Study on Non-Isothermal Transformation of Ti-6Al-4V in Solution Heating Stage

Abstract

In order to understand the non-isothermal transformation behavior of Ti-6Al-4V titanium alloy in the continuous heating stage of solution treatment, thermal dilatometry tests with heating rates of 0.1~0.8 °C/s were designed. The conversion between the expansion amount and the transformed volume fraction was realized by the lever principle, and the transformation characteristics of α + β → β were quantified based on the Kissinger-Akahira-Sunose (KAS) theory and the modified Johnson–Mehl–Avrami (JMA) model. The results show that the phase transformation kinetics curves present typical “S” patterns, and the element diffusion transformation controls the nucleation and growth of new grains during the transformation of α + β → β. The phase transformation interval gradually moves to high temperature regions with the increase of heating rates, and the phase transformation activation energy is 445.5 kJ·mol−1. The phase transformation process is divided into three stages according to the relationship between the Avrami exponent n and the transformed volume fraction fT. These three stages correspond to different stages of grain nucleation or growth.

Joining of Dissimilar Alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si Using Linear Friction Welding

“…Similar to the Ti-6242 alloy, phase transformations in Ti-64 are also temperature dependent and, with increasing temperature, the volume fraction of the primary-α phase would decrease due to the α + β → β transformation. Yu et al [59] studied this transformation for an equiaxed microstructure of primary-α grains with intergranular β and determined that at low heating rates (0.1 • C•s −1 ), noticeable transformation of the α phase to β begins only at 873 • C. Pederson et al [60] investigated phase transitions in Ti-64 with a bimodal microstructure using high temperature X-ray diffractometry and observed no change in the α/β fractions during isothermal holding at 610 • C, but the percentage of β increased from~4% at room temperature to 12% at 710 • C during isothermal holding for 2 h. Nonetheless reverse transformation on cooling to ambient temperatures, decreased the β fraction from 12% at 710 • C to~3% at room temperature-that is, slightly lower than the amount in the as-received microstructure [60]. For higher heat treatment temperatures, the β fraction increased considerably and, during isothermal holding at 800 • C, the microstructure contained 40% β after 2 h, but upon reverse transformation to room temperature, the amount of β was only~6% (i.e., roughly 2% higher than that in the as-received microstructure) [60].…”

Impact of Laser Marking on Microstructure and Fatigue Life of Medical Grade Titanium

“…6 and 7). The observation that cracks end near the martensitic zone in the HAZ strongly suggests that crack initiation has occurred as a consequence of a volume expansion associated with the β→α transition [19][20][21][22]. 2 The development of the HAZ can be discussed as follows.…”

Effect of Initial Microstructure on the Temperature Dependence of the Flow Stress and Deformation Microstructure under Uniaxial Compression of Ti-407

“…Ti-407 material was supplied in the form of a seamless rolled ring processed with a combination of temperatures above and below the β transus (T βt ), that is, in the β-phase field and in the (α + β) temperature region, respectively. T βt was determined by nonisothermal dilatometry, as this technique is an effective method for studying the phase transformation process [25,26] since the expansion curve and its first derivative can indicate the onset, development, and completion of the phase transformation [27]. The thermal dilatometry test was performed using a Linseis model L75 vertical dilatometer at a constant heating rate of 5 • C/min from room temperature to 900 • C. The thermal expansion of a cylindrical sample measuring 10 mm in diameter and 30 mm in length was plotted as a function of temperature, and its first derivative with respect to time was obtained to indicate the rate of expansion.…”