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The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys
Journal Pre-proof The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys
Bin Tang, Yudong Chu, Mengqi Zhang, Caisi Meng, Jiangkun Fan, Hongchao Kou, Jinshan Li PII:
S0254-0584(19)30942-3
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122125
Article Number:
122125
Reference:
MAC 122125
To appear in:
Materials Chemistry and Physics
Received Date:
29 May 2019
Accepted Date:
03 September 2019
Please cite this article as: Bin Tang, Yudong Chu, Mengqi Zhang, Caisi Meng, Jiangkun Fan, Hongchao Kou, Jinshan Li, The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122125
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Journal Pre-proof The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys
Bin Tang*,a,b, Yudong Chua, Mengqi Zhanga, Caisi Menga, Jiangkun Fana, Hongchao Koua, Jinshan Lia
aState
Key Laboratory of Solidification Processing, Northwestern Polytechnical University,
Xi’an 710072, China bShaanxi
Key Laboratory of High-Performance Precision Forming Technology and Equipment,
Northwestern Polytechnical University, Xi’an, Shaanxi, 710072, China
* Corresponding author. Bin Tang, Associate professor Address: No. 127, Youyi Xilu, Xi'an, Shaanxi, 710072, PR China. E-mail address: [email protected]; Tel/Fax: +86-29-88460294
Abstract: The ω phase transformation during isothermal aging and continuous heating process for three metastable β titanium alloys, Ti-5Al-5Mo-5V-3Cr (Ti-5553), Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) and Ti-7Mo-3Cr-3Nb-3Al (Ti-7333) were studied by dilatometry and transmission electron microscopy (TEM) methods. Results show that the ω phase transformation window of Ti-5553 and Ti-55531 alloys is about 275℃~350℃, while that of Ti-7333 alloy is 200℃~375℃. The ω particles produced during ageing process is larger than that produced during continuous heating process. A new developed Mo equivalent formula was used to discuss the effect of chemical composition on ω phase transformation for 1
Journal Pre-proof metastable β titanium alloys. It was found that metastable β titanium alloys with lower Mo equivalent have lower starting temperature of ω phase transformation and wider transformation window.
Keywords: ω phase transformation, metastable β titanium alloys, heat treatment, microstructural observation
1. Introduction Metastable β titanium alloys have remarkably less β-stabilizers contents than β titanium alloys, and are thus becoming an important development direction of high strength titanium alloys due to their high strength-density ratio, deeper hardenability and adjustable mechanical properties compared to the α + β and β titanium alloys [1-3]. Up to now, a lot of metastable β titanium alloys, such as Ti-15V-3Cr-3Al3Sn, Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-10V-2Fe-3Al (Ti-1023), have been applied in advanced aircrafts for springs and landing gears [4]. In addition, a metastable β titanium alloy, named Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy, has been developed to replace Ti-1023 alloy for Boeing 787 and Airbus A-350 due to its higher strength, wider processing window and deeper hardenability [5]. Subsequently, Ti-5Al-5Mo5V-3Cr-1Zr (Ti-55531) alloy was developed to get more comparable strength and better damage-tolerant properties [6]. Although the metastable β titanium alloys have important potential in high strength applications, the microstructure and performances are difficult to be controlled due to their inherent complexity, multicomponent and metastability. The occurrence of metastable phases [7-10], such as orthorhombic α'', hexagonal ω, hexagonal α', orthorhombic O', during the thermo-mechanical processing and components preparing processes, plays an important role in the microstructure evolution and performance change. The phase transformations induced by metastable phases results in the high microstructure sensitivity of metastable β titanium alloys. As one of the most important metastable phases in metastable β titanium alloys, ω phase, not only has great influence on the strength and ductility of the alloys [11, 12], but also can affect the precipitation of α phase upon continuous heating and duplex ageing [13, 14]. In the early studies, isothermal ω particles were thought to be harmful to ductility of titanium alloys [15, 16]. However, the metastable ω phase was found can act as a transition phase or a priority sites for the precipitation of nano-sized α phase, being beneficial to tensile strength [17]. Recently, the mechanism of ω-assisted α nucleation was investigated quantitatively by using phase-field simulation and high resolution scanning transmission electron microscopy [18]. Gao et al. reported that ω phase 2
Journal Pre-proof could enhance the yield strength of metastable β titanium alloy while retaining high ductility [19]. Although a lot of investigations about ω phase transformation have been carried out, the influence of chemical composition on ω phase transformation still needs further and deeper work. In the present work, the difference of ω phase transformation during continuous heating and aging processes in Ti-5553, Ti-55531 and a newly developed metastable β titanium alloy, Ti-7Mo-3Nb-3Cr3Al (Ti-7333) [20-22], has been studied. The transformation windows and morphology evolution of ω phase were investigated by dilatometry and transmission electron microscopy (TEM), respectively. Besides, the effect of chemical composition on ω phase transformation was discussed based on the newly developed Mo equivalent formula [23].
2. Materials and Methods Three kinds of metastable β titanium alloys, Ti-7333, Ti-5553 and Ti-55531, were used in present work. They were prepared by three times vacuum arc remelting (VAR) followed by multistep cogging and forging to round bar with diameter of Φ150 mm. The primary forging was carried out at the β phase field and the final forging was carried out at the α+β phase field. The β transus temperature of Ti-7333 alloy is approximately 850 ℃ measured by metallographic method, while the β transus temperatures of Ti-5553 and Ti-55531 alloys are close to 870 ℃. The actual chemical compositions of the three alloys are listed in Table 1. Table 1 Chemical composition of the three alloys used in present work (wt.%). element
Ti
Mo
Nb
Cr
V
Al
Zr
Ti-7333
Bal.
7.14
3.00
3.10
-
3.04
-
Ti-5553
Bal.
4.99
-
2.76
4.80
5.27
-
Ti-55531
Bal.
