| To solve the critical issues of the Magnet-hydro-dynamic(MHD)pressure drop,tritium permeation into blanket structure materials and high temperature corrosion of the structure materials by liquid lithium for Li/V blanket,a Ti/AIN bilayer coating on vanadium alloy substrate is proposed for the first time.AIN coating,as the top layer,can provide enough electrical resistivity,and realize the in situ formation and self-healing in Li.The metal Ti,as the interlayer,can react with the AIN layer to form Al-rich compound layer at the interface,which is expected to function as a tritium permeation barrier and further improve the tritium permeation resistance of the coating.Thus much effort should be paid on the investigation of a coating if it has the potential to solve the above three critical issues.In the present work,the bonding of between V4Cr4Ti alloy and Ti metal was firstly studied since the Ti interlayer plays an important role in combing the V-alloy and AIN coating.The diffusion bonding of the V4Cr4Ti alloy/Ti was conduced by using hot forging technique in the temperature range between 1150 ? and 950 ?.The interface,smooth and uniform in width,is cosisist of a needle-shape Widmanstatten structure zone and a single P-Ti phase zone.It showed that the largest hardness of 332 HV was measured in a narrow region between the Ti substrate and the joint,which is remarkably higher than that of the substrates(Ti of 190 HV and V4Cr4Ti alloy of 258 HV).The lowest hardness of only 182 HV was found in a region in the V4Cr4Ti substrate adjacent to the joint.The joint has a bonding strength higher than 165 MPa with a ductile fracture,indicating a good diffusion bonding can be realized between the two materials.Then the target V/Ti/AIN coating(V4Cr4Ti alloy is replaced by pure V metal)is prepared via combing vacuum hot pressing and powder-assisted nitriding method.The V/Ti diffusion-bonded interface prepared by hot pressing at 1200 ?is smooth.Since there was a creep behavior for the V and Ti metal took place during the pressing process,some Ti metal transferred from the middle part of the sample to the edge part.It promoted the formation of Ti solid solutions in the middle of V/Ti interface because of the accumulation of Ti metal.The bonding of Ti/Al couple at 550 ?.The formation of TiAl3 phase,the only interfacial reaction product,requires an incubation period,and the growth of TiAl3 layer follows the fitting equation:x = 4.3(t-0.76)0.44 in the present experimental conditions.It was found that when the holding time is increased to 5 h and 20 h,many Al-rich islands were formed in the reaction layers,mainly because A1 is the dominant diffusion component during the formation of the reaction layer.An AIN layer was not formed on the Al substrate via(Al+Mg+Al2O3)powder-assisted method at 580 ? because of the absence of the oscillating movements for the reaction chamber during the the nitriding.In addition,the Al substrate was alloyed to Al-Mg binary alloy.Finally,the effects of the Ti/AIN interfacial reaction and surface integrity on the deuterium permeation resistance of the coating are studied.For the"316L/Ti/AlN" bilayer coatings,some cracks were formed in the as-deposited coating before the permeation test,and the PRF of the as-deposited coating are only 1.2,2.1,3.0 and 1.3 at 600 ?,500 0?,350 ? and 200 ?,respectively.When the coating is heat treated at 600 ? for 7 h,high density of micro-cracks were formed in the AIN coating,however,the obtained PRF are 2.1,4.1,13.5 and 5.4,indicating the permeation resistance was improved by the heat treatment(interfacial reaction).For the "316L/Ti/AlN/Ti/AlN" multilayer coatings,the PRF of the as-deposited coating are as high as 560,274,75 and 3 at 600 ?,500 ?,350 ? and 200 ? respectively,which exhibits an exellent permeation resistance at high temperatures.For the coating heated at 700 ? with a heating rate of 1.5 ?/min,the PRF are increased to 536,325,90 and 4 at the test temperatures(except at 600 ?).However,when the annealed temperature is higher(760 ?)or the heating rate is increased to 2.5 ?/min,the PRF of the coating are decreased since many more cracks are developed.Based on the above results and analysis,it can be concluded that appropriate heat treatment(interfacial reaction)is conducive to improve the deuterium permeation resistance of the Ti/AIN coating;however,if large cracks are formed in the coating they would work as the shortcut for the deuterium during the permeation and significantly weaken the permeation resistance of the coating. |
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