Interphase precipitation occurring during solid-state phase transformations in micro-alloyed steels is generally studied through transmission electron microscopy, atom probe tomography, and ex situ measurements of Small-Angle Neutron Scattering (SANS). The advantage of SANS over the other two characterization techniques is that SANS allows for the quantitative determination of size distribution, volume fraction, and number density of a statistically significant number of precipitates within the resulting matrix at room temperature. However, the performance of ex situ SANS measurements alone does not provide information regarding the probable correlation between interphase precipitation and phase transformations. This limitation makes it necessary to perform in situ and simultaneous studies on precipitation and phase transformations in order to gain an in-depth understanding of the nucleation and growth of precipitates in relation to the evolution of austenite decomposition at high temperatures. A furnace is, thus, designed and developed for such in situ studies in which SANS measurements can be simultaneously performed with neutron diffraction measurements during the application of high-temperature thermal treatments. The furnace is capable of carrying out thermal treatments involving fast heating and cooling as well as high operation temperatures (up to 1200 °C) for a long period of time with accurate temperature control in a protective atmosphere and in a magnetic field of up to 1.5 T. The characteristics of this furnace give the possibility of developing new research studies for better insight of the relationship between phase transformations and precipitation kinetics in steels and also in other types of materials containing nano-scale microstructural features.

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The precipitation kinetics of vanadium carbides and its interaction with the austenite-to-ferrite phase transformation is studied in two micro-alloyed steels that differ in vanadium and carbon concentrations by a factor of two, but have the same vanadium-to-carbon atomic ratio of 1:1. Dilatometry is used for heat-treating the specimens and studying the phase transformation kinetics during annealing at isothermal holding temperatures of 900, 750 and 650 °C for up to 10 h. Small-Angle Neutron Scattering (SANS) and Atom Probe Tomography (APT) measurements are performed to study the vanadium carbide precipitation kinetics. Vanadium carbide precipitation is not observed after annealing for 10 h at 900 and 750 °C, which is contrary to predictions from thermodynamic equilibrium calculations. Vanadium carbide precipitation is only observed during or after the austenite-to-ferrite phase transformation at 650 °C. The precipitate volume fraction and mean radius continuously increase as holding time increases, while the precipitate number density starts to decrease after 20 min, which corresponds to the time at which the austenite-to-ferrite phase transformation is finished. This indicates that nucleation and growth are dominant during the first 20 min, while later precipitate growth with soft impingement (overlapping diffusion fields) and coarsening take place. APT shows gradual changes in the precipitate chemical composition during annealing at 650 °C, which finally reaches a 1:1 atomic ratio of vanadium-to-carbon in the core of the precipitates after 10 h.@en

The substitution of critical alloying elements in metals is a strategy to reduce the criticality of materials. Nano-steels are a novel grade of advanced highstrength steels that are suited for application in the chassis and suspension of cars and as fire-resistant steel in high-rise buildings. The high strength and ductility per unit mass make the nano-steels resource-efficient and reduce vehicle weight while maintaining crash worthiness. The excellent mechanical properties of certain nano-steels rely on the addition of small amounts (up to 0.1 wt.%) of Niobium as alloying element to the steel. Niobium is considered to be a critical raw material by the European Union due to its high economic importance as an alloying element in advanced, high-strength steel grades and due to the high supply risk related to the high degree of monopolistic production within the supply chain. This chapter describes the fundamental materials science that is needed for the substitution of the critical alloying element Niobium by Vanadium as an alloying element in nano-steels.@en

Nanosteels are used in automotive applications to accomplish resource-efficiency while providing high-tech properties. Quantitative data and further understanding on the precipitation kinetics in Nanosteels can contribute to fulfil this goal. Small-Angle Neutron Scattering measurements are performed on a Fe-C-Mn-V steel, previously heat-treated in a dilatometer at 650°C for several holding times from seconds to 10 hours. The evolution of the precipitate volume fraction, size distribution and number density is calculated by fitting the experimental Small-Angle Neutron Scattering curves. The effect of phase transformation on precipitation kinetics is also discussed. Complementary Transmission Electron Microscopy, Scanning Electron Microscopy and Inductively Coupled Plasma Optical Emission Spectroscopy measurements are performed to support the Small-Angle Neutron Scattering data analysis. @en

Understanding the intricate structure of dislocations in metals is a major issue in materials science. In this paper we present a comprehensive approach for the characterisation of dislocation networks, resulting in accurate quantification and significantly increasing the insight into the dislocation structure. Dislocation networks in metals consists of dislocation segments, pinned by microstructural obstacles. In the present paper a model is introduced that describes the behaviour of these dislocation segments in the pre-yield range of a tensile test on the basis of fundamental concepts of dislocation theory. The model enables experimental quantification of the dislocation density and segment length from the tensile curve. Quantitative results are shown and discussed on the development of the dislocation network as a function of increasing degree of plastic deformation, including validation and physical interpretation of the classical Taylor equation.

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The pre-yield deformation behaviour (i.e., at stresses below the yield stress) of two materials, pure iron and a low-alloy steel, and its anelastic nature are analysed at room temperature, before and after the dislocation structures are varied by plastic deformation. It is shown, based on tensile experiments, that this behaviour can be explained by limited reversible glide of dislocations without essential changes in the dislocation structure. Moreover, a physically-based model that characterises the dislocation structure by two variables, the dislocation density and the effective dislocation segment length, is used to quantitatively describe this deformation behaviour. The model validity is further evaluated by comparison with dislocation densities from X-Ray Diffraction measurements.@en