Relationship between hydrogen permeation and microstructure in Nb–TiCo two-phase alloys

• Relationship between microstructure and permeability is theoretically analyzed.
• Type and volume fraction of primary phase can be controlled by alloy composition.
• Permeability of Nb–TiCo with primary (Nb, Ti) phase does not obey the mixing rule.
• Permeability of Nb–TiCo with primary TiCo phase is higher than that of expected one.

The relationship between the microstructure and hydrogen permeability of as-cast Nb–TiCo alloys was investigated and is discussed on the basis of the mixing rule. The chemical compositions of these Nb–TiCo alloys may be expressed as (Nb4Ti47Co49)1−x(Nb90Ti8Co2)x. The alloys for which x = 0 and 1 exhibit essentially single TiCo and (Nb, Ti) phases, respectively. The microstructure of the alloy for which x = 0.3 is fully eutectic, and is composed of (Nb, Ti) and TiCo phases in the as-cast state. In the range of 0.3 < x < 1, the Nb–TiCo alloys present a primary (Nb, Ti) phase surrounded by a TiCo + (Nb, Ti) eutectic structure, and their hydrogen permeability values do not obey the mixing rule. In the region of 0 < x < 0.3, the alloys have a primary TiCo phase, and the hydrogen permeability values of these alloys are slightly higher than the values estimated using the mixing rule.