5.38
-
2.75
4.25
5.13
1.14
After multistep forging, specimens with dimension of Φ5 × 20 mm were cut from the round bars. Then the specimens were solution treated at 900 ℃ (β phase field) for 30 min in Tube Furnace to dissolve primary α phase followed by water quenching. In order to characterize the as-quenched microstructure, the specimens were polished with SiC papers from 240# to 2000# to remove scratches followed by etching at a solution of 10% HF + 10% HNO3 + 80% H2O. Besides, room temperature X-ray diffraction (XRD) was used to detect the phase constitution in the microstructure on the equipment of PHILIPS 3
Journal Pre-proof X’Pert MPD with Cu Ka radiation at 40 kV and 30 mA from 2θ = 20°-90°. By using optical microscopy (OM) and XRD, the microstructure of the as-quenched samples was found have single β phase.
Fig. 1 Microstructure and phase constitution of the three alloys after solution treated at 900 ℃ for 30 min and water quenching. (a) OM image for Ti-7333, (b) OM image for Ti-5553, (c) OM image for Ti-55531, (d) XRD patterns for three alloys.
The β -solution treated samples with 5 mm in diameter and 20 mm in length were used in the dilatometric tests, which performed on a dilatometer with inductive heating device (Netzsch DIL-402C). Then the thermal dilation of the β -solutionized Ti-7333, Ti-5553 and Ti-55531 samples during continuous heating and isothermal aging processes can be measured. For the continuous heating experiments, the samples were heated up at the rate of 1℃/min from room temperature to 900℃ in the high purity (≥99.999 %) argon atmosphere. For the isothermal aging experiments, the samples were heated up to 350℃ with the rate of 1℃/min followed by holding at 350℃ for 20h. After dilatometry tests, the samples were quenched to water. The ω phase was detected by TEM technique on a Tecnai G2 F30 transmission Electron microcopy operated at 200KV. The TEM samples were firstly cut from the investigated specimens by electricdischarge machining. Then the specimens were mechanically ground to ~ 50 μm thick and cut into Φ3 mm disks. At last, the thin foils used in TEM were prepared by twinjet polishing with an electrolyte of 63 vol.% methanol, 32 vol.% butanol and 5 vol.% perchlorate operating at 40 V and 233 K. 4
Journal Pre-proof 3. Results and discussion 3.1 Continuous heating In order to determine the transformation windows of ω phase for Ti-7333, Ti-5553 and Ti-55531 alloys during continuous heating process, dilatometry tests were carried out by using the as-quenched samples. Fig. 2 shows the dilatometry curves of Ti-7333, Ti-5553 and Ti-55531 alloys heated up with a rate of 1℃/min. Settefrati et al. [24] have reported that low heating rate (0.1℃/s) could lead to a transformation sequence including isothermal ω phase transformation for Ti-5553 alloy. Our previous work [25] also reported that the ω phase transformation in Ti-7333 alloy during continuous heating process took place at the heating rate of 1℃/min-10℃/min. Besides, the starting temperature of ω phase transformation increased with increasing the heating rates. Therefore, it is notable that the formation of ω phase is very sensitive to the heating rates. In present work, a low heating rate with 1℃/min was taken to ensure the occurrence of ω phase transformation during continuous heating process for the three alloys. In Ref. [25], the transformation sequence in Ti-7333 alloy under 1℃/min heating rate is β→β+ωiso→ β+ωiso+α→β+α. As shown in Fig. 2, the transformation sequences in Ti-5553 and Ti-55531 alloys are similar with that in Ti-7333 alloy, while the turning points on the dilatometry curves are different. It can be noted from Fig.2 that the temperature range of ω phase transformation in Ti-7333 alloy is about 200℃ ~ 375℃, which is broader than that of in Ti-5553 and Ti-55531 alloys (about 275℃~350℃).
Fig. 2 Dilatometry curves upon heating (heating rate of 1℃/min) for the Ti-7333 (red), Ti-5553 (black) and Ti-55531(blue) alloys.
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Journal Pre-proof Fig. 3 shows the TEM images and corresponding selected area electron diffraction (SAED) patterns of the as-quenched samples for the three metastable β titanium alloys. None ω particles was found in Fig. 3. In order to confirm the occurrence of ω phase transformation during continuous heating process, the as-quenched Ti-7333 and Ti-55531 alloys were heated up (with heating rate of 1℃/min) to 240℃ and 300℃ respectively then water quenched for microstructure observing. The SAED patterns and dark field TEM micrographs are shown in Fig. 4. It can be seen from Fig. 4(b) and Fig. 4(d) that, the reflections of ω phase can be observed obviously and the ω phase has classical orientation relationship with β matrix, {111}β//{0001}ω and <110>β//<11–20>ω. As shown in Fig. 4(a) and (c), the ω particles were found homogeneously distributed in the β matrix with an average size less than 10 nm. Besides, the density of ω particles shown in Fig. 4(a) is larger than that of in Fig. 4(c) while the particle size of former is smaller, indicating a coarsening process of ω phase during the continuous heating process.
Fig.3 TEM micrographs of the three alloys quenched from β solution treatment. (a) Ti-5553 alloy, (b) Ti-55531 alloy, (c) Ti-7333 alloy. The insets at the top right corner show the SAED patterns.
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Fig. 4 Dark field micrographs of ellipsoidal ω precipitates (a, c) and SAED patterns (b, d) along [011]β zone axis showing ω reflections when quenched from 240℃ for Ti-7333 alloy (a, b) and 300℃ for Ti-55531 alloy (c, d) during the continuous heating process, respectively. The white dash circles in (b) and (d) denote the selected spots for dark field micrographs.
