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