Solidification Path, Microstructures And Hydrogen Permeability Of Nb-Ni(Co)-Ti(Hf) Alloys

In recent years, the world’s energy strategy focuses on developing clean, non-toxic, harmless and pollution-free renewable new energy due to the exhaustion of fossil fuel and the increasingly serious environmental problems. Hygrogen gas has attracted much attention for its advantages as clean, non-toxic and harmless eaergy, which can be used in many fields of semiconductor industry, petrochemical industry, aerospace and fuel cells. The high hydrogen permeability and high resistance to hydrogen embrittlement of Pd and Pd-based alloys (particularly Pd-Ag alloys) allows them widely used as membranes for separation and purification of hydrogen gas in industry. However, Palladium is too expensive for large-scale industrial applications as hydrogen permeation membranes. The development of alternative membrane materials with higher hydrogen purification efficiency and lower cost is therefore strongly desired. From the viewpoint of the hydrogen permeability, some BCC (body-centered cubic) metals of group5, such as Nb, V and Ta, are promising candidates for their high hydrogen permeability, even larger than that of Pd. However, these pure metals and their single-phase alloys suffer from serious embrittlement druing the hydrogen permeation. Therefore, it is strongly desired to develop new alloys around group5B metals for hydrogen permeation which posseses both high hydrogen permeability and high resistance to hydrogen embrittlement.The ternary eutectic alloy microsegregation model and the corresponding algorithms coupling with Thermo-Calc software were built for predicting the solidification path of single-phase solidification, two-phase eutectic and three-phase eutectic solidification. The preidcted solidification paths for different Nb-Ni-Ti alloys correspond well with the experimental results. The compositional window with primary Nb(Ti, Ni) and eutectic {Nb(Ti，Ni)+TiNi} phases in Nb-Ni-Ti phase diagram was found. The hydrogen permeability of alloys in the window was characterized systematically. It is found that the variation of the hydrogen permeability of the alloys in this window is surprisingly high. High Nb content and Ni/Ti ratio lead to a high permeability. Nb55Ni20Ti25shows the highest permeability at673K, particularly2.9×108mol H2m-1s-1Pa-1/2. This is about1.8times higher than that of pure Pd. This work provides a new idea to design hydrogen permeable alloys by correlating the microstructures, solidification paths and hydrogen permeability.Based on the above developed Nb-Ni-Ti alloys, the element substitution method (Ti by Hf and Co by Ni) and directional solidification technology, were introduced to improve the hydrogen permeability. It is found that several new hydrogen permeable alloys, such as Nb-Ni(Co)-Ti, Nb-Ni-Ti(Hf) and Nb-Co-Hf, were developed. These alloys possess higher hydrogen permeability than that of Nb-Ni-Ti and also excellent resistance to hydrogen embrittlement. This can be attributed to the high hydrogen solubility and diffusivity in the eutectic microstructure. Typically, Nb40Hf30Co30alloy exhibits a pronounced high hydrogen permeability,4.96×108mol H2m-1s-1Pa-0.5at673K, which is3.5times higher than that of pure Pd. Alloys with directionally solidified microstructures are also found to possess significantly high hydrogen permeability because of the enhacement of hydrogen diffusion. Typically, Nb30Ti35Co35consisting entirely of eutectic exhibits the permeability,2.65×108mol H2m-1s-1Pa-0.5at673K. This permeability is further increased to3.58×108mol H2m-1s-1Pa-0.5by aligning eutectic grains perpendicularly to the membrane surface using directional solidification.Based on Fick diffusion law, the hydrogen permeation model for grouple5B alloy membrane was established and the "inflection point-tangent method" was deduced for calculation of the hydrogen diffusion coefficient and solubility coefficient for Nb-Ni(Co)-Ti，Nb-Ni-Ti(Hf) and Nb-Co-Hf system, in oder to investigate its influence on hydrogen permeability and hydrogen permeation mechanism. The main factors affecting the above two coefficients were also analyzed and summarized. Hydrogen diffusion coefficient is mainly related to two factors: one is the Nb phase distribution and its volume fraction, the other is related to the atomic radius size of the replacement elements. Hydrogen solubility coefficient is mainly depended on the gap size in the unit cell. The hydrogen permeation mechanism and the main factors influencing the hydrogen permeability of the above alloy membrane are elucidated by "resistance" model and "atomic dissolve" model.