EAPSI: Investigating Grain Boundary Strength of a Helium Implanted Engineering Alloy using Micron-Scale Tensile Testing

Howard Cameron B, Berkeley CA

Investigators

Abstract

According to a NPR report, 14 percent of the world?s electricity is supplied by nuclear power. The current fleet of 438 reactors spread across 30 nations in addition to the 67 new plants under construction in 15 countries contain a large breadth of specifications, design features, and structural materials. These structural materials must withstand ever increasing amounts of neutron radiation damage in addition to operating in a high temperature and pressure and corrosive degradation environment. Their safety and reliability remain of first importance as reactor lifetimes aspire to be extended. However, they become difficult and expensive to safely handle and test because they become highly radioactive. This radiation has been known to cause unwanted embrittlement and abrupt premature failure. This project focuses on simulating the high dose radiation effect of helium production by focusing a medium energy helium ion beam on a sample at reactor operation temperature. Unlike neutron irradiation in reactors, this technique does not cause the sample to become radioactive, but because the shallow penetration depth of the helium is limited to microns in depth, test samples must be manufactured on this length scale. Micro tensile tests will be performed to investigate the effect of the helium on the grain boundary strength of a structural material commonly used in many nuclear reactors. This research will be conducted in collaboration with Dr. Dhriti Bhattacharyya at the Australian Nuclear Science and Technology Organisation (ANSTO) in Lucas Heights, Australia who is a radiation damage specialist who has developed a novel in-situ micro-scale tensile testing technique to investigate nuclear structural materials. Micro-scale tensile testing specimens will be manufactured out of helium implanted engineering alloys using focused ion beam (FIB) milling techniques. These specimens will be tested inside of a scanning electron microscope (SEM) in a fashion that will effectively pull apart the material?s grain boundaries in order to test how the helium has effected its grain boundary strength, a strength determining feature crucial to the material remaining intact and reliability during extended service. In addition to providing quantitative mechanical stresses, this technique allows for real time observation of local deformation in real time, providing valuable information concerning the ultimate failure mechanism of reactor components. Selecting of specific types of grain boundaries to investigate and different microstructural features can be made using electron backscattered diffraction (EBSD) and post-test investigations can be performed using transmission electron microscopy (TEM) and EBSD to further understand the deformation mechanisms and the distribution of the helium, concerning whether it segregates along the material?s grain boundaries. The methods development of micro-tensile testing will have a wide variety of applications in materials research as a whole, and it will also allow a greater number of labs worldwide to safely conduct research and obtain better statistics with minimal radioactive sample material. This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Australian Academy of Science.