Fantao Kong

Dynamic Recrystallization of the Constituent γ Phase and Mechanical Properties of Ti-43Al-9V-0.2Y Alloy Sheet

A crack-free Ti-43Al-9V-0.2Y alloy sheet was successfully fabricated via hot-pack rolling at 1200 °C. After hot-rolling, the β/γ lamellar microstructure of the as-forged TiAl alloy was completely converted into a homogeneous duplex microstructure with an average γ grain size of 10.5 μm. The dynamic recrystallization (DRX) of the γ phase was systematically investigated. A recrystallization fraction of 62.5% was obtained for the γ phase in the TiAl alloy sheet, when a threshold value of 0.8° was applied to the distribution of grain orientation spread (GOS) values. The high strain rate and high stress associated with hot-rolling are conducive for discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), respectively. A certain high-angle boundary (HAGB: θ = 89° ± 3°<100>), which is associated with DDRX, occurs in both the recrystallized and deformed γ grains. The twin boundaries play an important role in the DDRX of the γ phase. Additionally, the sub-structures and sub-boundaries originating from low-angle boundaries in the deformed grains also indicate that CDRX occurs. The mechanical properties of the alloy sheet were determined at both room and elevated temperatures. At 750 °C, the alloy sheet exhibited excellent elongation (53%), corresponding to a failure strength of 467 MPa.

Effects of rolling deformation on microstructure and hardness of Ti-45Al-9Nb-0.3Y alloy

Abstract The microstructure evolution of as-rolled Ti-45Al-9Nb-0.3Y alloy as well as the nanohardness of β/B2 matrix was investigated by means of scanning electron microscopy (SEM) in backscattered electron mode (BSE) mode, transmission electron microscopy (TEM) and nanoindentation. This high Nb containing TiAl based alloy was rolled with 50%, 60%, 65% reduction, respectively. Omega phase precipitated in B2 phase with an orientation relationship of {110} β //{11 0} ω and 1> β // ω . Moreover, with the increase of deformation reduction, rod-like structure which was formed in γ grain transformed from (α 2 +γ) lamellae structure into α 2 phase only. Additionally, nanoinentation experiment revealed that the precipitation hardening of ω phase increased the hardness of β/B2 phase.

A Novel Composition Design Method for Beta-Gamma TiAl Alloys with Excellent Hot Workability

Beta-gamma TiAl alloys are promising light-weight structural materials for use in high-temperature applications. Disordered β phase at high temperature is beneficial to the hot workability of the alloys, while ordered β0 phase at room temperature is detrimental to the ductility of the alloys. However, we have not yet found a better way to quantitatively control β and β0 phase, which is key to improving the mechanical properties of beta-gamma TiAl alloys. In this paper, the effects of various β stabilizers on the contents of β0 and β phase were investigated. A quantitative composition design method for beta-gamma TiAl alloys was proposed. A room-temperature Mo equivalent ([Mo]eq-RT) was developed to estimate β0 phase content. Microstructural observations show that no β0 phase will precipitate in TiAl alloys when the value of [Mo]eq-RT is below 1. A high-temperature Mo equivalent ([Mo]eq-HT) was introduced to evaluate the hot workability of TiAl alloys. The relationship among alloy composition, β phase content, and hot workability was constructed. Isothermal compression tests indicate that the hot workability of TiAl alloys can be significantly improved when [Mo]eq-HT reaches above 1.3. The validity of [Mo]eq-RT and [Mo]eq-HT were verified by actual experiments. Two equivalence formulas provide important guidance for the composition design of beta-gamma TiAl alloys.