35 research outputs found

    Morphology of Proeutectoid Ferrite

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    Effect of Tempering on the Bainitic Microstructure Evolution Correlated with the Hardness in a Low-Alloy Medium-Carbon Steel

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    A low-alloy medium-carbon bainitic steel was isothermally tempered at 300 degrees C for up to 24 hours which led to a significant hardness decrease. In order to explain the decreasing hardness, extensive microstructural characterization using scanning and transmission electron microscopy, X-ray diffraction, and atom probe tomography was conducted. The experimental work was further supplemented by thermodynamic and kinetic simulations. It is found that the main underlying reason for the hardness reduction during tempering is related to dislocation annihilation, possibly also with corresponding changes in Cottrell atmospheres. On the other hand, cementite precipitate size, effective grain size of the bainite, and retained austenite fraction appear unchanged over the whole tempering cycle

    Evaluation and Modeling of the Rate of Formation of Lath Martensite in Fe-C Alloys, Extracted from Ultra-Rapid Quenching Experiments

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    Isothermal information is rarely available for the formation of martensite in Fe or Fe alloys due to a very high rate of transformation compared to the rate of heat conduction. Such information has now been extracted for lath martensite in some sets of Fe alloys from available information on ultra-rapid quenching but only at a single temperature for each alloy, related to its two MS temperatures. The temperature dependence could, thus, be studied only on binary sets of alloys. Those results have been applied to mathematical models based on the Arrhenius equation and illustrated with Arrhenius plots. For three sets of binary Fe alloys, a large group of rates came close to the rate of an almost pure and carbon-free Fe-C alloy. It illustrated that Cr, Ni, and Ru in low contents have relatively small effects on the rate of formation of lath martensite in Fe. It also demonstrated that the present measurements have considerable reproducibility. In contrast, a set of Fe-C alloys did not give a straight line in the Arrhenius plot. Using a new mathematical model based on the concept of the Arrhenius equation to express the effect of carbon, it was possible to predict the rate of formation of lath martensite for Fe-C alloys with fixed C content and their temperature dependencies which are not available experimentally due to the very high rate of formation.QC 20241025</p

    Comparing the deformation-induced martensitic transformation with the athermal martensitic transformation in Fe-Cr-Ni alloys

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    The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both these cases, there is a significant chemical driving force for the transformation from γ to α’-martensite (α’), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α’ that forms within individual shear bands if the stability of γ is low. Neighbouring α’ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favourable oriented with respect to the applied stress/strain field. The formation of α’ at individual shear bands is also rare, since no ε is present and instead α’ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.QC 20180522</p

    Comparing the deformation-induced martensitic transformation with the athermal martensitic transformation in Fe-Cr-Ni alloys

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    The microstructure of martensite formed athermally or via deformation in Fe-Cr-Ni alloys with different austenite (γ) stability has been investigated using microscopy. Two different types of microstructures, viz. blocky and banded structure, are observed after athermal and deformation-induced martensitic transformation (AMT and DIMT, respectively). The blocky structure form during AMT or DIMT if the stability of γ is low. In both these cases, there is a significant chemical driving force for the transformation from γ to α’-martensite (α’), and if it is not hindered by e.g. planar defects it can grow uninhibited into a blocky morphology without the necessity to nucleate new crystallographic variants to accommodate the transformation strains. On the other hand, the banded structure is due to the formation of ε-martensite (ε) during AMT, or the wider concept shear bands in the case of DIMT. The shear bands, and in particular ε, lower the nucleation barrier for α’ that forms within individual shear bands if the stability of γ is low. Neighbouring α’ units predominantly have a twin-related orientation relationship to accommodate the transformation strains. With increasing γ stability during DIMT, variant selection becomes pronounced with preferred formation of variants favourable oriented with respect to the applied stress/strain field. The formation of α’ at individual shear bands is also rare, since no ε is present and instead α’ forms at the intersection of shear bands for more stable γ. In conclusion, AMT and DIMT for low γ stability lead to similar microstructures, whereas the DIMT microstructure for high γ stability is distinct.QC 20180522</p

