Next Article in Journal
Study on the Influence Mechanism of Surface Morphology on Wear and Thermal Fatigue Performance of Laser-Treated Bionic Brake Drum
Previous Article in Journal
Experimental Study on Efficient Short Electric Arc Turning of Titanium Alloy
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Metals
Volume 15
Issue 2
10.3390/met15020123
metals-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Microstructure Evolution of Extruded TiAl Alloy During Vacuum Isothermal Superplastic Forging Process

by
Jintao Li
1,
Xiaopeng Wang
1,2,
Minyu Gong
1,
Zhenyu Guo
1 and
Fantao Kong
1,*
1
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
2
Center of Analysis and Measurement, Harbin Institute of Technology, Harbin 150001, China
*
Author to whom correspondence should be addressed.
Metals 2025, 15(2), 123; https://doi.org/10.3390/met15020123
Submission received: 27 December 2024 / Revised: 22 January 2025 / Accepted: 24 January 2025 / Published: 26 January 2025
Browse Figures
Versions Notes

Abstract

:
Vacuum isothermal forging is an ideal method for preparing high-performance TiAl alloy forgings, as it is carried out under the conditions of a uniform temperature field and oxygen isolation. The mechanical properties of TiAl alloys strongly depend on their microstructure, so it is important to study their microstructure evolution during the forging process to improve their properties. In this study, TiAl alloy forgings with different deformations were produced from the extruded billets by vacuum isothermal superplastic forging under lower temperatures and extremely low strain rate conditions. The results indicate that the streamlined structure in the extruded alloy was destroyed during the forging process. As the deformation increased, the dynamic recrystallization was more fully carried out, leading to a substantial decrease in remnant lamellar colonies and a significant increase in the γ phase, and the microstructure was transformed from nearly lamellar (NL) to near gamma (NG) structure. The proportion of high-angle grain boundaries (HAGB) increased with increasing deformation, while the grain size reduced from 20 μm to 4.6 μm. In addition, the streamlined features and textures exhibited a weakening trend with increasing deformation, leading to a decrease in the ultimate strength from 891 MPa to 722 MPa. To maintain the streamlined characteristics and retain strengthening effects, the forging deformation should not exceed 56.7%.

1. Introduction

Lightweight high-temperature structural materials have been a major focus for researchers in the aerospace and automotive industries [1,2], as they have both economic and environmental benefits [3,4]. TiAl alloys show a low density, only half of the Ni-based superalloys [5,6], and have high specific strength and modulus [7,8], sufficient oxidation resistance [9,10] combined with good creep and hot gas corrosion resistance [11], making them highly potential to replace heavy Ni-based superalloys in the temperature range of 650~900 °C [12,13]. In the past few decades, TiAl alloys have made great progress in both theory and application, with their successful application in components such as aircraft engines [14,15] and automotive exhaust valves [16,17]. However, the extensive engineering applications of TiAl alloys are severely limited by their poor room-temperature plasticity [18,19] and insufficient hot workability [20,21].
The refined microstructures can be obtained through thermomechanical treatments [22,23], which is of great significance for improving the mechanical properties of TiAl alloys [24,25]. Hot forging, including pack-forging, near-isothermal forging without canning, and vacuum isothermal forging, is a common method for decomposing the coarse as-cast microstructure. Pack-forging is usually carried out at higher temperatures [26,27]. The larger strain rates are usually used due to the rapid drop in temperature during the forging process. For TiAl alloys that are hard to deform, severe heat dissipation and high strain rates often lead to billets cracking. In contrast to pack-forging, containerless near-isothermal forging is generally conducted at lower temperatures [28]. The dies are also heated before forging, greatly delaying the temperature drop during the forging process. However, the lack of protective packaging causes severe oxidation of the billets during the forging process, so forgings can only be carried out at lower temperatures, which usually leads to insufficient dynamic recrystallization. Among the various hot working methods, vacuum isothermal forging is considered an ideal method for obtaining refined microstructures, as it maintains the uniformity of the temperature field during the forging process and ensures consistent overall performance of the forgings. It was reported that Leistritz company successfully prepared TNB-V4 compressor blades and TNM low-pressure turbine blades with a length of up to 200 mm by an isothermal forging process using extruded billets [14]. It was declared that the TNM alloy blades prepared by isothermal forging had been used for the Airbus A320neo PW1134G engine, which successfully made its maiden flight in 2014 [14]. One of the significant advantages of isothermal forging is the maintenance of constant temperature conditions, which allows the billets to be forged at extremely low strain rates. The TiAl alloys are superplastic forged in such a condition that is beneficial for reducing the deformation resistance of the alloys, fully utilizing their plastic deformation ability, greatly reducing the risk of cracking, and obtaining forgings with fewer defects. The significance of using extruded billets is that the extrusion process refines the grain size, improves the high-temperature deformation ability of the alloy, and allows the isothermal forging process to be carried out at temperatures much lower than that of traditional pack-forging. Low forging temperature can not only save costs but also limit the growth of dynamical recrystallization grains in the initial extruded billets, thereby improving the mechanical properties of forged blades [27]. Thus, the process route of vacuum isothermal superplastic forging using extruded billets at lower temperatures is reasonable. However, there are few reports on vacuum isothermal superplastic forging using extruded billets. It is of great significance to study the microstructure evolution of extruded TiAl alloys during the vacuum isothermal superplastic forging process to obtain specific microstructures for different application requirements.
In this study, TiAl alloy forgings with different deformation amounts were obtained by vacuum isothermal superplastic forging using the extruded alloy as the initial material. The microstructure evolution of TiAl alloys during the vacuum isothermal superplastic forging process is studied, which provides a possible pathway for preparing high-performance TiAl alloy forgings.

