The Microstructural Evolution, Tensile Properties, and Phase Hardness of a TiAl Alloy with a High Content of the β Phase

Abstract

In this paper, the microstructure, deformability, tensile properties, and phase hardness of the Ti–43Al–2Cr–0.7Mo–0.1Y alloy with a high β phase content were investigated. Microstructural analysis showed that the β phase precipitated not only at the colony boundaries but also inside the lamellae due to its high content. A high-quality forging stock was prepared through one-step noncanned forging. The total deformation reached above 80%, suggesting that the alloy has good hot deformability compared to other TiAl alloys. The deformed microstructure was composed of fine and equiaxed grains due to dynamic recrystallization. The high β phase content was shown to contribute to the decomposition of the initial coarse lamellae. Tensile testing showed that the alloy has good room-temperature ductility, even if the β phase content reaches above 20%. This is inconsistent with a previous study that showed that a large amount of the hard β phase is detrimental to the room-temperature ductility of TiAl alloys. Nanoindentation testing showed that the hardness of the β phase in the current alloy is about 6.3 GPa, which is much lower than that in the Nb-containing TiAl alloys. Low hardness benefits the compatible deformation among various phases, which could be the main reason for the alloy’s good room-temperature ductility. Additionally, the influence of various β stabilizers on the hardness of the β phase was also studied. The β phase containing Nb had the highest hardness, whereas the β phase containing Cr had the lowest hardness.

Characteristic effects of alloying elements on β solidifying titanium aluminides: A review

“…In the work of Cui et al. [ 19 ], it was noticed that the difference in hardness between the β and γ phases was close. It was used to explain the lower hardness value recorded for β-phase which contributes to the superior tensile properties (RT elongation of about 1.5%) of Ti–43Al–2Cr-0.7Mo-0.1Y alloy with satisfactory hot-workability owing to high β-phase present.…”

“…To achieve good oxidation resistance, creep, and high-temperature strength, Nb is usually added as alloying elements in TiAl-based alloys [ 15 , 25 , 99 ]. However, this causes increased hardness and leads to the formation of ω 0 -phase as an additional phase [ 19 , 24 , 99 ]. Typically, this ω 0 -phase precipitates in the β-matrix of β-solidifying TiAl-based alloys, but not the same for alloys where α 2 -phase precipitates [ 23 , 25 , 40 , 99 ].…”

“…However, in the study of Cui et al. [ 19 ], it was observed that the β-phase was not detrimental to the room-temperature ductility. Microalloying elements additions of B, Si and C were added to act as nucleation agents in TNM alloys during solidification and phase transformations.…”

“…Titanium aluminides are potential engineering materials where lightweight is required including strength retention at elevated temperatures [ 65 ]. Due to a density between 3.8 to 4.3 g/cm 3 , TiAl-based alloys are ideal candidates in aircraft applications [ 16 , 19 , 35 , 64 ]. For the past three decades, titanium aluminides intermetallic alloys have been of major interests especially in the aerospace and automotive industries.…”

“…Different cooling rates during annealing results in adjustments of the α 2 /γ colonies and lamellar spacings. Furthermore, it must be noted that the detrimental effects of the β-phase at the material's service temperatures could be easily reduced and/or completely removed with appropriate heat treatment techniques [16,19,20]. This detrimental effect emanates from the disordered A2 structure transforming at ambient temperature to the ordered B2 structure (β 0 -phase) which is brittle and harder than γ and α 2 phases.…”