    Indentation behavior of highly confined elasto-plastic materials

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    The effect of geometric confinement is well-known from hardness measurements of thin films on stiff substrates and has been modeled both phenomenologically and using e.g. Finite Element Analysis. However, these models are mainly focused on a specific experiment or a certain material family. In the present work, Finite Element Analysis is used to gain a better understanding of the interplay between geometric constraints in various microstructures and a wide range of materials properties. It is shown that a very simple model can be used to replicate thin film hardness data where the film is softer than the substrate as well as how materials properties alter the indentation behavior of materials confined in one to three dimensions. It is shown that qualitative agreement with nanoindentation of the metallic binder phase in the complex 3D-microstructure of a cemented carbide is achieved using an axisymmetric “pill-box” model with classical plasticity. It is also shown that the effect of higher-order confinement can be described by the Korsunsky thin film hardness model by re-optimizing the fitting parameters. QC 20200124</p

    Indentation behavior of highly confined elasto-plastic materials

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    The effect of geometric confinement is well-known from hardness measurements of thin films on stiff substrates and has been modeled both phenomenologically and using e.g. Finite Element Analysis. However, these models are mainly focused on a specific experiment or a certain material family. In the present work, Finite Element Analysis is used to gain a better understanding of the interplay between geometric constraints in various microstructures and a wide range of materials properties. It is shown that a very simple model can be used to replicate thin film hardness data where the film is softer than the substrate as well as how materials properties alter the indentation behavior of materials confined in one to three dimensions. It is shown that qualitative agreement with nanoindentation of the metallic binder phase in the complex 3D-microstructure of a cemented carbide is achieved using an axisymmetric “pill-box” model with classical plasticity. It is also shown that the effect of higher-order confinement can be described by the Korsunsky thin film hardness model by re-optimizing the fitting parameters. QC 20200124</p

    Effect of Si on bainitic transformation kinetics in steels explained by carbon partitioning, carbide formation, dislocation densities, and thermodynamic conditions

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    The effect of Si addition on the evolution of bainitic transformation, carbon diffusion, carbide formation, and dislocation density in steel was investigated using in-situ high-energy X-ray diffraction (HEXRD). Alloys Fe-0.4C-1.7Mn (in wt%) with 1–4 wt% Si were austenitized at 1273 K and then isothermally heat treated at 573, 623, and 673 K. According to the HEXRD results, increasing Si content reduces the bainitic transformation kinetics and causes the incompleteness of the bainitic transformation to occur at lower bainite volume fraction. This is because i) Si retards carbide formation, impeding the eutectoid bainitic transformation, and leads to the accumulation of carbon at the migrating interface; ii) Si leads to higher strain energy and more dislocations in the austenite that also hinders the migration of the interface. Carbide formation was observed to occur prior to the incomplete transformation stage. During further isothermal holding, the decrease in dislocation density due to dislocation annihilation had little effect on carbide formation or carbon diffusion. Finally, the Si content has a minor effect on the calculated T0_0, T0_0’, and WBs lines. The measured carbon content in carbon enriched austenite agrees well with WBs and T0_0 but not with T0_0’

    Martensite formation during incremental cooling of Fe-Cr-Ni alloys: An in-situ bulk X-ray study of the grain-averaged and single-grain behavior

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    The formation of martensite (ε and α′) in metastable austenitic Fe-18Cr-(10-11.5)Ni alloys was investigated in-situ during cooling. High-energy X-rays were used to study the bulk of the alloys. Both grain-averaged and single-grain data was acquired. ε played an important role in the formation of α′ with an indistinguishable difference in the martensite start temperature. The single-grain data indicated that stacking faults appear as precursors to ε. An analogy can be made with deformation-induced martensitic transformation, where the generation of nucleation sites would significantly lower the driving force required to overcome the energy barrier in low stacking fault energy Fe-Cr-Ni alloys
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