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Quantitative In-Situ TEM Studies of Small-Scale Plasticity in Irradiated and Unirradiated Metals


In this work, unirradiated and irradiated model body centered cubic (BCC) and face centered cubic (FCC) materials are investigated using advanced electron microscopy techniques to quantitatively measure local stresses and strains around defects, with the overarching goal of obtaining a fundamental understanding of defect physics.

Quantitative in-situ transmission electron microscopy (TEM) tensile tests are performed with Molybdenum-alloy nano-fibers, functioning as a model BCC structural material. Local true stress and strain around an active Frank-Read type dislocation source are obtained using quantitative load-displacement data and digital image correlation. A mixed Frank-Read dislocation source, b=a/2[-1-11](112) with a line direction 20° from a screw orientation and length 177 nm, is observed to begin operating at a measured local stress of 1.38 GPa. The measured local true stress values compare very well to estimated stresses using dislocation radius of curvature, and a line-tension model of a large bow-out configuration, with differences of only ~1%. The degree to which the local true stresses can be measured is highly promising. However, the ultimate failure mode of these fibers, sudden strain softening after dislocation starvation and exhaustion, cannot be captured at the typical camera frame rate of 30 frames per second. Thus, fibers are mechanically tested while under observation with the Gatan K2-IS direct electron detector camera, where the frame rate is an order of magnitude larger at 400 fps. Though the increase in frame rate adds to the overall understanding of the sudden failure, by definitively showing that the nano-fibers break rather than strain soften, the failure mechanism still operates too quickly to be observed. In the final investigation of this BCC model structural alloy, the mechanical behavior of heavily dislocated, but unirradiated, and He1+ and Ni2+ irradiated nano-fibers are compared. Remarkable similarities are found in the mechanical data, as the two defect conditions exhibit similar yield strengths, ultimate tensile strengths, and number and size of load-drops. This similarity implies that, even if materials contain dissimilar individual defects, the collective defect behavior can result in similar mechanical properties. Thus, the origin of mechanical properties can be ambiguous and caution should be taken when extrapolating to different size scales. Furthermore, such similarities highlight the importance of in-situ observation during deformation. These experiments provide a key test of theory, by providing a local test of behavior, which is much more stringent than testing behaviors averaged over many regions.

Advanced electron microscopy imaging techniques and quantitative in-situ TEM tensile tests are performed with Au thin-film as a model FCC structural material. These investigations highlight the various hurdles experimental studies must overcome in order to probe defect behavior at a fundamental level. Two novelly-applied strain mapping techniques are performed to directly measure the matrix strain around helium bubbles in He1+ implanted Au thin-film. Dark-field inline holography (DFIH) is applied here for the first time to a metal, and nano-beam electron diffraction (NBED) transient strain mapping is shown to be experimentally feasible using the high frame rate Gatan K2 camera. The K2 camera reduces scan times from ~18 minutes to 82 seconds for a 128x256 pixel scan at 400 fps. Both methods measure a peak strain around 10 nm bubbles of 0.7%, correlating to an internal pressure of 580 MPa, or a vacancy to helium ion ratio of 1V:2.4He. Previous studies have relied on determining the appropriate equation of state to relate measured or approximated helium density to internal bubble pressure and thus strain. Direct measurement of the surrounding matrix strain through DFIH and NBED methods effectively bypasses this step, allowing for easier defect interaction modeling as the bubble can be effectively simplified to its matrix strain. Furthermore, this study demonstrates the feasibility of fully strain mapping, in four dimensions, any in-situ TEM experiment. The final set of experiments with this model FCC structural material shows the attempted correlation of defect interactions and deformation behavior at the nano-scale. Experimental comparison of mechanical behavior from quantitative in-situ TEM tensile tests of focused ion beam (FIB) shaped, He1+ implanted, and FIB-shaped He1+ implanted Au thin-film show a wide range of behavior that could not be directly linked to irradiation condition. This is due to the large role that overall microstructural features, such as grain boundary orientation and texture, play in mechanical behavior at this size scale. However, these tests are some of the first to in-situ TEM mechanically strain single grain-boundaries free of FIB-damage. It is expected that, with well-defined grain orientations and boundaries, real conclusions can be made.

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