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Scholarly Works (3 results)

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UC Berkeley Electronic Theses and Dissertations (2017)

Investigation of FCC Ti in TEM Foils Formed Under Three Separate Conditions

by

Rachel E. Traylor

Doctor of Philosophy in Materials Science and Engineering

University of California, Berkeley

Professor Andrew M. Minor, Chair

A hexagonal-close packed (HCP) to face-centered cubic (FCC) phase transition has been observed in freestanding alpha-titanium thin foils under three separate conditions: Type I – upon focused ion beam (FIB) irradiation, Type II – during in situ heating in a transmission electron microscope (TEM), and Type III – during in situ TEM straining. FCC Ti has not been anticipated based on the equilibrium phase diagram, however, all FCC Ti phases were found to be stable after formation under ambient conditions. Investigations into the chemical nature of these anomalous FCC Ti phases have revealed that FIB-induced FCC Ti is actually a form of titanium hydride while thermally-induced is more closely related to oxygen contamination. Strain-induced FCC Ti is neither a hydride nor an oxide and readily undergoes FCC deformation twinning under an applied load. In situ TEM experimentation coupled with high-resolution nanobeam diffraction of titanium foils has suggested that this anomalous phase transformation is facilitated by HCP a-type dislocation nucleation, dissociation into partials, and propagation. For different reasons (primarily due to hydrogen, oxygen, and aluminum solutes for the type I, II, and III FCC investigations, respectively) it was suspected that HCP (not FCC) deformation twinning was suppressed in all samples where an HCP to FCC transformation was initiated. In addition, all TEM samples containing FCC Ti precipitates experienced stresses along the <0001>HCP in some form. Therefore, it is proposed that the series of transformations observed in the strain induced FCC Ti; HCP-to-FCC, and FCC-to-FCC twin help accommodate strains along the <0001>HCP when deformation twinning and (c+a) dislocation slip are not available.

    Cover page: FCC Ti
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    Pure Ti traditionally exhibits the hexagonal closed packed (HCP) crystallographic structure under ambient conditions and the body centered cubic (BCC) structure at elevated temperatures. In addition to these typical structures for Ti alloys, the presence of a face centered cubic (FCC) phase associated with thin films, interfaces, or high levels of plastic deformation has occasionally been reported. Here we show that small FCC precipitates form in freestanding thin foils during in situ transmission electron microscope (TEM) heating and we discuss the potential origins of the FCC phase in light of the in situ observations. This FCC phase was found to be stable upon cooling and under ambient conditions, which allowed us to explore its mechanical properties and stability via nanomechanical in situ TEM testing. It was found that FCC platelets within the HCP matrix phase were stable under mechanical deformation and exhibited similar mechanical deformation behavior as the parent HCP phase.
      Cover page: In situ TEM observation of FCC Ti formation at elevated temperatures
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history.
        Cover page: Direct imaging of short-range order and its impact on deformation in Ti-6Al
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