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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.

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.

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.

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...