Topic: Formation of Face-centered Cubic Phase in Titanium
Time: 2:30~4:30 pm, Jan 5th, 2015, Monday
Place: MSE205

Abstract: For solid materials with fixed chemical compositions, the phase of a material is determined by the evolution of free energies at varying temperature and under different external fields. Since every single phase has different physical properties, phase transition has been used to optimize materials properties in various applications. Titanium is a well-studied material with high strength-to-weight ratio and excellent corrosion resistance that is an important structural material for many industrial applications. Traditionally, pure Ti is found in the hexagonal closed packed (HCP) structure at ambient temperatures and body centered cubic (BCC) structure at elevated temperatures. Surprisingly, we show that Ti can actually undergo a thermally induced HCP to face centered cubic (FCC) phase transition in freestanding thin foils upon heating that was not anticipated in Ti phase diagrams. This FCC phase was found to be stable at ambient temperatures as well, which allowed us to explore the mechanical properties of this new phase using nanomechanical in situ TEM testing. It was found that FCC platelets within the HCP matrix phase formed strong barriers for dislocation propagation, significantly contributing to the work hardening capabilities of the dual-phase Ti. Combining multi-scale modeling, our results suggest a size-related formation mechanism of an unexpected dual-phase Ti which could be potentially used in the design of advanced Ti alloys.



 Qian Yu



 Center of Electron Microscopy and State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China; email
Yu earned her PhD degree in materials science and engineering from University of California at Berkeley in 2012. She was a postdoctoral researcher at National Center for Electron Microscopy at Lawrence Berkeley National Lab and UC Berkeley from 2012 to 2014. She joined the faculty of the Center for Electron Microscopy at Zhejiang University in 2014, where she is also a professor in the Department of Materials Science and Engineering. Yu is interested in materials characterization. Specifically her research interests focus on applying in situ electron microscopy techniques to probe into the correlations between structure and properties of materials.