2. Materials and Methods

The 58 mm × 48 mm × 38 mm square billets were cut from a TiAl alloy extruded rod (Figure 1b) using electric discharge machining and then polished by milling, as shown in Figure 1c. The nominal composition of the extruded alloy is Ti-45Al-4Nb-0.6Mo-0.1B.
Before forging, the billets were sprayed with glass lubricant on the surface, and then a vacuum isothermal superplastic forging was performed on a vacuum isothermal forging machine, as shown in Figure 1a. The vacuum isothermal forging machine used in this research was independently developed and designed by the research group. Vacuum monitoring sensors were installed in the vacuum chamber. During the forging process, the vacuum degree was maintained at 10−2~10−3 Pa. The temperature control during the experiment was achieved by collecting temperature information through multiple thermocouples at different locations. The deformation parameters are listed in Table 1. The forging direction (FD) was perpendicular to the extrusion direction (ED), as shown in Figure 1d. Figure 1e–g shows the TiAl alloy forgings with different reductions. The forgings exhibit a high surface quality without any cracks.
Phase structure analysis was carried out on an X-ray diffractometer (XRD) (Malvern Panalytical, Malvern, UK) with a test range of 20–90° and a scanning speed of 2°/min. The microstructure of the alloy before and after vacuum isothermal forging was analyzed using a 9XF-PC optical microscope (OM) (Shanghai optical instrument factory, Shanghai, China) and an electron probe JXA-8230 microscope (SEM) (Japan Electronics Co., Ltd., Tokyo, Japan). Electron backscatter diffraction (EBSD) characterizations were performed using a Zeiss Gemini 560 field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). The SEM and EBSD samples were first polished on 60 mesh and 2000 mesh sandpapers and then electropolished in a solution of CH3OH:C4H9OH:HClO4 = 6:3:1 for 75 s under a direct voltage of 30 V and a current of 1.1 A. Liquid N2 was added to the solution during electrolytic polishing to maintain the temperature at 233 K. The OM samples were etched in a solution of HF:HNO3:H2O = 1:2:10 for 10 s after electrolytic polishing. Image-Pro Plus 6.0 software was used to calculate the proportion of each phase. The dog-bone-shaped tensile samples with a gauge specification of 15 mm length, 5 mm width, and 2 mm thickness were prepared by electrical discharge machining and polished smoothly using 60 mesh and 2000 mesh sandpapers. The as-extruded and as-forged samples were subjected to room-temperature tensile tests using an electronic universal tensile testing machine (Instron-5569, Instron Corporation, Boston, MA, USA) with a tensile strain rate of 1 × 10−4 s−1.

