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

The microstructures and hydrogen permeability properties were investigated for Nb–NiTi eutectic alloys directionally solidified in an optical floating zone furnace. Rod-type eutectic microstructures with Nb rods aligned parallel to the growth direction are obtained with an alloy having a composition of Nb–41Ni–40Ti grown at relatively slow growth rates below 2.5 mm/h. The hydrogen permeability depends on the relative direction of aligned Nb rods within the membrane. The values for the specimens with Nb rods aligned perpendicular to the membrane surface are about an order of magnitude larger than those for the specimens with Nb rods parallel to the surface. The largest value for the former specimen is as high as 2.60 × 10−8 mol H2 m−1 s−1 Pa−1/2 (673 K), which is more than twice that reported for the as-cast Nb–NiTi eutectic alloy. No hydrogen embrittlement is observed between 573 and 673 K, indicating that the Nb–NiTi eutectic structure suppresses hydrogen embrittlement of Nb during hydrogen permeation.