| Pd and its alloys are dense metal membrane materials for high purity hydrogen separation,which has been used in industrials.However,due to the scarce and high cost of Pd,it is urgent to develop a new type metal membrane material.Nb-Ti-Co eutectic alloys for hydrogen separation designed based on the idea for sharing the functions are the potential alternatives for Pd and its alloys.Nb-Ti-Co eutectic alloys are composed of primary bcc-(Nb)phases and [bcc-(Nb)+ B2-Ti Co] eutectics.The primary bcc-(Nb)phases are mainly responsible for hydrogen permeability,while the eutectics mainly impart resistance to hydrogen embrittlement due to its excellent mechanical properties.Nowadays,it is usually to modulate the compositions of Nb-Ti-Co alloys to control the volume fractions of primary phases and eutectic structures for a balance between hydrogen permeability and hydrogen embrittlement resistance.However,the compositions of Nb-Ti-Co alloys for hydrogen separation are limited to a narrow region in the phase diagram.This is extremely limited further optimization of their hydrogen permeability.The Nb-Ti-Co eutectic alloys were selected in this thesis.Based on the idea of simultanously improving the hydrogen embrittlement resistance of primary bcc-(Nb)phases and maximumly strengthening the hydrogen permeability of eutectics,the new concept of modulation of targeted composition and structure is proposed.Through W targeted substituting nib and Fe substituting Co to modulate the transportation properties of primary phases and eutectics and then combining with the transportation model driven by hydrogen chemical potential gradient,a new type Nb(W)-Ti-Co(Fe)hydrogen permeable alloy is developed.Firstly,the solidification paths of Nb-Ti-Co ternary alloys were predicted based on the equilibrium phase diagram coupling with microsegregation.The compositional region of hydrogen permeable alloys with characterization of “primary bcc-(Nb)phases and eutectics” was explored.It is found that the prediction of solidification path was in good agreement with the reported experimental results,which provides the basis and foundation for moudulation of targeted strucutres.Based on the previous work,the Nb48Ti27Co25 alloy with much primary bcc-(Nb)phases was selected.The primary bcc-(Nb)phases of Nb48Ti27Co25 alloy were modulated through W targeted substituting Nb.With the increase of W content,the volume fraction of primary bcc-(Nb)phases increases,while volume fraction of bcc-(Nb)phases in eutectics decreases.And the W mainly dissolves into the primary bcc-(Nb)phases.Secondly,the Nb30Ti35Co35 alloy with fully eutectics was selected.The eutectics of Nb30Ti35Co35 alloy were molulated through Fe targeted substituting Co to investigate their microstructure evolution and hydrogen permeability.The Fe mainly dissolves into B2 type phases in eutectics,but can affect the compositions of bcc-(Nb)phases in eutectics.The fully eutectic structures were found in the alloys with Fe content less than 10 at%,but the primary bcc-(Nb)phases even Nb Fe phases precipitated at the Fe content more than 10 at%.Lastly,selecting the Nb48Ti27Co25 alloy,the structures of primary phases and eutectics were successfully modulated via W targeted substituting Nb and Fe targeted substituting Co,respectively.The hydrogen solubility,hydrogen permeability and hydrogen embrittlement resistance of the above Nb(W)-Ti-Co(Fe)alloys were systematically invesitigated in the temperature range of 523-673 K.The effects of W targeted substituting Nb and Fe targeted substituting Co on hydrogen transportation behaviors of Nb-Ti-Co alloys were also discussed.With the increase of W content,the hydrogen solubility of Nb(W)-Ti-Co alloys decreases,while the hydrogen embrittlement resistance enhances significantly.The complicated changes of hydrogen permeability were found.The hydrogen pemrebility of Nb(W)-Ti-Co alloys decreases with the increase of W content at temperatures above 573 K,but increases at first and then decreases at temperatures below 573 K.The optimized W content is 5 at%.In addition,the Fe substituting Co has little effect on the hydrogen solubility,but can improve the hydrogen permeability.The hydrogen permeability increases with the increase of Fe content,but the growth of hydrogen permeability obviously decreases at Fe content above 10 at%.The Fe substitution also has little effect on hydrogen embrittlement resistance at Fe content less than 5 at%,but the weakened hydrogen embrittlement resistance occurs at Fe content above 5 at%.The Nb(W)-Ti-Co(Fe)alloys can maintain the low hydrogen solubility after W and Fe targeted substitutions,but still obtain high hydrogen permeability in comparison with the original Nb48Ti27Co25 alloy.This may be closely related to the hydrogen diffusion behaviors of Nb(W)-Ti-Co(Fe)alloys.Thus,the Nb43W5Ti27Co20Fe5 alloy with higher hydrogen permeability and stronger hydrogen embrittlement resistance than that of Nb48Ti27Co25 alloy in the temperature ranging from 523 K to 673 K was developed.Considering that the intrinsic driving force is hydrogen chemical potential gradient,a transportation model was built to describe the hydrogen transportation of Nb(W)-Ti-Co(Fe)alloys,which the hydrogen absorption parameter and diffusion parameter were successfully uncoupled.An effective way to balance the hydrogen embrittlement resistance and hydrogen permeability via targeted substitution was found.Based on the transportation model,the hydrogen diffusion coefficinent of Nb-Ti-Co alloys were confirmed,which includes the intrinsic diffusion coefficient and apparent hydrogen diffusion coefficient.Then,the hydrogen diffusion behaviors of Nb(W)-Ti-Co(Fe)alloys were invesitigated systematically,and the complicated changes of hydrogen permeability were clarified clearly.W targeted substituting Nb can reduce the activation energies for hydrogen diffusion,which promotes the intrinsic hydrogen diffusion coefficient.Thus,the hydrogen permeability of Nb(W)-Ti-Co can be improved even at the low hydrogen solubility and simultaneously strong hydrogen embrittement resistance.Fe targeted substituting Co can maitain the originally low hydrogen solubility and improve intrinsic hydrogen diffusion coefficient of Nb-Ti-Co(Fe)alloys,which leads to the improved hydrogen permeability.Based on the transportation model driven by hydrogen chemical potential gradient and the first principle calculation,the hydrogen transportation mechanisms of the alloys were analyzed from the view point of the hydrogen gap dissolution and minimum energy path(MEP)for hydorgen diffusion.The internal mechanisms of reducing hydorgen solubility and simultaneously improving hydrogen diffusion after the mudulation of targeted structures via W/Nb and Fe/Co was revealed.According to the calculated results of absorption energy of hydrogen atom and charges gained from the surrounding atoms,the reduced binding energy between hydrogen atoms and interstices after the W targeted substituting Nb was found.Then,the minimum energy path for hydorgen diffusion was confirmed.The W substituting Nb can contribute to the optimized hydrogen diffusion path and the reduced hydrogen diffusion barrier. |
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