3. Results and Discussion

Figure 2 shows the XRD patterns of extruded and forged alloys with different deformation amounts. Both extruded and forged alloys are mainly composed of γ, α2, and β0 phases. The composition phases did not change after superplastic forging at a low strain rate, as no other phases were produced in the forged alloys. The microstructures of extruded and forged alloys with different deformation amounts are shown in Figure 3. The OM images show that the extruded alloy has a typical extruded streamlined structure, with lamellar colonies (white contrast) aligned in the extrusion direction. The uneven Al distribution caused by segregation during solidification is manifested by hot extrusion, resulting in a streamlined structure [29,30]. The streamline characteristics weaken after vacuum isothermal forging. Part of the extrusion streamline is still retained in the 45% deformation alloy, but the streamline width becomes narrower. There are basically no streamlines composed of lamellar colonies in the microstructure, with the deformation increasing to 62.5%. When the deformation further increases to 80%, it is difficult to observe streamlined features arranged along the extrusion direction in the microstructure. The change in streamlines indicates that the decomposition of the lamellar structure during the forging process leads to a decrease in the volume fraction and size of lamellar colonies.
The SEM microstructure image basically verifies the above change process. It can be seen in Figure 3b that the microstructure of the extruded alloy is an NL microstructure, mainly composed of most of the α2/γ lamellar colonies and a small amount of blocky γ phases distributed at the boundaries of the lamellar colonies. The average grain size of the lamellar colonies is about 40 μm, and the grain size of the γ phase is about 15 μm. The microstructure of the forged alloy consists of α2/γ lamellar colonies, blocky γ grains, and granular β0 phases distributed around the boundaries of lamellar colonies and γ grains, as illustrated in Figure 3d,f,h. The volume fractions of the lamellar colonies, γ grains, and granular β0 grains in the extruded and forged alloy microstructures were calculated, and the data are shown in Table 2.
The deformation occurs in the (α + β/β0 + γ) phase region due to the forging temperature being higher than the temperature of the α → α2 + γ eutectoid transformation. The deformation is mainly borne by the γ phase and the high-temperature β phase at this forging temperature. During the forging process, the boundary of lamellar colonies is prone to stress concentration due to differences in lamellar orientation, which can easily lead to dislocation accumulation. Therefore, the γ phase preferentially undergoes dynamic recrystallization at the boundaries of the lamellar colonies, and the main recrystallization mechanism is grain boundary sliding [26,31,32]. At this forging temperature, the β phase approaches a disordered BCC structure and undergoes dislocation slip along the <111> direction [33]. Consequently, the β phase has sufficient independent slip systems to coordinate the deformation between differently oriented lamellar colonies and γ grains to release local stress concentration while also providing space for the rotation of lamellar colonies [34,35]. The vacuum isothermal forging process has a uniform temperature field, a small strain rate, and a long high-temperature deformation time. Therefore, the β phase at the grain boundary undergoes spheroidization through a continuous dynamic recrystallization process, and it is distributed with equiaxed γ grains at the boundary of the lamellar cluster, forming a chain-like structure [36]. This granular β phase distributed along grain boundaries effectively suppresses the growth of γ grains and lamellar colonies during the cooling process. The 45% deformation forged alloy has a duplex (DP) microstructure, with a content of lamellar colonies equivalent to that of γ grains. However, the content of lamellar colonies is significantly reduced compared to the extruded alloy, and the size of lamellar colonies also decreases to about 25 μm. As the deformation increases, the volume fraction of lamellar colonies decreases, and the size of lamellar colonies further shrinks. The forged alloys with 62.5% deformation and 80% deformation exhibit NG microstructure, containing only a small amount of residual lamellar colonies. This suggests that the vacuum isothermal forging process is conducive to the decomposition of the lamellar structure.
The lamellar interfaces of residual lamellar colonies in forged alloys are mostly parallel to the extrusion direction, as illustrated in the SEM images. The proportion of the lamellar colonies in different orientations was statistically analyzed, as shown in Table 3. The reason for this result is that the yield stress of the lamellae strongly depends on the angle between the lamellar interface and the loading axis. When the loading axis is perpendicular to the lamellar interface (hard orientation II), the yield stress exhibits a peak value; when the lamellar interface is parallel to the loading axis (hard orientation I), the yield stress slightly decreases; when the lamellar interface is inclined at a moderate angle to the loading axis (soft orientation), the yield stress decreases to the minimum [37,38]. The yield strength of lamellar colonies in different orientations at 1100 °C is shown in Table 4. During the forging process, the soft-oriented lamellar undergoes sufficient dynamic recrystallization through phase boundary expansion under thermal coupling until complete decomposition [37,39]. The hard-oriented lamellae perpendicular to the forging direction have high deformation resistance, with most of them breaking under the action of force and only a small proportion undergoing dynamic recrystallization. Therefore, the hard-oriented lamellae are ultimately preserved.
