The present work is concerned with the effects of composition and heat treatment on the microstructure and corrosion behaviour of Zr based Nb alloys. The hydrogen uptake into the alloys during corrosion process was also investigated to understand the kinetics and mechanism of oxide formation of Zr based Nb binary alloys.
The structure of Zr based Nb alloys can consist of several different phases, each containing various concentrations of Nb and possessing different microstructures. The β-quenching of the 1 and 2.5 wt% Nb alloys resulted in the martensitic α'-Zr phase. ω-phase was formed with the martensitic α'-Zr by water quenching for the 5 wt% Nb alloy and β-Zr phase was retained for the alloys containing 10 wt% Nb or more. Through ageing of the quenched materials, unstable phases were decomposed into stable α-Zr and β-Zr phases. The morphology of β-Nb precipitates depended on the prior structure. Needle shaped β-Nb particles within the matrix between twin boundaries and equiaxed particles at twin boundaries were precipitated from the martensitic α'-Zr phase. However, agglomerated β-Nb particles were precipitated from the alloys having β-Zr phase. The Nb content of precipitated β-Nb phase was changed from 81.9 wt% to 93 wt% according to the initial Nb content of the alloy. The annealing process resulted in a mixed structure of α-Zr and β-Zr in the alloys with Nb contents less than 10 wt%. The β-Zr phase changed from a discrete form to a continuous film network along the α-Zr grain boundaries as the Nb content increased. The annealed 15 and 20 wt% Nb alloys consisted only of the β-Zr phase. The X-ray diffraction lines for the annealed 10 and 15 wt% Nb alloy were very broad and this line broadening was attributed to the non-uniform Nb distribution in the matrix.
The corrosion behaviour of the Zr-Nb alloys is clearly a function of both its Nb content and heat treatment. The formation of the martensitic α'-Zr phase by β-quenching treatment results in high weight gains at all test temperatures. Up to 5 wt% Nb, the corrosion resistance of the quenched alloy decreases linearly with increasing Nb contents. The corrosion rates of the specimens containing the metastable β-Zr phase were higher than that of the alloys having stable α-Zr and β-Nb phases, but lower than that of the alloys with martensitic α'-Zr. The corrosion rates of the specimens increased as the Nb content in the β-Zr phase deviates from the equilibrium value(~20 wt% Nb) and seemed to have similar weight gains irrespective of the amount of β-Zr phase if they form a continuous β-Zr network. Through ageing of the β-quenched specimens, the corrosion resistance was greatly improved due to the reduction of Nb supersaturation in the matrix phase. The agglomerated large β-Nb precipitates had detrimental effects on the corrosion resistance as compared to fine precipitates. Under the neutron irradiation(to $1.56×10^20$ fast neutrons/㎠ ＞ 1 MeV), the corrosion resistance of quenched specimens was sharply improved, but neutron flux had little or no effect on the aged ones. This neutron irradiation effect could be explained by the in-reactor ageing process.
The weight gain of the alloys in $H_2O$ steam exceeds that of the alloys in $D_2O$ steam, irrespective of Nb content and heat treatment. From the experimental results, it is suggested that the oxidation rate of Zr based Nb alloys is controlled by cathodic reduction of hydrogen ion. The Nb content raises the weight gain of the quenched alloys consisting of a martensitic α'-Zr phase, but it reduces the percentage hydrogen uptake into the α'-Zr phase. The weight gain of the aged alloys composed of α-Zr and β-Nb phases is exceeded by that of the quenched alloys, but the percentage hydrogen uptake into the aged alloys exceeds that into the quenched alloys. The results were discussed with respect to the aliovalent effects of Nb in matrix α- or α'-Zr matrix phase on the weight gain and percentage hydrogen uptake, respectively. The best alloy composition of Zr-Nb binary system is proposed as the aged binary alloy less than 5 wt% Nb from the view points of the weight gain, hydrogen uptake, and microstructure.
The Zr-Nb alloys showed uniform corrosion behaviour in the high temperature and high pressure water($H_2O$ and $D_2O$), there was no apparent rate law transition in the corrosion kinetics. The corrosion rates of the alloys at 400℃ under 10 MPa $H_2O$ steam could be expressed to the simplified form ΔW= $k·^n$, with the exponent n lying in the range 0.26-0.76. The variation of the n value seems to be related to the changes in the activation energy(30.1-54.4 kJ/mol) for the weight gain in $D_2O$ atmosphere. The experimental results indicate that the oxidation kinetics and mechanism of the Zr-Nb alloys changed according to the microstructure of the alloys.