Mo equivalent ([Mo]eq) is usually used to classify the titanium alloys. In this concept, the stability contribution of alloying elements can be quantitatively calculated. Therefore, it is easier to discuss the effect of chemical composition on phase transformation or microstructure evolution avoiding considering the complex interaction among the alloying elements. Conventionally, the Mo equivalent can be calculated by the typical formula proposed by Bania [26]. [Mo]eq = 1.0 Mo + 0.67 V + 0.44 W + 0.28 Nb + 0.22 Ta + 2.9 Fe +1.6 Cr +0.77 Cu + 1.11 Ni +1.43 Co + 1.54 Mn − 1.0 Al (wt.%) Recently, a new Mo equivalent formula was proposed by C. Dong et al [23]. [Mo]eq = 1.0 Mo + 1.25 V + 0.59 W + 0.28 Nb + 0.22 Ta + 1.93 Fe + 1.84 Cr + 1.50 Cu + 2.46 Ni + 2.67 Co + 2.26 Mn + 0.30 Sn + 0.47 Zr + 3.01 Si – 1.47Al (wt.%) 7
Journal Pre-proof In present work, Bania’s model and Dong’s model were used to calculated the Mo equivalent of the three alloys, respectively. Calculation results were shown in Table 2. By using a modified Mo equivalent formula, T. Gloriant et al. [27] found that the starting temperature of ω phase transformation increases with the Mo equivalent value. In present work, the ω phase precipitation starting temperature of Ti-7333 is about 200 ℃, which is lower than that (about 275 ℃) of Ti-5553 and Ti-55531. As listed in Table 2, the Mo equivalent value of Ti-7333 calculated by Bania’s model is larger than that of Ti-5553 and Ti55531,while the Mo equivalent value of Ti-7333 calculated by Dong’s model is smaller than that of Ti5553 and Ti-55531. As we have well known, the stability of β phase increases with the increasing of Mo equivalent value. Therefore, lower Mo equivalent tends to result in lower starting temperature of phase transformation. It indicates that the Dong’s model is more accurate than Bania’s model for discussing the starting temperature of ω phase transformation. Table 2 The Mo equivalent calculated by Bania’s model and Dong’s model alloys
Bania’s model (wt.%)
Dong’s model(wt.%)
Ti-7333 (wt.%)
9.9
8.95
Ti-5553 (wt.%)
7.35
9.42
Ti-55531 (wt.%)
7.31
9.89
3.2 Isothermal ageing In order to get a further understanding of the ω phase transformation during the low temperature ageing process for the three alloys, isothermal ageing tests were carried out in the dilatometer just after heating up to 350℃ with a rate of 1℃/min (Fig. 5). It can be seen that the three alloys dilate linearly at the initial stage owing to the thermal expansion of β phase during heating process. Note that the thermal expansion coefficient of Ti-7333 is higher than that of Ti-5553 and Ti-55531 alloys. By using the (dL/L0)% instead of dL/L0 value, the transition details in A-B segment of Fig. 2 were revealed. As shown in Fig. 5, two parts, denoted by A-B section and B-C section, were found in the segment shown in Fig. 2. Point A, B and C in Fig. 5 locates at 250℃, 309℃ and 350℃, respectively. The dilatometry exhibits decrease first and then increase. This phenomenon is supposed to be related to the ω phase precipitation during heating. In A-B section (Fig. 5), it reveals that β to ωiso transformation takes place associated with a linear contraction. In B-C section (Fig. 5), the β to ωiso transformation stops and the alloy continues to dilate. Additionally, the B-C section (Fig. 5) has a similar slope with that in initial linear expansion stage. 8
Journal Pre-proof According to the TEM observations shown in Ref. [25], the isothermal ω particles experienced a coarsening process in this section. After C point (Fig. 5), another isothermal β to ω transformation begins to occur during the subsequent 350℃ ageing process. Comparing the two isothermal ω phase shown in Fig. 4(a) and Fig. 6 of present work, the size of isothermal ω phase produced during isothermal ageing is larger than that of ω phase produced during the heating process (A-B section in Fig. 5). However, further investigations are needed for differentiating the two kinds of isothermal ω phase and the corresponding transformation mechanism. The curves of Ti-5553 and Ti-55531 changed slightly compared with that of Ti-7333. The dilatometry variation dL/L0 of Ti-5553 and Ti-55531 keeps increasing until 400 minutes.
Fig. 5 Dilatometry curves of Ti-7333(red), Ti-5553(black) and Ti-55531(blue) alloys measured during continuous heating and isothermal aging processes.
Fig.6 shows the SAED patterns taken along [113] β zone axis for Ti-7333 alloy aged for 4h and 8h at 350℃. The ω phase reflections were found locate at 1/3 and 2/3 (-1-21) β positions. Dark field images produced by corresponding ω phase reflections in Fig. 6(b) and (d) show that nano-sized ω particles (larger than 10nm) homogeneously distributed in β matrix. Comparing with Fig.6 (a), the ω particles are coarser than that of in Fig.6 (c). Similar results about the isothermal ω phase transformation in Ti-7333 alloy have been reported in our previous work [28]. In order to compare the isothermal ω phase transformation in Ti-7333 alloy and Ti-55531 alloy, the TEM observations were carried out along [113] β zone axis.
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Fig. 6 Dark field micrographs of ellipsoidal ω precipitates (a, c) and SAED patterns (c, d) along [113] β zone axis showing ω reflections of Ti-7333 alloy aged for 4h (a, b) and 8h (c, d) at 350℃. The white dash circles in (b) and (d) denote the selected spots for dark field micrographs.
Fig. 7 shows the SAED patterns taken along [113] β zone axis for Ti-55531 alloy. From Fig. 7(b), it can be observed that additional diffuse reflections locate at the 1/2 (-1-21) β positions except for the ω reflections at 1/3 and 2/3 (-1-21) β positions. It has been reported that these additional reflections located at 1/2 (-1-21) β positions belong to α precipitation [29, 30]. The intensity of ω reflection is higher than that of α reflections, indicating that only few α phase produced in this condition. The bright field image exhibits that the ω particles distribute uniformly with high density. After aging for 8h, it is apparent from Fig. 7(d) that the intensity of ω reflections decreases and overlaps with the α reflections, indicating a transformation from α to ω. Although there are some different opinions about the role of ω particles on the nucleation of α phase in the metastable titanium alloys [14, 31, 32], the ω-assisted α nucleation behavior has been commonly accepted.