Figure 4 presents the EBSD results of the extruded alloy and the forged alloys with different deformation amounts. As shown in Figure 4a1–d1, the color of most γ lamellae is concentrated in blue in the extruded alloy, and their orientations are close to <110>, while the colors of grains in the forged alloys are relatively dispersed. It can be seen from Figure 4a2–d2 that the proportion of the α2 phase in the extruded alloy is relatively high, reaching 17.4% (the α2 lamellae marked from 1 to 6 were not identified, so Image-Pro Plus 6.0 software was used to calculate the proportion of each phase). However, the proportion of the α2 phase significantly decreased after forging, which could be attributed to the occurrence of dynamic recrystallization during vacuum isothermal superplastic forging, which caused a remarkable decomposition of the lamellar structure. As the deformation increases, the content of the γ phase increases while the content of the α2 phase and β0 phase decreases. The increase in deformation causes part of the lamellar colonies in moderate and hard orientations to deflect and decompose into spherality under force and promotes the occurrence of α → α2 + γ and β → γ phase transitions. It can be seen from Figure 4a3–d3 that the proportion of high-angle grain boundaries in the microstructures after forging with different deformation amounts exceeds 90%. The low-angle grain boundaries continuously absorb dislocations, increase orientation differences, and ultimately form high-angle grain boundaries during dynamic recrystallization. The high proportion of the high-angle grain boundaries indicates that sufficient dynamic recrystallization occurred during the vacuum isothermal superplastic forging process. The proportion of high-angle grain boundaries increases with the increase in deformation (Figure 4a3–d3), while the grain size of the alloy gradually decreases. The grain size of the forged alloys is significantly refined compared to the extruded alloy. Additionally, the grain size of the alloy decreases to below 5 μm when the deformation increases to 80%.
Figure 5 shows the pole figures (PFs) and inverse pole figures (IPFs) of the γ phase in extruded alloy and forged TiAl alloys with different deformation amounts. It can be seen from the PFs in Figure 5a that the extruded alloy exhibits a strong texture in the γ phase, with a maximum polar density of 8.37. The maximum polar density significantly decreases after vacuum isothermal superplastic forging. As the deformation increases, the number of poles gradually increases, and the maximum polar density gradually decreases, indicating that the texture weakens with the increase in deformation. The IPFs in Figure 5 (b) demonstrate that the texture components of <111>⊥ED, <001>⊥ED, and <110>//ED exist in the extruded alloy. The texture strength of the alloy decreased after forging, and the texture types also changed. Along the forging direction, the <111>⊥ED texture weakens and almost disappears when the deformation reaches 62.5%. Between <011> and <010>, there is always a preferred orientation near the <032> orientation, which can be explained by mechanical twinning. In the tetragonal γ-TiAl, only four twinning systems are active because the other systems would destroy the ordered structure of γ-TiAl. These unidirectionally operating twinning systems cause an asymmetry of the polyhedron yield surfaces of the slip systems of dislocations [40,41]. The result is that dislocation slip causes an orientation rotation towards <302>, which becomes the stable orientation under compression deformation [42]. The increase in deformation does not lead to the enhancement of the <032> texture, indicating that the influence of deformation on twinning tendency is much smaller compared to deformation temperature. Along the extrusion direction, the <110>//ED texture generally disappears after forging. There are <032>//ED and <011>//ED textures in the alloy of 45% deformation. As the deformation increases, the grains rotate, causing the two textures to disappear, and a weak <010>//ED texture is produced in the alloy with 80% deformation. Due to the inequivalence between the <010> direction and the <001> direction in the γ phase, this texture is called the modified cubic texture [43]. This texture has also been found in hot-rolled TiAl alloy sheets [44]. Along the transverse direction (TD), the <112>//TD texture can be observed in the forged alloys. As the deformation increases, partial grains rotate their c-axis to align with the TD, resulting in the generation of the <001>//TD texture. Overall, the texture in the γ phase gradually decreased with the increase in deformation. Of perhaps greater significance for the texture development is that the new grains grow by bulging [44]. Therefore, the further refinement of grains caused by the increase in deformation is also one of the reasons for the gradual weakening of texture.
The fitted curve of the room-temperature strength variation tendency of extruded and forged samples is shown in Figure 6. It can be seen that the strength of the forged alloys is lower than that of the extruded alloy, and the strength shows a decreasing tendency from 891 MPa to 722 MPa with the increase in forging deformation. According to the microstructure analysis, it can be found that the volume fraction of lamellar colonies decreases significantly after forging, and the content of the lamellar colonies further decreases with the increase in deformation, resulting in a weakening of the streamline characteristics and a transition of microstructure from NL to NG. The change in strength is consistent with the microstructure transformation caused by the changes in extrusion streamlines. According to previous studies, the lamellar colonies with interfaces parallel to the extrusion direction exhibit excellent mechanical properties. Similar to PST single crystals with a 0° orientation, when the direction of tensile tests is parallel to the extrusion direction, the strain distribution ratio between the α2 and γ lamellae in these lamellar colonies is close to 1, indicating great deformation coordination between the two phases. Moreover, the mechanical twins and stacking faults that are beneficial for strength and plasticity are easily formed in lamellar colonies in this orientation. The dynamic recrystallization that occurs during the forging process leads to a significant decomposition of the lamellar structure and a remarkable reduction in the volume fraction of lamellar colonies parallel to the extrusion direction at the interface, resulting in the destruction of the extrusion streamlines. The adverse effects caused by the destruction of extrusion streamlines are stronger than the beneficial effects brought by grain refinement, so the strength and plasticity decrease with the increase in forging deformation. Therefore, it is of great significance to control the forging deformation amount to preserve the superior properties along the extrusion direction as much as possible. It can be found that the strength begins to decrease sharply at 56.7% deformation, making a tangent on the fitted curve in Figure 6. Thus, the deformation amount of 56.7% is the critical deformation at which the extrusion streamlines basically disappear. To maintain the extrusion streamlines, the deformation should be less than 56.7%.

