Effect Of Nb Addition On Tellurium Corrosion And Tellurium Embrittlement Behavior Of Alloys

As one of the recommended reactor types for the fourth-generation advanced nuclear energy system,the molten salt reactor has many advantages such as high thermal efficiency,inherent safety,thorium resource utilization and modular design convenience.However,early research on molten salt experimental reactors found that the fission product tellurium produced during service would diffuse into the grain boundary of the Hastelloy N alloy used in molten salt reactors to cause cracking（tellurium embrittlement）,which seriously threatens the service safety of construction,especially thin-walled parts.In order to overcome the tellurium embrittlement problem,relevant researchers have proposed an optimization scheme for Nb-added alloys,which has a significant improvement effect.Existing studies have shown that there are multiple paths for elements in the alloy to improve,including:（1）forming a protective layer（telluride or oxide）,（2）forming tellurium trapped carbides,（3）suppressing grain boundaries Tellurium diffusion.Whether Nb addition plays an improvement mechanism through one of the above paths remains to be studied.Further,on the basis of the mechanism study,whether the Ni-Nb binary alloy has application potential as a protective coating has yet to be confirmed.To this end,this paper studies the Te corrosion behavior of Nb-improved GH3535 alloy and Ni-Nb binary alloy in the near working condition.In a vacuum environment,the surfaces of Ni-Nb tensile specimens with different Nb contents（0、1、2、3、4 wt.%）and GH3535 tensile specimens with different Nb contents（0、1、2、4 wt.%）were vapor-deposited on a 300°C Te block（the full text refers to the alloy composition is all mass percentage）,and then etched at700°C for 250 h in high and low oxygen partial pressures for 250h,Through the detailed characterization and analysis of the generation and change of the cross-sectional corrosion products on the surface of the sample after Te corrosion,the intergranular distribution of Te and the intergranular cracks caused by Te,the mechanism of Nb addition to improve the alloy’s resistance to tellurium embrittlement was clarified.The main research results of this article are as follows:1.The effect of Nb content on alloy structure:（1）In Ni-Nb alloys,with the increase of Nb content in solid solution samples,Nb-rich precipitates at the grain boundaries of the alloy occur and grow.With the increase of Nb content in the aging sample,Nb-rich precipitates will also appear at the grain boundaries.（2）In Nb modified GH3535 alloy,As the Nb content increases,the grain size of the solid solution alloy gradually grows,and a small amount of large grains and a large number of small grains alternately appear.Because a large amount of NbC precipitates in the grain boundary of 4%Nb alloy restrict the growth of grains,many small grains appear;In the aging state of Nb to GH3535,the 0%Nb alloy showed granular and thin rod-shaped M₂C carbides at the matrix and grain boundaries.In the 1%Nb alloy,acicular MC carbides grow along the{1 0 0}plane of the substrate,and granular M₂C carbides containing Nb are formed at the grain boundaries.In the 4%Nb alloy,thick rod-shaped NbC carbides grow along the{1 1 1}plane of the substrate.As the Nb content in the alloy increases,the proportion of Nb element in MC or NbC carbides also increases.2.The effect of Nb content on the corrosion products on the surface of the alloy:（1）A continuous Ni₃Te₂ reaction layer was formed on the surface of the Ni-Nb alloy after being corroded by Te in the low hydroxyl tube and the high hydroxyl tube;when the Nb content increased to 1%,a small amount of NbO₂ appeared on the reaction layer.The difference is that the thickness of the reaction layer in the low hydroxyl tube increases with the increase of Nb content,and at the same time NbNi_2.38Te₃ long rod-shaped precipitates gradually appear in the reaction layer.In the high hydroxyl tube,the Ni3Te2 reaction layer on the surface of the sample gradually disappears as the Nb content increases,and a large amount of NbO₂ gradually appears in the matrix near the surface.（2）The GH3535 alloy produces a continuous Ni₃Te₂ reaction layer on the surface of the low hydroxyl tube.As the Nb content increases,it gradually thins and eventually disappears.At the same time,a long rod-shaped Mo₂C appears in the reaction layer,and many small particles of Cr₂O₃ appear on the reaction layer.The surface of the sample in the high hydroxyl tube is almost free of Ni₃Te₂,and the surface is evenly covered with a layer of Cr₂O₃.The oxygen partial pressure concentration seriously affects surface reaction products.3.The effect of Nb content on the intergranular diffusion distribution of Te element:（1）The penetration depth of Ni-Nb alloy Te along the grain boundary shows a decreasing trend with the increase of Nb content,and the more segregation of Nb at the grain boundary is,indicating that the addition of Nb can effectively resist the intergranular diffusion of Te,and the higher the Nb content,the better the effect.（2）The intergranular diffusion depth after Te corrosion in GH3535 alloy decreases with the increase of Nb content,and the intergranular diffusion depth of Te with 1%Nb is the smallest.Due to the precipitation of NbC,many fine crystals are produced.When the Nb content exceeds 2%,the Te corrosion depth of the sample grain boundary is very shallow.4.The effect of Nb content on alloy Te-induced intergranular cracks:（1）The Ni-Nb alloy samples no matter in the high oxygen partial pressure or the low oxygen partial pressure,as the niobium content increases,the average crack depth becomes shallower,which means that the oxygen partial pressure content has little effect on tellurium embrittlement.Only the 4%Nb sample under the experimental conditions of high oxygen partial pressure will have a shallower crack depth and more crack density,which is related to the appearance of NbO₂.（2）In Nb modified GH3535 alloy,it was found that oxidation will seriously affect the intergranular corrosion depth of Te.In the low hydroxyl tube,as the Nb content increases,the average crack depth of the sample decreases with the increase of the Nb content,and the crack density of the sample exceeding 2%Nb content increases sharply.After calculation of K value,1%Nb has the best anti-tellurium corrosion resistance.In summary,whether it is a GH3535 alloy containing Nb or a Ni-Nb binary alloy,adding the Nb content to 1%can provide a significant anti-tellurium embrittlement effect.Due to the fine-grained structure in the high-Nb GH3535 alloy,it has a V-shaped trend with increasing Nb content in crack statistics.Neither alloy failed to form a dense protective product layer.Even NbO₂ only played an auxiliary role in the highest Nb alloy.Therefore,the improvement effect of Nb has nothing to do with the formation of a protective product layer.Secondly,the Ni-Nb binary alloy also shows an improvement trend,so the improvement effect of Nb has nothing to do with the carbide distribution.Nb addition mainly acts by suppressing tellurium diffusion.