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Fig. 7 Bright field images of ellipsoidal ω precipitates (a, c) and SAED patterns (b, d) along [113] β zone axis showing ω reflections of Ti-55531 alloy aged for 4h (a, b) and 8h (c, d) at 350℃.
The alloy compositions not only affect the starting temperature of ω phase transformation but also influence the upper temperature limit ω phase. J.C. Williams et al. [10] found that the upper temperature limit of ω phase stability decreases with increasing Al content. In present work, the upper temperature limit of ω phase stability in Ti-7333 alloy is higher than that of in Ti-5553 and Ti-55531 alloys (see Fig. 2 and Fig. 5). Similarly, the alloy compositions also affect the duration time of stable ω phase. It can be seen from Fig. 6 and Fig. 7, the ω to α phase transformation occurs during aging for 4h at 350℃ in Ti55531 alloy whereas only ω phase was found in the β matrix of Ti-7333 alloy until aging for 8h at 350℃. The ω phase transformation during aging process in metastable β titanium alloys can be described as follows: The compositional and structural instabilities arise in the bcc lattice during water quenching from single β phase field. After subsequent ageing, the β phase in metastable β titanium alloys exhibits a tendency for phase separation, giving rise to solute-rich and solute-lean regions [33]. The ω phase tends to form in the solute-lean regions [34, 35]. Besides, it has been revealed that defects such as vacancy, local strain, compositional fluctuations play an important role in β to ω phase transformation. In our previous work [36], it was proposed that the nucleation of ω may associate with composition-dependent oscillating defect field induced by substitutional solutes, interstitial alloys and vacancies. E. Sukedai, et 11
Journal Pre-proof al. [37, 38] reported that the vacancies and their clusters induced by quenching and deforming can become nucleation sites for β to ω phase transformation, resulting in the enhancement of ω phase transformation when quenching from higher temperature or ageing at low temperature after deforming. The homogeneous distribution of ω phase in β matrix may be related to the lower energy barrier between ω and β phases, which resulting in high-frequency and homogeneous nucleation. With the nucleation and growth up of the ω particles the β-stablilizers like Mo, Nb, Cr, V are ejected from ω-affected regions. Then, the nucleation sites and transformation driving force decrease, and the growth of ω particles are suppressed due to the enriched β-stablilizers around ω particles [39]. It needs more time and higher temperature for the redistribution of solute elements to act as potential nucleation sites. Hence, the precipitation of ω phase during aging is not a continuous process.
4. Conclusions The ω phase transformation during continuous heating and isothermal aging in Ti-7333, Ti-5553 and Ti-55531 alloys was studied by using dilatometry and TEM techniques. The main results can be summarized as follows: (1) The ω phase transformation window of Ti-5553 and Ti-55531alloys is 275 ℃~350 ℃, while Ti7333 alloy has a broader transformation window of 200 ℃~375 ℃. (2) Dilatometry curves indicate that two kinds of isothermal β to ω phase transformation take place during the continuous heating and subsequent ageing processes, respectively. The ω particles produced during ageing process is larger than that produced during continuous heating process. (3) Based on the Dong’s Mo equivalent formula, the metastable β titanium alloys with lower Mo equivalent have lower starting temperature of ω phase transformation and wider transformation window.
Acknowledgements This work was financially supported by the Major State Research Development Program of China (No.2016YFB0701303), the National Natural Science Foundation of China (No.51301135) and the ‘‘111” Project (No. B08040).
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Journal Pre-proof [27] T. Gloriant, G. Texier, F. Prima, D. Laille, D.M. Gordin, I. Thibon, D. Ansel, Synthesis and phase transformations of beta metastable Ti-based alloys containing biocompatible Ta, Mo and Fe betastabilizer elements, Adv Eng Mater, 8 (2006) 961-965. [28] C.S. Meng, W.C. Ou, K. Hua, B. Tang, H.C. Kou, J.S. Li, Precipitation behavior of ω phase of Ti7333 titanium alloy during low temperature aging, The Chinese Journal of Nonferrous Metals, 23 (2013) s62-s66. [29] B. He, X. Cheng, J. Li, G.C. Li, H.M. Wang, omega-assisted alpha phase and hardness of Ti-5Al5Mo-5V-1Cr-1Fe during low temperature isothermal heat treatment after laser surface remelting, J Alloy Compd, 708 (2017) 1054-1062. [30] R. Banerjee, P.C. Collins, D. Bhattacharyya, S. Banerjee, H.L. Fraser, Microstructural evolution in laser deposited compositionally graded alpha/beta titanium-vanadium alloys, Acta Mater, 51 (2003) 3277-3292. [31] S. Azimzadeh, H.J. Rack, Phase transformations in Ti-6.8Mo-4.5Fe-1.5Al, Metall Mater Trans A, 29 (1998) 2455-2467. [32] F. Langmayr, P. Fratzl, G. Vogl, W. Miekeley, Crossover from Omega-Phase to Alpha-Phase Precipitation in Bcc Ti-Mo, Phys Rev B, 49 (1994) 11759-11766. [33] S. Banerjee, R. Tewari, G.K. Dey, Omega phase transformation - morphologies and mechanisms, Int J Mater Res, 97 (2006) 963-977. [34] A. Devaraj, S. Nag, R. Srinivasan, R.E.A. Williams, S. Banerjee, R. Banerjee, H.L. Fraser, Experimental evidence of concurrent compositional and structural instabilities leading to omega precipitation in titanium-molybdenum alloys, Acta Mater, 60 (2012) 596-609. [35] B.S. Hickman, The formation of omega phase in titanium and zirconium alloys: A review, J Mater Sci, 4 (1969) 554-563. [36] B. Tang, Y.W. Cui, H. Chang, H.C. Kou, J.S. Li, L. Zhou, Modeling of Incommensurate omega Structure in the Zr-Nb Alloys, Metall Mater Trans A, 43a (2012) 2581-2586. [37] E. Sukedai, M. Shimoda, H. Nishizawa, Y. Nako, Nucleation Behaviour of beta to omega Phase Transformations in beta-Type Ti-Mo Alloys, Mater Trans, 52 (2011) 324-330. [38] E. Sukedai, D. Yoshimitsu, H. Matsumoto, H. Hashimoto, M. Kiritani, beta to omega phase transformation due to aging in a Ti-Mo alloy deformed in impact compression, Mat Sci Eng a-Struct, 350 (2003) 133-138. 15
Journal Pre-proof [39] B. Tang, Y.W. Cui, H.C. Kou, H. Chang, J.S. Li, L. Zhou, Phase field modeling of isothermal beta -> omega phase transformation in the Zr-Nb alloys, Comp Mater Sci, 61 (2012) 76-82.