4. Conclusions

In this study, the TiAl alloy forgings with different deformations were prepared through vacuum isothermal superplastic forging using extruded square billets. The microstructure evolution of extruded alloy during vacuum isothermal superplastic forging was studied, and theoretical guidance was provided for the actual die-forging process of blade materials. The main conclusions are as follows:
(1)
The TiAl alloy forgings produced at an extremely low strain rate and a lower temperature exhibited a high surface quality without cracks.
(2)
The volume fraction of lamellar colonies and the grain size of the forged alloys were significantly reduced compared to the extruded alloy. The dynamic recrystallization process was promoted by the increase in deformation, leading to the rise in the proportion of equiaxed γ phase from 76.3% to 93% and the decrease in the volume fraction of remnant lamellar colonies from 70.26% to 8.32%. Therefore, the proportion of high-angle grain boundaries increased, and the grain size reduced from 20 μm to 4.6 μm. In addition, the type of the microstructure transitioned from NL to NG with the increased deformation.
(3)
During the forging process, the dynamic recrystallization occurred in soft-oriented lamellar colonies through phase boundary bulging until they decomposed completely, while hard-oriented lamellar colonies were only broken by force and retained at room temperature.
(4)
The γ grains of the forgings exhibited a preferred orientation in the <032> face parallel to the forging direction, showing a typical characteristic of mechanical twinning.
(5)
The extrusion streamlines and textures were gradually destroyed with the increase in deformation, leading to a reduction in strength from 891 MPa to 722 MPa. To maintain the streamlined features and hold its strengthening effects, the forging deformation should not exceed 56.7%.

