Microstructure Evolution and Texture Formation Mechanism of α Phase During Continuous Through-Transus Thermal Compression of TC4 Titanium Alloy
ZHAO Zhuoya, MENG Lingjian, LIN Peng(), CAO Xiaoqing()
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Cite this article:
ZHAO Zhuoya, MENG Lingjian, LIN Peng, CAO Xiaoqing. Microstructure Evolution and Texture Formation Mechanism of α Phase During Continuous Through-Transus Thermal Compression of TC4 Titanium Alloy. Acta Metall Sin, 2025, 61(5): 717-730.
Titanium alloy has emerged as the preferred structural material in the aerospace and marine industries because of its exceptional strength-to-weight ratio, corrosion resistance, and fatigue resistance. The primary application of titanium alloy in aerospace is evident in aeroengines, emphasizing the importance of developing lightweight, high-performance components to enhance engine reliability. Despite these advantages, challenges arise during hot processing because the alloy forms a strong texture, resulting in anisotropic mechanical properties. In addition, the formation of “macrozones”, areas with similar grain orientations during hot processing, further complicates matters by facilitating stress concentration during hot deformation, thereby increasing the likelihood of crack nucleation. Rapid crack propagation within “macrozones” reduces the service life of titanium alloy components, necessitating a thorough investigation of the formation mechanism and control methods for “macrozones”. This study delves into the microstructure evolution and texture formation of TC4 titanium alloy under various hot compression conditions, aiming to elucidate the role of weakening texture and “macrozone”. The microstructure and texture evolution of the α phase after α + β phase field, β phase field, and continuous through-transus thermal compression were examined in TC4 alloy through thermal compression tests, optical microscopy, electron backscatter diffraction, and reconstruction of the high-temperature β phase. The results indicate that specimens primarily comprised equiaxed α phase after compression in the α + β phase field. The activation of {}<> prismatic slip systems caused α phase rotation toward {}//forging direction (FD) orientation during deformation. With increased deformation and strain rate, α grains gradually rotated to {}//FD orientation. Cooling after holding or compression in the β phase field resulted in the development of lamellar α phase with a {}//FD texture. In the β phase field, 30% compression induced β grain rotation to //FD orientation, enhancing β phase//FD texture and promoting the formation of α phase{}//FD transformation texture during cooling. Increased deformation and strain rate facilitated dynamic recrystallization of the β phase, reducing β grain size, weakening α phase {}//FD texture, and refining α grain size. After continuous through-transus thermal compression, the dominant {}//FD texture formation occurred because of prismatic slip system activation. Under specific conditions, such as 30% compression in the β phase field and 30% compression in the α + β phase field at 0.01 s-1, inhibition of {}//FD texture formation was observed as α grains rotated to {}//FD orientation during dynamic precipitation. Increased deformation in the α + β phase field led to further rotation of α grains to {}//FD orientation, intensifying {}//FD texture. Conversely, holding in the β phase field (undeformed), then cooled to the α + β phase field and compressed at 0.01-1 to a 60% reduction resulted in weak variant selection during dynamic precipitation of α phase, with large deformation promoting dynamic recrystallization of α phase and yielding the weakest α phase {}//FD texture. And at a strain rate of 0.05 s-1, extensive deformation promoting dynamic recrystallization of α phase, and weakening {}//FD texture. Continuous through-transus thermal compression was identified as a method for obtaining lamellar-structured titanium alloys with weak texture. Subsequent mechanical property testing at room temperature (around 20 oC) revealed that the through-transus thermal compressed specimen at 1020 oC, 5 min (undeformed) + 920 oC, 60%, and 0.01 s-1 exhibited the weakest {}//FD texture intensity and the smallest grain size of α phase. This specimen demonstrated strong crack initiation and propagation resistance, resulting in the highest elongation.
Table 1 Deformation parameters of TC4 titanium alloys in different phase fields
Fig.1 Schematics of TC4 titanium alloy thermal compression processes (AC—air cooling) (a) α + β phase field compression (b) β phase field holding or compression (c) continuous through-transus compression
Fig.2 Schematics of sampling position (a) and tensile specimen size (b) of TC4 titanium alloy at room temperature (AD—axial direction, FD—forging direction, TD—transverse direction, RD—radial direction; unit: mm; 990 oC is the (α + β)/β transformation temperature)
Fig.3 OM image (a), EBSD orientation map (b), pole figure (PF) (c), and inverse pole figure (IPF) (d) of α phase in as-received TC4 titanium alloy
Fig.4 EBSD orientation maps, PFs, and IPFs of α phase in TC4 titanium alloys compressed in α + β phase field (a) sample No.1 (b) sample No.2 (c) sample No.3 (d) sample No.4
Fig.5 Schmid factor (SF) distributions of {0001}<> basal slip system, {}<> prismatic slip system, and {}<> pyramidal slip system of α phase in TC4 titanium alloys compressed in α + β phase field (a) sample No.1 (b) sample No.2 (c) sample No.3 (d) sample No.4
Fig.6 EBSD orientation maps, PFs, and IPFs of α phase in TC4 titanium alloys held or compressed in β phase field (a) sample No.5 (b) sample No.6 (c) sample No.7 (d) sample No.8
Fig.7 EBSD orientation maps of β phase in TC4 titanium alloys held or compressed in β phase field (a) sample No.5 (b) sample No.6 (c) sample No.7 (d) sample No.8
Fig.8 EBSD orientation maps and {0001} PFs of α-“macrozones” and {110} PFs of adjacent β grains in the areas I (a) and II (b) in Figs.6b and d
Fig.9 EBSD orientation maps, PFs, and IPFs of α phase in TC4 titanium alloys after through-transus thermal compression (a) sample No.9 (b) sample No.10 (c) sample No.11 (d) sample No.12 (e) sample No.13
Fig.10 SF distributions of {0001}<> basal slip system,{}<> prismatic slip system, and {}<> pyramidal slip system of α phase in TC4 titanium alloys after through-transus thermal compression (a) sample No.9 (b) sample No.10 (c) sample No.11 (d) sample No.12 (e) sample No.13
Sample No.
Tensile strength / MPa
Elongation / %
7
933.4
14.9
9
938.8
13.6
13
926.6
16.0
Table 2 Tension properties of TC4 titanium alloy at room temperature after compression under different processing methods with a total compression of 60% at 0.01 s-1
Fig.11 Orientation maps and {001} PFs of β grains with different orientations (a, b); and orientation maps, {0001} PFs, and IPFs of α grains (c, d) in the areas I (a, c) and Ⅱ (b, d) in Figs.6b and d
Fig.12 Orientation map and {001} PFs of β grains with different orientations (a, b); and orientation maps, IPFs and SF maps of {}<> prismatic slip system of α grains (c, d) in the areas I (a, c) and Ⅱ (b, d) in Fig.9e
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