Mo addition is widely used to retard ferrite transformation and increase hardenability of steels. It has been well recognized that Mo segregation at ferrite/austenite interface induces solute drag ...effect (SDE) and reduces migration rate of ferrite/austenite interface while retardation by Mo is less obvious in bainite transformation. However, element segregation, solute drag effects and its interface character dependence have not been clarified quantitatively. Therefore, in the present study, amount of Mo segregation, energy dissipation during interface migration and interface orientation relationship (OR) in ferrite and bainite transformations in Fe-0.4%C-0.5%Mo (mass%) alloy have been investigated quantitatively. Mo segregation at interfaces with non K-S OR is more significant than that at interface with near K-S OR. Amount of Mo segregation at interface with non K-S OR increases with increase in transformation time investigated while energy dissipation decreases. The amount of Mo segregation and energy dissipation measured at non K-S interface coincides well with SDE model using optimized segregation energy and interface thickness. On the other hand, energy dissipation at bainite/austenite interface is large in spite of negligibly small Mo segregation, which is inconsistent with the SDE model.
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We show that molecular dynamics (MD) simulations are capable of reproducing the drag of solute segregation atmospheres by moving grain boundaries (GBs). Although lattice diffusion is ...frozen out on the MD timescale, the accelerated GB diffusion provides enough atomic mobility to allow the segregated atoms to follow the moving GB. This finding opens the possibility of studying the solute drag effect with atomic precision using the MD approach. We demonstrate that a moving GB activates diffusion and alters the short-range order in the lattice regions swept during its motion. It is also shown that a moving GB drags an atmosphere of non-equilibrium vacancies, which accelerate diffusion in surrounding lattice regions.
Addition of small amount of Nb can strongly retard the ferrite growth kinetics. The origin has been attributed to the solute drag effect (SDE) by Nb segregation at the migrating ferrite/austenite ...interface. However, due to limitation of characterization techniques, the relations between elemental segregation, SDE and interface velocity have not been quantitatively clarified yet. Meanwhile, the strong affinity between Nb and C atoms can induce carbide precipitation at the interface, namely interphase precipitation, which may influence the Nb segregation behavior and make the issue even more complicated. Therefore, in this study, the interface information including amount of Nb segregation, energy dissipation, NbC precipitates and interface velocity in Fe-0.08C-(0.035, 0.061)Nb (mass%) model alloys is quantitatively investigated. It reveals that the energy dissipation at the migrating ferrite/austenite interface decreases with longer holding time or higher transformation temperature. The Nb atoms prefer to segregate at the non K-S interface rather than the near K-S interface. Amount of Nb segregation at the non K-S interface increases with longer time, while raising bulk Nb content or lowering transformation temperature does not lead to a notable increment of segregation. The relations between Nb segregation, energy dissipation, and interface velocity can be well reproduced by the SDE model with optimized parameters (i.e., segregation energy, interface thickness and trans-interface diffusivity). Occurrence of NbC interphase precipitation affects the transformation kinetics indirectly by weakening the SDE via consumption of Nb solutes in ferrite. In contrast, their pinning effect plays a marginal role.
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In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor ...dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO3. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects (‘solutes’) at grain boundaries can retard grain boundary migration.
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Solute atoms segregate and impose a retarding pressure, also known as solute drag pressure, at the grain boundary (GB) leading to reduced GB migration rates. The solute drag pressure ...depends critically on the segregation energy and the solute diffusivity across the GB. These parameters are, however, typically used as adjustable parameters to describe experimental observations. Here, we present an approach to analyze solute drag based on density functional theory (DFT) calculations. As an example, we apply the proposed approach to available experimental data for migration rates of the 30∘ GB in Au with Fe and Bi impurities at the ppm level. Based on the DFT calculations, Bi is identified as a strongly segregating element while Fe segregation is weak in comparison. The effective segregation energy for Bi is found to vary from −0.59 eV to −0.72 eV in the experimentally investigated temperature range of 500–610 K. Further, the activation energy for trans−GB diffusion of Bi is calculated with DFT to fall into the range of 0.5–0.6 eV. These DFT based values are consistent with those obtained by the conventional solute drag analysis of the experimental data using the Cahn−Lücke−Stüwe (CLS) model. The proposed approach is discussed in terms of its strengths for trend predictions as well as its quantitative uncertainties.
The roles of rare earth elements on the texture weakening of Mg alloy are critically assessed. Two main roles are (i) the changing of stacking fault energy of Mg and (ii) the enhancement of solute ...drag of grain boundaries and dislocations. Based on this analysis, alloy design concepts and optimum process conditions for Mg alloys are suggested to increase the formability of Mg alloys.
Grain boundary (GB) solute segregation has been used as a strategy to tailor the properties and processing pathways of a wide range of metallic alloys. GB solute drag results when segregated alloying ...elements exert a resistive force on migrating GBs hindering their motion. While GB segregation has been the subject of active research, a detailed understanding of solute drag and the migration kinetics of doped boundaries is still lacking, especially in technologically-relevant alloys. Through theoretical analysis, mesoscale modeling, and machine learning studies, we investigate GB segregation and solute drag and establish design maps relating drag effects to relevant alloy and GB properties, i.e., the complete alloy design space. We find that solute drag is dominant in immiscible alloys with far-from-dilute compositions in agreement with experimental observations of GB segregation in metallic alloys. Our analysis reveals that solute–solute interactions within the GB and the degree of segregation asymmetry greatly influence solute drag values. In broad terms, our work provides future avenues to employ GB segregation to control boundary dynamics during materials processing or under service conditions.
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The HfNbTaTiZr refractory high-entropy alloy was investigated on the grain growth kinetics and tensile properties. Grain growth at 1200–1350°C is rather slow. The activation energy is 389kJ/mol and ...the growth exponent is 3.5. The HfNbTaTiZr alloy has high strength, small work hardening and high ductility. Grain refining is found to enhance the tensile strength and ductility simultaneously.
•The HfNbTaTiZr alloy exhibits low rate and high activation energy of grain growth.•The slow grain boundary migration is a result of the solute-drag mechanism.•Grain refinement simultaneously increases tensile strength and ductility•The alloy with a small grain size has excellent tensile yield strength and ductility.
A computational 3D model that accounts for both nucleation and interface migration is a very useful tool to monitor and grasp the complexity of microstructure formation in low-alloyed steels. In the ...present study we have developed a 3D mixed-mode multigrain model for the austenite-ferrite and the austenite-ferrite-austenite formation capable of following diffusional phase transformations under arbitrary thermal routes. This new model incorporates the solute drag effect of a substitutional element (in this case Mn) and ensures an automatic change in transformation direction when changing from heating to cooling and vice-versa. An analytical solution for calculating the energy dissipation of solute drag together with multiple regression approximations for chemical potentials are proposed which significantly accelerate the computation. The modelling results are first benchmarked for an Fe-0.1C-0.5Mn (wt.%) alloy under different continuous cooling and isothermal holding conditions. The model revealed relatively large variations in transformation kinetics of individual grains as a result of interactions with neighboring grains. Then the model is applied to predict the transformation kinetics of a series of Fe-C-Mn alloys during cyclic partial phase transformations. The comparison with experimental dilatometer results nicely validates the predictions of this model regarding the change in overall transformation kinetics of the ferrite transformation as a function of the Mn content. New features of this model are its efficient algorithm to compute energy dissipation by solute drag, its capabilities of predicting the microstructural state for spatially resolved grains and the minimal fine tuning of modelling parameters. The code to implement this model is publicly available.
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