Lubomir Kral

Diffusion of Titanium and Nickel in B2 NiTi

Diffusion of both titanium and nickel was measured in the near stoichiometric Ni-49.4at.%Ti alloy with the B2 ordered structure. The radiotracer technique and the 44Ti and 63Ni isotopes were applied in the temperature interval from 900 to 1300 K. The penetration profiles were determined by precision parallel grinding or by ion beam sputtering at larger and smaller penetration depths, respectively. Titanium and nickel diffusivities were found to follow linear Arrhenius dependencies with the pre-exponential factors of 2.710-7 and 4.710-9 m2/s and the activation enthalpies of 205 and 143 kJ/mol, respectively. A vacancy mediated diffusion mechanism is suggested to provide diffusion of both nickel and titanium in the compound NiTi.

On the relationship between and conjugate twins and double extension twins in rolled pure Mg

Abstract The paper presents a new type of twin-like objects observed in rolled pure magnesium. They have and habit planes and their misorientations to the matrix are close to 56° and 63° about axis, respectively. The ad hoc performed theoretical analysis and atomic simulations allow to interpret the objects as double twins formed by the simultaneous action of two twinning shears with completely re-twinned volume of primary twin. The observed inclinations from the ideal misorientations for such double twins can be explained by the compliance of the strain invariant condition in the twin boundary. It seems plausible that, once the double twin is formed, its twin boundaries are hard to move by glide of twinning disconnections. If so, these twins represent obstacles for the motion of crystal dislocations increasing the hardness of the metal.

Desorption Kinetics of Hydrogen in Mg2NiH4 Hydride and in Hydrogeneated Eutectic (Mg/Mg2Ni)Hx

Mg-Ni alloys are perspective hydrogen-storage materials. In the present work, kinetics of hydrogen desorption was studied in Mg2NiH4 hydride and in hydrogenated eutectic (Mg/Mg2Ni)Hx. Time dependence of desorbed hydrogen was measured in temperature interval 552 –723 K and hydrogen diffusion coefficients were evaluated from obtained desorption curves. Results were interpreted in relation to phase composition investigated parallel by XRD. It was found that microtwinned low-temperature phase LT2 slows-down considerably the hydrogen desorption rate.

Kinetics of Phase Transformations in Mg2Ni-H System

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.

Diffusion of Carbon and Manganese in Fe-C-Mn

In this paper, the results of measurements of chemical diffusion of C in weldments of Fe- C-Mn alloys with content of 0-15 % Mn, are reported. Alloys were cast in induction furnace under protective Ar atmosphere. They were hot-forged to rods of diameter 15 mm, machined down to diameter 12 mm, and cut into pieces of 4 mm thickness. These cylindrical samples were metallographically polished. Diffusion couples, welded by electrical impulse, were sealed together with Ti-chips, into evacuated silica ampoules. After that, they were annealed at 1073-1373 K for 96-1.5 hours, respectively. Measurement of C redistribution was done in a normal direction to welding interface using electron-scanning microscope Philips SEM-505WDS equipped with Microspec WDX-2A. Diffusion coefficients of C were obtained and the relations between manganese and carbon were found.

Influence of long-term ageing upon the capacity of hydrogen storage in two novel Mg-Ni-In-C alloys

We have shown previously that the addition of In and C enhances the hydrogen desorption capacity and considerably improves the kinetics of hydrogen desorption from Mg-Ni based alloys, important from the perspective of hydrogen storage. In this paper, we present the results of tests of the long-term stability of the sorption behaviour of these materials. We studied how the capacity and sorption kinetics changed 5 years from the preparation of two chosen Mg-Ni-In-C alloys differing in indium concentration. Kinetic curves and PCT isotherms were measured at temperatures of 250–350◦C. It was found that long-term storage of the hydrogencharged samples led to no significant downgrade in hydrogen sorption kinetics. However, the absorption capacity somewhat decreased from about 5 wt.% H2 to about 3 wt.% H2 due to partial phase decomposition. Relatively intensive tentative oxidation during preparation, which had a slightly beneficial effect on the desorption rate of freshly-prepared samples, caused almost total blocking of hydrogen sorption in the aged samples. K e y w o r d s: hydrogen, hydrogen storage, magnesium alloys, indium

Interrelation between Hydrogen Desorption Kinetics and Structure of (Mg2Ni)Hx and Hydrogenated Eutectic(Mg/Mg2Ni)Hy

Mg-rich alloys of the binary system Mg-Ni are prospective hydrogen-storage materials. In the present study, desorption characteristics of hydrided Mg2Ni intermetalic and hydrided Mg/Mg2Ni eutectic mixture were investigated. Structure of experimental materials during the hydrogenation was observed by SEM. Three modifications of (Mg2Ni)Hx (x ~ 4) were prepared differing in the ratio of two low-temperature phases f = LT2/LT1: with (i) f >1, (ii) f ~ 1 and with (iii) f <1. Evolution of the ratio f during hydrogen desorption was checked by XRD. It was found that the micro-twinned phase LT2 is not desirable in hydrogen-storage materials containing Mg2Ni intermetallic. Diffusion coefficient of hydrogen in LT2 is about 20 times lower than in LT1.