Exploration of Novel Nano-scale Instabilities in Metastable Beta Titanium Alloys Using Transmission Electron Microscopy and Aberration-Corrected Scanning Transmission Electron Microscopy

Abstract

Recently, metastable beta titanium alloys have attracted considerable attention in the field of aerospace and bio-medical industries, due to the combination of attractive properties, such as high strength, low modulus, corrosion resistance and bio-compatibility. Compared with the most widely used alpha/beta titanium alloy, Ti-6Al-4V (wt.%), the microstructure in metastable beta titanium alloys can be manipulated by various thermo-mechanical processing routes that often involve the precipitation of fine scaled intragranular hcp structure alpha phase, with tailored fineness of the distribution of the precipitate phase depending on the given heat-treatment, generated by the influence of nano-scale structural and compositional instabilities. With the technological improvement in transmission electron microscopy, especially the probe aberration corrected scanning transmission electron microscopy, it has enabled the atomic resolution Z-contrast high angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) imaging, which is capable of characterizing the atomic column configurations with a sub-angstrom probe [1] and provides novel insights of nano-scale structures in titanium alloys. In our recent studies coupling aberration-corrected scanning transmission electron microscopy and 3D atom probe tomography, it has been shown that the nano-scale metastable hexagonal structure isothermal omega phase particles in the parent bcc structure beta phase can significantly affect the size, morphology and number density of alpha precipitates in Ti-5Al-5Mo-5V-3Cr (Ti-5553, wt.%) upon different heat treatments by providing additional driving force for super-refined alpha nucleation from the compositional and stress field associated with these nano-scale particles [2].