Microstructure And Hydrogen Transport Properties Of Nb-Pd-Ti-Co Hydrogen Permeable Alloys

The hydrogen permeable alloys Nb-Ti-Co exhibit excellent hydrogen permeation properties,but the embrittlement resistance of the alloys is insufficient because of the high hydrogen solubility of eutectic structure.During hydrogen permeation tests at high temperatures,the hydrogen flux of Nb alloy membranes drastically decreases over time.The decrease in hydrogen flux appears to be associated with the interdiffusion between palladium coating layers and Nb alloy substrates.In this paper,the substitution of Ti and Co by Pd formed Nb-Pd-Ti-Co alloys.The microstructure and hydrogen transport prot erties were studied.The effects of Pd on the properties of the alloys were investigated,and hydrogen permeation materials with high hydrogen permeation properties,good high temperature hydrogen permeation characteristics and excellent embrittlement resistance were obtained.The results of XRD,SEM and EDS analysis of Nb30 Pdx Ti35-x/2Co35-x/2(x=0,5,10,15,20)show that the main phases composition of the alloys are bcc-Nb phase and B2-Ti Co phase while the addition of Pd is not more than 10%.The microstructure of alloys is still full eutectic structure,and Pd mainly dissolved in the B2-Ti Co phase.The addition of Pd increases the lamellar distance of eutectic structure and coarsen the bcc-Nb and B2-Ti Co phases at grain boundaries.While the addition of Pd is more than 10%,the alloys produce primary B2-Ti Co and primary B2-Ti Co increases with the increase of Pd.The results of Hydrogen dissolution experiment show that the addition of Pd can effectively reduce the hydrogen solubility of the eutectic composites.The hydrogen permeability of alloys reduces with the increase of Pd.With the gradual replacement of Ti and Co in alloys,the hydrogen diffusion coefficient first decreases and then increases.Compared with the Nb30Ti35Co35 alloy,the alloy with Pd exhibits better hydrogen embrittlement resistance,that is,the addition of Pd can further improve the hydrogen embrittlement resistance of the alloy.After six hours of the high temperature hydrogen permeation experiment,the hydrogen flux of Nb30Ti35Co35 alloy decreased by 73.31%,but the Nb30Pd10Ti30Co30 only decreased by 38.72%.The addition of Pd can effectively reduce the interdiffusion between palladium coationg layers and Nb-Ti-Co alloy substrates,and significantly improve high temperature hydrogen permeation characteristics.The effect of Pd on the hypoeutectic hydrogen permeable alloys was investigated.Nb52 Pdx Ti25-25x/48Co23-23x/48(x=0,5,10)alloys remain double phase structure.The addition of Pd has no effect on the primary bcc-Nb phase,but increases the lamellar distance of eutectic structure and coarsen the bcc-Nb and B2-Ti Co phases at grain boundaries.While the addition of Pd is 5%,Pd is mainly dissolved in eutectic structure and hardly dissolved in primary Nb phase.While Pd is 10%,the Pd content in primary Nb phase is increased.The addition of Pd can reduce the hydrogen solubility of the Nb52Ti25Co23 alloy.The hydrogen permeability of Nb52Pd5Ti22.4Co20.6 alloy decreases slightly compared with Nb52Ti25Co23 alloy,but the hydrogen embrittlement resistance is improved.It indicates that the eutectic structure is regulated by the addition of Pd,and the ability to resist hydrogen embrittlement of the eutectic structure is improved.The hydrogen permeability of Nb52Pd10Ti19.8Co18.2 alloy is greatly reduced,which indicates that the addition of Pd can't be too much.After six hours of the high temperature hydrogen permeation experiment,the hydrogen flux of Nb52Ti25Co23 alloy decreased by 76.11%,but the Nb52Pd10Ti19.8Co18.2 only decreased by 49.87%.The addition of Pd can effectively improve high temperature hydrogen permeation characteristics of Nb52Ti25Co23 alloy.In these alloys,Nb52Pd5Ti22.4Co20.6 alloy has good comprehensive properties.At 673 K temperature,the hydrogen permeation coefficient is 2.78×10-8 mol H2 m-1s-1Pa-0.5,which is 1.74 times of pure Pd.Hydrogen embrittlement did not occur after cooling to room temperature.The hydrogen permeation coefficient decreased by 62.48% after 6 h high temperature hydrogen permeation experiment.