Microstructures and hydrogen permeability of directionally solidified Nb–Ni–Ti alloys with the Nb–NiTi eutectic microstructure

Microstructural stability and its effect on hydrogen permeability in equiaxed and directionally solidified eutectic Nb 30 Ti 35 Co 35 alloys

“…The highest apparent diffusivity is expected if all the bcc-Nb lamellae in the eutectic microstructure are contiguously arranged and perpendicular to the membrane surface. This has been testified by experimental results [10,13] and quantitatively by a rule of mixture based on effective medium theory [15]. In the present case, it is also found that a higher apparent hydrogen diffusion exists in the DS Nb 30 Ti 35 Co 35 samples compared to the EX samples, because the former have more contiguous bcc-(Nb, Ti) regions penetrating the membrane.…”

“…Directionally solidified (DS) Nb 30 Ti 35 Co 35 exhibits a much higher permeability than in the as-cast state with equiaxed (EX) microstructure [10]. A similar case was observed in DS eutectic Nb 19 Ti 40 Ni 41 [13]. The hydrogen permeability for the DS Nb 19 Ti 40 Ni 41 with bcc-Nb rods aligned perpendicular to the membrane surface is about an order of magnitude larger than that for the specimens with bcc-Nb rods parallel to the surface.…”

Changes in microstructures and hydrogen permeability of Nb30Hf35Co35 eutectic alloy membranes by annealing

“…In addition, the spatial distribution of BCC-Nb in the membrane is also important for hydrogen permeation, because this phase in the eutectic aligned perpendicular to the membrane surface can act as the pathways for hydrogen diffusion. If all eutectic grains perpendicular to the membrane surface so that no BCC-Nb grains are suspended in the membrane, the highest hydrogen permeability and diffusivity is expected, which has been testified by several researches [10,15,25]. In the present work, it should be noted that the as-cast Nb 30 Hf 35 Co 35 sample shows a higher hydrogen diffusion compared to the a-samples, which is attributable to the eutectic grains with more contiguous BCC-(Nb, Hf) regions penetrating the membrane, as seen in the inset in Fig.…”

Hydrogen permeability of glass-forming Ni-Nb-Zr-Ta crystalline membranes

“…In duplex phase alloys, such as those composed of a bcc primary phase embedded in a crystalline matrix, hydrogen is believed to diffuse through the permeable phase composed of bcc Group 5 elements with a dendritic solidified structure of which the hydrogen permeability values are approximately one order of magnitude higher than those of Pd-based alloys at 673 K. In addition to the composition, the diffusion rate is dependent on the morphology and continuity of the dendrite as well as the interface with the matrix [20]. The high hydrogen permeability values of the Ni42Nb28Zr25Ta5 crystalline alloy, which are similar to those of the Ni 30 Nb 40 Zr 30 alloy [21], are believed to result from the duplex structure of the cast alloy.…”