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Ultrafine duplex microstructure and excellent mechanical properties of TC4 alloy via a novel thermo-mechanical treatment
Journal of Alloys and Compounds 767 (2018) 617e621
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ultrafine duplex microstructure and excellent mechanical properties of TC4 alloy via a novel thermo-mechanical treatment X.J. Jiang a, b, G.Y. Chen a, X.L. Men a, X.L. Dong a, R.H. Han a, X.Y. Zhang b, R.P. Liu b, * a Hebei Provincial Key Laboratory of Traffic Engineering Materials, School of Materials Science and Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2018 Received in revised form 11 June 2018 Accepted 11 July 2018
Combined with the decomposition of martensitic phase and the globularization of close-packed hexagonal a phase, TC4 alloy with ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermo-mechanical technique of plastic deformation and aging treatment. The dual phase of a and martensitic is obtained by quenching from aþb phase region. Then, the ultrafine equiaxial microstructure is come from the globularization of deformed a phase, and the ultrafine acicular microstructure forms by the decomposition of deformed martensitic phase during aging treatment. Compared with lath microstructure, equiaxial microstructure and coarse duplex microstructure, the ultrafine duplex microstructure of TC4 alloy possesses ultrahigh strength of 1350 MPa and good elongation of 11%. © 2018 Elsevier B.V. All rights reserved.
Keywords: Titanium alloy Ti-6Al-4V Mechanical properties Microstructure
1. Introduction TC4 is an aþb titanium alloy, which exhibits an excellent combination of mechanical and physical properties such as good formability, low density, high specific strength, excellent corrosion resistance and high temperature strength retention for key applications in aerospace and medical apparatus, it also contributes to 80e90% of titanium usage on airframes including the fuselage, nacelles, landing gear, wings and empennage [1e5]. The initial lath microstructure associated with as-cast ingots may promote resistance to fatigue crack growth and high-temperature creep, but suffers from significant decreases in fatigue crack initiation resistance and plasticity compared to the more desirable equiaxial primary a (ap) microstructures [6]. Normally, to obtain equiaxial microstructure in Ti-6Al-4V alloy, a series of hot working and heat treatment steps are involved, and these thermo-mechanical treatments will inevitably lead to microstructure coarsening. Although the plasticity can be improved, the strength will decline [6e9]. According to the previous researches, severe plastic deformation (SPD) methods such as equal angular channel pressing (ECAP) and high pressure torsion (HPT) are efficient techniques to obtain
* Corresponding author. E-mail address: [email protected] (R.P. Liu). https://doi.org/10.1016/j.jallcom.2018.07.141 0925-8388/© 2018 Elsevier B.V. All rights reserved.
ultrafine nanoscale microstructure and excellent comprehensive mechanical properties [10e14]. However, SPD methods have many disadvantages, such as crack, heterogeneous microstructure and limited specimen size. In addition, the coarse grain TC4 alloy has limited room deformation ability. So the SPD methods are not suitable for industrial production. On the other hand, the formation of a UFG structure in TC4 alloy is possible at elevated temperatures. The transition from laboratory SPD methods to industrial technologies is connected with the development of new more efficient SPD modifications, a combination of SPD methods with conventional deformation and thermo-mechanical treatment (TMT) [15e17]. And the proposed approach to TMT of titanium alloys is well studied to obtain ultrafine equiaxial microstructure in TC4 alloy [18e20]. Duplex microstructure, which is widely used in titanium alloy, possesses both the good plasticity of equiaxial microstructure and high strength of lath microstructure [21e23]. Normally, to obtain duplex microstructure is similar to equiaxial microstructure, a series of thermo-mechanical treatments are needed. According to Weiss et al. [24], ap lath break-up during the deformation process of Ti alloy can be explained by two mechanisms. One is that during hot deformation, low and high angle boundaries are both formed across ap lath. Another possible mechanism of a lath break-up is that the localized shear and rotation of a lath can occur during hot deformation. To summarize, the equiaxial ap is formed as a result of
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deformation and recovery/recrystallization of initially course lathe. On the other hand, the secondary as lath is inherited from the transformation of the b/martensite matrix during annealing/cooling, which has been reported to follow the Burgers relationship, {0002}ajj{110}b and 〈11e20〉ajj〈111〉b with 12 possible a-orientations [25]. In these processes, the size of secondary as lath can be controlled by annealing temperature and cooling rate, however, the size of equiaxial ap is easy to grow up and coarsen. In this paper, TC4 alloy with ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermo-mechanical technique of plastic deformation and aging treatment. The dual phase of a and martensitic is obtained by quenching from aþb phase region. Then, the ultrafine equiaxial microstructure is come from the globularization of deformed a phase, and the ultrafine acicular microstructure forms by the decomposition of deformed martensitic phase during aging treatment. In addition, the excellent mechanical properties are obtained. Fig. 1. DSC curve of TC4 alloy.
