Evolution and mechanism of combustion microstructure of 600 ℃ high temperature titanium alloy
Authors: Wu Ming-Yu , Mi Guang-Bao , Li Pei-Jie , Huang Xu ,   Tribological Key Laboratory, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China,   Aviation Key Laboratory of Science and Technology on Advanced Titanium Alloys, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China
Published: June 2023

Oxides formed in the combustion process significantly affect the flame retardancy of titanium alloys, however, the evolution mechanism and formation mechanism of the combustion products of 600 ℃ high temperature titanium alloy remain uncertain. Frictional ignition method is employed in this paper to study the combustion behaviors of 600 ℃ high temperature titanium alloy, and the flame retardancy is evaluated according to the friction time, oxygen content and combustion state. <i>In-situ</i> observation of the burning phenomenon at the friction position and morphology after combustion is investigated, and the combustion states can be divided into oxidation stage, ignition stage and extended combustion stage. Further microstructure analysis is conducted subsequently by focus ion beam (FIB) and high resolution transmission electron microscope (HRTEM) to characterize the oxidation products with different valences in different zones of combustion microstructure. Al<sub>2</sub>O<sub>3</sub>, Ti<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> are observed as the main combustion products in the heat-affected zone, melting zone and combustion zone, respectively. Notably, TiO<sub>2</sub> is found to be formed by Ti<sub>2</sub>O<sub>3</sub> under the combustion condition, which is different from the TiO<sub>2</sub> transformed from the TiO mesophase under oxidation condition. This results in a lax structure composed of spherical Ti<sub>2</sub>O<sub>3</sub> particles and porous Ti matrix in the melting zone. Thermodynamic calculations including Gibbs free energy and decomposition pressure are discussed to explain the evolution mechanisms and formation mechanisms of different oxides. It is revealed that an Al content of 6% is insufficient to form a continuous protective Al<sub>2</sub>O<sub>3</sub> layer at the interface of the melting zone and heat affected zone. The difference in reaction path between TiO<sub>2</sub> formed by TiO and by Ti<sub>2</sub>O<sub>3</sub> can be ascribed to the formation of gaseous TiO phase. The sharp increase of TiO vapor pressure at about 1800 K reduces the stability of titanium oxide, thus causing the as-formed TiO to evaporate rapidly and forcing titanium to transform into TiO<sub>2</sub> via a more stable phase, Ti<sub>2</sub>O<sub>3</sub>. The formation of the porous structure composed of Ti<sub>2</sub>O<sub>3</sub> and Ti at the melting zone provides a path for the rapid internal diffusion of oxygen, which severely deteriorates the oxygen prevention capability of as-formed oxide layers. Besides, the TiO<sub>2</sub> synthesized from Ti-O melt in the combustion zone can hardly protect the inner structure. Therefore, the flame retardancy of 600 ℃ high-temperature titanium alloy is far from satisfactory.