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Precise description of the hydrogen charged state of mechanical test specimens including the spatial and energetic hydrogen distribution is considered to be essential for understanding the mechanisms of hydrogen embrittlement. Here, a simple hydrogen absorption model for hydrogen charging in liquids was developed for cylindrical specimens. The absorption model is based on an approximate solution of Fick's laws of diffusion and allows determining hydrogen diffusion coefficients and surface concentrations by measuring the total amount of absorbed hydrogen as a function of the cylinder height. Total hydrogen contents after charging of several cylinders consisting of quenched and tempered steel 42CrMo4 (1.7225) were measured by carrier gas hot extraction and the applicability of the absorption model on the measurement was demonstrated by mathematical fitting. The diffusion coefficient for this material was determined to be D = 3.8 10 − 11 m 2 / s and the hydrogens surface concentration increases with increasing charging time.

AISI 304L and AISI 316L are metastable austenitic stainless steels, which exhibit a good corrosion resistance due to their chromium contents but suffer from poor mechanical properties and therefore exhibit poor wear resistance. A slight hardening effect is gained by the formation of deformation-induced martensite either by cold-rolling or shot-peening. A considerable surface hardness could be achieved by plasma nitriding processes due to the formation of the so-called S-phase, the nitrogen diffusion zone. In industrial manufacturing processes cold-deformation could not be avoided and no final annealing is carried out to eliminate the deformation-induced martensite. Therefore, the knowledge about the behavior of the microstructure regarding the nitriding process and the resulting properties is fundamental. The S-phase is also assumed to improve the corrosion resistance if the plasma nitriding is performed below a critical temperature. Above this temperature chromium nitrides precipitate in the matrix and decrease the corrosion resistance. In this study, AISI 304L and AISI 316L were solution annealed and afterwards work hardened with a cold rolling degree of 38%. The surfaces were polished and shot-peened, respectively. The amount of deformation induced martensite depends on the material and this, in turn influences the nitriding behavior. It will be shown, that a highly deformed microstructure promotes the growth of the S-phase. Furthermore, the resulting surface hardness and corrosion resistances in saline solution were examined. The corrosion properties are highly dependent on the surface topography. Shot-peening increases the roughness and also induces crack like artifacts which accelerate corrosion kinetics. The results were evaluated statistically to derive correlations between the microstructure, the plasma nitriding behavior and the resulting properties.

Austenitic stainless steels exhibit good corrosion resistance due to their high chromium contents. Metastable austenitic stainless steels like 304L or 316L undergo a phase transformation to deformation-induced martensite by cold working, e.g. shot-peening. Furthermore, austenitic stainless steels suffer from quite poor mechanical properties which make them susceptible to wear. Therefore, plasma nitriding processes enhance the surface hardness up to the eight times of the original hardness value, thus improving the wear resistance. Additional, the corrosion resistance can be improved, too, if plasma nitriding is carried out below a critical temperature to avoid the precipitation of chromium nitrides. In this study, two austenitic steels – 304L and 316L – were used in a solution annealed condition with either polished or shot-peened surfaces. Furthermore, these samples were tested without plasma nitriding, with plasma nitriding or annealing and a combined nitriding and post annealing process. The microstructural properties were examined by optical microscopy (OM) and scanning electron microscopy (SEM), phases were analyzed by X-ray diffraction (XRD) and the corrosion properties were tested by means of potentiodynamic polarization testing in 5% NaCl solution. Statistical evaluations were carried out to identify significant effects between the microstructure and nitriding conditions and the corrosion properties. The results indicate, that plasma nitriding improves the corrosion resistance of both steels with polished surfaces, while shot-peening increases the corrosion rates. Surface hardness measurements show the positive influence of plasma nitriding to surface hardness. Furthermore, a correlation between specimen treatment and the corresponding microstructures, the nitriding and the annealing process and the corrosion resistance will be presented.

The metastable austenitic steels 1.4307 and 1.4404 are characterized by a fcc lattice with paramagnetic properties which can be transformed by cold forming to ferromagnetic bcc martensite with acicular structures via a defect-rich intermediate stage. However, using classical wetchemical etching methods, defects and bcc martensite can visually not be clearly distinguished. Usually, magnetic or X-ray diffraction methods are used to quantify the ferromagnetic fractions. However, they do not allow for imaging the bcc phase distribution in the microstructure. Even with EBSD measurements in defect-rich phases, which, for process-related reasons, involve high investigation costs and long measurement durations, this is often only possible to a limited extent. The present study presents fast and simple contrasting by means of ferrofluids. It allows for a precise and spatially resolved differentiation between (ferromagnetic) martensite and (paramagnetic) defect structures on metallographic cross-section by optical microscopy. Furthermore, it is shown that, after a ferrofluid treatment of metallographic cross-sections made of duplex steel, ferromagnetic ferrite areas already show evidence of grain orientations.

