| Zirconium alloys have been widely used as fuel cladding materials due to their low thermal neutron absorption cross section,excellent corrosion resistance and adequate mechanical properties.The corrosion resistance of fuel claddings has been considered as one of the most important properties to control the performance and the safety of nuclear reactors.It is well known that optimizing chemical compositions and adjusting manufacture processing can improve the performance of zirconium alloys.Up to now,however,the study on the mechanism of the specific role of alloying elements and manufacturing process on the corrosion process is still lack of depth.In addition,zirconium alloys must have certain mechanical properties when serving in the reactor.There are only sporadic reports on the discontinuous yielding behavior of zirconium alloy during deformation,and the explanation about this issue is still controversial.This work focuses on two kinds of self-developed Zr-Sn-Nb alloys.The effects of heat treatments and microalloying on the microstructure,the discontinuous yielding behavior and corrosion resistance of alloys have been systematically studied.Zr-0.5Sn-0.4Nb-0.3Fe-0.05 Cu alloys were annealed at 480?,500? and 550?for 5h,respectively.The recrystallization degree of the alloys increased with the increase of annealing temperature.The specimen was at the initial stage of recrystallization after ?-quenching and 480?/5h heat treatment.After annealing at500?/5h,partial recrystallization microstructure was obtained,while samples was in the state of complete recrystallization after annealing at 550?/5h.Tensile tests shown that the yield strength of the alloy decreased with the increase of recrystallization degree.A clear yield drop was observed in samples with complete recrystallization,and this behavior gradually disappeared with the reduction of recrystallization degree.Meanwhile,as the strain rate increased,the yield point phenomenon(YPP)became less prominent in the stress-strain curves.In the cyclic loading tensile test,the YPP behavior gradually disappeared and reappeared for Zr-0.5Sn-0.4Nb-0.3Fe-0.05 Cu alloy with the increase of prestrain.The initial yield drop was caused by the lack of mobile dislocations in crystals,whereas the latter one was attributed to the decomposition and recombination of dislocation core in crystals.The increased density of mobile dislocations introduced by pre-strain(<1%)contribute to the disappearance of yield drop,whereas the climb-dissociated partials introduced by pre-strain deformation(>9%)recombine onto the same prism plane to regain its moving ability,leading to the re-appearance of yield drop.With the decrease of recrystallization degree,the initial yield drop gradually faded away,while the latter one gradually strengthened.Additionally,with the increase of unloading degree and strain rate,the latter one gradually increased.After annealing at 550?/5h,the partially recrystallized microstructure was obtained for Zr-0.35Sn-0.75Nb-0.3Fe-0.15 Mo alloy,which corresponding to the yield plateau in the tensile curve at room temperature.After the trace addition of 0.1%Cr and 0.1%Cr+0.05%V,the yield plateau gradually disappeared.The mean grain size of matrix was reduced from 1.9 ?m to 1.7 ?m and 1.5 ?m by trace addition of 0.1%Cr and 0.1%Cr + 0.05%V to Zir alloy,respectively.The fraction of LAGBs increased from 50.2% to 53.9% and 60.8% in Zir-0.1Cr and Zir-0.1Cr-0.05 V,respectively.Besides,the volume fraction of second phase particles(SPPs)increased from 7.2% to11.1% and 11.8% in Cr-containing and Cr+V-containing alloys,respectively.The addition of Cr and V decreased the recrystallization degree of alloy,and increased the density of mobile dislocations in the materials,leading to the gradual disappearance of yield plateau in the tensile curves.The oxidation weight gain was obviously accelerated by the trace addition of0.05%Mn in Zr-0.35Sn-0.75Nb-0.3Fe-0.15 Mo alloy.With the addition of trace Mn,more defects(micro-pores of micro-cracks)were formed on the inner and outer surface,as well as the cross section of oxide films.With the increase of corrosion time,micro-cracks developed into long cracks,which lead to the early delamination of oxide film.Meanwhile,the mechanical strength of oxide films decreased with the trace addition of Mn.This is because the Mn addition promotes the Kirkendall effect,leading to more vacancy to form in the oxide.These vacancies therefore merged with others to form micro-pores,which provided short-circuit channel for the diffusion of O ions,thus accelerating corrosion rate.With the increase of corrosion time,the mechanical strength of oxide films gradually decreased.Zr-0.35Sn-0.75Nb-0.3Fe-0.15 Mo alloy was annealed at 550?/5h and 480?/5h respectively to obtain two kinds of recrystallization structures.The recrystallization degree of the former is higher than that of the latter,and the corrosion resistance of the former is higher than that of the latter.The better corrosion resistance is attributed to the reduction of oxide grain boundary area,since the grain boundaries are the most common short-circuiting paths for oxygen diffusion.Two mechanisms are responsible for crack formation in the oxide: phase transition from tetragonal to monoclinic and delayed oxidation of precipitates. |
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