Development of in-situ FIB/SEM techniques for microstructural characterization and evolution of polycrystalline metals

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Kane, Genevieve A.
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Electronic thesis
Materials engineering
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A fundamental understanding of materials processing through thermal and mechanical means is a significant factor when choosing materials to use across industries. The ability to control the failure of materials, or to choose a material strength that is appropriate for different applications, is useful across many industries. Current methods of testing for desired material properties include multiple time-consuming steps. For example – in order to inspect the strength of a metal, one may have to go through multiple heat treatments of the metal, inspection, sample preparation, mechanical testing, and final failure analysis. A faster and more reliable approach that could be universally used to help reduce the time of this testing would be valuable from an industry standpoint. In order to assist in the reduction of materials processing time, a more thorough understanding of the development of microstructure within material systems can be developed by observing microstructure evolution in real time under different thermal and mechanical processes. In this work, we achieve this in two ways: 1) The investigation of copper grain growth under uniform and non-uniform temperature gradients and 2) The study of Ti 6%Al 4%V (Ti 6-4) alloys during thermomechanical processing and cooling. To achieve this, the development of novel techniques, using a scanning electron microscope integrated with both thermal (Cu) and thermomechanical (Ti 6-4) processing techniques are created for each material system. To investigate grain growth in copper, an in-situ heater is employed to allow samples to reach temperatures from 300-500 °C for observation. This is then coupled with a micro-heater array allowing the observation of copper thin films deposited over ten heater array lines, capable of creating a temperature gradient across a sample. This novel technique allows up to a ~30 °C/cm temperature gradient across the sample. This allows in principle for investigation, not only of thin film copper grain growth at uniform temperatures over time, but for the modification of the gradients to optimize the grain structure. Ti 6%Al 4%V is a two-phase alloy commonly used in aerospace applications due to its ability to maintain high strength at service temperatures in excess of 500 °C. The resulting microstructures of Ti 6%Al 4%V span equiaxed, fully lamellar, martensitic and bi-modal microstructures. Each can be obtained through a combination of multiple deformation, recrystallization and annealing steps to obtain the final microstructure desired. The ability to deconvolute the effects of different processing steps (homogenization, deformation, recrystallization, and annealing) and combine them in new ways could reduce processing time, if the optimal microstructure can be achieved with fewer thermomechanical treatments. A stage that is capable of thermomechanical processing via localized heating and tensile testing for use inside of a scanning electron microscope was commissioned and modified, with capabilities of heating the sample above the Ti 6%Al 4%V β transus temperature of 995 °C, and straining the sample in ranges from 1x10-3 s-1 to 1x10-5 s-1. In addition, the capability of monitoring cooling rates in real time is developed. We aim to maintain uniform fields of view and imaging conditions in order to assess microstructure as thermomechanical deformation occurs. A series of experiments is developed to deconvolute the impact of cooling rate on lamellar microstructure formations. This leads to understanding the mechanisms of lamellar microstructure formation, and the optimization of the size of lamellae for future materials processing. Integrated indentation capabilities being developed will also allow for the potential of real time feedback into the microstructural properties of the lamellar structure, in order to further inform the optimal size of the microstructure. The resulting microstructure library leads to the ability to correlate multiple parameters with microstructure at different processing steps, resulting in a clearer understanding of the influence of those steps. This has the potential to contribute to the optimization of materials, significantly reduce the time needed to process Ti 6%Al 4%V and other alloys, and give insight on the fundamental growth of microstructure.
December 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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