| Vanadium alloy has been regarded as one of the most promising future fusion reactors structural materials,due to its reduced activation,good compatibility with liquid Li,excellent high-temperature mechanical properties and irradiation damage resistance.However,it is difficult for vanadium alloy to be worked plastically,attributed to its high melting point and active chemical property.Moreover,due to lack of solute transformation,the grains of as-cast alloy is rather coarse.Meanwhile,vanadium alloy,as a structural material of fusion reactors,has to be sharply irradiated by high-flux and high-energy(14 MeV)neutrons,causing microstructural evolution and weakened mechanical properties.Thus,it is very necessary to investigate evolution of the microstructures of vanadium alloy during deformation,heat-treatment and irradiation,in order to provide theoretical direction for improvement of processing technology and forecast of property change during serving.In the present research,the research target is V-4Cr-4Ti alloy which was prepared by the author.The microstructures of the cast,aged,deformed,annealed and/or irradiated alloy were observed by optical microscopy,scanning electron microscopy and transmission electron microscopy.In addition,hardness of the alloy treated above was tested by Vickers hardness instrument.At last,a large-size V-4Cr-4Ti ingot was thermally processed into some plates.Then,the tensile and impact properties of the plates were studied.The issues including recrystallization nucleation,grain refinement,and irradiation-induced microstructural evolution were emphatically discussed in the paper.Following prime novelty was acquired.Recrystallization nucleates preferentially at original boundaries of deformed grains.Based on this result,it was proposed that a recrystallization nucleation mechanism that long dislocation pile-up groups cause dislocation accumulation in front of grain boundaries and then induce recrystallization nucleation.Intersection of dislocations from different pile-up groups and dislocation accumulation near lath-like pre-precipitates can facilitate intragranular recrystallization nucleation respectively,proved in the related experiments.Irradiation-induced twinning in the alloy was found.The twinning originates form accumulation of supersaturated irradiation-induced vacancies which causes atom collapse and makes stacking faults formed.The shear induced the stacking faults is correspondingly twinning shear of BCC metals.The mechanism indicates that twinning could be used to absorb irradiation-induced point defects.In vanadium alloy,aging-induced lath-like precipitates can pin dislocations,leading a part of dislocations to survive after high-temperature ion irradiation.The irradiation-induced precipitates are greatly diff-erent from aging-induced precipitates in size,distribution,composition,orientation.Two plans of refining coarse grains of V-4Cr-4Ti alloy were proposed by deformation and recrystallization.Plan I:cold pre-deformation to a strain level not less than true strain 0.36 and following annealing at 1100～1200℃ for 60 min.PlanⅡ:hot deformation to true strain 1.2 under the condition 10 s-1+1150℃(ε+T)and following annealing at 1150℃ for 5 min.A 50 kg V-4Cr-4Ti alloy ingot was smelted.Total level of the impurities C,N,O in the ingot is 310 ppm,which is leading internationally.Its composition and microstructure are uniform in the ingot.The mechanical properties of hot processed plates of the alloy change with different heat-treatment technologies.The properties of the processed alloy almost keep up with international leading level. |
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