| Vanadium alloys are promising blanket structural materials for advanced fusion reactors because of their low neutron activation characteristics and excellent high temperature mechanical properties.One of the main issues concerning the design of blankets for fusion reactors is the tritium permeation through the structural materials into the circuit system.This would cause the losses of fusion fuels,tritium radiological hazard,and hydrogen/helium embrittlement of materials.The tritium permeation through structural materials must be depressed for enviromental safety and tritium self-sufficiency consideration.The tritium permeabilities of vanadium alloys are much higher than that of other candidate structural materials(RAFM steels and SiCf/SiC composite materials)and should be paid more attention.One of the most effective ways is to apply a coating named tritium permeation barrier(TPB)on structural materials.Aluminide coating is identified as the most promising TPB coating by China,EU and India researchers for the chemical stability,self-healing ability and excellent compatibility with structural materials.This type of coating is usually prepared by aluminizing and subsequent oxidation,forming an inner Al-rich alloy layer and an outer Al2O3 layer.At present,the coating on vanadium alloy substrate performs not as well as that on steel substrates.The possible reason can be that it's difficult to form high quality V-Al alloy layer and protective Al2O3 scale.For this reason,by using first-principles calculations and thermodynamics formalism,the composition and microstructure of V-Al alloy layer and the effects of alloying elements are analyzed;and the selective oxidation behavior of V-Al alloy and the effects of alloying elements are investigated.The mechanisms of alloying elements in the formation of aluminide coating on vanadium alloy substrate are revealed.The results are summarized as follows:1.The mechanism of alloying elements in the formation of Al-rich layer on V alloy surface are revealed.The diffusion of A1 in V is significantly hampered by Ti atoms,and no obvious impact of Cr atoms on A1 diffusion in V is found.In V substrate,Ti atoms occupy 1nn sites of vacancy and 2nn sites of Al due to strong attractive interactions between Ti and vacancy;Cr atoms occupy 2nn sites of vacancy and Inn sites of Al,due to weak repulsive interactions between Cr and vacancy.The migration barriers of A1 increase when Ti atoms occupy neighbor sites,which means the diffusion of A1 in V is inhibited by Ti atoms,and no obvious impact of Cr atoms on A1 diffusion in V is found.The results indicate that,during the process of heat treatment,Al diffusion into substrate is hindered by alloying element Ti,which makes Al participate more in the nucleation and growth of Al3V on the surface.Then the compositions and microstructures of Al-rich layer on V-Cr-Ti substrates are different with that of V substrates.As a result,although Al-rich layer on V and V-5Cr-5Ti substrates both present multilayered microstructures composed of Al3V,Al8V5 and V(Al)solid solution,for V-5Cr-5Ti substrates,Al3V layer are fine-grained and thicker,meanwhile the thickness of inner layer is reduced,the diffusion depth and the concentration of Al in the substrate is decreased.The results can be useful to the control of Al rich layer on V alloys substrates.2.The initial oxidation mechanism of V surface and the effects of alloying elements are revealed.The adsorption stability of O atoms on V(110)surface decrease with increasing oxygen coverage ?.When ??0.75 ML,on-surface and subsurface co-adsorption of O atoms are more favorable than on-surface adsorption,which means O atoms penetration into subsurface starts and the initial oxidation actuates.The alloying elements Al and Ti tend to segregate on V(110)surface and move outwards after relaxation;while Cr tend to not segregate on the surface and move inwards after relaxation.The oxygen coverage that actuates the initial oxidation decrease when Al and Ti doped on the surface,suggesting that Al and Ti bring forward the initial oxidation of V(110)surface.On the other hand,Cr has limited effects on the initial oxidation of V(110)surface.3.The surface phase diagrams for oxygen adsorption on V alloy surfaces are constructed,and selective oxidation behaviors of V alloys are analyzed.High oxygen coverages promote further surface segregation of Al on Al-segregated V(110)surface,resulting in the selective oxidation of Al.Similarly,oxygen can further promote the surface segregation of Ti,resulting in the selective oxidation of Ti.In contrast,oxygen does not promote the surface segregation of Cr,and can not induce the selective oxidation of Cr.The surface phase diagrams can be used to predict the selective oxidation behaviors of V-Al alloys,and they provide guidance for the preparation of TPB coatings on vanadium alloys.4.The initial oxidation mechanism of Al3V surface and the effects of alloying elements are discussed.Al-terminated(110)surface is the most stable surface.The adsorption stability of O atoms increase with increasing oxygen coverage.It can be inferred from DOS analysis and the distance between O and other atoms that at the initial stages the oxidation of Al atoms and internal-oxidation of V atoms occur,this means the protecting Al2O3 scale can be formed after oxidation.Ti and Cr atoms prefer to occupy V sites in 2nd layer,the substitution of these atoms can both improve the adsorption stability of oxygen on the surface,while the mechanisms are different.The interactions between Ti and O atoms are strong,so the interactions between Al and O atoms are weakened after Ti substitution,that means Ti and A1 can both be oxidized to form mixed oxides.On the other hand,the interactions between Al and O atoms are slightly enhanced after Cr substitution,so the oxidation of A1 atoms are improved,that means the protective Al2O3 scale can be formed.The results can provide theoretical guidance for improving processing techniques concerned with aluminide TPB coatings on V alloys substrates,and can also be instructive for the study of the oxidation behavior of vanadium alloys and the preparation of protective oxide film on the surface of vanadium alloys. |
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