Nb-Ti-Co氢分离合金近共晶点处的显微组织及其渗氢性能

MICROSTRUCTURE AND HYDROGEN PERMEATION CHARACTERISTIC OF NEAR EUTECTIC Nb-Ti-Co HYDROGEN SEPARATION ALLOY

探寻了Nb-Ti-Co氢分离合金的共晶点成分并利用Bridgman定向凝固实验对其进行验证,研究了近共晶点处9种合金的显微组织、氢渗透性能及氢脆现象,并与贵金属Pd的氢渗透性能进行比较.结果表明,Nb-Ti-Co三元合金中完全由共晶Nb[Ti,Co)+TiCo相构成的合金成分为Nb_(31)Ti_(35)Co_(34);当Bridgman定向凝固实验的抽拉速率为5μm/s时,共晶组织中的2相呈现岀规则的共生生长.9种合金中完全由共晶相构成的合金在673.5 K具有最大的氢渗透系数2.7×10^(-8)mol/(m·s·Pa^(0.5)),是相同条件下Pd的氢渗透系数的1.72倍;Nb含量相同时,随着Ti/Co比值的降低,氢渗透系数逐渐减小.氢渗透过程中,合金膜内部的初生TiCo相作为裂纹源首先萌生裂纹,而后以此发生二次裂纹现象并逐渐向膜边缘扩展;当TiCo相体积分数小于5%时,共晶Nb(Ti,Co)+TiCo相抵消原有初生TiCo相上的裂纹源,使得合金膜具有良好的抗氢脆性能.

Because of the high hydrogen permeability and high resistance to hydrogen embrittlement of Pd- based alloys, Pd-Ag alloys have been used as hydrogen permeation membranes for separation and purification of hydrogen gas. However, Pd is too expensive for large-scale industrial applications as hydrogen permeation mem- branes. The development of alternative membrane materials with higher hydrogen purification efficiency and lower cost is therefore strongly desired. From the viewpoint of the hydrogen permeability, some bcc metals such as Nb, V and Ta are promising candidates since their predicted hydrogen permeability are larger than that ofPd. Nb-Ti-Co ternary alloy has been considered as the best candidate because of lower cost, good thermal stability, the high hy- drogen permeability and high resistance to hydrogen embrittlement. In this work, the eutectic composition in Nb-Ti- Co alloy was probed and verified through Bridgman directional solidification experiment. Microstructures, hydro- gen permeation properties and hydrogen embrittlement of near eutectic Nb-Ti-Co alloy were investigated and com- pared with those of pure Pd. The results indicate that the eutectic composition in Nb-Ti-Co alloy is Nb31Ti35Co34, and its solidification microstructures contain only the Nb（Ti, Co） and TiCo phase, which shows regular coupled growth by Bridgman directional solidification at the rate of 5 μm/s. The eutectic alloy shows the higher hydrogen permeability of 2.7 × 10.8 mol/（m s Pa0.5） at 673.5 K, which is 1.72 times higher than that of Pd alloy. The hydrogen permeability decreases with the decrease of Ti/Co ratio at the same Nb content. A slight crack first appears in the Ti- Co phase, which is the source of cracks in the membrane, and then the cracks propagate along the edge of the mem- brane. When the volume fraction of TiCo phase is less than 5%, the eutectic Nb（Ti, Co）＋TiCo phases may offset the crack in the TiCo phase, thus the membrane exhibits large resistance to the hydrogen embrittlement.