Quasi-dynamic visualization of crack propagation and bridging evolution in Si3N4 and SiC ceramics by mechano-luminescence

The propagations of macro-scale cracks were visualized in Si₃N₄ and SiC ceramics using a mechano-luminescence (ML) of SrAl₂O₄:Eu,Dy. The bridging zones behind the crack tip were also clearly detected in the crack path within realistic time frames. The ML made it possible to detect cracks propagating within a speed range of 200 m/s to 250 m/s, thereby realizing so-called quasi-dynamic R-curves. The magnitudes and shapes of the bridging stress distributions were found to change with the advancing cracks, continuing to change as the applied load changes, even after the cessation of crack propagation. The effective toughening then commenced and the applied stress intensity factors increased dramatically up to ∼120 MPa√m in Si₃N₄ and 150 MPa√m in SiC. The K_Tip values expected on the basis of the instantaneous bridging stress distributions obtained from ML observations deviated greatly from those measured using the conventional crack tip lengths; rather, the values support the results obtained by using bridging tips in quasi-dynamic crack propagations.     

Optimization of processing conditions and characterization investigations of ceramic injection molded and sintered ZrO2/WC composites

In this investigation, 3 mol% Y₂O₃ stabilized ZrO₂-based composites reinforced with 10 vol.%, 20 vol.% and 40 vol.% WC (named as 3Y-TZP/10WC, 3Y-TZP/20WC and 3Y-TZP/40WC) were fabricated by using injection molding and sintering. Mechanical properties of these composites varied due to WC addition and dwelling time. Density, strength and toughness decreased with shorter dwelling time and increasing WC content however a significant enhancement in fracture toughness was obtained by 3Y-TZP/20WC composite which had 9.2 MPa m^1/2 toughness. Severe unlubricated wear tests which were performed under 55 N normal load and 45 km sliding distance showed that 3Y-TZP/20WC composite had the lowest wear rate and wear volume values which are 2 × 10⁻⁸ mm³/(N m⁻¹) and 0.05 mm³, respectively.     

Einfluß von Korrosion und mechanischer Belastung auf das Rißwachstum niedriglegierter,ferritischer Stähle in sauerstoffhaltigem Hochtemperaturwasser

Influence of corrosion and mechanical loading on the crack growth of low-alloyed ferritic steels in oxygenated high temperature water The mathematical and with regards to the contents main features of the mostly developed analytical model for corrosion-assisted crack growth are presented and the crack growth velocities resulting for low-alloyed ferritic materials in high-temperature water are given. Experimentally determined crack growth velocities for the ferritic material 20 MnMoNi 5 5 with two different sulfur contents as well as for the similar material 22 NiMoCr 3 7 in deionized, oxygenated (0.4 and 8 ppm O₂) high temperature water at 240°C are compared with calculated ones. The constant load experiments at different level were performed on compact tension specimens with a thickness of 50 mm (2T-CT-specimens). The experimental results show, that up to a stress intensity factor K_I of 60 MPa $ \sqrt m $ the corrosion-assisted crack advance is neither dependent on the oxygen content of the medium and the K₁-value, nor on the sulfur content of the steel. A deviation up to 3 magnitudes compared to the calculated values exists. Furthermore, an increasing crack growth velocity with decreasing test duration is observed. Between 60 and 75 MPa $ \sqrt m $ the crack growth velocity increases by several magnitudes also independent of the above mentioned parameters. Above 75 MPa $ \sqrt m $ fracture of the specimen occurs soon after loading. In this region the experimentally derived crack growth velocity fits well with the analytical model. A possible explanation for the deviation between experimental and analytical results could be seen in low-temperature creep processes at the crack tip. Results of preliminary investigations on low-temperature creep processes of the material 20 MnMoNi 5 5 in air at 240°C are presented.     

Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C

New Pourbaix diagrams were calculated at 25, 75 and 95 °C for the Nb-H₂O system. The species and were considered. Potentiodynamic polarization and mass loss experiments (14 days) were conducted in concentrated H₂SO₄ (20, 40 and 80 wt%) and HCl (20 and 38 wt%) solutions at 75 and 95 °C. Nb forms a metastable pentoxide (Nb₂O₅) in H₂SO₄ and HCl solutions which dissolves as . Corrosion rates decrease between the 40% and the 80% H₂SO₄ solutions. SEM micrographs show generalized pitting in the 20% and 40% H₂SO₄ solutions. Mass loss corrosion rates did not exceed 306 μm/yr. Corrosion rates estimated by Tafel extrapolation were within two orders of magnitude of those measured by mass loss and it is shown that this finding is consistent with the thickening of the oxide.     

