Hydrogen Solubility And Diffusivity In Nb-Ti(Hf)-Co Eutectic Alloys

Recently, one kind of new hydrogen permeable alloys such as Nb-Ti-Ni、Nb-Ti-Co、Nb-Hf-Ni and Nb-Zr-Ni has been developed, which exhibits an excellent balance between hydrogen permeability and embrittlement resistance. Thses alloys are based on appropriately alloying by Ti、Ni、Co and Hf in group 5B metals such as Nb and V, which results in an eutectic structure. In this work, phase and microstructure of as-cast Nb30Ti35-xHfxCo35(x=0, 10, 17.5) alloys were firstly investigated by SEM and XRD. Then, a Sieverts-type apparatus was used to characterize the hydrogen solubility of as-cast Nb30Ti35-xHfxCo35(x=0, 10, 17.5) alloys. Finally, the diffusion behaviour of hydrogen in these alloys was investigated by a time-lage method.As-cast Nb30Ti35-xHfxCo35(x=0, 10, 17.5) alloys consist of a fully eutectic structure. Typically, Nb30Ti35Co35 and Nb30Ti25Hf10Co35 consist of [bcc-Nb + B2-Ti Co] and [bcc-Nb + B2-(Ti, Hf)Co], respectively, and Nb30Ti17.5Hf17.5Co35 consists of [bcc-Nb + orthorhombic-(Hf, Ti)Co]. It should be noted that the(Ti, Hf)Co originates from the partitial replacement of Ti by Hf in B2-Ti Co lattice, but the(Hf, Ti)Co originates from the partitial replacement of Hf by Ti in orthorhombic-Hf Co lattice. Therefore, the B2-(Ti, Hf)Co remains ductile as B2-Ti Co, but the orthorhombic-(Hf, Ti)Co is brittle intrinsically as orthorhombic-Hf Co. From this point, the content of Ti in hydrogen permeable Nb30Ti35-xHfxCo35 alloys should be lower than that of Hf to improve the embrittlement resistance.The dissolved hydrogen content in as-cast Nb30Ti35-xHfxCo35(x=0, 10, 17.5) alloys increases with decreasing temperature, which indicates an exothermic reaction of the present hydrogen dissolution. The hydrogen solubility in the alloys increases with the content of Hf. Typically, the hydrogen solubility coefficient of Nb30Ti17.5Hf17.5Co35 at 573 K is 17.75 mol H2 m-3 Pa-1/2. This is 1.3 times that of Nb30Ti35Co35 and 4.5 times that of pure Pd. The XRD results of hydriding Nb30Ti35Co35 and Nb30Ti25Hf10Co35 alloys show that the hydrogen dissolution in bcc-Nb in their microstructure induces the formation of Nb-H hydride. There is no hydrogen dissolution in B2-Ti Co and B2-(Ti, Hf)Co. In contrary, there is significant hydrogen dissolution in both bcc-Nb and orthorhombic-(Hf, Ti)Co for Nb30Ti17.5Hf17.5Co35. This induces the formation of Nb-H and(Hf, Ti)Co H hydrides. Based on the microstructure and hydrogen solubility of Nb30Ti35-xHfxCo35(x=0, 10, 17.5) alloys, it can be concluded that Nb30Ti25Hf10Co35 exbibits high potential as hydrogen permable alloy to reach an excellent balance between hydrogen permeability and embrittlement resistance.The apparent diffusivity DH and lattice diffusivity D*H of hydrogen in Nb30Ti35Co35 and Nb30Ti25Hf10Co35 were derived by a time-lag method. It shows that the substitution of Hf by Ti induces an increase of distance between institutial sites in bcc-Nb lattice and thus a decrease of hydrogen diffusivity. The apparent hydrogen diffusivity increases with increasing temperature and dissolved hydrogen content. Typically, the value of DH increases to 12.76×10-10 m2 s-1 when dissolved hydrogen content increasing from 0.1242 to 0.24654, which is 3.2 times larger. The lattice hydrogen diffusivity remains constant at one certain temperature.