interface and interface friction is assumed to be equal to the available chemical driving force, is applied to theoretically explain the stagnant stage. A kinetics transition from a fast growth mode without diffusion of Mn and Si inside the austenite-bainitic ferrite interfaces to a slow growth mode with diffusion inside the interface is predicted. The stagnant stage is caused by the transition to a slow growth mode. The Gibbs energy balance approach describes the experimental observations very well.