Mechanoluminescent determination of the mode I stress intensity factor in SrAl2O4:Eu2+,Dy3+

The use of mechanoluminescence (ML) has enabled a new technique for the determination of the mode I stress intensity factor (SIF) in SrAl₂O₄:Eu²⁺,Dy³⁺ (SAO). The stress rate fields and its cumulative isostress contour patterns in terms of ML in the vicinity of a crack tip during compact tension were evaluated based on the two different stress criteria of the hydrostatic and deviator in triggering the trap-releasing processes of the ML mechanism. The magnitude and shape of the crack tip stress field predicted by the deviator stress criterion rather than by the hydrostatic stress criterion showed good agreement with the experimental ML evidence in a determination of the SIF from the ML intensity. The analysis derived from the deviator stress criteria not only allowed the possibility of characterizing the fracture toughness in a range of modes under static and dynamic crack propagation, but also provided a better understanding of the ML mechanism of the trap-releasing process in SAO.     

Activities and immiscibility in the system Cu-Rh

The thermodynamic activity of rhodium in solid Cu-Rh alloys is measured by the electromotive force method in the temperature range from 1050 to 1325 K with a solid-state cell:

Use of a dislocation-based boundary element model to extract crack growth rates from depth distributions of intergranular stress corrosion cracks

A dislocation-based boundary element model was used to simulate intergranular stress corrosion crack propagation in virtual microstructures. A Monte Carlo approach was used in which the propagation of approximately 100 cracks was calculated for different Voronoi generated microstructures. At every simulation step the model gave the position of the crack tip together with stress intensity factors K_I and K_II. Using a simple power-law-type crack growth rate $\mathit{da}/\mathit{dt}=D_p{K}^{m_p}$, the depth of each particular crack can be calculated knowing the time the samples were exposed to the stress and corrosive environment. Existing experimental data giving crack depth distributions for Alloy 600, and XM-19 and 304 stainless steel are investigated and the best-fit crack growth law established. Alloy 600 in a light water reactor environment and XM-19 in high-temperature water both lead to m_p = 3. While for 304 stainless steel in the more aggressive K₂S₄O₆/H₂SO₄ (pH 2) an exponent m_p = 0.8 was found.     

The effect of iodine content and specimen orientation on stress corrosion crack growth rate in Zircaloy-4

The influence of specimen orientation, stress intensity factor (K_I), and iodine concentration on the iodine-induced stress corrosion cracking growth rates in Zircaloy-4 was investigated in iodized methanol solutions at ambient temperature. When K_I is lower than 20 MPa.m^1/2, the intergranular and mixed intergranular/transgranular crack propagation rates increase linearly with (K_I − K_I,th), K_I,th being the onset of propagation stress intensity factor. The increase in iodine content induces an increase of the crack growth rate for a given K_I, and a decrease of the K_I,th. The specimen orientation is a second order parameter. A crack propagation law, depending on iodine content, is proposed.     

The corrosion behavior of Co-Mo alloys in H2-H2O-H2S environments

The corrosion behavior of Co alloyed with up to 40 wt.% Mo alloys was studied in H₂-H₂O-H₂S gas mixtures over the temperature range between 600C and 900C. The parabolic rate constants for corrosion decreased with increasing amounts of Mo. The compositions of all gas atmospheres fall in the sulfide(s stability region of the ternary M-O-S phase diagrams at all temperatures investigated. All the corrosion scales were composed of sulfides, while no oxide was detected. The sulfide scales formed were duplex at all temperatures except at 900C. The outer layer consisted primarily of cobalt sulfide, while the inner layer was complex and heterophasic, the phases formed being highly composition dependent. MoS₂ predominated in the inner layer for all alloys. However, a metallic Mo layer was formed in the innermost layer of Co-40 Mo. Activation energies were different for all alloys, increasing with increasing Mo content. Identical kinetics were observed for Co-30Mo corroded at 700–800C. A Chevrel-phase Co_1.62Mo₆S₈ was present in scales formed on the samples exhibiting the temperature-independent kinetics. A possible model in which Co_1.62Mo₆S₈ forms preferentially in H₂-containing mixed gas is suggested. Alloys corroded at 900C formed a lamellar-structure scale which contained Co and CoMo₂S₄ layers perpendicular to the alloy surface. A eutectoid decomposition of an unknown Co-Mo sulfide may be responsible for the presence of the lamellar structure.     

