| Nuclear energy is one of the new energy that can replace fossil energy,due to the large energy density,high efficiency,and able to large scale industry application.Steam generator is the main component of the pressurized water reactor(PWR)nuclear power plant.In order to meet the harsh service condition,the steam generator tube materials experienced austenitic stainless steel,Inconel 600 and Inconel 690 alloy.Alloy 690 is considered to be the best material for steam generator heat transfer tube of the third generation PWR nuclear power plant.However,after long time servicing,intergranular corrosion and intergranular stress corrosion cracking may occur in steam generator tubes.The main reason of intergranular failures is the precipitation of the Cr-rich carbides at grain boundaries,which would lead to chromium depletion at the grain boundaries,and reduce the corrosion resistance of the grain boundary.Therefore,in order to prolong the service life and reliability of the heat transfer tube of the steam generator,it is necessary to study the precipitation behavior of grain boundary carbide for Inconel 690 alloy.The scanning electron microscopy(SEM),electron backscatter diffraction techniques(EBSD),high resolution transmission electron microscopy(HRTEM)and in-suit SEM tensile technique were used to study the effect of grain boundary character,triple junction type and aging temperature on the precipitation morphology evolution of the grain boundary carbide in Alloy 690.The following conclusions can be drawn:(1)By 5%of a single step cold reduction and subsequent annealing at 1100?for 5 min,the propotion of low?CSL grain boundaries in Alloy 690 was increased to more than 75%.The radios of?3-?3-?9,?3-?9-?27 and?3-?27-R triple junctions increased significantly.Hence,The diversity of triple junction type was increased significantly.(2)There are some differences of carbide precipitation morphology at same character grain boundary near different triple junction at same sample.The plate like carbide precipited near the?3_i and?9 grain boundaries,however,the plate like carbide is not affected by triple junction type.The sizes of carbide precipitated at?3_c,?27 and random grain boundaries are difference in different triple junction,the general trends of carbide sizes are:smallest at the triple junctions composed of three low?CSL grain boundary,and then two low?CSL grain boundary,one low?CSL grain boundary and zero low?CSL grain boundary.(3)The aging temperature affects the precipitation morphology of grain boundary carbides significantly.The higher the aging temperature,the larger the carbide precipitation.However,there are some differences between carbide precipitated at grain boundaries with different characters.After aging at 1000?for1 h,there is no carbide precipitated at?3_c grain boundary,and no plate like carbide precipitated at?3_i and?9 grain boundary.The size of carbide precipitated at?27and random grain boundary is changed in the samples aged at higher temperature.(4)During aging at the higher temperature,the precipitation and dissolution process of grain boundaries carbide only involves the transformation of carbide to the matrix.The transition zone formed at the interface between carbide and matrix when the sample aged at lower temperature will not be found.(5)Under the in-situ SEM tensile test,grain boundary carbide was migrated with grain boundary.The relationship between grain boundaries characters and the slip lines can be concluded as:the slip line can pass through the coherent twin boundary,but can't pass through?9,?27 and random grain boundary.When the deformation reaches to 40%,in order to coordinate the plastic deformation,multiple slips will occur in all grains.(6)Compared to the?3_c and?3_i grain boundaries,?9,?27 and random grain boundaries were more easily to form microcrack.But the initiation of grain boundary microcrack not only related to the type of grain boundary,but also related to the type of nearby grain boundaries.The higher the?value of the nearby grain boundary,the more easily the microcracks occur at grain boundary.In additional,the triple junctions composed of 3 low?CSL grain boundaries have a good ability to resist the formation of the microcracks than other triple junctions. |
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