Microstructure And Hydrogen Permeability Of Nb-Ti(Zr)-Co Alloys

Microsturcture and hydrogen permeability of as-cast Nb-Ti(Zr)-Co alloys induced by the substitution of Ti by Zr were investigated in this work. This involves 18 alloys, which can be divided into four types, termed as ―S-type‖、―A-type‖、―B-type‖ and ―C-type‖. Their phase and microstructure were investigated by SEM and XRD firstly, and thus the hydrogen permeable alloys in Nb-Ti(Zr)-Co system were determined which consist of an appropriate structure. Then, the correlation among composition, microstructure, hydrogen permeability, solubility and diffusivity was built.For Nb-Ti(Zr)-Co alloys with an equal ratio between Ti(Zr) and Co content, i.e. S-type alloys, the as-cast microstructure mainly consists of primary bcc-Nb and eutectic [bcc-Nb + B2-(Ti, Zr)Co]. Additionally, brittle(Zr, Ti)2Co appears at the boundaries of eutectic grains. The increase of Zr content induces higher volume fraction of primary bcc-Nb and brittle(Zr, Ti)2Co, but a lower one of bcc-Nb in the eutectic. Correspondingly, the eutectic morphology changes from lamellae to rods.For Nb-Ti(Zr)-Co alloys which satisfy the following conditions, they consist of the appropriate microstructure for hydrogen permeation, i.e.,(1) the ratio between Ti(Zr) and Co content lower than 1 but larger than 0.6;(2) the Nb content larger than 30at%;(3) the substitution of Ti by Zr limited in the range of 5at%Zr. The above A-type alloys consist of primary bcc-Nb and eutectic [bcc-Nb+B2-(Ti, Zr)Co] or fully eutectic. Among them, Nb35Ti20Zr5Co40(A1) is the eutectic composition. The substitution of Ti by Zr induces higher volume fraction of primary phase, but the decrase of the ratio between Ti(Zr) and Co content decreases volume fraction of primary phase. When the ratio between Ti(Zr) and Co content lower than 0.6, the primary bcc-Nb changes to primary B2-(Ti, Nb, Zr)Co or Nb6Co7(B-type alloys). When the Zr content reaches to 10at% in A-type alloys(C-type alloys), they mainly consist of primary bcc-Nb/Nb6Co7 and eutectic [bcc-Nb+B2-(Ti, Nb, Zr)Co]. In comparison with the A-type alloys, the C-type alloys consist higher fraction of primary bcc-Nb, but the bcc-Nb in eutectic structure reduces, and the B2-(Ti, Zr)Co in eutectic changes to B2-(Ti, Nb, Zr)Co.The hydrogen solubility and permeability are characterized for three hydrogen permeable alloys determined above, i.e., Nb35Ti23Zr5Co37、 Nb40Ti20Zr5Co35 and Nb35Ti18Zr10Co37. It shows that the hydrogen solubility increases with increasing Zr content, but the hydrogen diffusivity decreases. Nb35Ti18Zr10Co37 exhibits the highest hydrogen permeablility, typically 3.5×10-8 mol H2 m-1 s-1 Pa-0.5 at 673 K. This is 2.19 times that of pure Pd.