Author Contributions

Conceptualization, F.K.; methodology, F.K., X.W. and J.L.; validation, J.L., M.G. and Z.G.; formal analysis, J.L. and X.W.; investigation, X.W., M.G. and Z.G.; resources, F.K. and X.W.; data curation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and F.K.; visualization, J.L. and F.K.; supervision, F.K.; project administration, F.K.; funding acquisition, F.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Science Center for Gas Turbine Project (P2022-A-IV-001-003).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Sokolovsky, V.S.; Stepanov, N.D.; Salishchev, G.A. The Effect of Cellular Reaction on Mechanical Behavior and Microstructure Evolution of β-Solidified γ-TiAl Based Alloy during Hot Deformation. Intermetallics 2025, 179, 108653. [Google Scholar] [CrossRef]
  2. Xu, R.; Li, M.; Zhao, Y. A Review of Microstructure Control and Mechanical Performance Optimization of γ-TiAl Alloys. J. Alloys Compd. 2023, 932, 167611. [Google Scholar] [CrossRef]
  3. Klein, T.; Usategui, L.; Rashkova, B.; Nó, M.L.; San Juan, J.; Clemens, H.; Mayer, S. Mechanical Behavior and Related Microstructural Aspects of a Nano-Lamellar TiAl Alloy at Elevated Temperatures. Acta Mater. 2017, 128, 440–450. [Google Scholar] [CrossRef]
  4. Liu, Z.; Yang, J.; Ma, Y.; Ye, J.; Liu, Y. The Impact of Directional Annealing on the Microstructure and 900 °C Tensile Properties of γ-TiAl Alloy. Intermetallics 2025, 179, 108648. [Google Scholar] [CrossRef]
  5. Usategui, L.; López-Ferreño, I.; Echániz, T.; Sainz-Menchón, M.; Musi, M.; Clemens, H.; López, G.A. Emissivity Measurements Conducted on Intermetallic γ-TiAl-Based Alloys for Aeronautical Applications. J. Mater. Res. Technol. 2023, 27, 3170–3179. [Google Scholar] [CrossRef]
  6. Li, M.; Li, J.; Zhou, T.; Xiao, S.; Chen, Y.; Xu, L.; Hu, L.; Shi, L. The Investigation of Microstructure Evolution, Deformation Behavior and Processing Performance of the High Niobium Containing TiAl Alloys. Intermetallics 2021, 138, 107336. [Google Scholar] [CrossRef]
  7. Liang, Z.; Xiao, S.; Chi, D.; Zheng, Y.; Xu, L.; Xue, X.; Tian, J.; Chen, Y. Compressive Creep Behavior of High Nb Containing TiAl Alloy: Dynamic Recrystallization and Phase Transformation. Intermetallics 2023, 163, 108067. [Google Scholar] [CrossRef]
  8. Ganesan, H.; Scheider, I.; Cyron, C.J. Understanding Creep in TiAl Alloys on the Nanosecond Scale by Molecular Dynamics Simulations. Mater. Des. 2021, 212, 110282. [Google Scholar] [CrossRef]
  9. Zhang, C.; Zhang, S.; Pan, Y.; Xu, W.; Singh, H.; Liu, B.; Lu, D.; Wang, H.; Zhang, J.; Lu, X. Effect of Sn Addition on the Mechanical Properties and High-Temperature Oxidation Resistance of Intermetallic TiAl Alloys by First Principles Study and Experimental Investigation. J. Mater. Res. Technol. 2022, 21, 3666–3677. [Google Scholar] [CrossRef]
  10. Cao, S.; Han, J.; Wang, H.; Xiao, S.; Xu, L.; Chen, Y. Effects of Cycle Heat Treatment on the Microstructure and Mechanical Property of As-Cast γ-TiAl Alloy. Mater. Sci. Eng. A 2022, 857, 144053. [Google Scholar] [CrossRef]
  11. Ren, G.; Dai, C.; Mei, W.; Sun, J.; Lu, S.; Vitos, L. Formation and Temporal Evolution of Modulated Structure in High Nb-Containing Lamellar γ-TiAl Alloy. Acta Mater. 2019, 165, 215–227. [Google Scholar] [CrossRef]
  12. Kim, Y.W.; Kim, S.L. Advances in Gammalloy Materials–Processes–Application Technology: Successes, Dilemmas, and Future. JOM 2018, 70, 553–560. [Google Scholar] [CrossRef]
  13. Chen, G.; Peng, Y.; Zheng, G.; Qi, Z.; Wang, M.; Yu, H.; Dong, C.; Liu, C.T. Polysynthetic Twinned TiAl Single Crystals for High-Temperature Applications. Nat. Mater. 2016, 15, 876–881. [Google Scholar] [CrossRef]
  14. Janschek, P. Wrought TiAl Blades. Mater. Today Proc. 2015, 2, S92–S97. [Google Scholar] [CrossRef]
  15. Sun, Z.; Zhu, L.; Mo, X.; Nan, H.; Ding, X. Microstructure Characterization and Properties of Graphene Oxide-Reinforced TiAl Matrix Composites. Metals 2021, 11, 883. [Google Scholar] [CrossRef]
  16. Tetsui, T. Application of TiAl in a Turbocharger for Passenger Vehicles. Adv. Eng. Mater. 2001, 3, 307–310. [Google Scholar] [CrossRef]
  17. Gebauer, K. Performance, Tolerance and Cost of TiAl Passenger Car Valves. Intermetallics 2006, 14, 355–360. [Google Scholar] [CrossRef]
  18. Genc, O.; Unal, R. Development of Gamma Titanium Aluminide (γ-TiAl) Alloys: A Review. J. Alloys Compd. 2022, 929, 167262. [Google Scholar] [CrossRef]
  19. Yang, Y.; Feng, H.; Wang, Q.; Chen, R.; Guo, J.; Ding, H.; Su, Y. Improvement of Microstructure and Mechanical Properties of TiAl−Nb Alloy by Adding Fe Element. Trans. Nonferrous Met. Soc. China 2020, 30, 1315–1324. [Google Scholar] [CrossRef]
  20. Zheng, G.; Tang, B.; Zhao, S.; Wang, W.Y.; Chen, X.; Zhu, L.; Li, J. Evading the Strength-Ductility Trade-off at Room Temperature and Achieving Ultrahigh Plasticity at 800 °C in a TiAl Alloy. Acta Mater. 2022, 225, 117585. [Google Scholar] [CrossRef]
