Introducing ω and O′ nanodomains in Ti-6Al-4V: The mechanism of accelerating α → β transformation kinetics via electropulsing

Abstract

Conventional kinetics theory for diffusion-controlled phase transformation shows that the reverse transition should lag behind the temperature rise through rapid heating, i.e., overheating is required. In this work, we found that the β-transus temperature decreased by ∼50 °C during studying the α → β transformation in Ti-6Al-4V alloy via electropulsing treatment (EPT). The calculation suggests that the acceleration of transformation kinetics cannot be fully explained by Joule heat and athermal effects of the electromigration effect and electron wind theory. The microstructural evolution during EPT was systematically investigated utilizing scanning electron microscope (SEM), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), transmission Kikuchi diffraction (TKD), and transmission electron microscope (TEM). Microscopic analysis shows that the nano-sized ω and O′ phases formed in the β phase, which causes large numbers of lattice distortion regions. The defects are conducive to accelerating the bulk diffusion of alloying elements in β. Moreover, the nanodomains limited the growth of martensite, therefore nanocrystalline martensite formed after quenching. These findings develop the understanding of the destructive effect of current on metallic crystal, which will help to guide microstructural regulation in titanium and other alloys.