Phase Transformation Of Ti-55531Titanium Alloy During Continiuous Heating Process

Ti-55531alloy is a near-β titanium alloy with high strength, high toughness and fine hardenability(δb≥1200MPa, KiC≥55MPa·m1/2, Hardening surface thickness≥250mm), which has a significant value in aerospace industry, especially making framing member of heavy aircraft. Mechanical property of Titanium alloy is closely related to microstructure, while microstructure is determined by phase transformation during heat treatment process. As a newly developed Titanium alloy, Ti-55531has all the complex phase transformation in near-β titanium alloy. Isothermal heat treatment has been a mature method studying phase transformation. However, there are a lot of restrictions to isothermal heat treatment, such as repeatedly and long-time aging, incontinuity of temperature etc. Considering the problems above, Continiuous heating method was introduced in this paper. And both X Ray Diffraction, microstructure observation were conducted in this research. We successfully investigated over dilatation, sequence, mechanism, thermodynamics and dynamics of Ti-55531phase transformation. The main conclusions were as follows,1) Phase transformation sequence of Ti-55531during continiuous heating was confirmed as:Below192℃:ωathâ†'β;192～347℃:βâ†'>ωiso;347～376℃:ωisoâ†'α+β and βâ†'α;409～648℃:βâ†'α;648～831℃αâ†'β; Over831:full β.2)The ωiso in Ti-55531was proved to behave as nucleation sites for nanoscale a phase precipitation:The orientation relationship (210)ω//(002)α between α and ω was found, indicating that the nucleation of a phase taken place in the core of ωiso particles.3)The function of βâ†'ωiso activation energy and fraction was built, and the average activation energy of βâ†'ωiso in Ti-55531was calculated as81.6KJ/mol.4)The KJMA model to non-isothermal regime was built, and an expression of the local Avrami exponent as a function of the phase volume fraction was obtained. Consequently, the phase transformation mechanism of ωiso phase was investigated. Results showed that in the initial stage, the mechanism of3D nucleation and diffusion-controlled grain growth dominates the transformation, and in the main part of the whole transformation process, the phase transformation mechanism is3D growth at a near-0nucleation rate.