Austenitic stainless steels are widely used in medical and food industries because of their excellent corrosion resistance. However, they suffer from weak wear resistance due to their low hardness. To improve this, plasma nitriding processes have been successfully applied to austenitic stainless steels, thereby forming a thin and very hard diffusion layer, the so-called S-phase. In the present study, the austenitic stainless steels AISI 304L and AISI 316L with different microstructures and surface modifications were used to examine the influence of the steel microstructure on the plasma nitriding behavior and corrosion properties. In a first step, solution annealed steel plates were cold-rolled with 38% deformation degree. Then, the samples were prepared with three kinds of mechanical surface treatments. The specimens were plasma nitrided for 360 min in a H2–N2 atmosphere at 420 °C. X-ray diffraction measurements confirmed the presence of the S-phase at the sample surface, austenite and body centered cubic (bcc)-iron. The specimens were comprehensively characterized by means of optical microscopy, scanning electron microscopy, glow discharge optical emission spectroscopy, X-ray diffraction, surface roughness and nano-indentation measurements to provide the formulation of dependencies between microstructure and nitriding behavior. The corrosion behavior was examined by potentio-dynamic polarization measurements in 0.05 M and 0.5 M sulfuric acid and by salt spray testing.

Powder X-ray diffraction experiments of sillenites Bi12M O20 (M=Si, Ge, Ti) were performed with synchrotron radiation at the ESRF, Grenoble, at pressures up to 39 GPa (M=Si), 50 GPa (M=Ge), and 37 GPa (M=Ti), respectively. These three sillenites were investigated for the first time over such a large pressure range and they show no phase transition up to the highest pressures achieved. Birch–Murnaghan equations of state of third order were fitted to the pressure dependence of the lattice parameters. The resulting bulk moduli B0 and their pressure derivatives B ′ are B 0 = 63 ( 1 ) GPa with B ′ = 6.6 ( 3 ) (M=Si), B 0 = 63.0 ( 5 ) GPa with B ′ = 5.90 ( 7 ) (M=Ge), and B 0 = 48 ( 1 ) GPa with B ′ = 9.4 ( 5 ) (M=Ti). In the case of Bi12SiO20 and Bi12TiO20 the equation of state fits were restricted to pressures below 15 GPa, because the experimental data were affected by non-hydrostatic stress at higher pressures. The equation of state of neon was redetermined ( V 0 = 11.7 ( 6 ) cm 3 / mol , B 0 = 5 ( 1 ) GPa , B ′ = 5.5 ( 3 ) GPa ) and its use as an internal pressure standard for diffraction experiments is discussed. A second cubic phase could be identified in our Bi12TiO20 sample. This was attributed to eulytite-type Bi4Ti3O12, which has B 0 = 50.9 ( 8 ) GPa and B ′ = 6.9 ( 3 ) .

In situ synchrotron X-ray diffraction was used to observe the reaction induced by laser heating of a mixture of tantalum and boron in a diamond anvil cell. Laser heating at pressures of 12 and 24 GPa resulted in the formation of TaB 2 . The bulk modulus of TaB 2 ( B 0 = 341(7) GPa) was determined from a fit of a second-order Birch–Murnaghan equation of state to the p − V data. Density functional theory based calculations complemented the experimental observations and were used to obtain the full tensor of elastic stiffness coefficients. The choice of the most appropriate exchange-correlation functional for the description of elastic properties is discussed.

Tantalum nitrides were formed by reaction of the elements at pressures between 9(1) and 12.7(5) GPa and temperatures >1600–2000 K in the laser-heated diamond anvil cell. The incorporation of small amount of nitrogen in the tantalum structure was identified as the first reaction product on weak laser irradiation. Subsequent laser heating led to the formation of hexagonal β -Ta2N and orthorhombic η -Ta2N3, which was the stable phase at pressures up to 27 GPa and high temperatures. No evidence was found for the presence of ɛ -TaN, ϑ -TaN, δ -TaN, Ta3N5-I or Ta3N5-II, which was predicted to be the stable phase at P > 17 GPa and T = 2800 K, at the P , T -conditions of this experiment. The bulk modulus of η -Ta2N3 was determined to be B 0 = 319 ( 6 ) GPa from a 2nd order Birch–Murnaghan equation of state fit to the experimental data, while quantum mechanical calculations using the density functional theory gave a bulk modulus of B 0 = 348.0 ( 9 ) GPa for a 2nd-order fit or B 0 = 339 ( 1 ) GPa and B ′ = 4.67 ( 9 ) for a 3rd-order fit. The values show the large incompressibility of this high-pressure phase. From the DFT data the structural compression mechanism could be determined.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.