Origin and Characterization of the Local Stress State at Dislocation Channel - Grain Boundary Interaction Sites and its Role in Irradiation Assisted Stress Corrosion Cracking
Johnson, Drew
2020
Abstract
The objective of this study was to quantitatively measure the stress state present at the interaction sites between dislocation channels and grain boundaries in irradiated stainless steel. The full geometry of grain boundaries was determined using electron backscatter diffraction and mechanical serial sectioning techniques such that calculated stresses could be resolved in a direction acting normal to the grain boundary plane. The full stress tensor surrounding dislocation channel – grain boundary interaction sites were calculated prior to straining in a high temperature aqueous environment Grain boundary susceptibility to intergranular fracture was characterized based on boundary misorientation, offset angle with respect to the loading axis, Schmid Factor, and stress magnitude acting at the point of intersection between individual dislocation channels and grain boundaries. Results show that the magnitude of the tensile stress acting perpendicular to the grain boundary is strongly correlated with the cracking susceptibility of the boundary, however a complete picture of the mechanisms responsible for irradiation stress corrosion cracking is still unknown. Molecular dynamics simulations and a discrete dislocation array model were used to benchmark the calculated stress values made using cross correlation at individual dislocation channel – grain boundary interaction sites. Both models, while approaching the problem using different methods and length scales, were able to accurately describe the stresses that were observed experimentally at both discontinuous and continuous channel interaction sites. Discontinuous channel interactions were observed to generate a large stress field at the grain boundary, while a similar stress amplification was not observed at continuous channel sites. Comparing the cross correlation stress measurements with cracking behavior after straining in a simulated reactor environment allowed for the direct linkage of tensile and shear stress magnitude with crack initiation, which is a first of a kind measurement. This work was able to determine a pseudo-threshold for crack initiation in a Fe13Cr15Ni alloy, where below this stress no cracks were observed. Above this threshold, cracks were observed to form and the cracking fraction of analyzed sites increased as the total stress acting normal to the grain boundary increased. The findings of this work indicate that the stress acting normal to the grain boundary, which is caused by both the impinging dislocation channel and the applied stress during deformation, drives the crack initiation process as long as the material is subjected to a high temperature corrosive environment. This helps provide additional insight into the mechanisms which are fundamental to the IASCC process.Subjects
IASCC Local Stress HREBSD Cracking in irradiated material Localized Deformation and Dislocation Channeling
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