High temperature steam oxidation of Al_3Ti-based alloys for the oxidation-resistant surface layer on Zr fuel claddings

摘要

We investigated the feasibility to apply Al_3Ti-based alloys as the surface layer for improving the oxidation resistance of Zr fuel claddings under accident conditions. Two types of Al_3Ti-based alloys with the compositions of Al-25Ti-10Cr and Al-21Ti-23Cr in atomic percent were prepared by arc-melting followed by homogenization annealing at 1423 K for 48 h. Al-25Ti-10Cr alloy showed an L1_2 quasi-single phase microstructure with a lot of needle-shaped minor phase and pores. Al-21Ti-23Cr alloy consisted of an L1_2 matrix and Cr_2Al as the second phase. Al _3Ti-based alloys showed an extremely low oxidation rate in a 1473 K steam for up to 7200 s when compared to Zircaloy-4. Both alloys exhibited almost the same oxidation rate in the early stage of oxidation, but Al-25Ti-10Cr showed a little lower oxidation rate after 4000 s than Al-21Ti-23Cr. The difference in the oxidation rate between two types of Al_3Ti-based alloys was too marginal to distinguish the oxidation behavior of each alloy. The resultant oxide exhibited almost the same characteristics in both alloys even though the microstructure was explicitly distinguished from each other. The crystal structure of the oxide formed up to 2000 s was identified as Al _2O_3 in both alloys. The oxide morphology consisted of columnar grains whose length was almost identical to the average oxide thickness. On the basis of the results obtained, it is considered that Al _3Ti-based alloy is one of the promising candidates for the oxidation-resistant surface layer on Zr fuel claddings.