Doctor of Philosophy
The nickel-based Alloy 600, also known as Inconel 600 has been found to be susceptible to stress corrosion cracking (SCC) in high temperature aqueous environments. Despite extensive research, the mechanisms by which this process occurs remain in question. It is known that SCC results from the simultaneous effects of a chemically corrosive environment and a tensile stress. Many studies have been conducted on the microscopic chemical changes associated with SCC. There have been fewer studies of the microscopic stress/strain process. The main objective of this thesis is to use the new x-ray based diffraction technique – polychromatic x-ray microdiffraction (PXM) to study changes of the microstructure and elastic/plastic deformation introduced by SCC in Alloy 600, thus leading us to an understanding of the mechanisms of SCC. More traditional techniques such as neutron diffraction and electron backscatter diffraction (EBSD) were also used in this thesis. Neutron diffraction measures information down to a depth resolution of a few mm, while EBSD is considered to be a surface measurement (1-2 μm). By comparison, PXM examines strain to a depth of several grains in alloy 600 (~ 60 μm), which is particularly appropriate for detecting information from intergranular interactions under typical stress corrosion conditions. The capability of measuring strain directions (compressive or tensile) is another advantage of PXM in SCC study. Besides the microstructure and elastic strain information, the local plastic deformation can also be examined by assessing the streaking/splitting of the Laue spots and modeling of the Laue diffraction images. In this thesis, PXM was first validated by comparing the data to those from neutron diffraction and EBSD for a simple case – uniaxially stressed tensile specimens. Then studies were carried out on C-ring samples before and after cracked by SCC. Torsional stressing of an Alloy 600 C-ring specimen results in significant tensile strain in the stress axis direction. Intergranular SCC (IGSCC) was observed in Alloy 600 C-ring specimens electrochemically corroded in a high temperature caustic solution. Tensile strains with respect to the stress axis and plastic strain could be identified in regions ahead of the crack tip. These regions iv correspond to triple junctions where crack arrest occurs and re-initiation requires a buildup of the strains. Keywords Alloy 600, Polychromatic X-ray Microdiffraction (PXM), neutron diffraction, Electron Backscatter Diffraction (EBSD), Stress Corrosion Cracking (SCC), deviatoric strain, plastic deformation, dislocation
Chao, Jing, "Studies of Microscopic Scale Strains in Nickel Alloys Resulting from Mechanical and Chemical Forces" (2012). Electronic Thesis and Dissertation Repository. Paper 900.