Environment effect on internal fatigue crack propagation studied with in-situ X-ray microtomography

In the present study, physically short fatigue cracks initiated from an artificial internal notch under vacuum conditions are observed in smooth specimens made of titanium alloy. The crack initiated from such a notch is not in contact with the surface during the majority of its propagation. Another batch of artificially notched specimens in the same alloy having their internal notch linked with the surface to bring air to the initiated short crack were tested too. All the specimens were ultrasonically cycled with synchrotron micro-computed tomography acquisitions which were regularly acquired from crack initiation detection to final failure without demounting the specimen. The number of cycles to failure, the internal crack growth rate and the crack growth mechanisms of internal cracks are compared for propagation under either air conditions or in the core of the alloy (i.e. under vacuum environment). It is shown that the environment plays a key role in the explanation of the very low crack growth rate of internal short crack loaded at very low stress amplitude leading to the gigacycle regim

Kinetic data and mechanistic model for initial sintering of TiO₂ anatase powder

International audienceMeasurements of surface area reduction of TiO2 anatase powder (of initially 100 m2g-1) are carried out at various partial pressures of water and oxygen and at 823 K. The kinetic equation for the experimental rates is obtained. We have developed a mechanistic model involving six consecutive elementary steps, in which hydroxyl species play a dominant role. No geometric assumption is required. The general expression of the deduced theoretical rates has the same form as the experimental rates. The comparison between the experimental rates with the predicted ones points out that the rate-limiting step is the surface diffusion of hydroxyl species. This model can easily be used for any other compounds, for particle growth and porosity decrease

Influence of aditives on the thermal properties of TiO₂ anatase: surface area stabilization and transformation into rutile

Pas publiéInternational audienceHigh surface area anatase (100m2g-1-1) was modified by addition of La3+, Al3+,Zr4+,Ti4+, Ta5+ and Nb5+ ions by impregnation method. Differential thermal analysis exhibits two successive exothermic phenomena attributed to surface area decrease and anatase-rutile transformation. Kinetic measurement of each of them have been performed. The additives slow down these two processes. Lanthane and zirconium are found to be particulary active for surface area stabilization. Low-surface area anatase (11m2.g-1) was prepared and doped according to the same method. Its transformation into rutile is also inhibited but the various influence of the additives were found to be different compared to the high-surface doped anatase

Initial sintering of submicrometer titania anatase powder

International audienceMeasurements of surface area reduction of TiO2 anatase powder (of initially 100 m2* g−1) were carried out for various partial pressures of water and oxygen at 823 K. The kinetic equation obtained for the experimental rates has the formulation r=k[PH2O]1/[PO2]1/β where alpha and β are equal to 2 and -12, respectively. A mechanistic model involving six consecutive elementary steps was developed, in which hydroxyl species play a dominant role. No geometrical assumption is required. The general expression of the deduced theoretical rates has the same form as previously given. The comparison between the experimental and the predicted rates points out that the rate-limiting step is the surface diffusion of hydroxyl species. This model can easily be used for any other compounds, for particle growth, and for porosity elimination