Preparation And Properties Of V-5Cr-5Ti Surface V-Al/Al₂O₃ Barrier Coating

Vanadium alloys are the only material that is both non-ferromagnetic and ductile of candidate low activation structural materials, considering as the promising option for advanced fusion reactor structural material applications. However, their hydrogen permeabilities are well above any other candidate structural materials, easily leading to the losses of D and T fuels, tritium radioactivity pollution and hydrogen/helium embrittlements of materials. Therefore, it must prevent V-alloy directly contacting with hydrogen isotope. Coatings coated on the surface of structural materials, with a low permeability for tritium, named tritium permeation barrier ?TPB? are one of the most effective methods to minimize tritium permeation of structural materials. M-Al/Al₂O₃ composite coating was recognized as the most promising TPB with excellent comprehensive properties.In this thesis, a compact V-Al/Al₂O₃ coating with high TPRF and well adhesion to V-5Cr-5Ti was firstly obtained through systematic investigation of preparation procedures, coating structure and coating performance, employing the processes "Ionic liquid aluminum electroplating+ Heating aluminizing+ Selective oxidation", which was a promising TBPs preparation technique with engineering application. Thesis results provided the scientific basis for further complex V-alloy breeder parts TBPs, including preparation technology optimizing and device design for industrialization-scale production. Not only is it significant for tritium self-sufficiency, economy, safety and environmental friendliness of the fusion reactor, but it could be referenced for the surface protection of V-alloy. The results are summarized as follows:1) An Al-coating with dense structure, uniform thickness and well adhesion was firstly obtained on the surface of V-5Cr-5Ti with ionic liquid ?AlCl₃-EMIC, mole rate 2:1? electroplating technology. The effect of electrochemical etching pretreatment, current densities, deposition time and temperature on aluminum electrodeposition was investigated. The adhesion of coating improved effectively by electrochemical etching pretreatment. The fitted etching mehod was potentiostatically anodic polarization at 1.25 V ?Vs. Al? for 15min. At the room temperature, the average grain size of the crystallites decreased with the increasing current density ?4-32 mA/cm²?, but the deposition current efficiency increased slightly as the current density increased to 20 mA/cm², and then rapidly decreased. The Al-coating became more integrity and compact as the deposition time extended. When raising the temperature of electrolyte, the grain size of the crystallites became larger, leading lower of coating density, therefore, the room temperature was better for deposition. The optimal current densities for Al-electrodeposition in ionic liquid ?AlCl₃-EMIC, mole rate 2:1? were 12-20 mA/cm², with maxium current efficiency to 83%, and maxium deposition rate to 21?m/h.2) The influence of heat treatment parameters on aluminide coating structure was systematically investigated for the Al-coated V-5Cr-5Ti, obtaining V-Al layer with controllable of structure, phase and thickness. For the aluminizing process, the phase transformation is as following: Al₃?V, Cr, Ti??Al₈?V, Cr, Ti?₅?V?Al, Cr, Ti?. The aluminide coatings were consisted of a double-layer or a single-layer structure. The inner layers were highly homogeneous, free of porosity with Al₈?V, Cr, Ti?₅ or V?Al, Cr, Ti? solid solution, whereas the outer layers ?surface? were irregular and porous with Al₃?V, Cr, Ti? or Al₃?V, Cr, Ti?+Al₈?V, Cr, Ti?₅. The inner layers structure only depended on the temperature and time when Al source was enough supplied. The thicknesses of the inner layer increased with the raising temperature and time, whiles that of the outer layer decreased. According to the Al content requirement for selective oxidation and the structure characteristic of aluminide coatings, the aluminized coating, consisting of a compact and homogenous inner layer of Al₈?V, Cr, Ti?₅ and a thin outer layer could be the fitted aluminides for subsequent selective oxidation. The corresponding aluminized condition is at 950? for 2 h under Ar protection, with 20?m Al-coating on V-5Cr-5Ti. The growth of the Al₈?V, Cr, Ti?₅ layer is diffusion-controlled in the temperature range from 750 to 950?, and the growth kinetic can be described as:X=4.21×105 exp ?1.45×105/R? t1/2.3) The effect of O partial pressure, oxidation temperature and time on selection oxidation behavir of Al₈?V, Cr, Ti?₅ coating was studied, formed a compact ?-Al₂O₃ on the surface. The oxidation of Al₈?V, Cr, Ti?₅ layer under low O partial pressure ?10 Pa? at 950?for 0.5-1h can obtained a dense ?-Al₂O₃ with thickness from 270 to 390nm. After oxidation, due to the selective oxidation of Al and the Al diffusion to substrate, the phase of the surface layer had transformed from Al₈?V, Cr, Ti?₅ to V?Al,Cr,Ti? solid solution. The final obtained V-Al/Al₂O₃ coating was consisted of a compact homogenous V-Al layer ??m level? and ?-Al₂O₃ film ?100nm level?.4) The tritium resistivity of V-Al/Al₂O₃ coating was evaluated by dynamic gas phase permeation method. The D permeabilities were reduced by 2-3 order of magnitudes, under 350-500?. The rate-controlling mechanism for hydrogen permeation through V-Al/Al₂O₃ barriers was area defect model, which the barrier is nominally considered impermeable while all hydrogen transport occurs through a limited number of cracks and defects. This suggested that the integrity of V-Al/Al₂O₃ is the determinant for the tritium resistivity.