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Dmytrakh I.M.,NASU Karpenko Physico Mechanical Institute
Strain | Year: 2011

The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico-Mechanical Institute of NASU. The model scheme of the pre-fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non-propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions. © 2010 Blackwell Publishing Ltd.

Kravets I.B.,NASU Karpenko Physico Mechanical Institute
Radioelectronics and Communications Systems | Year: 2012

Paper presents theoretical results of modeling periodically correlated random processes. We compare the known parametric models: periodic autoregression model of moving average, parametric model of coherent representation and parametric model of harmonic representation. Dependences of properties of correlation and spectral functions related to different models of periodically correlated random processes on their parameters are studied. Main differences between approximations of characteristics of the considered models are revealed. © 2012 Allerton Press, Inc.

Demyanyshyn N.M.,NASU Karpenko Physico Mechanical Institute
Ukrainian Journal of Physics | Year: 2015

The relaxation of static photoelasticity in lead germanate (Pb5Ge3O11) crystals has been studied. The anisotropy of relaxation amplitudes and their anomalies near the phase transition point were revealed. The photoelastic relaxations and peculiarities in their temperature behavior are explained as manifestations of the piezocaloric mechanism. © N.M. DEMYANYSHYN, 2015.

Denys R.V.,Institute for Energy Technology of Norway | Denys R.V.,NASU Karpenko Physico Mechanical Institute | Yartys V.A.,Institute for Energy Technology of Norway | Yartys V.A.,Norwegian University of Science and Technology
Journal of Alloys and Compounds | Year: 2011

The present work gives the data of systematic studies of the influence of magnesium on the crystal structure and hydrogenation behaviour of the PuNi 3-type La1-xMgxNi3 (x = 0-0.67) intermetallic alloys. Synchrotron X-ray diffraction studies revealed that substitution of La in LaNi3 by Mg proceeds in an ordered way, only within the Laves type layers of the hybrid crystal structures build from the MgZn2- and CaCu5-type slabs. When completed, it leads to the formation of LaMg2Ni9 (2MgNi2 + LaNi 5). Gradual increase of Mg content is accompanied by a linear decrease of the volumes of the unit cells. Interestingly, a substantial contraction takes place also for the Mg-free CaCu5-type slabs. Hydrogen interaction with the La1-xMgxNi3 alloys was investigated by in situ synchrotron X-ray, neutron powder diffraction and pressure-composition-temperature studies. In the whole substitution range, La1-xMgxNi3 alloys form intermetallic hydrides with H/M ratio ranging from 0.77 to 1.16. Magnesium influences structural features of the hydrogenation process and determines various aspects of the hydrogen interaction with the La1-xMgxNi3 intermetallics causing: (a) more than 1000 times increase in equilibrium pressures of hydrogen absorption and desorption for the Mg-rich LaMg 2Ni9 as compared to the Mg-poor La2.3Mg 0.7Ni9 and a substantial modification of the thermodynamics of the formation-decomposition of the hydrides; (b) an increase of the reversible hydrogen storage capacities following increase of Mg content in the La1-xMgxNi3 to ∼1.5 wt.% H for La2MgNi9; (c) improvement of the resistance against hydrogen-induced amorphisation and disproportionation; (d) change of the mechanism of the hydrogenation from the anisotropic to isotropic one. Thus, optimisation of the magnesium content provides different possibilities in improving properties of the studies alloys as hydrogen storage and battery electrode materials. © 2010 Elsevier B.V. All rights reserved.

Denys R.V.,Institute for Energy Technology of Norway | Denys R.V.,NASU Karpenko Physico Mechanical Institute | Yartys V.A.,Institute for Energy Technology of Norway | Yartys V.A.,Norwegian University of Science and Technology | Webb C.J.,Griffith University
Inorganic Chemistry | Year: 2012

Reversible hydrogen storage capacity of the La 3-xMg xNi 9 alloys, charged by gaseous hydrogen or by electrochemical methods, reaches its maximum at composition La 2MgNi 9. As (La,Mg)Ni 3-3.5 alloys are the materials used in advanced metal hydride electrodes in Ni-MH batteries, this raises interest in the study of the structure-properties interrelation in the system La 2MgNi 9-H 2 (D 2). In the present work, this system has been investigated by use of in situ synchrotron X-ray and neutron powder diffraction in H 2/D 2 gas and by performing pressure-composition-temperature measurements. The saturated La 2MgNi 9D 13.1 hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group R3̄m (No.166); a = 5.4151(1) Å; c = 26.584(2) Å; V = 675.10(6) Å 3). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi 5 (CaCu 5 type) and LaMgNi 4 (Laves type). These are occupied by D to form LaNi 5D 5.2 and LaMgNi 4D 7.9. The LaNi 5D 5.2 slab has a typical structure observed for all reported LaNi 5-containing hybrid structures of the AB 5 + Laves phase types. However, the Laves type slab LaMgNi 4D 7.9 is different from the characterized individual LaMgNi 4D 4.85 hydride. This results from the filling of a greater variety of interstitial sites in the La 2MgNi 9D 13/LaMgNi 4D 7.9, including MgNi 2, Ni 4, (La/Mg) 2Ni 2, and (La/Mg)Ni 3, in contrast with individual LaMgNi 4D 4.85 where only La 2MgNi 2 and Ni 4 interstitials are occupied. Despite a random distribution of La and Mg in the structure, a local hydrogen ordering takes place with H atoms favoring occupation of two Mg-surrounded sites, triangles MgNi 2 and tetrahedra LaMgNi 2. A directional bonding between Ni, Mg, and hydrogen is observed and is manifested by a formation of the NiH 4 tetrahedra and MgH 6 octahedra, which are connected to each other by sharing H vertexes to form a spatial framework. © 2012 American Chemical Society.

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