Solid State Phenomena Vol. 138

Abstract: The present paper deals with the application of the transient techniques for thermophysical analysis of the structural changes in materials. The technique has been applied for study of equilibrium transitions as well as for kinetic transitions. A special methodology has been developed to study kinetic transitions like crystallization, melting, etc. in a “pseudo-equilibrium states” by the help of porous structures. The paper includes three different issues: the transient methods for measuring thermodynamic and transport parameters, data analysis and application of the pulse transient method for measurements of materials in thermodynamic equilibrium, pseudoequilibrium and in non-equilibrium (quasi-equilibrium) states. Equilibrium transitions in CsPbCl3 and CsPbBr3 single crystals, kinetic transitions of freezing and thawing water in porous stones and non-equilibriums states in E-glass and Al2O3 ceramics during sintering have been studied.

Abstract: Numerous solid-state transformations occur in metal iodides. These transformations can be classified into three categories: polymorphic transformations, polytypic transitions and molecular solids. Many of the modifications of metal iodides involve metastable phases transforming into stable phases. Revisions to the In-I and Th-I phase diagrams are made based on data found in the literature.

Abstract: A new characterization method, "Macroscopic Composition Gradient (MCG) Method" is proposed to investigate the phase transformations near the phase boundaries, such as the solubility limit, order/disorder line and so on. Since the macroscopic composition gradient in the alloy is prepared so as to step over the phase boundary, the morphological transition of critical phenomena at the phase boundary can be observed by means of analytical transmission electron microscopy. By utilizing this method, the critical minimum size of stable precipitate in the vicinity of edge of miscibility gap is experimentally determined for the Ni3Si in Ni-Si, Ni3Al in Ni-Al, Cu4Ti in Cu-Ti and Co in Cu-Co binary alloy systems. The results are as follows: The critical nucleus size shows a steep increase up to several tens of nm in a very narrow composition range less than 0.3at% from the phase boundary. The Gibbs-Thomson relation and the conventional nucleation theory statistically rationalize such the composition dependence of nucleus size change. However, the nucleus formation is kinetically never rationalized by the conventional nucleation theories. The phase decomposition of supersaturated solid solution progresses by a mechanism of spinodal phase decomposition, even in the composition range near the solubility limit, i.e. a so-called Nucleation- Growth region. Such the phase decomposition behavior is never rationalized by the Boltzmann- Gibbs free energy, which is based on the extensive entropy. The experimental facts obtained here are explained by Tsallis's non-extensive entropy. It should be noted that the present experiments can systematically be conducted in the composition range very near the solubility limit where they has hardly been examined in the past. The MCG method proposed here is considered to open a new way to investigate the microstructure evaluation, particularly for the critical phenomena near the phase boundary.

Abstract: The response of the relative resistivity changes to the isochronal annealing was measured in MgSc, MgScMn and MgYNdScMn alloys. The derivatives of the annealing curves were fitted by Gaussian functions to determine and separate the temperature ranges of the phase transformations taking place. The Austin-Rickett kinetics is argued to justify this procedure for diffusion driven transformations. TEM observation verified the conclusions drawn from the procedure.

Abstract: Hydrogen is suggested as a promising fuel of the near future for the utilization in automotive and mobile applications. Therefore, safe and effective hydrogen storage systems need to be developed. One of the possibilities, suitable especially for mobile applications, is the storage of hydrogen in the form of light-metal hydrides. In this work we studied microstructure and hydrogen absorption and desorption kinetics in selected Mg-Ni alloys. Hydrogen saturation was carried out by the cathodic polarization in alkaline water-based solution. It was confirmed that hydrogen could be stored in the Mg2Ni intermetallic phase forming Mg2NiH0.3 phase using this technology. MgH2 hydride is also formed when the temperature of 90 °C is applied. The total content of hydrogen in the material after saturation is approx. 0.7 wt. % according to the thermogravimetry analysis. This low value is caused probably by the surface oxidation, blocking further hydrogen diffusion. Thermal hydrogen desorption tests showed that the Mg2NiH0.3 phase is able to release hydrogen even at temperatures lower than 100 °C.

Abstract: Kinetics of hydrogen desorption from Mg2NiH4 was studied. Experimental material was prepared by two techniques – by melting and casting and by ball-milling and compacting into pellets. Experimental materials were hydrogen charged at elevated temperature and pressure. The pellets were charged in two different regimes resulting in structures with high fraction of twinned low-temperature phase LT2 and with low fraction of LT2. It was made an attempt to measure diffusion coefficients of hydrogen and its temperature dependence both in high-temperature (HT) and in low-temperature (LT) phases of Mg2NiH4. The measurement was carried out in temperature interval from 449 K to 576 K by the volumetric method. It was found that the LT2 slows-down the desorption rate considerably.

Abstract: Specific phase transitions to the compound-like impurity nanosegregation structures at dislocations and grain boundaries in metals and their influence on diffusion-assisted processes are considered, mainly, on the basis of the thermodynamic analysis of the related experimental data. The following systems and aspects are in detail considered: (1) the hydride-like nanosegregation of hydrogen at dislocations and grain boundaries in palladium and their influence on the apparent characteristics of hydrogen solubility and diffusivity in palladium; (2) the physics of the anomalous characteristics of diffusion of Fe and other transition impurities in crystalline Al at elevated temperatures, the role of the compound-like nanosegregation (CLNS) of Fe and the others at dislocations and grain boundaries in Al, analysis of the Mössbauer and diffusion data on CLNS of Fe at grain boundaries and dislocations in Al; (3) some new physical aspects of internal oxidation and nitridation of metals (for Cu-0.3%Fe alloy/Cu2O surface layer, and for (Ni-5%Cr) alloy / N2 gas), the role of the compound-like impurity nanosegregation at dislocations and grain boundaries, study results on the deviations from the classical theories predictions and their interpretation. The possibility is considered of nanotechnology applications of the study results for creation of nanostructured metals with compound-like nanosegregation structures at grain boundaries, in order to obtain specific physical and mechanical properties of such a cellural-type nanocomposites. In particular, it can be complex hydride-like, carbide-like, nitride-like, carbide-nitride-like, oxide-like or intermetallide-like nanosegregation structures at grain boundaries of nanostructured metals.

Abstract: The mechanisms of formation of activated states upon diffusion of impurity interstitial atoms in metals have been identified, which allows formulation of the theoretical criterion for reliability of corresponding experimental data. This criterion is valid for almost all experimental results gained using precise techniques for bcc, fcc, and hcp metals. In case of hydrogen, oxygen, and nitrogen, most adequate are the models of potential diffusion barrier that are based on the elasticity theory. For carbon and boron, a less energy-taking mechanism is realized which involves the electron system rearrangement in a crystal during the diffusion jump. The accuracy of the theoretical criterion is in line with the capacities of modern precision techniques.

Abstract: The possibilities of Fisher’s model development and generalization are considered, as this model in its classical form contradicts some experimental results. Particularly, it does not explain such a phenomenon, observed in Mössbauer studies, as the transfer of grain-boundary diffusing atoms into near-boundary crystallite areas at temperatures when the volume diffusion is admittedly frozen. Different models explaining this phenomenon are briefly reviewed, the greatest attention being paid to a model accounting for the presence of equilibrium-composition near-boundary layers. It is demonstrated that based on the results of grain boundaries investigations one can conclude that at relatively low temperatures (< 0.35 – 0.40 Tm) the diffusant pumping from a grain boundary proceeds much faster than volume diffusion, and possible reasons for that are considered.