When 11:30 AM - 1:30 PM Dec 14, 2015
Where 2906 Cooley Building, Baer Room
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The Role of Dislocation Channeling in IASCC Initiation of Neutron Irradiated Austenitic Stainless Steel


Kale Stephenson
Thesis Defense

Gary Wise, advisor.

 

The objective of this study was to understand the role of dislocation channeling in the initiation of irradiation-assisted stress corrosion cracking (IASCC) of neutron irradiated austenitic stainless steel. Constant extension rate tensile (CERT) experiments in a simulated normal water chemistry (NWC) environment (288°C, 2 ppm dissolved oxygen, 0.2 μS/cm) compared cracking behavior amongst a set of stainless steels after irradiation in the BOR-60 fast reactor at 320 °C. Based on the results, several alloys with a range in IASCC susceptibility were selected for study, including a commercial purity 304L stainless steel irradiated to 5.5, 10.2, and 47.5 dpa and two high purity stainless steels, Fe-18Cr-12Ni and Fe-18Cr-25Ni, irradiated to ~10 dpa. CERT test results determined that irradiation hardening correlated strongly with increased IASCC, and the average height of steps formed by dislocation channels intersecting the sample surface was larger on conditions with high IASCC susceptibility. Despite the results of these experiments, limited knowledge was gained in regards to the crack initiation mechanism as samples were strained to complete failure in the CERT test.

 

To better understand the key factors affecting IASCC initiation, a miniature four-point bend test was developed and performed in NWC conditions on samples created from irradiated tensile bars used in the CERT tests. Four-point bend tests were performed in small stress or strain increments to characterize the development of dislocation channeling and observe intergranular cracks with limited propagation. For the same alloys, the four-point bend test produced the same relative IASCC susceptibility as constant extension rate tensile (CERT) experiments, which allowed the prior knowledge from CERT results to be used as a prediction tool for IASCC behavior in the four-point bend test.

 

IASCC initiation was induced by the dissolution of MnS inclusions in high temperature water, which created an occluded local environment with a high potential for crack initiation. The intersection of dislocation channels with these sites provided the stress necessary to initiate an intergranular crack. The stress necessary to initiate IASCC decreased with increasing dose, due to a reduction in the stress to form dislocation channels. The HP Fe-18Cr-12Ni alloy was less susceptible to IASCC than the CP 304L alloy due to the lack of MnS inclusions, and no cracking was observed in the high Ni alloy. The IASCC resistance of the high Ni alloy was attributed to its propensity for dislocation channel transmission across grain boundaries, which relieved stress accumulation and likely prevented the critical grain boundary stress required to initiate cracking from being achieved.