A study on the hot workability of a novel TNM-RE alloy (RE = Y, La, Ce)

The hot deformation behavior of a novel TNM-RE alloy (RE = Y, La, Ce) was studied using a hot simulation machine (Gleeble-3800), and microstructural evolution was also characterized. Finally, 3D forging was carried out on isothermal forging equipment. It is shown that the as-cast lamellar colony size is about 20 ∼ 30 μm, which is refined by the formation of rare Earth oxides and borides at grain boundaries inhibiting grain growth. The peak stress of the TNM-RE alloy deformed at 1200 °C/0.01 s−1 is about 97 MPa, which is governed by the lamellar colony size and the B2 phase. Based on microstructure observation, it is found that the lamellar is bent and elongated to coordinate plastic deformation, where dynamic recrystallization nucleates preferentially, and full dynamic recrystallization is obtained at 1220 °C/0.01 s−1. The TNM-RE alloy was forged by 3D isothermal forging method, and fine grains with a size of 10 ∼ 20 μm were obtained by controlling the process parameters. The novel TNM-RE alloy shows an excellent hot workability.


Introduction
TiAl alloys are lightweight high-temperature structural materials that are increasingly used in aerospace industries and automobile sectors because of their excellent creep and oxidation resistance, high specific strength and modulus at elevated temperatures [1][2][3][4]. However, the low ductility and poor fracture toughness leading to reduced hot workability of TiAl alloys directly restricts their development and application. To this end, researchers have been focusing on the alloy design and thermal treatment optimization to obtain uniform and fine grains, in order to achieve improved mechanical properties.
Adding β phase stabilizing elements such as Nb, Mo, V, Cr, Mn, etc can change the solidification path of TiAl alloys from α phase solidification to β phase solidification. β phase solidification is used to reduce or eliminate alloying segregation, refine microstructure, and improve hot workability of TiAl alloys [4][5][6]. At the same time, it is found that B2 phase (low-temperature ordered β phase) is formed after adding β phase stabilizing elements, which can promote stable deformation and improve hot workability at high temperature. Therefore, many novel β-γ TiAl alloys were developed with hot workability being better than that of traditional TiAl alloys [7][8][9]. By using conventional forging and rolling, it has been found that the developed Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy optimized with the β phase stabilizing elements shows improved hot deformability, and cracks can hardly be found during the hot processing [10,11]. Grain boundary sliding could be the dominant mechanism contributing to superplastic elongation, while minor B addition can be of paramount importance in modifying the grain characteristics, such as significant grain refinement and changing the α-phase morphology from lamellar to globular [12]. According to recent studies, the (TiB + TiC) reinforcements can be the possible nucleation sites for the α-and β-phases and thus minimizing the deformation steps and decreasing the cost. Therefore, the Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe alloy with an excellent deformability and a high strength, via the (TiB + TiC) reinforcement, can achieve both less deformation steps and high mechanical properties [13,14]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
As illustrated by Chen et al [9] in an earlier work, Ti-43Al-V plate was rolled using canned forging and rolling technology. Rare Earth elements such as Y, La, Ru, etc are used for grain refinement and mechanical property improvement: e.g., Ru improves the strength and ductility simultaneously through solid solution strengthening and refinement strengthening [15,16]. Refining grains and lamellae is promoted by Y addition, and uniform distribution of Nb, Mo and the breaking Y compounds might be profitable to prevent oxidation of TiAl alloy, which lead to an improved hot deformability of a Ti-45Al-5Nb-0.3Y alloy [17][18][19]. Minor La and Ce addition has a refinement on the microstructure of TiA1 alloys, which presents in the TiAl alloy dominantly in the form of Al-RE constituents uniformly distributed in grain boundaries in the shape of discontinuous network. In addition, the bending strength of TiAl alloy is reinforced by the addition of appropriate alloying elements La and Ce [20]. Thermal simulation experiments are usually used to study the flow behavior and microstructural evolution, which is the basis of thermal processing parameters setting. Hot deformation temperatures of TiAl alloys are selected above T e (eutectoid transformation temperature, >1100°C) to reduce deformation resistance and avoid cracking. In addition, the hot deformation should be carried out below T α (γ → α transition temperature, <1300°C) to limit the content of hard α phase, improving deformability and dynamic recrystallization (DRX). Also, a large number of experiments show that the hot deformation window of 1150 ∼ 1250°C under various strain rates is suitable for appropriate hot working. The hot processing of TiAl alloy is canned forging using traditional pressing machine. While, the deformation dead zone cannot be avoided during repeated upsetting deformation, resulting in uneven microstructure. In the present work, a novel TNM-RE alloy (RE = Y, La, Ce) was designed by optimizing the chemical composition using proper β phase stabilizers and rare Earth elements, aiming at an excellent hot workability. 3D isothermal forging was applied to eliminate deformation dead zone, and uniform fine grains were properly obtained.

