Abstract
As-forged WSTi6421 titanium alloy billet after β annealing was investigated. Abnormally coarse grains larger than adjacent grains could be observed in the microstructures, forming abnormal grain structures with uneven size distribution. Through electron backscattered diffraction (EBSD), the forged microstructure at various locations of as-forged WSTi6421 titanium alloy billet was analyzed, revealing that the strength of the β phase cubic texture generated by forging significantly influences the grain size after β annealing. Heat treatment experiments were conducted within the temperature range from Tβ-50 °C to Tβ+10 °C to observe the macro- and micro-morphologies. Results show that the cubic texture of β phase caused by forging impacts the texture of the secondary α phase, which subsequently influences the β phase formed during the post-β annealing process. Moreover, the pinning effect of the residual primary α phase plays a crucial role in the growth of β grains during the β annealing process. EBSD analysis results suggest that the strength of β phase with cubic texture formed during forging process impacts the orientation distribution differences of β grains after β annealing. Additionally, the development of grains with large orientations within the cubic texture shows a certain degree of selectivity during β annealing, which is affected by various factors, including the pinning effect of the primary α phase, the strength of the matrix cubic texture, and the orientation relationship between β grain and matrix. Comprehensively, the stronger the texture in a certain region, the less likely the large misoriented grains suffering secondary growth, thereby aggregating the difference in microstructure and grain orientation distribution across different regions after β annealing.
Damage-tolerant titanium alloys have been extensively used in the aerospace industry because of their outstanding fracture toughness and resistance against fatigue crack propagatio
Generally, the formation mechanism of AGS is considered as the discontinuous or abnormal growth of the original β grain
The experiment material supplied by Western Super-conductor Materials Technology Co., Ltd is a forged billet of WSTi6421 titanium alloy, as shown in

Fig.1 Schematic diagram (a) and appearance (b) of sampling locations and reference coordinate system for β-annealed and as-forged samples
As shown in
The macrostructures of β-annealed samples are shown in

Fig.2 Macrostructures at center region (a), quarter thickness region (b), and surface region (c) of β-annealed sample
The orientation distribution maps of the surface region, quarter thickness region, and center region of the as-forged sample are shown in

Fig.3 Orientation distributions (a–f) and pole figures (g–l) of α phase (a–c, g–i) and β phase (d–f, j–l) at different locations of as-forged samples: (a, d, g, j) surface region, (b, e, h, k) quarter thickness region, and (c, f, i, l) center region
Orientation distribution functions (ODFs) of the cross-sections of α phase at φ2=0° and φ2=30° based on the Bunge-Euler angles (φ1, Φ, φ2) are shown in

Fig.4 Microstructure analyses of α phase in center region of as-forged samples: (a) ODF cross-section views and orientation density extremum; (b) schematic diagrams of grain distribution of orientation density extremes; (c) IPF of orientation density extreme; (d) IPF of residual components; (e) comparison of orientation density extremes with band contrast map; (f) pole figures of the regions in Fig.4c and 4d
In summary, the cubic texture of the α phase predominantly appears in the secondary α phase and it is associated with the Burgers orientation relationship with the residual β phase cubic texture. This result suggests that the β phase in the center region develops a cubic texture during forging. At the center region, the texture strength is higher, and AGS forms after β annealing. Conversely, at the quarter thickness region, the texture strength is lower, and AGS does not form after β annealing. This indicates that the strength of the cubic texture caused by β phase during forging significantly influences the microstructures after β annealing.
The macrostructures of the samples after β annealing at 920, 940, 960, 970, and 980 °C are shown in

Fig.5 Macrostructures of samples after β annealing at different temperatures: (a) 920 °C, (b) 940 °C, (c) 960 °C, (d) 970 °C, and (e) 980 °C

Fig.6 Schematic diagram of sampling positions for microstructure ob-servation of samples after β annealing at different temperatures

