Alloy Design For Molten Salt Reactor Operated At 800℃ Based On GH3539 Alloy

GH3535 alloy is the only successfully used structural alloy for molten salt reactor,with the maximum allowable temperature of 704℃.The operating temperature of the molten salt reactor needs to be further elevated beyond 800℃to achieve high energy efficiency processes,such as hydrogen production.However,all the commercial alloys cannot meet the performance requirements of the structural alloy for molten salt reactors operated above 800℃.Amongst the possible scenarios,one new GH3539alloy seems to be the most potential candidate alloy for molten salt reactor operated above 800℃.Previous studies show that GH3539 alloy possesses excellent high-temperature mechanical properties and good molten salt corrosion resistance.However,the ductility at 650℃-750℃and high-temperature oxidation resistance of GH3539alloy still need improvement.The purpose of this thesis is to optimize the composition of GH3539 alloy,and GH3539Zr,GH3539Nb and Cr-free-GH3539 alloys are proposed.The mechanical properties,molten salt corrosion resistance,irradiation resistance,and high-temperature oxidation resistance of the alloy are significantly improved.The major conclusions are as follows.（1）The addition of Zr into the GH3539 alloy causes constitutional undercooling and a reduction of solid-liquid interfacial energy due to the enrichment of Zr at the solid-liquid interface during the solidification process.The grains are refined and the number of carbides increases in the as-cast and the final microstructure.When the alloy receives stress loading at intermediate temperature,the effects of grain refinement strengthening and precipitation strengthening lead to an enhancement of the weak grain boundaries,and more dislocation can be accommodated at the grain boundaries.The larger fraction of dynamic recrystallization generated at a certain degree of stain contributes to the significant improvement of elongation.In the high-temperature oxidation process of GH3539 alloy,Zr is solid-soluted in the intermediate oxide layer Ni WO₄,and Ni²⁺is preferentially replaced by Zr⁴⁺,increasing the formation energy oxygen vacancies and then hindering their formation.The resistance to high-temperature oxidation of the alloy is therefore improved.The optimum Zr content in GH3539 alloy is supposed to be 0.03-0.1 wt.%.（2）The addition of Nb into the GH3539 alloy causes constitutional undercooling and an aggravation of the enrichment of C element at the solid-liquid interface during the solidification process.The grains are refined and the number of carbides increases in the as-cast and the final microstructure.The effects of solid solution strengthening,work hardening,grain refinement strengthening,and second phase strengthening caused by Nb addition greatly improve the strength of the alloy.Additionally,the excellent elongation of the alloy is maintained due to grain refinement and an increased proportion of twin boundaries.In the high-temperature oxidation process of GH3539alloy,Nb is solid-soluted in the intermediate oxide layer Ni WO₄ and Ni²⁺is replaced by Nb⁴⁺or Nb⁵⁺,hindering the formation of oxygen vacancies and improving the resistance to high-temperature oxidation of the alloy.When the Nb content exceeds 4wt.%,the number of grain boundaries is large enough to make Cr rapidly diffuse to the surface of the alloy to form a continuous and dense Cr₂O₃ oxide layer,leading to a significant improvement in the high-temperature oxidation resistance of the alloy.The optimum Nb content in GH3539 alloy is supposed to be 3-4 wt.%.（3）For Ni-xW-6Cr alloys,the increase of W content hinders the accumulation of dislocation loops in the alloys and improves the irradiation resistance;The increase of W content substantially deteriorates the corrosion resistance of the Ni-x W-6Cr alloys to molten salts.This is because the activities and chemical potentials of W and Cr increase concurrently with the increase of W content.During the molten salt corrosion process,the contents of Cr and W in the outermost alloy surface should be zero,and the larger chemical potential in the alloys with higher W means a higher diffusion rate of Cr and W,resulting in a greater diffusion force for Cr and W from the inner matrix to the surface and accelerating their corrosion by molten salt;Moreover,the addition of Cr into the Ni-28W-0.4Hf alloy brings Cr WO₄ in the intermediate oxide layer Ni WO₄and leads a transition of the structure of the intermediate oxide scale from a single phase to a mixed phase.Cr WO₄ contains many oxygen vacancies,resulting in a decrease in the high-temperature oxidation resistance.Based on these findings,a new Cr-free-GH3539 alloy was developed.The mechanical properties,high-temperature oxidation resistance,and molten salt corrosion resistance of Cr-free-GH3539 alloy were effectively improved by reducing or even completely removing the Cr content from the GH3539 alloy and increasing the W content,and adding Nb,Hf,or other reactive elements to the alloy to replace the anti-high-temperature oxidation effect of Cr.