UC Berkeley

Abstract

In-Situ Micromechanical Testing in Extreme Environments

By

Amanda Sofia Lupinacci

Doctor of Philosophy in Engineering - Materials Science and Engineering

University of California, Berkeley

Professor Andrew M. Minor, Chair

In order to design engineering applications that can withstand extreme environments, we must first understand the underlying deformation mechanisms that can hinder material performance. It is not enough to characterize the mechanical properties alone, we must also characterize the microstructural changes as well so that we can understand the origin of material degradation. This dissertation focuses on two different extreme environments. The first environment is the cryogenic environment, where we focus on the deformation behavior of solder below the ductile to brittle transition temperature (DBTT). The second environment is the irradiated environment, where we focus on the effects that ion beam irradiation has on both the mechanical properties and microstructure of 304 stainless steel. Both classes of materials and testing environments utilize novel in situ micromechanical testing techniques inside a scanning electron microscope which enhances our ability to link the observed deformation behavior with its associated mechanical response.

This dissertation presents the development of a novel in situ cryogenic micromechanical testing apparatus in a SEM. This technique makes it possible to directly link the observed deformation behavior of solders with the mechanical response below the DBTT. This dissertation also presents a novel method for linking the observed changes in mechanical properties of ion irradiated steels with changes in the defect density by the use of both nanoindentation, laue microdiffraction and in-situ micromechanical testing.

Characterizing plasticity mechanisms below the DBTT is traditionally difficult to accomplish in a systematic fashion. Here, we use a new experimental setup to perform in situ cryogenic mechanical testing of pure Sn micropillars at room temperature and at -142 °C. Subsequent electron microscopy characterization of the micropillars shows a clear difference in the deformation mechanisms at room temperature and at cryogenic temperatures. At room temperature, the Sn micropillars deformed through dislocation plasticity while at -142 °C they exhibited both higher strength and deformation twinning. Two different orientations were tested, a symmetric (100) orientation and a non-symmetric (45&hibar;1) orientation. The deformation mechanisms were found to be the same for both orientations. This approach was also extended to a more complex solder alloy that is commonly used in industry, Sn96. In the case of the solder alloy more complex geometries were also utilized to obtain better understanding of the deformation mechanisms in the presence of a preexisting defect, such as a crack. Similar twinning behavior is observed in the Sn96 alloy however, it was also found that grain boundaries and second phase intermetallics can act as sources for dislocation motion and generation.

The observations and analysis in this thesis give novel insight into the deformation mechanisms of two materials in extreme environments. In the case of cryogenic testing we have demonstrated the preference for Sn based solders to undergo deformation twinning as a primary deformation mechanism below the DBTT of the alloy. Prior to this study, deformation mechanisms of pure Sn below the DBTT were not well characterized due to a lack of experimental methods for doing so. In this study, we have developed a novel in situ cryogenic apparatus that has made it possible to systematically study plasticity mechanisms down to -142 °C. Through EBSD and TEM characterization of the twinning behavior at cryogenic temperatures, we have gained a greater understanding of the deformation mechanisms that are active below the DBTT. Furthermore, we have extended this technique to a more complex alloy, Sn96 as well as more complex geometries (clamped beam geometry).

In the case of ion irradiated stainless steel, which is a surrogate for neutron irradiations, we have correlated the mechanical property response that is associated with various irradiation conditions with the defects induced during irradiation. Due to the fact that ion beam irradaion has a limted penetration depth, nanoindetation and micro compression testing needed to be deployed. Through both nanoindentation and Laue micro diffraction we have shown a direct correlation between the observed radaition induced increase in hardness and the radaition induced increase in FWHM. We found that 10dpa ion beam irradaion is enough to saturate the material in hardening as well as lattice strain which is related to the defects present.. It was found that the 304SS shows no FWHM gradient assocatied with the ion beam irradiation The use of micropillars also enabeld us to characterize the dramatic change in yield strength due to ion beam irradiation while a detaield charactreisation of each micro pillar in regards to the critical resolved shear strenght is conducted. It was found that the radiation envrionment causes not just an increace in critical resolved shear stress but the material also looses its workhardenability completely. It is also shown that post testing TEM investigation of the pillars on 304 stainless steel apears not possible due to the fact that FIB damage induces a stress induced phase transfomration and therefore a detailed crystallogprahic examination using TEM cannot be conducted.

In summary this thesis demonstrate novel small scale mechanical testing techniques on engineering applications which opens the gate to utilize basic scientific tools on engineering problems.