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Journal Pre-proof Highlights 1. The ω phase precipitation starting temperature of Ti-7333 is lower than that of Ti-5553 and Ti-55531. 2. The ω phase transformation window of Ti-7333 is wider than that of Ti-5553 and Ti-55531. 3. Two kinds of isothermal ω transformation take place during the continuous heating and ageing processes.
4. Metastable β titanium alloys with lower Mo equivalent are beneficial for the ω phase transformation.
Bin Tang, Yudong Chu, Mengqi Zhang, Caisi Meng, Jiangkun Fan, Hongchao Kou, Jinshan Li PII:
S0254-0584(19)30942-3
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122125
Article Number:
122125
Reference:
MAC 122125
To appear in:
Materials Chemistry and Physics
Received Date:
29 May 2019
Accepted Date:
03 September 2019
Please cite this article as: Bin Tang, Yudong Chu, Mengqi Zhang, Caisi Meng, Jiangkun Fan, Hongchao Kou, Jinshan Li, The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122125
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Journal Pre-proof The ω phase transformation during the low temperature aging and low rate heating process of metastable β titanium alloys
Bin Tang*,a,b, Yudong Chua, Mengqi Zhanga, Caisi Menga, Jiangkun Fana, Hongchao Koua, Jinshan Lia
aState
Key Laboratory of Solidification Processing, Northwestern Polytechnical University,
Xi’an 710072, China bShaanxi
Key Laboratory of High-Performance Precision Forming Technology and Equipment,
Northwestern Polytechnical University, Xi’an, Shaanxi, 710072, China
* Corresponding author. Bin Tang, Associate professor Address: No. 127, Youyi Xilu, Xi'an, Shaanxi, 710072, PR China. E-mail address: [email protected]; Tel/Fax: +86-29-88460294
Abstract: The ω phase transformation during isothermal aging and continuous heating process for three metastable β titanium alloys, Ti-5Al-5Mo-5V-3Cr (Ti-5553), Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) and Ti-7Mo-3Cr-3Nb-3Al (Ti-7333) were studied by dilatometry and transmission electron microscopy (TEM) methods. Results show that the ω phase transformation window of Ti-5553 and Ti-55531 alloys is about 275℃~350℃, while that of Ti-7333 alloy is 200℃~375℃. The ω particles produced during ageing process is larger than that produced during continuous heating process. A new developed Mo equivalent formula was used to discuss the effect of chemical composition on ω phase transformation for 1
Journal Pre-proof metastable β titanium alloys. It was found that metastable β titanium alloys with lower Mo equivalent have lower starting temperature of ω phase transformation and wider transformation window.
Keywords: ω phase transformation, metastable β titanium alloys, heat treatment, microstructural observation
1. Introduction Metastable β titanium alloys have remarkably less β-stabilizers contents than β titanium alloys, and are thus becoming an important development direction of high strength titanium alloys due to their high strength-density ratio, deeper hardenability and adjustable mechanical properties compared to the α + β and β titanium alloys [1-3]. Up to now, a lot of metastable β titanium alloys, such as Ti-15V-3Cr-3Al3Sn, Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-10V-2Fe-3Al (Ti-1023), have been applied in advanced aircrafts for springs and landing gears [4]. In addition, a metastable β titanium alloy, named Ti-5Al-5Mo-5V-3Cr (Ti-5553) alloy, has been developed to replace Ti-1023 alloy for Boeing 787 and Airbus A-350 due to its higher strength, wider processing window and deeper hardenability [5]. Subsequently, Ti-5Al-5Mo5V-3Cr-1Zr (Ti-55531) alloy was developed to get more comparable strength and better damage-tolerant properties [6]. Although the metastable β titanium alloys have important potential in high strength applications, the microstructure and performances are difficult to be controlled due to their inherent complexity, multicomponent and metastability. The occurrence of metastable phases [7-10], such as orthorhombic α'', hexagonal ω, hexagonal α', orthorhombic O', during the thermo-mechanical processing and components preparing processes, plays an important role in the microstructure evolution and performance change. The phase transformations induced by metastable phases results in the high microstructure sensitivity of metastable β titanium alloys. As one of the most important metastable phases in metastable β titanium alloys, ω phase, not only has great influence on the strength and ductility of the alloys [11, 12], but also can affect the precipitation of α phase upon continuous heating and duplex ageing [13, 14]. In the early studies, isothermal ω particles were thought to be harmful to ductility of titanium alloys [15, 16]. However, the metastable ω phase was found can act as a transition phase or a priority sites for the precipitation of nano-sized α phase, being beneficial to tensile strength [17]. Recently, the mechanism of ω-assisted α nucleation was investigated quantitatively by using phase-field simulation and high resolution scanning transmission electron microscopy [18]. Gao et al. reported that ω phase 2
Journal Pre-proof could enhance the yield strength of metastable β titanium alloy while retaining high ductility [19]. Although a lot of investigations about ω phase transformation have been carried out, the influence of chemical composition on ω phase transformation still needs further and deeper work. In the present work, the difference of ω phase transformation during continuous heating and aging processes in Ti-5553, Ti-55531 and a newly developed metastable β titanium alloy, Ti-7Mo-3Nb-3Cr3Al (Ti-7333) [20-22], has been studied. The transformation windows and morphology evolution of ω phase were investigated by dilatometry and transmission electron microscopy (TEM), respectively. Besides, the effect of chemical composition on ω phase transformation was discussed based on the newly developed Mo equivalent formula [23].