  21. Liu, N.; Niu, J.; Chen, Y.; Wang, X.; Wang, J.; Xiang, H.; Wei, D.; Chen, G. Effect of In-Situ Post-Heating on the Microstructure and Tensile Performance of TiAl Alloys Produced via Selective Electron Beam Melting. Mater. Sci. Eng. A 2023, 885, 145585. [Google Scholar] [CrossRef]
  22. Erdely, P.; Staron, P.; Maawad, E.; Schell, N.; Klose, J.; Mayer, S.; Clemens, H. Effect of Hot Rolling and Primary Annealing on the Microstructure and Texture of a β-Stabilised γ-TiAl Based Alloy. Acta Mater. 2017, 126, 145–153. [Google Scholar] [CrossRef]
  23. Niu, H.Z.; Tong, R.L.; Chen, X.J.; Zhang, T.B.; Zhang, D.L. Rapid Decomposition of Lamellar Microstructure and Enhanced Hot Workability of an As-Cast Triphase Ti–45Al–6Nb–1Mo Alloy via One-Step Alpha-Extrusion & Annealing. Mater. Sci. Eng. A 2021, 801, 140438. [Google Scholar] [CrossRef]
  24. Yang, G.; Xu, X.; Sun, T.; Xu, S.; Gui, W.; Zeng, J.; Mu, Y.; Liang, Y.; Lin, J. A Refined Fully Lamellar TiAl Alloy Extruded at α-Phase Region: Microstructure and Mechanical Properties. Mater. Sci. Eng. A 2023, 888, 145804. [Google Scholar] [CrossRef]
  25. Gupta, R.K.; Kumar, V.A.; Babu, R.R.; Rao, A.G. Development of Ductile Γ+α2 Titanium Aluminide through Ingot Metallurgy Route, Thermomechanical Processing and Characterization. Mater. Sci. Eng. A 2017, 703, 124–136. [Google Scholar] [CrossRef]
  26. Zhang, S.Z.; Kong, F.T.; Chen, Y.Y.; Liu, Z.Y.; Lin, J.P. Phase Transformation and Microstructure Evolution of Differently Processed Ti–45Al–9Nb–Y Alloy. Intermetallics 2012, 31, 208–216. [Google Scholar] [CrossRef]
  27. Tang, B.; Cheng, L.; Kou, H.; Li, J. Hot Forging Design and Microstructure Evolution of a High Nb Containing TiAl Alloy. Intermetallics 2015, 58, 7–14. [Google Scholar] [CrossRef]
  28. He, W.; Tang, H.; Liu, H.; Jia, W.; Liu, Y.; Yang, X. Microstructure and Tensile Properties of Containerless Near-Isothermally Forged TiAl Alloys. Trans. Nonferrous Met. Soc. China 2011, 21, 2605–2609. [Google Scholar] [CrossRef]
  29. Zhang, J.; Jing, Y.; Fu, M.; Gao, F. Microstructure Optimization of Ingot Metallurgy TiAl. Intermetallics 2012, 27, 21–25. [Google Scholar] [CrossRef]
  30. Carneiro, T.; Kim, Y.-W. Evaluation of Ingots and Alpha-Extrusions of Gamma Alloys Based on Ti–45Al–6Nb. Intermetallics 2005, 13, 1000–1007. [Google Scholar] [CrossRef]
  31. Fukutomi, H.; Nomoto, A.; Osuga, Y.; Ikeda, S.; Mecking, H. Analysis of Dynamic Recrystallization Mechanism in γ-TiAl Intermetallic Compound Based on Texture Measurement. Intermetallics 1996, 4, S49–S55. [Google Scholar] [CrossRef]
  32. Koeppe, C.; Bartels, A.; Clemens, H.; Schretter, P.; Glatz, W. Optimizing the Properties of TiAl Sheet Material for Application in Heat Protection Shields or Propulsion Systems. Mater. Sci. Eng. A 1995, 201, 182–193. [Google Scholar] [CrossRef]
  33. Appel, F.; Paul, J.D.H.; Oehring, M. Gamma Titanium Aluminide Alloys: Science and Technology, 1st ed.; Wiley: Hoboken, NJ, USA, 2011; ISBN 978-3-527-31525-3. [Google Scholar]
  34. Singh, V.; Mondal, C.; Kumar, A.; Bhattacharjee, P.P.; Ghosal, P. High Temperature Compressive Flow Behavior and Associated Microstructural Development in a β-Stabilized High Nb-Containing γ-TiAl Based Alloy. J. Alloys Compd. 2019, 788, 573–585. [Google Scholar] [CrossRef]
  35. Jiang, H.; Tian, S.; Guo, W.; Zhang, G.; Zeng, S. Hot Deformation Behavior and Deformation Mechanism of Two TiAl-Mo Alloys during Hot Compression. Mater. Sci. Eng. A 2018, 719, 104–111. [Google Scholar] [CrossRef]
  36. Li, J.; Liu, Y.; Wang, Y.; Liu, B.; He, Y. Dynamic Recrystallization Behavior of an As-Cast TiAl Alloy during Hot Compression. Mater. Charact. 2014, 97, 169–177. [Google Scholar] [CrossRef]
  37. Zhang, W.-J.; Lorenz, U.; Appel, F. Recovery, Recrystallization and Phase Transformations during Thermomechanical Processing and Treatment of TiAl-Based Alloys. Acta Mater. 2000, 48, 2803–2813. [Google Scholar] [CrossRef]
  38. Inui, H.; Kishida, K.; Misaki, M.; Kobayashi, M.; Shirai, Y.; Yamaguchi, M. Temperature Dependence of Yield Stress, Tensile Elongation and Deformation Structures in Polysynthetically Twinned Crystals of Ti-Al. Philos. Mag. A 1995, 72, 1609–1631. [Google Scholar] [CrossRef]
  39. Cao, G.H.; Klöden, B.; Rybacki, E.; Oertel, C.-G.; Skrotzki, W. High Strain Torsion of a TiAl-Based Alloy. Mater. Sci. Eng. A 2008, 483–484, 512–516. [Google Scholar] [CrossRef]
  40. Mecking, H.; Hartig, C.; Kocks, U.F. Deformation Modes in γ-TiAl as Derived from the Single Crystal Yield Surface. Acta Mater. 1996, 44, 1309–1321. [Google Scholar] [CrossRef]
  41. Bartels, A.; Hartig, C.; Willems, S.; Uhlenhut, H. Influence of the Deformation Conditions on the Texture Evolution in γ-TiAl. Mater. Sci. Eng. A 1997, 239–240, 14–22. [Google Scholar] [CrossRef]
  42. Bartels, A.; Kestler, H.; Clemens, H. Deformation Behavior of Differently Processed γ-Titanium Aluminides. Mater. Sci. Eng. A 2002, 329–331, 153–162. [Google Scholar] [CrossRef]
  43. Liu, R.; Liu, D.; Tan, J.; Cui, Y.; Yang, R.; Liu, F.; Withey, P.A. Textures of Rectangular Extrusions and Their Effects on the Mechanical Properties of Thermo-Mechanically Treated, Lamellar Microstructure, Ti–47Al–2Cr–2Nb–0.15B. Intermetallics 2014, 52, 110–123. [Google Scholar] [CrossRef]
