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Chromium (Cr) was concentrated as chromium spinel in the vanadium slag produced by pre-oxidizing vanadium-containing hot metal in Pan-steel in China. Due to the chromium-rich Hongge ores used, chromium concentration in this slag was much higher than that in common vanadium slags. To propose a process for effective Cr extraction from the slag, mechanisms of Cr oxidation in Pan-steel vanadium slag under sodium roasting conditions were studied experimentally and with thermodynamic simulations. Chromium slags without vanadium were synthesized in the laboratory to mimick mineralogical characteristics of Pan-steel vanadium slag and characterized by XRD, SEM/EDS and TG-DSC techniques. Conditions for Cr extraction from the chromium slag after sodium roasting by leaching with water were studied and optimized. Results showed that Cr spinel was encapsulated in the silicate phase. During sodium roasting, Cr spinel was oxidized and decomposed after oxidation and decomposition of the olivine phase. Sodium carbonate began to react with chromium oxide, which was produced by decomposition of Cr spinel, at 537°C and large amount of sodium chromate appeared after 800°C. However, multi-component liquid phase (mixture of Na2Cr2O4, Na2CrO4 and Na2CO3) was produced after 1 000°C, which suppressed the conversion of Cr(III) to Cr(VI) by inhibiting oxygen supply. A conversion about 90% was obtained when the chromium slag was roasted at 1 000°C for 2 h with Na2CO3 addition in stoichiometric ratio to total amount of Cr2O3 and SiO2. 96% of Cr in the leaching liquid was recovered as chromium sesquioxide in fine powder form and high purity by reducing and precipitating.

Laboratory X-ray diffraction contrast tomography (LabDCT) has recently been developed as a powerful technique for non-destructive mapping of grain microstructures in bulk materials. As the grain reconstruction relies on segmentation of diffraction spots, it is essential to understand the physics of the diffraction process and resolve all the spot features in detail. To this aim, a flexible and standalone forward simulation model has been developed to compute the diffraction projections from polycrystalline samples with any crystal structure. The accuracy of the forward simulation model is demonstrated by good agreements in grain orientations, boundary positions and shapes between a virtual input structure and that reconstructed based on the forward simulated diffraction projections of the input structure. Further experimental verification is made by comparisons of diffraction spots between simulations and experiments for a partially recrystallized Al sample, where a satisfactory agreement is found for the spot positions, sizes and intensities. Finally, applications of this model to analyze specific spot features are presented.

A novel de-Phosphorisation & V-extraction process (de-P & V-extraction) is proposed in which lime is added during the production of vanadium slag. This is expected to promote both phosphorus removal and environment-friendly vanadium extraction from the vanadium-containing hot metal simultaneously. The influence of CaO addition on the chemical components and mineralogical morphology of vanadium-containing phase in vanadium slags has been investigated by XRD and SEM/EDS. Results show that the increase in CaO contents changes both species of vanadium-containing phase and the morphologies of vanadium-containing spinels in vanadium slags. Vanadium concentrates as (Mn,Fe,Mg)V2O4 in the spinel phase in vanadium slag with CaO content less than 10 mass%. When CaO content is higher than 12.89 mass%, vanadium exists both as (Mn,Fe,Mg)V2O4 in dendritic-like spinel and as Ca3V2(SiO4)3 in goldmanite in the matrix. Precipitation of perovskite results in the dendritic-like shape of spinels while V 3+ diffusing away from spinel lattice leads to the formation of goldmanite Ca3V2(SiO4)3. Thermodynamic calculation of phase equilibrium has been conducted to understand the phase evolution. This work demonstrates the influence of CaO addition on the existence form of vanadium-containing phase in vanadium slag and thus provides the theoretical foundation for the proposed novel de-P & V-extraction process.

We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540°C, 550°C, and 565°C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe 2 Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail.

When metals are mechanically loaded at elevated temperatures for extended periods of time, creep damage will occur in the form of cavities at grain boundaries. In the present experiments it is demonstrated that in binary iron-tungsten alloys creep damage can be self healed by selective precipitation of a W-rich phase inside these cavities. Using synchrotron X-ray nano-tomography the simultaneous evolution of creep cavitation and precipitation is visualized in 3D images with a resolution down to 30 nm. The degree of filling by precipitation is analysed for a large collection of individual creep cavities. Two clearly different types of behaviour are observed for isolated and linked cavities, where the isolated cavities can be filled completely, while the linked cavities continue to grow. The demonstrated selfhealing potential of tungsten in iron-based metal alloys provides a new perspective on the role of W in high-temperature creep-resistant steels.

Laboratory diffraction contrast tomography (LabDCT) is a novel technique for non-destructive imaging of the grain structure within polycrystalline samples. To further broaden the use of this technique to a wider range of materials, both the spatial resolution and detection limit achieved in the commonly used Laue focusing geometry have to be improved. In this work, the possibility of improving both grain indexing and shape reconstruction was investigated by increasing the sample-to-detector distance to facilitate geometrical magnification of diffraction spots in the LabDCT projections. LabDCT grain reconstructions of a fully recrystallized iron sample, obtained in the conventional Laue focusing geometry and in a magnified geometry, are compared to one characterized by synchrotron X-ray diffraction contrast tomography, with the latter serving as the ground truth. It is shown that grain indexing can be significantly improved in the magnified geometry. It is also found that the magnified geometry improves the spatial resolution and the accuracy of the reconstructed grain shapes. The improvement is shown to be more evident for grains smaller than 40 µm than for larger grains. The underlying reasons are clarified by comparing spot features for different LabDCT datasets using a forward simulation tool.