Materials and methods
The TNM-RE alloy (RE = Y, La, Ce) having a nominal composition of Ti-44Al-6Nb-1Mo-0.3(B, Y, La, Ce) was fabricated by vacuum arc remelting (VAR) method. The raw materials were aluminum particle (99.99 wt%), sponge titanium (99.7 wt%), niobium particle (99.95 wt%), molybdenum particle (99.95 wt%), boron powder (99.9 wt%), yttrium (99.99 wt%), and La-50Ce master alloy (wt%) respectively. The nominal composition of B, Y elements was 0.1 at.% respectively, and the La, Ce was 0.05 at.% respectively. Before the melting process, the chamber was firstly vacuumized to 10 −3 Pa and then the chamber was filled by argon to a required pressure. The ingot has a dimension of 90 mm in diameter and 450 mm in height. Hot compression test specimens were extracted by wire-electrode cutting from the ingot, with a dimension of 8 mm in diameter and 12 mm in height. The samples for isothermal forging tests were also obtained from the ingot, with a dimension of 80 mm in diameter and 120 mm in height.
Gleeble-3800 thermo-simulation machine was used for hot compression. The test temperatures were selected as 1160°C, 1180°C, 1200°C and 1220°C, and the strain rate was determined as 0.01 s −1 with the engineering strain selected as 60%. The temperature during deformation was measured by a digital Pt-Rh thermocouple. The heating rate was 5°C s −1 and the soaking time was 3 min. The high-temperature microstructure was preserved by using water quenching after compression.
The 3D isothermal forging method was carried out on an isothermal forging equipment to obtain fine and uniform grains. The ingot was deformed along X direction firstly, and rotated 90°and deformed along Y direction, then rotated 90°and deformed along Z direction, as shown in figure 1. The process was repeated twice to obtain homogenous microstructure. The strain and compression strain rate were determined to be 30% and 1 mm s −1 each pass, respectively. The TiAl alloy was isothermally forged to prevent temperature loss during forging. Moreover, the specimens were canned using stainless steel within thermal insulations. The forging temperature was 1200 ∼ 1240°C and the strain rate was 0.01 s −1 based on the results from the hot compression tests. An air cooling was conducted after forging of the specimens.
Both optical microscopy (OM, Leica MPS 30) and scanning electron microscopy (SEM, Gemini SEM 500) equipped with electron backscattered diffraction (EBSD) were conducted for the observation of the deformed and forged specimens. The specimens prepared by mechanical grinding, mechanical polishing, and etching with a Koller solution (5 vol% HNO 3 + 5 vol% HF + 90 vol% H 2 O) for 5 ∼ 8 s. X-ray diffraction (XRD) (2θ = 20°∼ 90°, 5°/min) was adopted to analyze the phases of the alloy. For EBSD observation, a vibration polishing step was used for the forged samples. The scanning step size of 1 μm was selected for data collection, and the data was further processed with Channel 5 software.

