Studies on the β → α Phase Transition Kinetics of Ti–3.5Al–5Mo–4V Alloy under Isothermal Conditions by X-ray Diffraction

Abstract

The β → α phase transition kinetics of the Ti–3.5Al–5Mo–4V alloy with two different grain sizes was investigated at the isothermal temperature of 500 °C. A method to estimate the function of the precipitate fraction of the α phase with different aging times was developed based on X-ray diffraction analysis. The value of the α precipitate fraction increased sharply at first, then increased slowly with the aging time, and finally reached equilibrium. The value of the α precipitate fraction was higher in the alloy aged for the same time at a higher solution temperature, while the size of the α precipitate was smaller at a higher solution temperature. The β → α phase transition kinetics under isothermal conditions were modeled in the theoretical frame of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. The kinetic parameters of JMAK deduced different transformation mechanisms. The mechanism of the phase transition in the first stage was dominated by mixed transformation mechanisms (homogeneously nucleated and acicular-grown α structure, and grain boundary-nucleated and grown α precipitate), while the second stage was the growth of the fine α precipitate, which was controlled by slow diffusion. As the aging time increased, the hardness of the Ti–3.5Al–5Mo–4V alloy increased sharply. After the hardness of the alloy reached a plateau, it began to decline. The hardness of the alloy was always higher at a higher solution temperature.

Microstructure, Texture Evolution, and Aging Behavior of a Cold‐Rolled Ti–B20 Alloy

“…The specimen was cut into plates required for cold rolling by a wire‐cut electrical discharge machine (EDM). Based on previous research on the Ti–B20 alloy, [ 25,26 ] the specimen was solution treated (ST) at 850 °C/1 h to eliminate forging stress, obtaining a uniform single β ‐phase; afterward, the specimen was cooled to room temperature by water quenching (WQ). Furthermore, the plates were subjected to deformation degrees of 10%, 30%, 50%, and 70% of CR at room temperature by taking a correctly sized specimen and performing an aging treatment for different times at 500 °C in the salt bath furnace.…”

“…However, recent investigations on the new β ‐Ti alloy Ti–B20 chiefly focus on a single heat treatment; there are few studies on the microstructure and texture evolution characteristics that form during cold deformation and the effect of cold rolling on the aging behaviors of Ti–B20 alloys during aging treatment. [ 25,26 ] To enhance the mechanical properties of Ti–B20 alloys, the objective of this work is to investigate the microstructure and texture evolution characteristics and aging behavior of CR Ti–B20 alloys using X‐ray diffraction (XRD), scanning electron microscopy (SEM), electron back scattering diffraction (EBSD), and transmission electron microscopy (TEM). The relationship between the microstructures and mechanical properties of the Ti–B20 alloy is analyzed.…”

Effects of Double-Ageing Heat Treatments on the Microstructure and Mechanical Behaviour of a Ti-3.5Al-5Mo-4V Alloy

“…As a new type of metastable β-titanium alloy, the phase transformation mechanism of the Ti-3.5Al-5Mo-4V alloy is similar to that of the same type of metastable β-titanium alloy; however, the particularity of its critical value molybdenum equivalent inevitably leads to its phase transformation mechanism being different from that of a general metastable β-titanium alloy. At the same time, the current research on Ti-3.5Al-5Mo-4V alloys has mainly focused on thermal deformation and single-stage ageing heat treatment [ 23 , 24 ]. To improve the mechanical properties of Ti-3.5Al-5Mo-4V alloys and expand their application in aerospace and other fields, it is necessary and significant to study the rule of microstructure evolution during the double-stage aging process.…”