| Zr-2.5Nb alloy has excellent radiation stability and corrosion resistance,so it is widely used as pressure tube and structural material for heavy water reactors.The service environment of Zr-2.5Nb alloy pressure tube in the reactor is extremely harsh,and the service time is as long as 30 years,so the performance requirements of Zr-2.5Nb alloy tube are very strict.The corrosion resistance and mechanical properties of Zr-2.5Nb alloy are particularly sensitive to its microstructure,and the processing methods play a decisive role for its microstructure.The processing methods of Zr-2.5Nb alloy tube mainly include vacuum consumable arc melting,forging,β-phase quenching,hot extrusion,cold rolling and finished annealing.Among them,finished annealing plays a decisive role for the microstructure of Zr-2.5Nb alloy tube,so it is necessary to study the effect of finished annealing process parameters on the microstructure of Zr-2.5Nb alloy tube.In the existing studies,there is still a lack of in-depth understanding of the precipitation mechanism and growth behavior of the second phase particles during the annealing process of Zr-2.5Nb alloys.The above problems have become urgent problems to be solved in the research of Zr-2.5Nb alloys for nuclear reactors.In this paper,the Zr-2.5Nb alloy tube after cold rolling is taken as the research object,and the samples are annealed at different temperatures for the same time.The microstructure of the cold-rolled Zr-2.5Nb alloy after annealing at different temperatures is analyzed by scanning electron microscope（SEM）,X-ray diffractometer（XRD）,high-resolution transmission electron microscope（HRTEM）and other testing techniques.On this basis,the in-situ heating TEM study of the cold-rolled Zr-2.5Nb alloy realizes the real-time observation of the second phase change during the annealing process of the Zr-2.5Nb alloy,so as to understand the precipitation mechanism and growth behavior of the second phase.After that,the mechanical properties and the corrosion resistance of Zr-2.5Nb alloy before and after annealing are tested by micro hardness,tensile and autoclave static corrosion test methods,and the effects of annealing process parameters on its corrosion resistance and mechanical properties are explored.The main findings are as follows:（1）The unannealed cold-rolled Zr-2.5Nb alloy contains elongated close-packed hexagonalα-Zr and surrounding filamentous body-centered cubicβ-Zr,and contains a large number of dislocation entanglements inside area.There areα-Zr,β-Zr and a small amount of metastableωphase precipitated fromβ-Zr in the sample structure after annealing at 300℃for 24 hours.After annealing at 400℃for 24 hours,there areα-Zr,ωphase andβ-Zr enriched Nb element in the sample structure,and the number of dislocations in the sample after annealing is significantly reduced.The microstructure of the sample after annealing at 500℃for 24 hours includesα-Zr andβ-Nb,because the Nb element in theωphase diffuses to theβenr phase,and theωphase gradually transforms intoα-Zr,while the Nb element inβenr gradually is enriched,and after reaching a certain level,theβ-Nb precipitates are formed.The structure of the sample after annealing at 600℃for 24 hours isα-Zr andβ-Zr.Due to the recrystallization of the sample during the annealing process,the grains ofα-Zr andβ-Zr are both equiaxed,and the dislocations in the sample are also almost disappeared.The best annealing temperature is 500℃.（2）When in-situ heating in Zr-2.5Nb alloy is affected by factors such as thickness effect,β-Nb will be enriched and precipitated inα-Zr through the diffusion of Nb element after heating at 500°C,which is different from the result of normal annealing.At the same time,the orientation relationship betweenβ-Nb andα-Zr is clarified as（110）β-Nb∥（10（?）0）α-Zr,[1（?）0]β-Nb∥[0001]α-Zr.（3）Combining the test results of the mechanical properties and the corrosion resistance of the Zr-2.5Nb alloy before and after annealing with the microstructure results,the following conclusions are drawn:The cold-rolled Zr-2.5Nb alloy has high tensile strength and hardness,but poor plasticity,which is caused by work hardening.After annealing at 300℃for 24 hours,the tensile strength and hardness of Zr-2.5Nb alloy samples increase significantly,and the plasticity is improved.This is caused by the combined effect of the dispersion strengthening of theωphase and the solid solution strengthening of the Nb atoms in theβ-Zr.After annealing at400℃for 24 hours,the hardness and tensile strength of the Zr-2.5Nb alloy are lower than those of the sample annealed at 300℃,but their plasticity and corrosion resistance are better than those of the sample annealed at 300℃.This is due to the weakening of the work hardening effect due to the reduced dislocation density.After annealing at 500℃for 24 hours,the hardness and tensile strength of the Zr-2.5Nb alloy sample are poor,but the plasticity is good.This is due to a significant softening of the material caused by a higher degree of recovery of the sample.Andβ-Nb precipitated from the sample after annealing at 500℃plays a good role in enhancing the corrosion resistance,so the corrosion resistance of the Zr-2.5Nb alloy sample after annealing at 500℃is the best.The hardness,tensile strength,yield strength and corrosion resistance of Zr-2.5Nb alloy sample annealed at 600℃are the lowest among the samples annealed at various temperatures,but the plasticity is the best.This is due to the complete recrystallization of the Zr-2.5Nb alloy samples.And the recrystallizedβ-Zr will greatly damage the corrosion resistance of Zr-2.5Nb alloy samples,so the corrosion resistance of Zr-2.5Nb alloy after annealing at 600℃is the worst.In brief,the annealing temperature of 500℃is beneficial for improving the overall performance of Zr-2.5Nb. |
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