Sintered stainless steels: Fatigue crack propagation resistance under hydrogen charging conditions

Monophasic and multiphasic (two and three phases) sintered stainless steels were prepared both considering premixes of AISI 316LHC and AISI 434LHC stainless steels powders and using a prealloyed duplex stainless steel 25% Cr, 5% Ni, 2% Mo powder. Their fatigue crack propagation resistance was investigated both in air and under hydrogen charging conditions (0.5 M H₂SO₄ + 0.01 M KSCN aqueous solution; applied potential = −700 mV/SCE), considering three different stress ratios (R = 0.1; 0.5; 0.75). Fatigue crack propagation micromechanisms were investigated by means of fracture surface scanning electron microscope (SEM) analysis.For all the investigated sintered stainless, fatigue crack propagation resistance is influenced by hydrogen charging and an increase of crack growth rates dependent on the steel microstructure is obtained. Experimental results also allow to identify the sintered stainless steel obtained from the prealloyed 25% Cr, 5% Ni, 2% Mo powder as the most resistant to fatigue crack propagation in air and under hydrogen charging conditions.     

High temperature thermo-physical properties and thermal shock behavior of metal–diborides-based composites

The thermo-mechanical properties and thermal shock resistance of two metal diborides composites, HfB₂ with 20 vol.% SiC and HfB₂ with 20 vol.% SiC and 10 vol.% AlN, were investigated. Results showed that the composite HfB₂–SiC–AlN had a higher specific heat capacity than that of composite HfB₂–SiC. However, the occurrence of a Si–Al–O phase in the grain boundaries increased the heat flow resistance at grain boundaries, resulting in decrease thermal diffusivity and thermal conductivity. The calculated thermal shock resistance parameters indicated that the addition of AlN increased the resistance against crack initiation and crack propagation of composite, corresponding to the increase in critical thermal shock temperature difference from 400 to 600 °C.     

Thermodynamic properties of SmFeO3(s) and Sm3Fe5O12(s)

The enthalpy increments and the standard molar Gibbs energy (G) of formation of SmFeO₃(s) and Sm₃Fe₅O₁₂(s) have been measured using a Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. A λ-type transition, related to magnetic order-disorder transformation (antiferromagnetic to paramagnetic), is apparent from the heat capacity data at ∼673 K for SmFeO₃(s) and at ∼560 K for Sm₃Fe₅O₁₂(s). Enthalpy increment data for SmFeO₃(s) and Sm₃Fe₅O₁₂(s), except in the vicinity of λ-transition, can be represented by the following polynomial expressions:

Stability, validity, and sensitivity to input parameters of the slip-dissolution model for stress-corrosion cracking

The slip-dissolution model of stress-corrosion crack growth is reviewed and developed from several points of view: the differences between ‘discontinuous’ (Vermilyea) and ‘continuous’ (Ford, Andresen, Shoji) versions of the model; stability and possible multiplicity of predicted crack velocities for given mechanical parameters, and the sensitivity of the predicted crack velocity to small variations in the electrochemical and mechanical parameters. We find that for relatively steep anodic current decays on the bare metal surface at the crack tip (for example, i ∼ t^−m with m = 0.8), the output of the continuous type of model is extremely sensitive to the strain hardening exponent and to the location or cutoff distance in the logarithmic plastic strain distribution at which the crack-tip strain rate is calculated. Difficulties also appear because this distance is likely to be a function of other parameters such as yield stress. The handling of loading rate () effects in Shoji’s treatment appears unrealistic, leading to a much too weak dependence of the crack-tip strain rate on . However, irrespective of how is introduced, dual crack velocity solutions are found for negative ; the stability of these is discussed.     

Synthesis and crystallization behavior of 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) nanosized powders prepared using a simple co-precipitation process

The synthesis and crystallization behavior of 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) nanopowders prepared using a simple co-precipitation process at 348 K and pH = 7 were investigated using differential scanning calorimetry/thermogravimetry (DSC/TG), an X-ray diffractometer (XRD), the Raman spectra, transmission electron microscopy (TEM), selected area electron diffraction (SAED), and an energy dispersive spectrometer (EDS). The activation energy of tetragonal ZrO₂ crystallization from 3Y-TZP freeze-dried precursor powders using a non-isothermal method, namely, 169.2 ± 21.9 kJ mol⁻¹, was obtained. The growth morphology parameter n was approximated as 2.0, which indicated that it had a plate-like morphology. The XRD, Raman spectra, and SAED patterns showed that the phase of the tetragonal ZrO₂ was maintained at 1273 K. The crystallite size of 3Y-TZP freeze-dried precursor powders calcined at 1273 K for 5 min was 21.3 nm.