Influence of Quenching Parameters on the Stability of the β Solid Solution in a High-Strength Titanium Alloy

Abstract

Changes in the structure and phase composition of the high-strength Ti–5Al–5Mo–5V–3Cr alloy after quenching in water and in air from the temperatures of heating corresponding to the β field (in the range of 860–1000°C) and upon subsequent continuous heating have been studied using the methods of optical, scanning, and transmission electron microscopy, X-ray diffraction, microprobe, and thermal analyses, and hardness. It has been shown that the temperature of heating to a significant extent influences the structure of the Ti–5Al–5Mo–5V–3Cr alloy after cooling. In the case of a significant overheating above the temperature of the polymorphic transformation (T_pt +100–150°C), a large amount of vacancies arise, which activate the diffusion processes and promote the appearance of “precipitates” upon cooling in air in the form of single or intersecting packets of very thin plates (with a thickness of 3–5 μm) enriched in Al, Mo, Cr and with a bcc lattice similar to that of the β matrix. Upon subsequent continuous heating, this leads to the inhibition of the decomposition of the metastable matrix β solid solution with the formation of intermediate phases by a homogeneous mechanism and activates the diffusion β → α transformation (as compared to quenching in water), which occurs by the heterogeneous mechanism of decomposition.