The effect of Bi addition(0.05 wt%-0.50 wt%)on the microstructure of zirconium alloys,including T5(Zr-0.7 S n-1.0 Nb-0.3 Fe-0.1 Cr),S5(Zr-0.8 Sn-0.35 Nb-0.4 Fe-0.1 Cr),Zr-4(Zr-1.5 Sn-0.2 Fe-0.1 Cr) and Zr-1 Nb,was investigated by transmission electron microscopy(TEM),energy-dispersive spectroscopy(EDS) and selected area electron diffraction(SAED).Results show that with the increase in Bi content,h-Zr(Fe,Cr,Nb)2,o-Zr(Fe,Sn,Bi)_(2) and Zr-Fe-Cr-Nb-Sn-Bi second-phase particles(SPPs) precipitate successively in the T5+xBi and S5+xBi alloys;in the Zr-4+xBi alloys,h-Zr(Fe,Cr)2,o-Zr(Fe,Sn,Bi)2,Zr-Fe-Cr-Sn-Bi and Zr-Fe-Cr-Bi SPPs are detected successively.While as for Zr-1 Nb+xBi alloys,Bi-free SPPs appear.The addition of Bi promotes the precipitation of SPPs with Sn in the alloys.The concentration of Bi dissolved in α-Zr matrix increases with the decrease in Sn content in the alloys.Adding reasonable Bi has little influence on the solid solution content of Nb in α-Zr matrix.
In order to optimize the microstructure and composition of N18 zirconium alloy （Zr-1Sn-0.35Nb-0.35Fe-0.1Cr, in mass fraction, %）, which was developed in China in 1990s, the effect of microstructure and composition variation on the corrosion resistance of the N18 alloy has been investigated. The autoclave corrosion tests were carried out in super heated steam at 400 ~C/10.3 MPa, in deionized water or lithiated water with 0.01 mol/L LiOH at 360 ~C/18.6 MPa. The exposure time lasted for 300-550 days according to the test temperature. The results show that the microstructure with a fine and uniform distribution of second phase particles （SPPs）, and the decrease of Sn content from 1% （in mass fraction, the same as follows） to 0.8% are of benefit to improving the corrosion resistance; It is detrimental to the corrosion resistance if no Cr addition. The addition of Nb content with upper limit （0.35%） is beneficial to improving the corrosion resistance. The addition of Cu less than 0.1% shows no remarkable influence upon the corrosion resistance for N18 alloy. Comparing the corrosion resistance of the optimized N18 with other commercial zirconium alloys, such as Zircaloy-4, ZIRLO, E635 and Ell0, the former shows superior corrosion resistance in all autoclave testing conditions mentioned above. Although the data of the corrosion resistance as fuel cladding for high burn-up has not been obtained yet, it is believed that the optimized N18 alloy is promising for the candidate of fuel cladding materials as high burn-up fuel assemblies. Based on the theory that the microstructural evolution of oxide layer during corrosion process will affect the corrosion resistance of zirconium alloys, the improvement of corrosion resistance of the N18 alloy by obtaining the microstructure with nano-size and uniform distribution of SPPs, and by decreasing the content of Sn and maintaining the content of Cr is discussed.
B.X. ZhouM. Y. YaoZ.K. LiX.M. WangJ. ZhouaC.S. LongQ. LiuB.F. Luan