2. Materials and Methods Three kinds of metastable β titanium alloys, Ti-7333, Ti-5553 and Ti-55531, were used in present work. They were prepared by three times vacuum arc remelting (VAR) followed by multistep cogging and forging to round bar with diameter of Φ150 mm. The primary forging was carried out at the β phase field and the final forging was carried out at the α+β phase field. The β transus temperature of Ti-7333 alloy is approximately 850 ℃ measured by metallographic method, while the β transus temperatures of Ti-5553 and Ti-55531 alloys are close to 870 ℃. The actual chemical compositions of the three alloys are listed in Table 1. Table 1 Chemical composition of the three alloys used in present work (wt.%). element
Ti
Mo
Nb
Cr
V
Al
Zr
Ti-7333
Bal.
7.14
3.00
3.10
-
3.04
-
Ti-5553
Bal.
4.99
-
2.76
4.80
5.27
-
Ti-55531
Bal.
5.38
-
2.75
4.25
5.13
1.14
After multistep forging, specimens with dimension of Φ5 × 20 mm were cut from the round bars. Then the specimens were solution treated at 900 ℃ (β phase field) for 30 min in Tube Furnace to dissolve primary α phase followed by water quenching. In order to characterize the as-quenched microstructure, the specimens were polished with SiC papers from 240# to 2000# to remove scratches followed by etching at a solution of 10% HF + 10% HNO3 + 80% H2O. Besides, room temperature X-ray diffraction (XRD) was used to detect the phase constitution in the microstructure on the equipment of PHILIPS 3
Journal Pre-proof X’Pert MPD with Cu Ka radiation at 40 kV and 30 mA from 2θ = 20°-90°. By using optical microscopy (OM) and XRD, the microstructure of the as-quenched samples was found have single β phase.
Fig. 1 Microstructure and phase constitution of the three alloys after solution treated at 900 ℃ for 30 min and water quenching. (a) OM image for Ti-7333, (b) OM image for Ti-5553, (c) OM image for Ti-55531, (d) XRD patterns for three alloys.
The β -solution treated samples with 5 mm in diameter and 20 mm in length were used in the dilatometric tests, which performed on a dilatometer with inductive heating device (Netzsch DIL-402C). Then the thermal dilation of the β -solutionized Ti-7333, Ti-5553 and Ti-55531 samples during continuous heating and isothermal aging processes can be measured. For the continuous heating experiments, the samples were heated up at the rate of 1℃/min from room temperature to 900℃ in the high purity (≥99.999 %) argon atmosphere. For the isothermal aging experiments, the samples were heated up to 350℃ with the rate of 1℃/min followed by holding at 350℃ for 20h. After dilatometry tests, the samples were quenched to water. The ω phase was detected by TEM technique on a Tecnai G2 F30 transmission Electron microcopy operated at 200KV. The TEM samples were firstly cut from the investigated specimens by electricdischarge machining. Then the specimens were mechanically ground to ~ 50 μm thick and cut into Φ3 mm disks. At last, the thin foils used in TEM were prepared by twinjet polishing with an electrolyte of 63 vol.% methanol, 32 vol.% butanol and 5 vol.% perchlorate operating at 40 V and 233 K. 4
Journal Pre-proof 3. Results and discussion 3.1 Continuous heating In order to determine the transformation windows of ω phase for Ti-7333, Ti-5553 and Ti-55531 alloys during continuous heating process, dilatometry tests were carried out by using the as-quenched samples. Fig. 2 shows the dilatometry curves of Ti-7333, Ti-5553 and Ti-55531 alloys heated up with a rate of 1℃/min. Settefrati et al. [24] have reported that low heating rate (0.1℃/s) could lead to a transformation sequence including isothermal ω phase transformation for Ti-5553 alloy. Our previous work [25] also reported that the ω phase transformation in Ti-7333 alloy during continuous heating process took place at the heating rate of 1℃/min-10℃/min. Besides, the starting temperature of ω phase transformation increased with increasing the heating rates. Therefore, it is notable that the formation of ω phase is very sensitive to the heating rates. In present work, a low heating rate with 1℃/min was taken to ensure the occurrence of ω phase transformation during continuous heating process for the three alloys. In Ref. [25], the transformation sequence in Ti-7333 alloy under 1℃/min heating rate is β→β+ωiso→ β+ωiso+α→β+α. As shown in Fig. 2, the transformation sequences in Ti-5553 and Ti-55531 alloys are similar with that in Ti-7333 alloy, while the turning points on the dilatometry curves are different. It can be noted from Fig.2 that the temperature range of ω phase transformation in Ti-7333 alloy is about 200℃ ~ 375℃, which is broader than that of in Ti-5553 and Ti-55531 alloys (about 275℃~350℃).
Fig. 2 Dilatometry curves upon heating (heating rate of 1℃/min) for the Ti-7333 (red), Ti-5553 (black) and Ti-55531(blue) alloys.
