Influence of Fe on the Hydrogen Storage Properties of LaCeNi Alloys.

LaNi5 intermetallic compounds with a hexagonal CaCu5 type structure can react reversibly with hydrogen. The element substitutions in LaNi5 can significantly change the hydrogenation properties, allowing to tune them to a large extent. It could be very advantageous to partially replace Ni or La with other elements to reduce the cost of this alloy as well as the equilibrium pressure of absorption and desorption. The hydrogen storage properties of ball-milled AB5 alloys containing the elements La, Ce (A-rare elements) and Ni, Fe (B-transition metals) were studied in this paper. Although the substitution of Ni (atomic radius 1.49 Å) with Fe atom (atomic radius 1.56 Å) increased the unit cell volume from 86.4149 to 87.947 5 Å3 of the LaNi5 phase, its hydrogen storage capacity was still close to the value 1.4 wt %. The enthalpy (ΔH) of hydride formation for hydrogen absorption and desorption of the experimental alloys was in the range of 29-32.6 kJ/mol. A very favorable effect of Fe on the sorption properties was found in the significant reduction of the equilibrium pressure of absorption and desorption. These studied experimental Fe-containing alloys were able to store hydrogen at 300 K and with pressure under 0.1 MPa. The fastest sorption kinetics of hydrogen was found in alloys with FeNi phase particles located on the surface of the powder. However, if the FeNi phase was segregated at the grain boundaries, it acted as a barrier limiting the growth of the hydride phase. This led to a decrease of the hydride sorption kinetics.

Creep Resistance of S304H Austenitic Steel Processed by High-Pressure Sliding.

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500-700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.