The Rehbinder effect in tests of superalloys in contact with molten salts

The results of the Rehbinder effect manifestation during testing of superalloys in contact with corrosive media containing Na2SO4 + NaCl are discussed. We present the experimental study of the effect of salts containing chlorine and sulfur on the mechanical properties and long-term strength of single crystal and powder nickel-based superalloys at high temperatures. The practical value of the work is associated with the possible operation of gas turbine parts in conditions of ingress of the particles containing chlorine, sulfur and sodium into the flow path. A simplified (compared to a previously used) procedure of testing wrought alloys for long-term strength in molten salts is developed. A comparative study of the mechanical properties and long-term strength of a single crystal superalloy in the initial state (previously damaged by corrosion of different duration) and being in contact with salts during testing is carried out. Due to the large scatter of experimental data, the method of lower envelopes has been proposed and implemented to determine the guaranteed values of the long-term strength. It is shown that the guaranteed values of the long-term strength at different temperatures and test durations can decrease by 2.5 – 5.0 times in the presence of salts. Metallographic studies of the nature of damage and destruction of samples are carried out. The revealed decrease in the long-term strength of heat-resistant alloys in contact with salts is interpreted as the Rehbinder effect and not as a manifestation of the effect of stress corrosion cracking. A methodology for using the obtained test results with a duration of up to several thousand hours in highly aggressive environments is proposed to predict the long-term strength in relation to long-term operation (tens of thousands of hours) under conditions of relatively low salt loads.

COMPARATIVE CHARACTERISTICS OF MECHANICAL AND ADSORPTION PROPERTIES OF ACTIVATED CHARCOALS APPLIED FOR ADSORBERS OF NPP AIR DISCHARGE SYSTEMS

This paper presents the results of studies on the strength and adsorption characteristics of granular activated charcoals of the brand ISQ DON GW4, С40/4 EXTRA KJ, and Norit RKJ4. The ultimate strength of samples was measured in different states: initial (H2O vapor saturation as a result of long-term storage), dehydrated, I2 vapor saturated and H2O + I2 vapor saturated. A significant (25…40%) decrease in the activated charcoal fracture stress due to the iodine vapor saturation, concerned with a Rehbinder effect, was observed. The degree of the adsorption decrease in the sorbent mechanical stability should be taken into account when selecting sorbents for filters of NPP air discharge purification systems (ventilation systems).

Novel Unexpected Reconstructions of (100) and (111) Surfaces of NaCl: Theoretical Prediction

We have predicted stable reconstructions of the (100) and (111) surfaces of NaCl using the global optimization algorithm USPEX. Several new reconstructions, together with the previously reported ones, are found. For the cleaved bare (100) surface, pure Na and pure Cl are the only stable surface phases. Our study of the (111) surface shows that a newly predicted Na3Cl-(1 × 1) reconstruction is thermodynamically stable in a wide range of chlorine chemical potentials. It has a sawtooth-like profile where each facet reproduces the (100) surface of rock-salt NaCl, hinting on the preferred growth of the (100) surface. We used Bader charge analysis to explain the preferable formation of this sawtooth-like Na3Cl-(1 × 1) reconstruction of the (111) surface of NaCl. We find that at a very high chemical potential of Na, the polar (and normally absent) (111) surface becomes part of the equilibrium crystal morphology. At both very high and very low chemical potentials of Cl, we predict a large decrease of surface energy and fracture toughness (the Rehbinder effect).

Phenomena of crystal splitting during growth as a result of the Sternberg-Punin and Rebinder effects

The causes of growth blocking, splitting, twisting of crystals are considered. These phenomena are the result of the combined effect of the Punin effect (total autodeformation due to intersectorial heterometry, appearing when impurities are unequally captured by different faces or their parts) and the Rehbinder effect (a sharp decrease in the strength of the surface layer of a growing crystal several times covered by a film of mother solution containing active substances).