  44. Brokmeier, H.-G.; Oehring, M.; Lorenz, U.; Appel, F.; Clemens, H. Neutron Diffraction Study of Texture Development during Hot Working of Different Gamma-Titanium Aluminide Alloys. Metall. Mater. Trans. A 2004, 35, 3563–3579. [Google Scholar] [CrossRef]
Metals 15 00123 g001
Figure 1. (a) Vacuum isothermal forging machine; (b) extruded rod; (c) extruded square billets; (d) schematic diagram of the direction of the samples; TiAl alloy forgings with reduction of (e) 45%, (f) 62.5%, and (g) 80%, respectively.
Figure 1. (a) Vacuum isothermal forging machine; (b) extruded rod; (c) extruded square billets; (d) schematic diagram of the direction of the samples; TiAl alloy forgings with reduction of (e) 45%, (f) 62.5%, and (g) 80%, respectively.
Metals 15 00123 g001
Metals 15 00123 g002
Figure 2. XRD patterns of as-extruded TiAl alloy and as-forged TiAl alloys with different deformations.
Figure 2. XRD patterns of as-extruded TiAl alloy and as-forged TiAl alloys with different deformations.
Metals 15 00123 g002
Metals 15 00123 g003
Figure 3. The microstructures of (a,b) as-extruded alloy and as-forged alloys with (c,d) 45% reduction, (e,f) 62.5% reduction, and (g,h) 80% reduction, respectively: (a,c,e,g) OM images; (b,d,f,h) SEM images in BSE mode.
Figure 3. The microstructures of (a,b) as-extruded alloy and as-forged alloys with (c,d) 45% reduction, (e,f) 62.5% reduction, and (g,h) 80% reduction, respectively: (a,c,e,g) OM images; (b,d,f,h) SEM images in BSE mode.
Metals 15 00123 g003
Metals 15 00123 g004
Figure 4. The EBSD analysis of (a1–a4) as-extruded alloy and as-forged alloys with (b1–b4) 45% reduction, (c1–c4) 62.5% reduction, and (d1–d4) 80% reduction, respectively: (a1,b1,c1,d1) IPF maps; (a2,b2,c2,d2) phase maps; (a3,b3,c3,d3) misorientation angle maps; (a4,b4,c4,d4) grain size maps.
Figure 4. The EBSD analysis of (a1–a4) as-extruded alloy and as-forged alloys with (b1–b4) 45% reduction, (c1–c4) 62.5% reduction, and (d1–d4) 80% reduction, respectively: (a1,b1,c1,d1) IPF maps; (a2,b2,c2,d2) phase maps; (a3,b3,c3,d3) misorientation angle maps; (a4,b4,c4,d4) grain size maps.
Metals 15 00123 g004
Metals 15 00123 g005
Figure 5. (a) Pole figures and (b) inverse pole figures of as-extruded alloy and as-forged alloys with different reductions.
Figure 5. (a) Pole figures and (b) inverse pole figures of as-extruded alloy and as-forged alloys with different reductions.
Metals 15 00123 g005
Metals 15 00123 g006
Figure 6. The fitted curve of the room-temperature strength variation tendency of extruded and forged samples.
Figure 6. The fitted curve of the room-temperature strength variation tendency of extruded and forged samples.
Metals 15 00123 g006
Table 1. The deformation parameters of the billets.
Table 1. The deformation parameters of the billets.
SamplesDeformation Parameters
Deformation Temperature (°C)Strain Rate (s−1)Deformation Amount (%)
111500.00145
211500.00162.5
311500.00180
Table 2. The volume fractions of lamellar colonies, γ grains, and β0 grains.
Table 2. The volume fractions of lamellar colonies, γ grains, and β0 grains.
SamplesVolume Fractions (%)
(α2/γ) Lamellar Coloniesγ Grainsβ0 Grains
As-extruded70.2629.220.52
As-forged, 45%44.7152.293.0
As-forged, 62.5%16.0480.893.07
As-forged, 80%8.3288.712.97
Table 3. The proportion of the lamellar colonies in different orientations.
Table 3. The proportion of the lamellar colonies in different orientations.
SamplesThe Angles Between the Lamellar Interface and the Extrusion Direction
0–10°10–60°60–90°
As-extruded48.94%27.66%23.4%
As-forged, 45%62.96%37.04%0
As-forged, 62.5%68.75%31.25%0
As-forged, 80%71.43%28.57%0
Table 4. The yield strength of lamellar colonies in different orientations at 1100 °C; data from [38].
Table 4. The yield strength of lamellar colonies in different orientations at 1100 °C; data from [38].
The Angle Between the Lamellar Boundaries and the Loading Axis
0°31°90°
Yield stress (MPa)18070260
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Li, J.; Wang, X.; Gong, M.; Guo, Z.; Kong, F. Microstructure Evolution of Extruded TiAl Alloy During Vacuum Isothermal Superplastic Forging Process. Metals 2025, 15, 123. https://doi.org/10.3390/met15020123

AMA Style

Li J, Wang X, Gong M, Guo Z, Kong F. Microstructure Evolution of Extruded TiAl Alloy During Vacuum Isothermal Superplastic Forging Process. Metals. 2025; 15(2):123. https://doi.org/10.3390/met15020123

Chicago/Turabian Style

Li, Jintao, Xiaopeng Wang, Minyu Gong, Zhenyu Guo, and Fantao Kong. 2025. "Microstructure Evolution of Extruded TiAl Alloy During Vacuum Isothermal Superplastic Forging Process" Metals 15, no. 2: 123. https://doi.org/10.3390/met15020123

APA Style

Li, J., Wang, X., Gong, M., Guo, Z., & Kong, F. (2025). Microstructure Evolution of Extruded TiAl Alloy During Vacuum Isothermal Superplastic Forging Process. Metals, 15(2), 123. https://doi.org/10.3390/met15020123

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop