Shiwei Tian

The presence of the brittle β/B2 phase in TiAl alloys often deteriorates their mechanical properties, posing a significant challenge for manufacturing large-sized, high-performance sheets. To address this issue, this study systematically investigates the synergistic effect of pack rolling and subsequent heat treatment on the microstructure evolution and mechanical properties of a Ti-44Al-4Nb-1.5Mo-0.1B-0.1Y alloy. Sheets with two different deformation levels (R7: 69.8% and R11: 83.0% reduction) were prepared via pack rolling. This was followed by a series of heat treatments at different temperatures (1150–1350 °C) and cyclic heat treatments at 1250 °C (3, 6, and 9 cycles). The results demonstrate that the higher deformation level (R11) promoted extensive dynamic recrystallization, resulting in a uniform microstructure of equiaxed γ, α₂, and β phases, while the lower deformation (R7) retained a significant fraction of deformed γ/α₂ lamellae. Heat treatment at 1250 °C was identified as optimal for transforming the microstructure into fine lamellar colonies while effectively reducing the β/B2 phase. Cyclic heat treatment at this temperature further decreased the β-phase content to 4.1% after 9 cycles. The elimination mechanism was determined to follow the β→ α → γ + α₂ phase transformation sequence, driven by the combined effect of rolling-induced defects and cyclic thermal stress. Cyclic heat treatment at this temperature was particularly effective in generating a high density of nucleation sites within the lamellar colonies, leading to significant refinement of the lamellar structure. Consequently, the R11 sheet subjected to 9 cycles of heat treatment exhibited a 15.5% increase in tensile strength and an 8.3% improvement in elongation compared to the hot-isostatically pressed state. This enhancement is primarily attributed to the significant refinement of lamellar colonies and the reduction in interlamellar spacing. This work presents an effective integrated processing strategy for fabricating high-performance TiAl alloy sheets with superior strength and toughness.

The austenite grain growth and recrystallization behaviors of three pipeline steels with different Nb contents were investigated through reheating and thermal simulation compression experiments. The initiation conditions for dynamic and sub-dynamic recrystallization of austenite were analyzed, and sub-dynamic recrystallization equations in Avrami form were established. The influences of Nb content and deformation conditions on the evolution of grain size during austenite recrystallization was examined. The findings indicate that the austenite grain size of the three steels increases gradually with higher reheating temperatures, while the average grain size decreases with increasing Nb content. Sub-dynamic recrystallization initiation temperatures for the B150-steel, B145-steel, and 73-steel were found to be 920 °C for 10 s, 940 °C for 30 s, and 960 °C for 30 s, respectively. During high-temperature deformation, Nb in solid solution hindered recrystallization by impeding grain boundary and dislocation movement. At lower deformation temperatures, Nb(C, N) precipitation pinned grain boundaries and dislocations and consumed substantial free energy, thus competing with recrystallization. As Nb content increased, strain-induced precipitation became more pronounced, resulting in more effective inhibition of recrystallized grain growth.

The hot deformation behavior of TiAl alloy is analyzed by combining the true stress–strain curve and microstructure analysis. The results show that the flow stress of TiAl alloy presents the characteristics of work hardening-dynamic softening, it increases with the increase of strain rate and decreases with the increase of temperature. Based on the true stress–strain curves, the flow stress of TiAl alloy under different deformation conditions is predicted by hyperbolic sinusoidal formula. In addition, the effects of deformation temperature and strain rate on the microstructure evolution of TiAl alloy are revealed. In the process of hot compression deformation, dynamic recrystallization and γ→α, β→α phase transformation occurs. During tensile deformation, TiAl alloy exhibits super-plasticity, which is mainly due to grain rotation and coordinated deformation of β phase.

TiAl alloys undergo cyclic temperature changes during use, the process of which can be simulated by the thermal shock test. A systematic investigation of the thermal shock behavior of four Mo‐containing TiAl alloys is conducted. The increase in Mo content from 1.0% to 4.0% causes the gradual decrease in the volume fraction of γ/α₂ lamellar colony, while the volume fraction of equiaxed γ and β_o phases gradually increases. At the same time, the thermal shock resistance of the TiAl alloys decreases as the Mo content increases. After thermal shock, cracks often occur within the lamellae and extend in a zigzag manner for TiAl−1.0Mo and TiAl−1.5Mo alloys. Their thermal shock resistance is enhanced by crack deflection, bridging, and microcrack shielding. For TiAl−2.0Mo and TiAl−4.0Mo alloys, cracks occur at the grain boundaries or within the γ phase and extend straight, with the result that these two alloys have worse thermal shock resistance than the other two alloys due to the limited effect of microcrack shielding. In addition, the microstructure stability of TiAl alloys after thermal shock is discussed, and there is a critical value of Mo content between 3.13% and 5.67%, which inhibits the β_o → ω phase transition.

Herein, the hot compression deformation behavior and microstructure instability of the different Mo‐containing Ti₂AlNb‐based alloys are investigated in the temperature range of 900−1150 °C and strain rate range of 0.001−1 s⁻¹. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the microstructure evolution during the deformation process. The results reveal that Mo element addition can refine the lamellar α₂/O phase of Ti₂AlNb‐based alloy. In addition, it has a certain influence on the phase‐transition temperature of the alloy, which leads to the narrowing of the O + B2 + α₂ phase region. During the hot compression process, the spheroidization of the lamellar structure in Ti₂AlNb−0.5Mo alloy is mainly through the transformation of O→B2 + α₂, whereas the microstructure of Ti₂AlNb−2Mo alloy is mainly converted from the O phase to the B2 phase. In addition, it is found that the high content of Mo element causes the segregation of Al element and leads to poor plasticity and stability of the Ti₂AlNb−2Mo alloy. As such, the hot deformation performance of the Ti₂AlNb−0.5Mo alloy is significantly better, which provides theoretical guidance for the composition design of the Ti₂AlNb‐based alloy.