Results and discussion
3.1. As-cast microstructure Figure 2 shows XRD spectrum of the as-cast TNM-RE alloy, which demonstrates the alloy is mainly composed of α phase, γ phase and B2 phase. XRD is failed to detect the diffraction peaks of borides and rare Earth oxides due to the low content of B and rare Earth elements. Figure 3 shows the as-cast microstructure of TNM-RE alloy prepared by VAR method. It can be found that the microstructure is near lamellar with a size of about 20 ∼ 30 μm of the lamellar colonies. After addition of β stabilizing element, the β solidification acts as the solidification path, changing from α solidification, and consequently fine grains with random orientation are obtained based on the Blackburn relationship between α phase and β phase. In this study, 6% Nb and 1% Mo were added and the solidification process of the alloy was complete β solidification, and therefore the grains of the alloy are fine thanks to the grain refinement effect resulting from β solidification. Also, trace elements B, Y, La and Ce are also benefit to grain refinement. White bright blocks and linear precipitations are observed at lamellar colony boundary which can effectively inhibit grain growth and refine grains which can be seen in figures 3(c) and (d). γ blocks are observed at lamellar colony boundary accompanied by B2 phase formation, resulting in a microstructure of mixed (γ + B2) phases, as  shown in figure 3(d). At low temperature, B2 phase forms resulting from the ordering of the disordered β phase, while at high temperature, the B2 phase will transform again to the disordered β phase [21]. The β phase transforms to α phase at higher temperatures, while β phase is retained at grain boundary due to the incomplete transformation. As a result, during cooling process γ phase precipitates from β phase, and the mixed (γ + B2) microstructure is obtained. Normally, the solidification path of the alloy is L → L + β → β + α → α → Lamellar (α + γ) → Lamellar (α 2 + γ) when the content of Al is below 45% [22]. However, the incomplete transformation of β → α occurs in the present alloys due to a high content of β phase stabilizing elements, and as a result β phase becomes residual. Besides, a high content of β phase stabilizing elements reduce both T β (β → α transition temperature) and T α (α → γ transition temperature) of the TiAl alloys. The detail transus temperatures of this alloy are not measured, which will be part of a future study.
The chemical composition of the white block and linear precipitates were analyzed by energy dispersive x-ray spectroscopy (EDS), as shown in figure 4. Results show that the white and bright block precipitates are rare Earth oxides rich in Ce, La, Y. The oxide is formed through chemical reaction at high temperature, which inhibits grain growth and thus refining the grains during solidification. The high Ti content is attributed to formation of TiB, and the linear precipitates are determined to be boride. It has been reported that a certain amount of B can facilitate the formation of linear borides at grain boundaries and inhibits grain growth of TiAl alloys [23]. Figure 5(a) shows the tensile behavior of TNM-RE alloy deformed at 1160 ∼ 1220°C under strain rate of 0.01 s −1 in terms of true stress-strain curves. The data show typical DRX softening characteristics. At the initial deformation stage, the dislocation density increases rapidly, resulting in significant work hardening. DRX softening occurs with increasing strain, and work hardening competes with dynamic recovery and DRX softening. Before reaching the peak of the flow stress, work hardening effect is dominate. When the peak of the flow stress is reached, the DRX softening effect becomes dominant and the flow stress decreases with increase of strain. Finally, a dynamic balance is obtained between work hardening and DRX softening [24][25][26][27]. The peak stress is decreasing with an increase of temperature because that the DRX degree is larger at higher temperature as shown in figure 5(b). A decrease from 134 MPa to 41 MPa can be seen for the flow stress as the deformation temperature increases from 1160°C to 1220°C. Compared with Ti-43.5Al-4Nb-1Mo-0.1B alloy and other typical β-γ TiAl alloys, the TNM-RE alloy shows a lower flow stress. For example, the peak stresses are 140 MPa and 130 MPa for Ti-43.5Al-4Nb-1Mo-0.1B alloy and Ti-45Al-5.4V-3.6Nb-0.3Y alloy, respectively, when deformed at 1200°C/0.01s −1 , while the peak stress of TNM-RE alloy is 97 MPa when deformed at the same deformation condition [28][29][30][31]. The lower peak stress is mainly attributed to the grain refinement and formation of B2 phase. The peak stress is decreasing with the decrease of lamellar colony size, and with the increase of the size of B2 phase. That is because the refined grains provide more DRX nucleation sites during hot deformation, and as a result DRX is improved and the peak stress is decreased [32]. Besides, B2 phase possesses more undisturbed slip systems and promotes the motion of dislocations, which contributes to coordinating deformation at elevated temperature [33].

