[en] Engineering ceramics have excellent properties such as high strength, high hardness and high heat resistance compared with metallic materials. To apply the ceramic in fast reactor environment, it is necessary to evaluate the sodium compatibility and the influence of sodium on the mechanical properties of ceramics. In this study, the influence of high temperature sodium on the mechanical properties of sintered ceramics of conventional and high purity Al₂O₃, SiC, SiAlON, AlN and unidirectional solidified ceramics of Al₂O₃/YAG eutectic composite were investigated by means of flexure tests. Test specimens were exposed in liquid sodium at 823K and 923K for 3.6Ms. There were no changes in the flexural strength of the conventional and high purity Al₂O₃, AlN and Al₂O₃/YAG eutectic composite after the sodium exposure at 823K. On the contrary, the decrease in the flexural strength was observed in SiC and SiAlON. After the sodium exposure at 923K, there were also no changes in the flexural strength of AlN and Al₂O₃/YAG eutectic composite. In the conventional and high purity Al₂O₃ and SiC, the flexural strength decreased and signs of grain boundary corrosion were detected by surface observation. The flexural strength of SiAlON after the sodium exposure at 923K increased instead of severe corrosion. In the specimens those showed no changes in the flexural strength, further exposure in sodium is needed to verify whether the mechanical properties degrade or not. For SiAlON, it is necessary to clarify the reason for the increased strength after the sodium exposure at 923K. (author)

[en] In order to study the mechanical properties of tetragonal polycrystal zirconia stabilized with ceria various powder compositions with different CeO² content were made. Modulus of rupture for those compounds was measured. Tetragonal retained phase was determined for samples of CeO₂-ZrO₂ ceramics with and without superficial mechanical treatment. The experimental results allowed us to evaluate the effects of CeO₂ content and sintering temperature in the mechanical properties and tetragonal transformed phase (t→ m) in ceramics of CeO₂-ZrO₂ systems. (author)

[en] The effect of free chromium oxide on the change of properties of lanthanum chromite ceramics La_0.95Ca_0.05CrO₃ at high temperature is studied. The stability of properties is estimated by changes in porosity mass losses and electric conductivity after thermal treatment at 1600-1700 dec C. It is established that to increase the stability of ceramics properties it is advisable to use molten materials with the minimum content of free chromium oxide in the initial charge

[en] The room temperature slow crack growth resistance in air (approx. 50% relative humidity) and in water for large cracks in MgO-partially stabilized zirconia (PSZ) improves with increase in critical fracture toughness, Ksub(Ic). Ageing the as-fired PSZ at 1400 C for 8 h results in decreasing Ksub(Ic) from 8.5 MPa msup(1/2) to 6 MPa msup(1/2). The ageing treatment also promotes the growth of eutectoid decomposition products on grain boundaries that is accompanied by a decrease in the dependence of the change in Region 1 crack velocity with a change in the applied stress intensity. Calculated times to failures are markedly decreased in the aged as compared to the as-fired PSZ ceramic. (author)

[en] A considerable decrease of elasticity modulus in Tb₂Ti₂O₇ in the low-temperature range is found. It is shown that responsible for the effect are electron-deformation interaction and structure specificity of Tb³⁺ ion low energy levels in the Tb₂Ti₂O₇ crystal lattice field. The estimation of critical temperature value of the cooperative Yahn-Teller effect in Tb₂Ti₂O₇:T_c ≅ 0.1 K is given

[en] An etch-pitting technique has been used to compare the dislocation distribution around unrecovered, i.e. with a load still applied to the indenter, and recovered impressions in single crystal MgO. Thus it is shown that, immediately upon removing normal loads greater than 100 g (0.98 N), additional slip lines are developed on the surface surrounding the actual indentation. Reversed plastic flow, in previously undeformed material adjacent to the region which has become work-hardened during the formation of the impression, is considered to be responsible for this phenomenon. It is suggested that this effect is caused by elastic recovery in the work-hardened region as the load is removed. Whilst this appears to have a negligible effect on the accuracy of hardness measurements, it could be an important contribution to cumulative processes of deformation and fracture as in comminution and wear. (author) 23 refs., 3 figs

[en] Ceramic test specimens with controlled surface flaws introduced by Knoop indentation are being increasingly used in fracture mechanical investigations. Such artificially induced cracks must have the same characteristics as cracks of natural origin. A problem of transferability is caused by the existence of a damaged surface zone which acts as a wedge. In the theoretical part the effect of wedging on crack opening behaviour is treated. In the experimental part first the removal of residual stress effects due to wedging is described and finally the application of Knoop cracked specimens to lifetime measurements under static load is demonstrated. The experimental part deals with aluminum oxide. (author) 7 refs., 6 figs

[en] A new application of a.c. impedance technology for monitoring crack propagation was discovered in this study, and this methodology was patented. By excluding ranges of frequency, a single frequency was chosen to monitor crack propagation of sensitized 304 stainless steel in thiosulfate solution. The behavior of the phase angle from the impedance data gives interesting information regarding crack growth. Generally, the phase angle tends to rise if the crack propagates continuously. Computer simulation showed that the phase shift was caused by the variation of the capacitance in the equivalent circuit. ((orig.))

[en] Plates of SiC, TiB₂ and Al₂O₃ were impacted edge-on with steel projectiles at striking velocities between 20 m/s and 1000 m/s. Fracture propagation in the specimen was visualised by means of a Cranz-Schardin camera within the first twenty microseconds. The phenomenology of damage is discussed and compared for the different types of ceramics. The velocities of single cracks and crack fronts were measured. The experiments were supplemented by ceramographical investigations of recovered fragments. Void nucleation and coalescence could be identified as the failure mechanism in impacted TiB₂. (orig.)

[en] Under certain high strain rate conditions, plastic deformation can be assumed to be adiabatic, and a significant temperature increase can occur at large strains. In this study, tantalum and tantalum-tungsten alloys were subjected to high shear strains at high strain rate using a specially-designed stepped specimen in a Hopkinson bar. Upon completion of the deformation, the region is cooled to below one-half of the temperature achieved due to the adiabatic heating in less than one millisecond. Microstructural analysis by transmission electron microscopy revealed that the highly deformed shear-band region consisted of a gradual decrease in dislocation cell size with increasing strain. An analysis based on dynamic recovery (enabled by the adiabatic temperatures rise associated with plastic deformation), is used to predict the recovered microstructure. (orig.)