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Characterization of Nano-scale Instabilities in Titanium Alloys Using Aberration-Corrected Scanning Transmission Electron Microscope

Due to the refined nature of microstructures that can be effectively manipulated by the application of various thermal/mechanical processes, metastable beta titanium alloys have attracted considerable attention in recent days. Usually, such refinement involves the precipitation of the intragranular hcp structure alpha phase. In authors recent studies, it has been shown that the size, morphology and number density of these alpha precipitates in Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) can be significantly influenced by the nano-scale structural and compositional instability present in parent bcc structure beta phase, more specifically in this alloy, the metastable hexagonal structure isothermal omega phase [1-3]. In these latter studies, it was found that either the compositional and/or stress field associated with the isothermal omega phase may contribute to an increased driving force for alpha nucleation [3]. Recent technological improvement in TEM’s, spectroscopy detectors and cameras, specifically probe aberration corrected STEM instruments, have enabled atomic resolution z-contrast high angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) imaging capable of characterizing atomic column configurations with a sub-angstrom probe [4] and provides novel insights of new nano-scale instabilities in titanium alloys.