Solid State Phenomena Vol. 137

Abstract: Junction disclinations are important elements of the structure of nanostructured metals produced by severe plastic deformation (SPD). Effect of these defects on the formation energy of vacancies in grain boundaries (GBs) is studied by means of atomistic computer simulations. Estimates based on the calculations of vacancy formation energies suggest that at least two orders of magnitude increase of the GB diffusion coefficient can be expected due to junction disclinations in nanostructured metals.

Abstract: The following experimental data are presented and discussed: a) the explosion-like nucleation of micro- (meso) cracks; b) the dependence of the scale of local “microcatastrophe” on the relation between the released strain energy and the dissipative properties of the nearest environment; c) the determinative influence of the dissipative properties on the rate of redistribution of local stresses. The presented data evidence that every event of defect nucleation that takes place at either micro- or mesoscale level in the heterogeneous solid examines the energy absorbtion properties of the whole system. Future localization of the macroscopical failure is of “accidental type”, that is results from random, active formation of the ensemble of micro- and mesocracks in different localities as a consequence of the decrease of the dissipative capability of surrounding volume.

Abstract: The advantages of the OMI algorithm to compute the logarithmic decrement and the resonant frequency from free decaying oscillations is reported. The OMI algorithm is proved to be the best solution in the computation of the logarithmic decrement and the resonant frequency for high damping levels.

Abstract: Internal friction peaks observed in single or polycrystals are clearly due to a dislocation relaxation mechanism. Because a sample observed by transmission electron microscopy (TEM) often exhibits in the same time various dislocation microstructures (isolated dislocations, dislocation walls, etc.) it is very difficult to connect the observed relaxation peak with a particular dislocation microstructure. Using isothermal mechanical spectroscopy (IMS), it is easier to compare, for instance, the evolution of a relaxation peak with measurement temperature to the microstructural evolution observed by in-situ TEM at the same temperatures. IMS was used to study a relaxation peak in a 5N aluminium single crystal firstly 1% cold worked and then annealed at various temperatures. TEM experiments performed in the same material at various temperatures equal to the temperatures used for the damping experiments made possible to link this internal friction peak with a relaxation effect occurring inside dislocation walls. In two other experiments in a 4N aluminium polycrystal and in a metal matrix composite with SiC whiskers, it is shown that the observed relaxation peaks are connected to the motion of dislocations inside polygonization boundaries in the first case and in dislocation pile-ups around each whisker in the second one. Theoretical models proposed to explain such relaxation peaks due to a dislocation motion inside a dislocation wall or network are discussed.

Abstract: High temperature plasticity of fine-grained ceramics (ZrO2, Al2O3, etc) is usually associated with a grain boundary sliding process. The aim of the present research is then to improve the high-temperature mechanical strength of polycrystalline zirconia (3Y-TZP) through the insertion of multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw), which are susceptible to pin the grain boundaries. The effect of these nano-sized particles on grain boundary sliding has been studied by mechanical spectroscopy.

Abstract: An influence of Cd content on the kinetics of a spontaneous low-temperature structure transformation in In-rich In - Cd alloys has been investigated using acoustic, resistivity and DSCtechniques. It is established that increase of the concentration of Cd leads to an essential increase of a driving force of the transition that results in an increase of the transition rate and in decrease of the relaxation time. The low-temperature instability of acoustic, resistivity and thermal properties is caused presumably by a decomposition of the solid solution that has features of a phase transition of the 1st order. The main empirical activation parameters of the transformation are derived. The activation energy amounts to U0 = 0.43 eV, the attempt period is τ 0 =5 × 10-9 s. The probable border of the decomposition in the phase diagram of the In-Cd system is established.

Abstract: Acoustic properties of polycrystalline Cr samples of 99.99 % purity are investigated at frequencies of longitudinal vibrations f ~ 75 kHz and of bending vibrations at 0.6 < f < 1.5 kHz when thermocycling within the temperature interval 5 − 330 K. Special attention is paid to the acoustic anomalies in the vicinity of the magnetic phase transitions: the Néel point and the spin-flip transition. In the as-received samples, a significant hysteresis of the acoustic properties has been found for the first time. The effect of heat treatments on internal friction and elastic properties of chromium polycrystals is also investigated. The data obtained are compared with the effect of small preliminary deformation and а long-time ageing at room temperature on the ultrasound anomalies in Cr single crystal. The observed behavior of the acoustic properties may be caused by changes in the antiferromagnetic domain structure in Cr polycrystals under the action of thermoelastic stresses arising from quenching, preliminary plastic deformation or thermocycling.