Fig.7 SEM microstructures at position A (a–d, g), position B (e, h), and position C (f, i) of samples after β annealing at different temperatures: (a) as-forged; (b) 920 °C; (c) 940 °C; (d–f) 960 °C; (g–i) 970 °C
In summary, with the increase in the heat treatment (β annealing) temperature, the transformation of α phase occurs in two stages. Initially, the secondary α phase is transformed, leading to the early formation of the β phase during heat treatment. Subsequently, the primary α phase is transformed, during which the α phase size progressively decreases. At this juncture, the remaining primary α phase serves as the secondary phase, pinning the β grain boundaries. Near the surface region of as-forged sample, the β grains can more easily overcome the pinning effect of the residual primary α phase and grow rapidly at the temperatures between 960 and 970 °C. Near the center region of as-forged sample, higher temperatures are necessary for the β grain growth, resulting in the less formation of β grains and the macrostructure variation of the annealed samples.
Applying the Burgers orientation correlation, the β phase subjected to annealing at 940, 960, and 980 °C is meticulously reconstructed. EBSD orientation distributions and quality distributions of the β phase at position A, B, and C in samples annealed at 940 and 960 °C are shown in

Fig.8 EBSD orientation distributions (a–f) and quality distributions (g–i) of β phase reconstruction at position A (a, d, g), position B (b, e, h), and position C (c, f, i) of samples after annealing at 940 °C (a–c) and 960 °C (d–i)

Fig.9 Macrostructure of sample after annealing at 980 °C (a); β phase reconfiguration orientation distributions of position I (b), position II (c), position III (d), and position IV (e) in Fig.9a; grain boundary misorientations along Line 1 (f), Line 2 (g), Line 3 (h), and Line 4 (i) in Fig.9b–9e, respectively
As depicted in
As shown in Fig.
After β annealing at 980 °C, it is noted that the misorientation of β grains formed near the center region is smaller and the grains reduce, signifying a macrostructural change. This variation is attributed to the cubic texture generated during the forging process. After β annealing, the center region exhibits a markedly small number of high-angle grains and small misorientation, compared with those of other regions. Consequently, the strength of the β phase cubic texture formed during forging plays a crucial role in the development of β annealing structures. Theoretically, when the size of the high-angle β grains approaches that of the β subgrains in the matrix, the misorientation increases, which increases the grain boundary migration energy, facilitating the absorption during heat treatment. Therefore, the strength of the cubic texture is intensified with the increase in heat treatment temperature. This phenomenon aligns with the theoretical predictions. However, experiment data suggest that the growth of large orientation grains within the cubic texture is selective. It is influenced by various factors, such as the pinning effect of the primary α phase, the cubic texture strength in the matrix, and the orientation relationship between the β grains and the matrix. Some β grains with smaller misorientation manage to overcome the pinning effect, thereby gaining a size advantage within the matrix. β grains oriented closely to [110] are the most likely to be assimilated by the matrix, and with the increase in cubic texture strength, β grains near [111] orientation are also prone to assimilation. Consequently, the development of AGS near the center region after β annealing can be characterized by not only the texture strength but also the diminished survival ratio of large misorientation β grains within the strong cubic texture, thereby amplifying the disparities in structure and grain orientation distribution in different regions after β annealing.
1) In the β-annealed microstructure, ACGs with sizes signi-ficantly larger than those of surrounding grains can be observed, particularly in the center region along ST direction. These ACGs are interspersed among fine grains, forming AGS.
2) The formation of β phase during forging is a prerequisite for the development of AGS, and the strength of the cubic tex-ture affects the evolution process of the β-annealed structure.
3) During the phase transformation process, the cubic tex-ture of the β phase produced during forging is inherited ini-tially by the secondary α phase and subsequently by the newly formed β phase. The cubic texture of the β phase produced during forging is essential for the formation of AGS, and tex-ture strength influences the microstructures after β annealing.
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