Hot deformation behavior
The microstructures of the TNM-RE alloy deformed at 1160 ∼ 1220°C are shown in figure 6. At a relatively low deformation temperature of 1160°C, the deformation microstructure is mainly composed of residual lamellar and partial dynamic recrystallized grains at the lamellar colony boundary as shown in figure 6(a). The lamellar is elongated and bent during deformation, and dynamic recrystallization is insufficient at a low deformation temperature. The degree of dynamic recrystallization increases with the increase of temperature suggesting that a higher deformation temperature promotes dynamic recrystallization, as shown in figures 6(b)-(d). The grain boundary is the main nucleation sites of dynamic recrystallization for TiAl alloy, and grain refinement can provide more nucleation sites and promote the dynamic recrystallization, leading to low peak stress. Last but not least, the coordinating deformation of grain boundaries promotes the plastic deformation and hinders cracks under higher temperatures. B2 grains are relatively soft at high temperatures and thus the flow stress is lower. The co-existence of B2 phase and dynamic recrystallization grains at the lamellar colony boundary indicates that the B2 phase promotes dynamic recrystallization. The low hot deformation resistance of TNM-RE alloy is mainly attributed to the fine grains and the B2 phase, which promotes dynamic recrystallization and hot workability.  High temperature forging is a major hot working approach of processing the TiAl alloy ingot, and the deformation temperature is a dominating parameter in the process to determine the eventual performance of the alloy. Cracking and unstable deformation can be effectively eliminated under appropriate deformation temperature. Based on the hot deformation results presented in figures 5 and 6, the optimized parameters were determined as: hot deformation temperature of >1200°C, strain rate of 0.01 s −1 , and strain of 60%, for obtaining fine and uniform grains for 3D isothermal forging.

3D isothermal forging
TiAl alloy is easy to crack due to its poor performance in plastic deformation and the temperature loss during forging, and therefore canned forging is usually used for TiAl alloys. The canned material is stainless steel, and thermal insulations are filled to ensure isothermal forging and to prevent temperatures loss during forging and transferring. The hot deformation temperature, strain rate and strain level were determined based on hot compression results (as described above) to obtain fine and uniform grains. The initial microstructure is fully deformed by 3D forging method to minimize the intensity of the forging texture and thus optimizing the uniformity of the grains. The 3D forging microstructures at different positions are shown in figure 7, which all present a duplex microstructure. The grains are mainly equiaxial and lamellar colonies without obvious forging morphology. The microstructure is uniform from the center to the surface, and the average grain size is  10 ∼ 20 μm, indicating that the deformation dead zone can be effectively eliminated in 3D forging process. The 3D isothermal forging method shows the feasibility of obtaining fine grains (10 ∼ 20 μm) without apparent cracks, and can be an optimum hot processing approach of the TiAl alloys.
The EBSD analyses of the 3D forged TNM-RE alloy are shown in figure 8. The phase map indicates that the TiAl alloy was refined after forging, consisting of equiaxed γ grain (yellow), B2 (blue) and residual lamellar colonies (consisting of α 2 phase (red) and γ phase (yellow)) with a size of ∼20 μm, as shown in figure 8 (a). The inverse pole figure (IPF) maps are shown in figure 8(b) overlapped with 2 ∼ 15°low-angle grains boundaries (LAGBs) as white lines and >15°high-angle grain boundaries (HAGBs) as black lines. It is obvious that LAGBs are dominant, and the majority of HAGBs distribute in the B2 phase region, while LAGBs prefer to exist in the γ phase region. In addition, γ and α 2 grains are randomly oriented, while the B2 exhibits a strong texture of horizontal direction close to {111} and normal direction close to {100}, as shown in figures 8(d)-(f). A possible reason is that the B2 phase is soft and deforms more easily at higher temperature due to more independent slip systems activated. Figure 8(c) shows the local misorientation map. The γ phase and residual lamella colonies show a higher misorientation value, suggesting that a larger plastic strain can be retained in the γ phase and the residual lamella colonies.

Conclusions
In this paper, the hot deformation behavior of the novel TNM-RE alloy was investigated along with the microstructural evolution, and the 3D isothermal forging was carried out t for this alloy. Some major conclusions can be drawn as follows: (1)B and trace rare Earth elements can refine TiAl alloys by the formation of borides and rare Earth oxides, and the as-cast lamellar colony size of the TNM-RE alloy is 20 ∼ 30 μm.
(2)The TNM-RE alloy shows a lower peak stress compared to traditional TiAl alloys. This is because that fine grains and B2 phase promote dynamic recrystallization and improve hot workability.
(3)The 3D isothermal forging microstructure is a duplex microstructure with an average grain size of 10 ∼ 20 μm. 3D isothermal forging is a feasible hot working approach for obtaining uniform fine grains for TiAl alloys.

Data availability statement
The data generated and/or analysed during the current study are not publicly available for legal/ethical reasons but are available from the corresponding author on reasonable request.