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In-situ Neutron Diffraction and Small-Angle Neutron Scattering (SANS) are employed for the first time simultaneously in order to reveal the interaction between the austenite to ferrite phase transformation and the precipitation kinetics during isothermal annealing at 650 and at 700 °C in three steels with different vanadium (V) and carbon (C) concentrations. Austenite-to-ferrite phase transformation is observed in all three steels at both temperatures. The phase transformation is completed during a 10 h annealing treatment in all cases. The phase transformation is faster at 650 than at 700 °C for all alloys. Additions of vanadium and carbon to the steel composition cause a retardation of the phase transformation. The effect of each element is explained through its contribution to the Gibbs free energy dissipation. The austenite-to-ferrite phase transformation is found to initiate the vanadium carbide precipitation. Larger and fewer precipitates are detected at 700 than at 650 °C in all three steels, and a larger number density of precipitates is detected in the steel with higher concentrations of vanadium and carbon. After 10 h of annealing, the precipitated phase does not reach the equilibrium fraction as calculated by ThermoCalc. The external magnetic field applied during the experiments, necessary for the SANS measurements, causes a delay in the onset and time evolution of the austenite-to-ferrite phase transformation and consequently on the precipitation kinetics.

In this paper a new insight into fundamentals of static recrystallization, precipitation and their interaction during sub-critical annealing of three cold-rolled low-carbon microalloyed steel grades is presented. The grades under investigation are a base grade containing V as a microalloying element, a Ti+ grade containing Ti as microalloying element added into the base grade, and a Ti+Mn+ grade containing additional Mn added into the Ti+ grade. The cold-rolled steels are sub-critically annealed inside a muffle furnace to simulate industrial continuous annealing parameters in order to investigate the interaction between recrystallization and precipitation across transient stages of the annealing process as a function of temperature and time. The Zener pinning of precipitates and solute drag force of Mn on the recrystallization process are calculated and compared with measured values obtained from experimental studies on the recrystallization kinetics. Results suggest that the recrystallization kinetics is fastest in the base grade. For the Ti+ grade, fine (< 15 nm) (Ti,V)(C/N) particles retard the recrystallization kinetics. For the Ti+ and Ti+Mn+ grades, solute drag effect of Mn solute atoms for dwell time longer than 2 min at annealing temperature of 800 oC is negligible.

In the present study, cold-rolled low-carbon steel is annealed at three different conditions: 700 oC for 0 s, 800 oC for 0 s and 800 oC for 2 min at the heating rate of ~10 oC/s. Recrystallization behaviour on sample surface is studied using a heated stage Scanning Electron Microscopy and Electron Backscattered Diffraction. For the lower annealing temperature of 700 oC with no dwell, almost no recrystallization is observed and microstructure resembles the as-received deformed material with the exception of occasional sub-micron sized nuclei. For the annealing conditions of 800 oC 0 s and 800 oC 2 min, onset and evolution of recrystallization is observed in-situ as a function of the initial as-cold rolled texture. Slower recovery rate of alpha fibre than gamma fibre is observed and confirmed by lower drop in average geometrically necessary dislocation (GND) density for un-recrystallized alpha fibres (1.1E+14 m-2 for 700 oC 0 s , 1.4E+14 m-2 for 800 oC 0 s and 4.5E+14 m-2 for 800 oC 2 min) than for un-recrystallized gamma fibre grains (3.0E+14 m-2 for 700 oC 0 s , 6.2E+14 m-2 for 800 oC 0 s and 9.8E+14 m-2 for 800 oC 2 min) during annealing. Strong gamma texture in recrystallized matrix is found for annealing conditions of 800 oC 0 s and 800 oC 2 min. From TEM characterisation it was shown that sub-grain boundaries are decorated with fine precipitates (diameter d < 15 nm) of titanium-vanadium carbides (Ti,V)C for the annealing condition of 700 oC 0 s, which suggests that these precipitates play a major overall role in retardation of the recrystallization kinetics.

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.

A new generation of low-carbon microalloyed High Strength Low Alloy (HSLA) steels has been developed to utilize a combination of single-phase ferritic microstructures and optimized interphase precipitation to provide high level strength and exceptional formability. The interphase precipitation reaction is a transient process lending itself strongly to take advantage of in-situ characterization techniques. The austenite/ferrite interface kinetics during isothermal transformation at 1003 K is measured using HT-CSLM, the pre-exponential effective mobility constant was found to be mobility 0.822 (m J)/(mole s). The V interphase precipitation is characterised using TEM at isothermal transformation temperatures of 923 and 973 K as having inter-sheet spacing of 22±7 and 32±9 nm respectively. Interphase precipitation inter-sheet-spacing is simulated using a revised Quasi-Ledge model and qualitatively predicts the observed trends observed for inter-sheet spacing. The results of in-situ characterisation and modelling suggest that it is possible to optimize the strengthening potential of the precipitation processes by controlling the thermal processing of microalloyed HSLA.

In-situ characterization techniques have been applied to elucidate the influence of γ/α transformation upon the extent of interphase precipitation in a low-carbon, vanadium-HSLA steel. Electron Back-scattered diffraction analyses of the γ/α orientation relationship with continuous cooling at 2 and 10 K/s suggest that the proportion of ferrite likely to hold interphase precipitation varies little with cooling rate. However, TEM analyses show that the interphase precipitation refines with increasing cooling rate in this cooling range. With cooling rates in excess of 20 K/s, interphase precipitation is increasingly suppressed due to the increasingly diffusional-displacive nature of the Widmanstatten γ/α transformation that is activated. The present study illustrates that the extent and dimensions of interphase precipitation can be controlled through controlled cooling.

Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630 °C and 650 °C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.

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.

This paper discusses practical EBSD strategies for identification and partitioning of phase constituents in DP and TRIP microstructures including martensite, retained-austenite, bainite, intercritical and epitaxial ferrite. EBSD data is complemented with nano-indentation analysis, providing evidence of indentation-induced phase transformation of retained-austenite in TRIP steel and micro-crack initiation at the interface between ferrite and mechanically transformed martensite.

In-situ material characterization methods such as High Temperature Confocal Laser Scanning Microscopy and hot-stage Electron Back Scattered Diffraction allow direct observation of microstructural processes during simulated thermal treatment. In this paper V-bearing micro-alloyed low carbon steel with and without Mo addition is studied using a combination of in-situ methods. It is concluded that addition of 0.2 wt.% Mo into V-bearing steel restricts austenitic grain growth as well as limitation of formation of both Allotriomorphic and Widmanstatten ferrite through a coupled solute drag effect.

Functionalized oligo(cyclohexylidene)s form an attractive class of molecular building blocks for the development of organic materials for (opto)electronic applications because of their longrange electron transfer properties over several molecular units. 3] The compounds are built from 1,4-linked cyclohexylidene units bridging an electron donating and accepting moiety via an alternating ± ± orbital topology. The photoinduced electron transfer process in oligo(cyclohexylidene)s in relation to their orbital topology has been the subject of various studies. 4±6] He photoelectron spectroscopy (PES), ab initio calculations, and natural bond orbital (NBO) analysis on the basic unit of this class of compounds, 1,1 -bicyclohexylidene (BCH), 7] and of its end-functionalized derivatives have shown that throughobservation that the mixed endo/exohedral addition patterns lead to energetically favorable products.

High-resolution (2+1) resonance-enhanced excited-state photoelectron spectroscopy on supersonic jet expansions of pure C2H4 and C2D4 has been employed to investigate the spectroscopy and dynamics of the lowest excited 11B3u(π,3s) Rydberg state, as well as the 11B1g(π,3py), 11B2g(π,3pz), and 21Ag(π,3px) Rydberg states of ethylene. In combination with ab initio calculations, this approach has for the 11B3u(π,3s) Rydberg state confirmed the majority of previous assignments in the two-photon excitation spectrum but, at the same time, has demonstrated that the hypothesis of a vibronic coupling between this state and the Jahn−Teller distorted 11B2g(π*,3s) state should be discarded. The resonances on which this hypothesis was based are in the present study shown to arise from excitation of the ν3(ag) vibrational mode. For the 3p Rydberg manifold, the present study has for the first time been able to determine unambiguously the 0−0 transitions to all three (π,3p) Rydberg states and to disentangle the vibrational ...

The lowest excited singlet states of 4-(dimethylamino)-4‘-cyanostilbene (DCS), 4-(dimethylamino)-4‘-nitrostilbene (DANS), and 4-di(hydroxy-ethyl)amino-4‘-nitrostilbene (DANS-diol) have been investigated by high-resolution fluorescence excitation and dispersed emission spectroscopy on samples seeded in supersonic expansions. Using ab initio calculations of the harmonic force fields of the electronic ground state as a starting point for the analysis of vibrational modes and frequencies, detailed vibrational assignments of the rich line structure of the S1 ← S0 transition in these three push−pull stilbenes are reported. From the experimental and theoretical analysis, it becomes apparent that the excitation spectra are dominated by low-frequency vibrational modes of the stilbene-like backbone, albeit that a characterization in terms of trans-stilbene vibrational terminology is not very appropriate. The fluorescence excitation and emission spectra show significant Franck−Condon factors for transitions involvin...

The singlet states of the 3p Rydberg manifold of the structurally rigid amines 1-azabicyclo[2.2.2]octane and 1-azaadamantane seeded in supersonic expansions have been investigated by means of fluorescence excitation spectroscopy. Under these conditions a better spectral resolution could be obtained than in previous studies, enabling us to resolve and identify unambiguously the transitions to the second and third excited singlet states. These states are assigned as the 11E and 31A1 states, associated with excitation of a lone-pair electron to the 3px,y and 3pz Rydberg orbitals, respectively. Analysis of the vibrational energies and intensities of vibronic transitions observed in the excitation spectra of both states leads to the conclusion that the equilibrium geometry and vibrational properties of the molecule in these two states resemble closely those of the lowest excited singlet state, which derives from excitation of a lone-pair electron to the 3s Rydberg orbital, and those of their ionic core. Excita...

The spectroscopy and photophysics of tetramethylethylene and 1,1‘-bicyclohexylidene have been investigated using excited-state photoelectron spectroscopy in combination with multiphoton excitation. Vibronic coupling within the full manifold of the excited singlet states has been shown to play a dominant role in determining the spectroscopic properties of these compounds. Although this vibronic coupling inhibits the determination of excited state excitation energies by excitation spectroscopy, it enables at the same time their observation in the photoelectron spectra. As a result, a large number of previously unobserved Rydberg states could be located and assigned. For both molecules ionization from the Rydberg states is observed to be heavily perturbed by ionization from the underlying quasi-continuum of the (π,π*) valence state. The photoelectron spectra of 1,1‘-bicyclohexylidene reveal that the state around 55000 cm-1 (6.82 eV), which has previously been assigned as a second valence state, does not show...