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Journal Pre-proof Fig. 3 shows the TEM images and corresponding selected area electron diffraction (SAED) patterns of the as-quenched samples for the three metastable β titanium alloys. None ω particles was found in Fig. 3. In order to confirm the occurrence of ω phase transformation during continuous heating process, the as-quenched Ti-7333 and Ti-55531 alloys were heated up (with heating rate of 1℃/min) to 240℃ and 300℃ respectively then water quenched for microstructure observing. The SAED patterns and dark field TEM micrographs are shown in Fig. 4. It can be seen from Fig. 4(b) and Fig. 4(d) that, the reflections of ω phase can be observed obviously and the ω phase has classical orientation relationship with β matrix, {111}β//{0001}ω and <110>β//<11–20>ω. As shown in Fig. 4(a) and (c), the ω particles were found homogeneously distributed in the β matrix with an average size less than 10 nm. Besides, the density of ω particles shown in Fig. 4(a) is larger than that of in Fig. 4(c) while the particle size of former is smaller, indicating a coarsening process of ω phase during the continuous heating process.
Fig.3 TEM micrographs of the three alloys quenched from β solution treatment. (a) Ti-5553 alloy, (b) Ti-55531 alloy, (c) Ti-7333 alloy. The insets at the top right corner show the SAED patterns.
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Fig. 4 Dark field micrographs of ellipsoidal ω precipitates (a, c) and SAED patterns (b, d) along [011]β zone axis showing ω reflections when quenched from 240℃ for Ti-7333 alloy (a, b) and 300℃ for Ti-55531 alloy (c, d) during the continuous heating process, respectively. The white dash circles in (b) and (d) denote the selected spots for dark field micrographs.
Mo equivalent ([Mo]eq) is usually used to classify the titanium alloys. In this concept, the stability contribution of alloying elements can be quantitatively calculated. Therefore, it is easier to discuss the effect of chemical composition on phase transformation or microstructure evolution avoiding considering the complex interaction among the alloying elements. Conventionally, the Mo equivalent can be calculated by the typical formula proposed by Bania [26]. [Mo]eq = 1.0 Mo + 0.67 V + 0.44 W + 0.28 Nb + 0.22 Ta + 2.9 Fe +1.6 Cr +0.77 Cu + 1.11 Ni +1.43 Co + 1.54 Mn − 1.0 Al (wt.%) Recently, a new Mo equivalent formula was proposed by C. Dong et al [23]. [Mo]eq = 1.0 Mo + 1.25 V + 0.59 W + 0.28 Nb + 0.22 Ta + 1.93 Fe + 1.84 Cr + 1.50 Cu + 2.46 Ni + 2.67 Co + 2.26 Mn + 0.30 Sn + 0.47 Zr + 3.01 Si – 1.47Al (wt.%) 7
Journal Pre-proof In present work, Bania’s model and Dong’s model were used to calculated the Mo equivalent of the three alloys, respectively. Calculation results were shown in Table 2. By using a modified Mo equivalent formula, T. Gloriant et al. [27] found that the starting temperature of ω phase transformation increases with the Mo equivalent value. In present work, the ω phase precipitation starting temperature of Ti-7333 is about 200 ℃, which is lower than that (about 275 ℃) of Ti-5553 and Ti-55531. As listed in Table 2, the Mo equivalent value of Ti-7333 calculated by Bania’s model is larger than that of Ti-5553 and Ti55531,while the Mo equivalent value of Ti-7333 calculated by Dong’s model is smaller than that of Ti5553 and Ti-55531. As we have well known, the stability of β phase increases with the increasing of Mo equivalent value. Therefore, lower Mo equivalent tends to result in lower starting temperature of phase transformation. It indicates that the Dong’s model is more accurate than Bania’s model for discussing the starting temperature of ω phase transformation. Table 2 The Mo equivalent calculated by Bania’s model and Dong’s model alloys
Bania’s model (wt.%)
Dong’s model(wt.%)
Ti-7333 (wt.%)
9.9
8.95
Ti-5553 (wt.%)
7.35
9.42
Ti-55531 (wt.%)
7.31
9.89
3.2 Isothermal ageing In order to get a further understanding of the ω phase transformation during the low temperature ageing process for the three alloys, isothermal ageing tests were carried out in the dilatometer just after heating up to 350℃ with a rate of 1℃/min (Fig. 5). It can be seen that the three alloys dilate linearly at the initial stage owing to the thermal expansion of β phase during heating process. Note that the thermal expansion coefficient of Ti-7333 is higher than that of Ti-5553 and Ti-55531 alloys. By using the (dL/L0)% instead of dL/L0 value, the transition details in A-B segment of Fig. 2 were revealed. As shown in Fig. 5, two parts, denoted by A-B section and B-C section, were found in the segment shown in Fig. 2. Point A, B and C in Fig. 5 locates at 250℃, 309℃ and 350℃, respectively. The dilatometry exhibits decrease first and then increase. This phenomenon is supposed to be related to the ω phase precipitation during heating. In A-B section (Fig. 5), it reveals that β to ωiso transformation takes place associated with a linear contraction. In B-C section (Fig. 5), the β to ωiso transformation stops and the alloy continues to dilate. Additionally, the B-C section (Fig. 5) has a similar slope with that in initial linear expansion stage. 8
Journal Pre-proof According to the TEM observations shown in Ref. [25], the isothermal ω particles experienced a coarsening process in this section. After C point (Fig. 5), another isothermal β to ω transformation begins to occur during the subsequent 350℃ ageing process. Comparing the two isothermal ω phase shown in Fig. 4(a) and Fig. 6 of present work, the size of isothermal ω phase produced during isothermal ageing is larger than that of ω phase produced during the heating process (A-B section in Fig. 5). However, further investigations are needed for differentiating the two kinds of isothermal ω phase and the corresponding transformation mechanism. The curves of Ti-5553 and Ti-55531 changed slightly compared with that of Ti-7333. The dilatometry variation dL/L0 of Ti-5553 and Ti-55531 keeps increasing until 400 minutes.
