The Effects Of Grain Boundary Engineering,Ageing Treatment And Cold Working On The Inward Diffusion Of Te Into Hastelloy N

Hastelloy N was used as the main structural material for the molten salt reactor?MSR?owing to the resistance to high temperature,chemically aggressive fluoride molten salts and intense radiation fluxes.After the MSR being operated successfully for 4 years in 1960s,Hastelloy N was examined systematically in Oak Ridge National Laboratory?ORNL?.The alloy presented intergranular brittle fracture resulting from the attack along grain boundaries?GBs?by fission product tellurium?Te?.The properties related grain boundary can be improved by the grain boundary engineering?GBE?by changing the grain boundary character distribution?GBCD?in the materials which have face centered cubic?FCC?structure with low stacking fault energy?SFE?.In this paper,the effects of primary carbide distribution on the evolution of GBCD in Hastelloy N were focused on.The Hastelloy N sample was cold rolled with the reduction of 50%in thickness and annealed at 1177?for 30min to optimize the GBCD of Hastelloy N.Then the effects of GBE,ageing treatment and cold working on the diffusion of Te into Hastelloy N were discussed.The results are as follows:?1?The proportion of low?CSL GBs can be enhanced to more than 70%in Hastelloy N after GBE.Due to particle stimulated nucleation?PSN?and inhibition of GB migration caused by the primary carbides during recrystallization,more fine grains and random GBs were formed around the primary carbides,which are detrimental to GBE.The proportion of low?CSL GBs for the specimen with more intensive and larger primary carbides was lower than that for the specimen with more dispersive and finer primary carbides.?2?There were three main paths of Te diffusion into Hastelloy N:GBs,some low-index crystallographic planes in the interior of grains and surface corrosion.Furthermore,intergranular diffusion mainly occurs in the cold working samples.?3?Except?3 GBs,the others would become the diffusion paths when Te was diffusing along GBs into the alloy.The diffusion depth became deeper as the experimental time increased.The proportion of?3 GBs was higher in the samples with GBE,which greatly reduced the GB diffusion.There are a large amount of secondary carbides on GBs in the aged samples,which effectively suppressed the formation of compounds of Te with Cr,Mo,Ni and other elements,greatly retarding GB diffusion.In the cold worked samples,a large amount of Te was assumed to diffuse along some low-index crystallographic planes in the interior of grains,which resulted in the suppression of GB diffusion.As a result,GBE,ageing treatment and cold working can effectively alleviate the Te diffusion along GBs into the alloy.?4?The diffusion along some low-index crystallographic planes can be often observed in the cold worked samples.The diffusion depth became deeper as the experimental time increased.The diffusion of Te along some low-index crystallographic planes in the interior of grains was more obvious in the cold worked samples without GBE than that in the samples with GBE.The diffusion of Te along some low-index crystallographic planes in the interior of grains was also largely alleviated in the ACW samples.It may be due to a large amount of secondary carbides precipitated on GBs,which consumed a lot of Mo,Cr,Ni elements and thus suppressed the formation of the compounds of Te with these elements.Therefore,GBE and ageing treatment can effectively alleviate Te diffusion along some low-index crystallographic planes in the interior of grains.?5?The surface corrosion thickness of the alloy was thicker as the experimental time increased.In the solution samples and aged samples without cold working,Te was diffusing along the GBs into that alloy.Comparing with the samples without GBE,the proportion of low?CSL GBs was higher in the samples with GBE,resulting in the fewer paths of the diffusion along GBs.Therefore,the surface corrosion layer was thicker in GS and GA samples with GBE.?6?When Te was diffusing along the GBs,the zones where the compounds of Cr₃Te₄ were formed were likely to peel.Te,which diffused along some low-index crystallographic planes in the interior of grains,formed the compound of Ni,Mo and Cr elements and distributed alternately.The compound of Ni₃Te₂,Ni_2.86Te₂,Cr₃Te₄,MoTe₂ and NiTe₂ were observed in the area.