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

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

“…In a general view taking into account the values of F, K and D, it is concluded that the lower permeability caused by the microstructural change from a lamellar to the duplex microstructure is mainly attributed to the significant reduction of hydrogen diffusivity after annealing. The similar case was observed in NbeTieCo [11], NbeWeMo [18] and CreMo steel [19] alloys.…”

“…Such a microstructural change during annealing is attributed to the simultaneous coarsening of the both phases. Similar coarsening phenomenons were also observed in eutectic Nb 19 Ti 40 Ni 41 [10] and Nb 30 Ti 35 Co 35 [11] after annealing. A Hf 2 Co phase additionally appears in a-samples at 1273 K for 168 h, Fig.…”

“…Finally, we emphasize the superiority and difference of the as-cast (or a-) Nb 30 Hf 35 Co 35 alloy, respect to the previous ones in the field. Firstly, the as-cast Nb 30 Hf 35 Co 35 alloy exhibits a stable permeability and also possesses pronounced high hydrogen permeability of 3.3 Â 10 À8 mol H 2 m À1 s À1 Pa À0.5 , compared with the as-cast Nb 19 Ti 40 Ni 41 and Nb 30 Ti 35 Co 35 alloys [10,11]. High permeability will promote this alloy to meet the US DOE performance targets [27].…”

“…Wang et al [10] investigated the microstructural change and its effect on hydrogen permeability in as-cast eutectic Nb 19 Ti 40 Ni 41 alloy by annealing, and found that the initial fully lamellar eutectic microstructure turned into a duplex phase by annealing at 1373 K for 168 h. Correspondingly, its hydrogen permeability at 573 K, F 573 K , is about one half of the as-cast state. Subsequently, Li et al [11] studied the microstructural stability and its effect on hydrogen permeability in eutectic Nb 30 Ti 35 Co 35 alloys. The similar transformation of the eutectic morphology is also found in the as-cast samples after annealing, which is accompanied by a decrease in hydrogen permeability.…”

“…A further study about this problem will accelerate its potential implication in industry. If this alloy exhibits a high F value compared to the eutectic Nb 30 Ti 35 Co 35 [11] and Nb 19 Ti 40 Ni 41 [10] alloys after annealing, then we may use the rapid quenching technique to prepare thin Nb-HfCo amorphous membrane. However, amorphous alloys are usually thermally unstable and their desirable properties may lost by crystallization under different annealing temperatures [8,14].…”