Recovery And Recrystallization Behavior Of Pure Tungsten And Yttrium Oxide Particles Strengthening Tungsten Rolled Plate At High Temperature Operation

Tungsten material is one of the most promising main candidates of plasma-facing materials(PFMs)in thermonuclear fusion reactors.Tungsten material has to face the synergy of high thermal loads,high-flux plasma bombardment and neutron irradiation.Plastic deformation tungsten materials will occur recovery,recrystallization and grain growth during operating long-term at high-temperature service environment.In particular,recrystallization can significantly change the microstructure and texture of tungstenbased materials,result in a decrease in mechanical properties and thermal conductivity,restore the brittleness of tungsten-based materials,increase the ductile-brittle transition temperature,and further reduce the fatigue life.In addition,recrystallization will also reduce thermal shock resistance and radiation resistance,accelerate material performance degradation,and will seriously affect the safe operation and service life of the fusion reactor.Therefore,according to the requirements of the design,test and engineering application of fusion reactor PFMs,it is of great theoretical significance and engineering practical value to conduct in-depth and systematic theoretical analysis and experimental study on thermal stability of tungsten materials.In this paper,based on the relevant research progress and preliminary work of tungsten-based materials,the thermal stability of commercial pure tungsten plates produced by two different companies and the thermal stability of yttrium oxide reinforced tungsten-based materials developed and prepared by the research group were studied in depth.The main researchs and results are as follows:(1)Three pieces of pure tungsten warm-rolled plates with different deformation amount were subjected to isothermal annealing at 1250 ℃.The microstructure and texture in the rolled sate and recrystallized sate were characterized in detail.The effect of rolling thickness reduction on the thermal stability of pure tungsten rolled plate at high temperature was systematically analyzed.There are certain dynamic recrystallized nuclei/grains in the deformed microstructure of the three tungsten plates after the last rolling pass.With the increase of rolling thickness reduction,the number of recrystallized nuclei/grains increases,the average size of subgrains decreases,and the stored energy increases gradually.Under isothermal annealing at 1250 °C,the recrystallization rate also increases with the increase in stored energy and the number of recrystallized nuclei/grains.As the rolling thickness reduction increases,the average grain size of the fully recrystallized microstructure decreases but the average aspect ratio remains almost constant.(2)The microstructure and texture of three pure tungsten plates manufactured by three different rolling processes(unidirectional rolling and cross-rolling with a same medium rolling thickness reduction and cross-rolling high rolling thickness reduction)under rolling and annealing conditions were characterized in detail,so as to evaluate the application prospect of pure tungsten plates as PFMs in fusion devices.Compared with the cross-rolled tungsten plate with high rolling thickness reduction and the unidirectional rolling plate,although the cross-rolled plate with medium rolling thickness reduction has a larger grain size,but at the same time with moderate mechanical strength and more excellent thermal stability than other two plates.Therefore,the cross-rolling tungsten plate with medium rolling thickness reduction has more application prospects in the fusion device.(3)Isothermal annealing experiments were performed on yttrium oxide particle dispersion-strengthening tungsten prepared by two different rolling processes at different temperatures.The distribution characteristics(size,shape,and distribution)of oxide yttrium particles are characterized in detail,and the mechanism of the influence of yttrium oxide particles on the recrystallization process of tungsten-based materials for fusion reactors during steady-state high-temperature service is analyzed.Large yttrium oxide particles can accelerate the nucleation of recrystallization through mechanism of particle stimulated nucleation.The small yttrium oxide particles hinder the migration of the high angle boundary by the Zener pinning force during the growth of the recrystallized nuclei/grains.The yttrium oxide particles distributed inside the grains pin the movement of dislocations and substructure boundaries,which hinder the occurrence of dynamic recovery and reduce the misorientation and stored energy inside the grains.Compared with the pure tungsten plates,the addition of yttrium oxide particles in the tungsten matrix combined with the composite thermomechanical treatment process can significantly improve the thermal stability of the tungsten-based material.(4)The rolling process will cause the inhomogeneity of the microstructure and texture of the rolled plate along the thickness direction,which in turn affects its recrystallization process.The grains and substructures of different tungsten plates along the thickness direction are characterized in detail.The difference characteristics of microstructure heterogeneity were described quantitatively,and the recrystallized microstructure heterogeneity and recrystallized rate heterogeneity caused by the above nonhomogeneous deformed microstructure were compared and investidated.The grain size of warm rolled plate is uniform along the thickness direction,while the substructure shows great heterogeneity.The central layer of rolled plate contains more potential recrystallized nuclei,larger subgrains and large misorientation within the grain,which can significantly accelerate the recrystallization nucleation and reduce the thermal stability of tungsten.This result is similar to the pure tungsten plate prepared by different rolling methods.Although the yttrium oxide particle dispersion-strengthening tungsten plate manufactured by the combination of warm rolling and intermediate recrystallization annealing has an inhomogeneous microstructure.The large grain size and low stored energy in central layer region effectively prevent the recrystallization rate.