Electron Microscopy Study on the Precipitate Transition Behavior in Metastable beta-Ti Alloys

Abstract

Microstructural evolution in β Titanium alloys is an important factor that governs the properties exhibited by them. Exact understanding of complex phase transformations in these alloys is vital to tailor their microstructures and in turn their properties to our advantage. Apart from the effect of thermal treatment on these alloys, the focus of this work is to study is the nucleation and growth of α precipitates triggered by the compositional instabilities in the β matrix, instilled in them during non equilibrium heat treatments. The present work is an effort to investigate such a phenomenon. For this, advanced nano-scale characterization tools such as High Resolution STEM, High Resolution TEM, EFTEM and EDS have been used to determine the structure, distribution and composition of the non equilibrium instabilities such as ω, and also to investigate the subsequent nucleation of stable α. Here studies have been conducted metastable β-Titanium alloys of Ti-15wt%Mo alloy, an attempt is made to understand the stability of the ω phase and factors governing the nucleation of the α-phase at low to intermediate temperatures. For the first time, atomically resolved HRTEM results showed the presence of nano-scale α regions within the ω precipitates. These studies are revalidated by conventional selected area diffraction and EDS results. Also TEM dark field and selected are diffraction studies are conducted to understand the effect of quenching and subsequent aging of ω precipitates. Using HAADF-STEM techniques, the elemental partitioning involved in coarsening of ω is investigated in detail. Thus in this work, very early stages of phase nucleation in titanium alloys are successfully probed at an atomic resolution. It was shown experimentally that there is mixed-mode transformation occurring during the growth of these precipitates that is diffusion controlled. Finally by calculating the free energy composition diagrams for the Ti-Mo alloy system, it can be suggested that the  precipitation in the  matrix occurs in the following sequence. The calculated G-X plots obtained in the present work, therefore, show that there is necessity of considering the  phase should be nucleated at the core of the  phase particles. It would be useful to create a phase transformation model with various alloying elements and predict omega morphology and distributions. Furthermore, with the compositional information collected, the effect of omega as a heterogeneous  nucleation site can be modeled.