Microstructure and hydrogen permeability in Nb–Ti–Co multiphase alloys

Microstructure and hydrogen permeability (Φ) of as-cast Nb–Ti–Co alloys have been investigated to find out alloys combining high hydrogen permeability and large resistance to the hydrogen embrittlement by scanning electron microscopy (SEM) and by the gas permeation technique, respectively. Hydrogen permeable alloys are obtained near the line connecting the TiCo compound and pure Nb, and about 50 mol% Nb and less. These alloys are classified into three groups, i.e. (1) the primary TiCo phase and the eutectic {TiCo + (Nb, Ti)} phase, (2) TiCo + (Nb, Ti) + the eutectic {TiCo + (Nb, Ti)} phase and (3) the primary (Nb, Ti) + the eutectic {TiCo + (Nb, Ti)} phase. The present work indicates that the eutectic phase suppresses the hydrogen embrittlement of the Nb–Ti–Co alloys. The eutectic Nb30Ti35Co35 alloy exhibits large resistance to the hydrogen embrittlement and the highest Φ of 2.64 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 1.7 times higher than that of pure Pd.