Microstructure And Hydrogen Permeability Of Nb-Ti-Ni(Fe) Hydrogen Separation Alloys

Hydrogen energy,as a renewable clean energy,has a broad utilization prospect.At present,membrane sparation technology is the most important way to obtain high purity hydrogen.The 5B group metal in the transition element has high hydrogen permeability,Nb,for example,whose hydrogen permeability coefficient is almost 19times higher than that of pure Pd at 673K,is expected to replace Pd as a new type of hydrogen permeation membrane material in membrane sparation technology.The Nb-Ti-Ni alloy is designed to enhence the embrittlement resistance ability of alloy during hydrogen permeation,which has a good performance in hydrogen permeability and embrittlement resistance.During the design of Nb-Ti-Ni hydrogen sparation alloys,researchers regulate Nb content in alloys to obtain the balance of hydrogen permeability and embrittlement resistance.However,this method has several limitations:the regulation range is narrow,and the alloys need to avoid the appearance of NiTi₂ phase,Nb₈Ti₃Ni₉ phase and other brittle phases.It is necessary to explore a new approach to develop the hydrogen sparation alloys with high hydrogen permeability and high embrittlement resistance.The researchs found that the eutectic of Nb-Ti-Fe alloy exhibits an excellent hydrogen permeability and its microstructure is very similar to the Nb-Ti-Ni alloy.Therefore,the Fe element could be used to regulate the eutectic phase of Nb-Ti-Ni alloy,and the general performance of alloy during hydrogen permeation will be enhanced.As-cast Nb-Ti-Ni?Fe?alloys consists of primary?Nb,Ti?phase and eutectic{TiNi?Fe?+bcc-?Nb,Ti?}?the atomic ratio of Fe/Ni below 0.9?and NbFe phase?the atomic ratio of Fe/Ni greater than 0.9?.The volume fraction is decreased?primary?Nb,Ti?phase?and increased?bcc-?Nb,Ti?in eutectic structure?with increasing Fe content.Besides,the substitution of Ni by Fe resulting in the spheroidization of primary?Nb,Ti?phase and the refinement of eutectic phase.The lamellar spacing of Nb₄₄Ti₂₈Ni₁₈Fe₁₀ alloy eutectic phase is only 0.06?m.Thus,the substitution of Ni by Fe deflects the eutectic ditch in the Nb-rich region of Nb-Ti-Ni phase diagram to the bcc-Nb phase region.The solidification path of the alloy?the atomic ratio of Fe/Ni below 0.9?is as follows:the primary?Nb,Ti?phase precipitate firstly during soldification,as the temperature decreases,binary eutectic reaction occures around the primary?Nb,Ti?phase and eutectic{TiNi?Fe?+bcc-?Nb,Ti?}phase precipitate until the end of solidification.The substitution of Ni by Fe has a minor effect on the hydrogen absorption behavior,only slightly reduces or not decreases the hydrogen concentration in alloys,but enhances the hydrogen diffusivity at high temperature above T_int.The B-T^-1 line for alloy intersects at a certain temperature,which is defiend in this paper as the intersection temperature,T_int.The mobility for hydrdgen diffusion is larger for Fe-mixed alloys than non-Fe alloys.The values of T_int for the systems studied in this paper are estimated to be 552K(Nb₄₀Ti₃₀Ni_30-xFe_x),564K(Nb₄₄Ti₂₈Ni_28-xFe_x)and 611K(Nb₅₆Ti₂₃Ni_21-xFe_x),respectively.The intersection temperature T_int is determined by the balance of the alloying effects on the activation energy and the pre-exponential factor.The substitution of Ni by Fe increases the hydrogen permeation flux of alloys,except Nb₄₄Ti₂₈Ni₁₈Fe₁₀ alloy,in which hydrogen flux was decreased at experimental temperature.The hydrogen flux becomes more sensitive to the temperature change for the Fe-mixed alloys than non-Fe alloys.The substitution of Ni by Fe also increases the hydrogen permeability???of eutectic phase,except Nb₄₄Ti₂₈Ni₁₈Fe₁₀ alloy.The?value of eutectic phase in Nb₄₄Ti₂₈Ni₂₃Fe₅ alloy?the atomic ratio of Fe/Ni is 0.22?is 69% higher than that of Nb₄₄Ti₂₈Ni₂₈ alloy,which reaches the maximum increase range.Besides,the substitution of Ni by Fe reduces the duration of alloys in thermal cycling test.Nb₅₆Ti₂₃Ni₁₆Fe₅ alloy has the shortest duration of 100 minutes,in which hydrogen embrittlement fracture occurs when cooling to 572K.As a result,the substitution of Ni by Fe enhances the hydrogen permeability at high temperature above the intersection temperature,T_int,but with minor embrittlement resistance penalty.As-cast Nb₄₄Ti₂₈Ni₂₃Fe₅ exhibits the best combined ability of hydrogen permeability,3.91×10^-8 mol H₂ m^-1s^-1Pa^-0.5,two times higher than that of pure Pd,and excellent embrittlement resistance,175 minutes in thermal cycling test,as compared to other alloys.