Research On The Effect Of Alloyed Dilute Silicaon And Processing Procedures On Microstructure And Corrosion Behavior In Zr-Sn-Nb Alloys

For excellent nuclear performance,zirconium alloys are used as nuclear reactor fueling cladding materials.At present,only the second generation of zirconium alloys can be produced by our nation.The third generation of zirconium alloys are acquired via imports.Designing high performance zirconium alloys with independent intellectual property rights is a top priority task.The study on the mechanism of corrosion kinetics transition of zirconium alloys in current research is still lack of depth.Therefore,exploring corrosion mechanism deeply is very significant.Zr-0.5Sn-0.35Nb-0.35Fe-0.02 Si-0.08 O,Zr-0.5Sn-0.35Nb-0.35Fe-0.05Cu-0.08 O and Zr-0.5Sn-0.35Nb-0.35Fe-0.08 O alloys prepared via various procedures were investigated by multiple characterization and analysis techniques including scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),X-ray diffraction(XRD),micro-hardness test,autoclave corrosion tests,tensile tests and quantitative metallography.System work had conducted on the microstructures,tensile properties and corrosion behavior at 360°C/18.6MPa pure water and 0.01mol/L LiOH aqueous solution.The mechanism of zirconium alloys was discussed.The main experimental results and conclusions were as follow:1.Partial microstructure with fine and uniform distributed second phase particles(SPPs)were obtained in Zr-0.5Sn-0.4Nb-0.3Fe-0.02Si-0.08 O via conventional rolling/annealing procedure or second ?-quenching procedure.The alloy with equiaxed grains and sub-grains(size: 200-300 nm)was obtained via conventional rolling/annealing procedure.The remnant laths with width of 200 nm could be observed in the alloy processed by second ?-quenching procedure.Both the conventional rolled/annealed samples and the second ?-quenched samples possessed proper corrosion resistance and hydrogen absorption,superior to low tin Zircaloy-4(Zr-4)alloy and consistent with or superior to M5 alloy.Furthermore,the alloys had adequate tensile properties via both procedures,which could be used as fuel cladding candidate.2.Adding a trace of Si into Zr-0.5Sn-0.4Nb-0.3Fe-0.08 O alloy could refine SPPs,decrease the degree of recrystallization and enhance the strength of alloy.During corrosion process,large stress was produced at oxide/metal interface due to the great volume difference between oxide and the consuming metal matrix.This stress led to the deformation of metal matrix and produced undulated interface which associated formation of cracks.The evolution of undulated interface was slower in the alloy with higher strength.However,a trace of alloyed Si would not modify the susceptibility of the oxide at a noticeable extent.Therefore,the corrosion kinetics transition of the Si-containing alloy was postponed.3.Before corrosion kinetics transition,almost all cracks appeared above the crests of undulated interface.With the growth of oxide,more cracks were presented in the oxide.The length of cracks augments as the increase of the amplitude of undulations.When these long cracks linked up and split oxide into two layers,the corrosion kinetics transition took place.Both the SPPs and the transformation of from tetragonal phase to monoclinic phase may provide susceptible regions for the formation of cracks.The driving force for cracking was the tensile stress which was perpendicular to the interface and above the crests of undulations stemmed from the local stress redistribution by the formation of undulations.After corrosion kinetics transition,the thickness of oxide which interacted with metal matrix reduced and the mechanical energy at interface decreased correspondingly.Hence,the undulations became mild again.4.Adding low-temperature pre-deformation between ?-quenching and hot rolling had significant influence on the microstructure of Zr-0.5Sn-0.4Nb-0.3Fe-0.05Cu-0.08 O alloy.The processing of low-temperature pre-deformation introduced dislocations,increased the nucleation sites,reduced the initial diameter of SPPs and obtained homogeneous microstructure with a refined grains and uniformly distributed SPPs after hot rolling.The different microstructural features between pre-deformed samples and directly hot rolled ones,including grain size,texture and distribution of SPPs,were inconspicuous after final annealing,but homogeneous microstructure obtained in pre-deformed samples at an early stage of fabrication was beneficial for optimizing the conventional manufacturing route and enhancing the controllability of microstructure of zirconium alloys.5.The corrosion behavior of Zr-0.5Sn-0.4Nb-0.3Fe-0.05Cu-0.08 O was sensitive to the variation of microstructure.For partial recrystallized samples,the corrosion rate constant increased with the decrease in degree of recrystallization and corrosion kinetics transition was postponed with the increasing tensile strength.Lateral cracks were associated with corrosion kinetics transition and presented above the crests with high amplitude at undulated interface.The alloy with higher strength had slower evolution of undulated interface and thus possess better corrosion resistance.6.The weight gain of zirconium alloys mainly obeyed parabolic law.When the corrosion rate was high enough,the deformed metal was consumed before the undulations became sharp.As the corrosion rate decreased,the speed of oxide/metal interface moving toward the metal substrate was slow down correspondingly.The undulations became sharp due to the stress took longer time to act on the deformed region.The local metal substrate reached a reasonable level of deformation(i.e.undulations reached a reasonable level of amplitude),cracks were generated and stress relieved.Therefore,the elongation of sample could hardly be the rigorous limitation on corrosion transitions.