Study On Microstructure And Hydrogen Transport Behaviors Of Nb(V)-Ti-Co Alloys

Hydrogen separation technology is now widely used in hydrogen purification,and Pd-based metal hydrogen separation membranes are commercially available.However,its cost,scarcity,and durability limitations make it unsuitable for large-scale commercial applications.Because of their excellent hydrogen permeability and mechanical properties,5B group metals are expected to be a viable alternative to Pd and its alloys.Among them,the Nb-Ti-Co and V-Ti-Co hydrogen separation alloys have high hydrogen permeation and hydrogen embrittlement resistance and are considered to be very promising hydrogen separation alloys for research.However,so far,the persistent properties of the above-mentioned alloy films need to be improved and the reasons for the different hydrogen seepage properties in the two alloy systems are still unclear.Based on this,a series of research works on Nb（V）-Ti-Co hydrogen separation alloys have been carried out in this paper,focusing on their phase diagram,microstructure,hydrogen transport behaviour,mechanical properties and hydrogen embrittlement mechanism,as follows:1.Based on the previous research of this group,the microstructure,mechanical properties and hydrogen permeation properties of Nb-Ti-Co alloys near the eutectic point were explored,and the results showed that all the alloys were composed of the primary phase（Ti Co phase/bcc-（V,Ti）phase）and eutectic{bcc-（V,Ti）+Ti Co}phase,and the eutectic phase in the alloys gradually decreased with the change of Nb content.The Nb₅₅Ti_22.5Co_22.5 alloy contains the most bcc-（V,Ti）phase.Furthermore,the alloy’s hydrogen uptake increases with increasing temperature and with increasing Nb content.Correspondingly,the alloy’s hydrogen permeation performance improves with increasing bcc-（V,Ti）phase,with the Nb₅₅Ti_22.5Co_22.5 alloy having the best hydrogen permeation performance of 3.64×10^-8mol·H₂·m^-1·s^-1·Pa^-0.5 at 673 K,which is approximately 2.3 times that of pure Pd under comparable experimental test conditions.2.On the basis of the above work,V was added to the Nb-Ti-Co alloy to form a new（V,Nb）-Ti-Co alloy,and its microstructure evolution,hydrogen permeation properties and hydrogen dissolution properties were systematically studied.The results show that with the addition of V,impurity phases（Co₂（V,Nb）₃,Co₇Nb₆）gradually appear in the alloy,and with the gradual replacement of Nb,the hydrogen absorption of the alloy decreases,while the hydrogen permeation properties first decrease and then increase.At 673 K,the hydrogen permeability of the Nb₃₅Ti_32.5Co_32.5 alloy was 2.77×10^-8 mol·H₂·m^-1·s^-1·Pa^-0.5,which is approximately 1.7 times that of pure Pd.In addition,combined with the PCT curves of（V,Nb）-Ti-Co alloys,the DBTC interval of the alloys was initially explored in addition to that of about 0.36 H/M.This value widens the gap with the DBTC of V-based single-phase alloys,which provides a basis and reference for the design of alloys with high hydrogen embrittlement resistance.3.The phase diagram of the V-Ti-Co alloy and the microstructure,mechanical properties,corrosion properties,and hydrogen permeation properties of the V_23.5+xTi₅₀Co_26.5-x（x=-6,-3,0,3,6,9）alloy were investigated based on the study of the（V,Nb）-Ti-Co alloy system.The alloy phase diagram contains eight phase regions（bcc-（Nb,Ti）,Ti Co,Sigma（Co V）,fcc,Ti Co₂（h）,Ti Co₂（c）,Ti Co₃ and Ti₂Co）and six equilibrium solidification reactions:U₁（L+Ti Co→Ti Co₂（h）+Ti Co₂（c））,U₂（L+sigma（Co V）→bcc-（V,Ti）+Ti Co）,U3（L+Ti Co→Ti Co2（h）+sigma（Co V））,U₄（L+Ti Co₃→Ti Co₂（h）+fcc）,P₁（L+Ti Co+bcc-（V,Ti）→Ti₂Co）,E₁（L→Ti Co₃+Ti Co₂（h）+sigma（Co V））.When the V content of the V23.5+x Ti50Co26.5-x alloy is 23.5 at%,the eutectic bcc-（V,Ti）+Ti Co structure forms,and the precipitation of incipient Ti Co and bcc-（V,Ti）phases occurs when the content is less than or greater than 23.5 at%.Among the six alloys,V23.5Ti50Co26.5 has the best mechanical and corrosion resistance properties,which are primarily due to the alloy’s high eutectic content.Furthermore,the alloy has the best hydrogen permeation properties,with theΦvalue of 4.05×10^-8 mol·H₂·m^-1·s^-1·Pa^-0.5 at 673 K,which is 2.5 times greater than pure Pd in the same situation.