Fig. 5 Dilatometry curves of Ti-7333(red), Ti-5553(black) and Ti-55531(blue) alloys measured during continuous heating and isothermal aging processes.
Fig.6 shows the SAED patterns taken along [113] β zone axis for Ti-7333 alloy aged for 4h and 8h at 350℃. The ω phase reflections were found locate at 1/3 and 2/3 (-1-21) β positions. Dark field images produced by corresponding ω phase reflections in Fig. 6(b) and (d) show that nano-sized ω particles (larger than 10nm) homogeneously distributed in β matrix. Comparing with Fig.6 (a), the ω particles are coarser than that of in Fig.6 (c). Similar results about the isothermal ω phase transformation in Ti-7333 alloy have been reported in our previous work [28]. In order to compare the isothermal ω phase transformation in Ti-7333 alloy and Ti-55531 alloy, the TEM observations were carried out along [113] β zone axis.
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Fig. 6 Dark field micrographs of ellipsoidal ω precipitates (a, c) and SAED patterns (c, d) along [113] β zone axis showing ω reflections of Ti-7333 alloy aged for 4h (a, b) and 8h (c, d) at 350℃. The white dash circles in (b) and (d) denote the selected spots for dark field micrographs.
Fig. 7 shows the SAED patterns taken along [113] β zone axis for Ti-55531 alloy. From Fig. 7(b), it can be observed that additional diffuse reflections locate at the 1/2 (-1-21) β positions except for the ω reflections at 1/3 and 2/3 (-1-21) β positions. It has been reported that these additional reflections located at 1/2 (-1-21) β positions belong to α precipitation [29, 30]. The intensity of ω reflection is higher than that of α reflections, indicating that only few α phase produced in this condition. The bright field image exhibits that the ω particles distribute uniformly with high density. After aging for 8h, it is apparent from Fig. 7(d) that the intensity of ω reflections decreases and overlaps with the α reflections, indicating a transformation from α to ω. Although there are some different opinions about the role of ω particles on the nucleation of α phase in the metastable titanium alloys [14, 31, 32], the ω-assisted α nucleation behavior has been commonly accepted.
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Fig. 7 Bright field images of ellipsoidal ω precipitates (a, c) and SAED patterns (b, d) along [113] β zone axis showing ω reflections of Ti-55531 alloy aged for 4h (a, b) and 8h (c, d) at 350℃.
The alloy compositions not only affect the starting temperature of ω phase transformation but also influence the upper temperature limit ω phase. J.C. Williams et al. [10] found that the upper temperature limit of ω phase stability decreases with increasing Al content. In present work, the upper temperature limit of ω phase stability in Ti-7333 alloy is higher than that of in Ti-5553 and Ti-55531 alloys (see Fig. 2 and Fig. 5). Similarly, the alloy compositions also affect the duration time of stable ω phase. It can be seen from Fig. 6 and Fig. 7, the ω to α phase transformation occurs during aging for 4h at 350℃ in Ti55531 alloy whereas only ω phase was found in the β matrix of Ti-7333 alloy until aging for 8h at 350℃. The ω phase transformation during aging process in metastable β titanium alloys can be described as follows: The compositional and structural instabilities arise in the bcc lattice during water quenching from single β phase field. After subsequent ageing, the β phase in metastable β titanium alloys exhibits a tendency for phase separation, giving rise to solute-rich and solute-lean regions [33]. The ω phase tends to form in the solute-lean regions [34, 35]. Besides, it has been revealed that defects such as vacancy, local strain, compositional fluctuations play an important role in β to ω phase transformation. In our previous work [36], it was proposed that the nucleation of ω may associate with composition-dependent oscillating defect field induced by substitutional solutes, interstitial alloys and vacancies. E. Sukedai, et 11
Journal Pre-proof al. [37, 38] reported that the vacancies and their clusters induced by quenching and deforming can become nucleation sites for β to ω phase transformation, resulting in the enhancement of ω phase transformation when quenching from higher temperature or ageing at low temperature after deforming. The homogeneous distribution of ω phase in β matrix may be related to the lower energy barrier between ω and β phases, which resulting in high-frequency and homogeneous nucleation. With the nucleation and growth up of the ω particles the β-stablilizers like Mo, Nb, Cr, V are ejected from ω-affected regions. Then, the nucleation sites and transformation driving force decrease, and the growth of ω particles are suppressed due to the enriched β-stablilizers around ω particles [39]. It needs more time and higher temperature for the redistribution of solute elements to act as potential nucleation sites. Hence, the precipitation of ω phase during aging is not a continuous process.
4. Conclusions The ω phase transformation during continuous heating and isothermal aging in Ti-7333, Ti-5553 and Ti-55531 alloys was studied by using dilatometry and TEM techniques. The main results can be summarized as follows: (1) The ω phase transformation window of Ti-5553 and Ti-55531alloys is 275 ℃~350 ℃, while Ti7333 alloy has a broader transformation window of 200 ℃~375 ℃. (2) Dilatometry curves indicate that two kinds of isothermal β to ω phase transformation take place during the continuous heating and subsequent ageing processes, respectively. The ω particles produced during ageing process is larger than that produced during continuous heating process. (3) Based on the Dong’s Mo equivalent formula, the metastable β titanium alloys with lower Mo equivalent have lower starting temperature of ω phase transformation and wider transformation window.
Acknowledgements This work was financially supported by the Major State Research Development Program of China (No.2016YFB0701303), the National Natural Science Foundation of China (No.51301135) and the ‘‘111” Project (No. B08040).
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Journal Pre-proof Highlights 1. The ω phase precipitation starting temperature of Ti-7333 is lower than that of Ti-5553 and Ti-55531. 2. The ω phase transformation window of Ti-7333 is wider than that of Ti-5553 and Ti-55531. 3. Two kinds of isothermal ω transformation take place during the continuous heating and ageing processes.
4. Metastable β titanium alloys with lower Mo equivalent are beneficial for the ω phase transformation.