Chemical and structural analysis of grain boundaries in Inconel 690 for corrosion resistance

Abstract

Stress Corrosion Cracking (SCC) is a failure mechanism that results from the combination of tensile stress, corrosive environment and material susceptibility; it is frequently an intergranular attack. Material-environment combinations for SCC readily exist in nuclear power plants, and are critical for to the longevity of the reactor components. Inconel 690 (alloy 690 UNS N06690) is an alloy that has been put into service in the nuclear industry over the past 20 years due to its relatively good resistance to SCC. A new generation of nuclear plants is likely to be built in the US and the life of existing and new nuclear plants are expected to extend to 60-80 years. The study of alloy 690, as well as other structural metals, is important in order to understand, predict, and avert costly and dangerous failures that could occur due to SCC later in the life of the plants. The microstructure of an alloy has an important effect on its corrosion and SCC behavior. In particular, high energy grain boundary structures in austenitic Ni-base alloys and stainless steels have been shown to have greater SCC susceptibility. This thesis studies the fundamental structural and chemical properties of grain boundaries in alloy 690, to better understand the SCC resistances and susceptibilities of different grain boundary structures. In order to investigate the grain boundaries based on their structure, an integrated approach was developed to allow for site-specific chemical and mechanical characterization.

(cont.) The chemical analysis, which was the focus of this thesis, was accomplished using a Transmission Electron Microscope (TEM) for imaging and a Scanning Transmission Electron Microscope (STEM) with Energy Dispersive X-ray Spectroscopy (EDS) for elemental analysis. TEM samples from selected grain boundaries were prepared in a site-specific manner using a Focused Ion Beam (FIB). The mechanical analysis of the grain boundaries was accomplished through nanoindentation by a collaborator in the same research group. To identify grain boundaries of interest, for TEM sample creation by FIB or nanoindentation, the surface crystallographic structure was mapped using Orientation Image Microscopy (OIM). Microindents on the surface were utilized as fiduciary markers in the navigation of the surface. The three structures examined were low 1, low angle, and high angle grain boundaries. Boundaries were characterized in a: 1) solution annealed state, 2) Thermomechanically Processed (TMP) state consisting of a 5% compression followed by annealing at 10000 C with a water quench, 3) TMP state consisting of a 5% compression followed by annealing at 9500 C with a furnace cooling. Chemical composition differences, major element segregation or precipitation, were not found at grain boundaries in the solution annealed material or the TMP material that was water quenched. Cr-carbide precipitation was observed at the grain boundaries in the furnace cooled samples. The structural character and distribution of the carbides was dependent on structure of the host grain boundary.

(cont.) Low E grain boundaries exhibited a thin band of Cr-carbide on the boundary that was approximately 50 nm thick. On low angle grain boundaries, coarsened Cr-carbides were observed in semi-continuous form; with an average size of 230 nm. On high angle grain boundaries, further coarsening of the carbides resulted in a discontinuous distribution with an average precipitate size of 430 nm. Cr depletion occurred in the vicinity of the carbides; depletion was the most severe on high angle grain boundaries, down to 20wt-%. The suspected cause of the varying degree of coarsening of the Cr-carbides was the differences in diffusivity that control the kinetics of precipitation at the grain boundary. The "mean field" model for the coarsening of a distribution of carbides was used for quantitatively comparing the diffusivity of Cr at the high and low angle grain boundaries. The result indicated that diffusivity of Cr at high angle grain boundaries was an order of magnitude higher than at low angle grain boundaries, at the temperature of Cr-carbide formation 600-950' C. High angle grain boundaries have been shown to be the most susceptible to corrosion and SCC previously. The results of this work suggest that the higher diffusivity of Cr at the high angle boundaries of alloy 690 could contribute to SCC susceptibility through two mechanisms: 1) The coarser carbides, formed because of higher diffusivity, can more easily initiate microcracks if they are present. 2) The higher diffusivity leads to greater Cr redistribution, which could leave the boundary in a chemical state more prone to corrosion.