Study Of Grain Boundary Segregation And Precipitation In Ni-Cr-Fe Alloys

Nuclear power has the features of low pollution and high efficiency. It eases theenergy crisis, and reduces the carbon emissions at the same time. Steam generatorsare the main components of pressurized water reactor (PWR) nuclear power plants.To meet the harsh environmental conditions, austenitic stainless steels have beenused as steam generator tube materials in PWRs earlier. However, the austeniticstainless steels generally damaged by severe stress corrosion cracking (SCC).Thenthe corrosion resistant Inconel-600 alloy was used as an alternative material. But,SCC also appeared in Alloy 600. Alloy 690 is considered as the best steam generatortube materials for the third generation PWRs.But, prolonging service lifetime and improving performance of steam generatortubeshave been demanded by the nuclear energy industry. The steam generatortubesmade of Alloy 690 may also failure in the late service life. The alloys whichonce were used as steam generator tube materials are all Ni-Cr-Fe alloys. Thesealloys generally meet similar problem, e. g. corrosion along the grain boundary is amain failure reason of these alloys. Therefore, it is necessary to study the grainboundary related properties of Ni-Cr-Fe alloys, e.g. Alloy 690 and 304 austeniticstainless steel. This study is helpful to further improve the service life of Ni-Cr-Fealloys.The atom probe tomography, high resolution transmission electron microscopy,scanning electron microscopy and electron backscatter diffraction techniques wereused to study the grain boundary segregation, carbide precipitation, and their effectson intergranular corrosion resistance in Alloy 690 and 304 austenitic stainless steel.The following conclusions can be drawn:(1) Solute or impurity atoms have different tendency to segregate at differentregions in the same grain boundary both in Alloy 690 and 304 austenitic stainlesssteel. The segregated atoms do not randomly distribute in the grain boundary region.There is a ⁷.2 nm concentration fluctuation periodicity. The periodic distribution ofsegregated atoms is also the reflection of the structure of grain boundaries. (2) Before the Cr23C6 nucleated at grain boundary, C atoms and Cr atomsco-segregated to the grain boundaries in Alloy 690 and 304 autenitic stainless steel.The C and Cr atoms co-segregated at one side of the grain boundaries while not thegrain boundary core regions, the grain boundaries lying on the high indexed crystalplane of that side of grain. The number of excess Cr atoms in the C-Crco-segregation zone is more than that demanding for the nucleation of carbide, butthe number of excess C atoms is too low to nucleate carbide. If the concentration ofC is more than 0.3 at. %, the C-Cr co-segregation will form at the grain boundary.When the concentration of C is less than 0.2 at. %, the C-Cr co-segregation zone cannot be detected at grain boundaries.(3) Carbides nucleate on the grain which grain boundary plane lying on the highindex crystal plane, and have coherent orientation relationship with it. Carbides growfaster into the grain which does not have coherent orientation relationship with it. Thephase interface between carbides and matrix can be divided into flat and curvatureparts.Most of incoherent phase interfaces are curvature and not lying on typicalcrystal planes.More and more flat interfaces form at the coherent interface duringprolonging the aging time. These flat phase interfaces generally lye on {111}, {002},{011} crystal planes and other low index crystal planes.(4) The Cr atoms are observed to be periodically segregate on low index crystalplanes near the coherent phase interfaces. The periodic segregation leads to atransition phase forming near the curvature coherent phase interfaces. The transitionphase is less than 30 nm in width. This transition phase has hexagonal close packstructure with the lattice constants:And the transition phase has (111)γ// (0001)_hcp, (1-1-1)γ// (11-2-1)_hcp, (200)γ//(11-22)_hcp orientation relationship with the matrix.(5) Not only the Cr concentration level, but also the Cr concentration gradientsin the Cr depletion zone neat the grain boundary are the key factors that influencethe resistance to intergranular corrosion of the grain boundary. The grain boundaryengineering (GBE) treatments can improve the resistance to intergranular corrosion of the materials significantly. The weight losses of GBE specimens weresignificantly reduced than that of Non-GBE specimens. The intergranular corrosiongenerally penetrates along the outer random grain boundaries of the grain clusters inGBE specimens. The grain clusters are hard to drop, and the lower layer of thematrix can be protected.