Small-Scale Mechanical Testing and Size Effects in Extreme Environments

Ion irradiation is often used to simulate the effects of neutron irradiation due to reduced activation of materials and vastly increased dose rates. Nonetheless, the low penetration depth of ions requires the development of small-scale mechanical testing techniques, such as nanoindentation and microcompression, in order to measure mechanical properties of the irradiated material. Nanoindentation is a widespread and useful method for evaluating mechanical properties at the sub-micron length scale. However, the Indentation Size Effect (ISE) where hardness increases with decreasing penetration depth remains a major obstacle to obtain meaningful macroscopic mechanical properties from small volume testing. In situ microcompression testing, although requires extensive sample preparation, has served as a novel small-scale testing tool for evaluating mechanical properties due to the capability to directly observe deformation mechanisms while probing well-defined volumes and producing real-time stress-strain curves. Nevertheless, Sample Size Effect (SSE), the commonly observed phenomenon where the strength of materials increases with decreasing sample size, has remained a remarkable drawback of this technique. Although ISE and SSE phenomena have been studied extensively at room temperature, the influence of temperature on both phenomena is currently not clear./nThis work emphasizes the development of high temperature small-scale mechanical testing techniques and systematically addresses the independent influence of irradiation and temperature on ISE and SSE phenomena in an austenitic Fe-Cr-Ni alloy (800H) to establish the baseline for nanoindentation testing and microcompression testing for metal alloys, especially for ion-irradiated alloys in extreme environmental conditions. The 800H steel sample was irradiated with 70 MeV Fe9+ at 450 C to the total dose of 20.68 dpa. Cross-sectional indents performed perpendicular to the irradiated edge confirms the SRIM calculation of 6.2 μm penetration depth. FIB-fabricated TEM foil containing the unirradiated and the irradiated areas of the sample suggests the validity of characterization studies of irradiated samples prepared by FIB procedures. All micromechanical tests were conducted up to 300 C, which is well below the irradiation temperature, to quantify the effect of temperature on size effects without the influence of dislocation density./nEBSD was performed to search for two grains of known orientations large enough to contain all indents for ISE studies in both the unirradiated (<111> and <131>) and the irradiated (<111> and <113>) 800H at all temperatures. A <111> grain orientation was purposely chosen to be the same for direct comparisons before and after irradiation. An interesting observation is that grain orientation has a strong influence on indentation size effect. It was found that in all cases, the ISE is less pronounced at high temperatures due to the increase of the plastic zone size. For the same grain orientation, the indentation size effect is less pronounced in the irradiated 800H at all test temperatures./nIn order to allow straightforward cross-comparisons between indentation and microcompression testing techniques, micro-pillars were fabricated in a <111> grain containing both the unirradiated and the irradiated volumes at room temperature and at 300 C. In situ characteristics of the microcompression testing revealed the change in deformation mechanism during compression after irradiation. TEM lift-out of the compressed micro-pillars were fabricated for microstructural characterization. The influence of irradiation and temperature on sample size effect were independently investigated. The yield stress of micro-pillars increase with irradiation and with decreasing size. For the first time, the temperature dependence of sample size effect is observed in fcc metals. This implies that bulk strength rather than crystal structure determines the behavior of sample size effect at elevated temperatures./nIn summary, this dissertation unravels the fundamentals behind indentation size effect and sample size effect at high temperatures and correlates microstructures with mechanical properties. It was discovered that mechanical testing methods determine how size effect behavior is influenced by temperature. Due to significantly less pronounced indentation size effect, high temperature nanoindentation has been proven to help bridge the gap between small-scale mechanical testing and bulk testing. The clear difference in the microstructures of the unirradiated and the irradiated regions in the FIB-manufactured TEM lift-out foil justifies FIB as a proper TEM preparation method. In situ microcompression and TEM lift-outs of the compressed micro-pillars demonstrate the change in deformation mechanisms resulting from ion irradiation.

Advisor: Minor, Andrew M