2. Experiment For the present experiment, the TC4 alloy ingots (the chemical composition is shown in Table 1) used in the present study was provided by BAOTi Group, China. Specimens with sizes approaching 10 mm 10 mm 50 mm were machined. Differential scanning calorimetry (DSC) was used to determine the transition temperature at a cooling rate of 10 C/min. According to the result of DSC, as shown in Fig. 1, the button-shaped specimens with an average of 12 mm thickness were cut and homogenised at 1200 C for 2 h, and then followed by solution treatment at 950 C for 2 h and subsequent quenching in water. Then, the specimens were cold rolled to obtain the strain of 40%, and subsequent aging and annealing treatments were carried out at 600 C, 800 C, 950 C, 1000 C for 2 h, separately, as shown in the flow chart of Fig. 2. The following test samples were all taken from the rolled ingots along the rolling direction (RD). The phases of the specimens were confirmed by conventional X-ray diffraction (XRD) with Cu Ka radiation (D/max2500/PC), and the quantitative ratio of a and b phases in each state were determined by using Jade5 software. The microstructure of the heat treated plated samples was examined using optical microscopy (OM) and transmission electron microscopy (TEM), the TEM specimens (3 specimens were collected from the edge to the center in each state) were prepared via twin-jet electrochemical polishing in a solution containing 10% perchloric acid and 90% methanol at 14 V and 35 C. The selected OM and TEM photos in this paper are all representatives of uniform microstructures. Boneshaped plate specimens with an original gauge length of 21 mm and a cross-sectional dimension of 2 mm 3 mm were prepared for the tensile tests. Uniaxial tensile tests were performed on an Instron 5982 testing machine at a strain rate of 5 104 s1, and 5 samples of each state were tested in tension (the measurement error is within 2%).
Fig. 2. Process flow chart.
3. Results and discussion The XRD patterns of TC4 alloy after thermo-mechanical treatment are shown in Fig. 3. It is clear that the phases are similar, the XRD patterns are mainly a/a0 and b phases peaks, no intermetallic phase and/or other martensitic phase peaks is observed. However, Fig. 3. The XRD patterns of TC4 alloy after different thermo-mechanical processes. Table 1 Chemical composition of TC4 alloy.
As-received
Ti (wt%)
Al (wt%)
V (wt%)
O (wt%)
N (wt%)
Bal.
5.8
4.2
0.13
0.007
the proportion of phases is different. The b phase is nearly 47% in solution treated specimen, and others are a0 martensite phase and a phase, because there is no difference in the crystal structure of a0 martensite phase and a phase, which are all close-packed
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
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Fig. 4. Optical micrographs of TC4 alloy: (a) solid solution treatment, (b) deforming, (c) 800 C aging, (d) 950 C annealing, (e) 1000 C annealing.
hexagonal structure, so a0 martensite phase and a phase are uniformly marked as a phase in XRD patterns. After 600 C aging, a0 martensite phase decompose into a phase and b phase, then b phase increases to 50%. When aging temperature increases to 800 C, a part of b phase transforms to a phase, so b phase decreases to 43%. As heat treatment (annealing) temperature is further improved to the phase transition temperature, more b phase
transforms to a phase in the cooling process, and 40% and 34% b phase are preserved during 950 C and 1000 C, respectively. The microstructure of the heat treated plated samples are shown in Figs. 4 and 5. The lath primary a phase is observed in solution treated specimen as shown in Fig. 4a, this kind of a phase is directly transformed from b phase during heat preservation in aþb two phases region. Obasi [25] indicated that the b/a phase
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Fig. 5. TEM microstructure of TC4 alloy: (a) solid solution treatment, (b) deforming, (c) 600 C aging.
Fig. 6. Stress-strain curves of TC4 alloy after different thermo-mechanical processes.
transformation in Ti alloys is governed by the so-called Burgers orientation relationship {0002}a jj {110}b and 〈11e20〉a jj 〈111〉b with 12 possible a-orientations that can transform from a single parent b grain. So the basketweave morphology would appear. In €tten a (aWGB) was also appeared near the grain addition, Widmansta boundary. This phenomenon has been reported by Sun [26] in TA15 alloy, he pointed out that the aWGB nucleated through surface instability and the protuberance of grain boundary a and the growth of aWGB started from a small protuberance and spread into a b grain with a sectorial morphology, to become lath instead of spiculate or oblate cuboid in shape. After deforming, the lath a phase is distorted (Fig. 4b). According to Weiss et al. [24], there are two mechanisms for a lath break-up during the deformation process of titanium alloy. One is that both low and high angle boundaries across a lath are formed, another is that the localized shear and rotation of a lath occurs. Then, equiaxial microstructure, coarse duplex microstructure and lath microstructure form after subsequent heat treatment at 800 C, 950 C, 1000 C for 2 h, separately, as shown in Fig. 4c to Fig. e. The equiaxial microstructure is come from static globularization and growth of distorted
Table 2 Mechanical properties of TC4 alloy after different thermo-mechanical processes. Heat treatment temperature
Microstructure
Yield strength/MPa
Tensile strength/MPa
Elongation/%
600 800 950 1000
ultrafine duplex equiaxial coarse duplex lath
1256 900 1009 1103
1350 958 1070 1180
11 17 12 8
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
primary a phase at 800 C aging. When heat treatment temperature is up to 950 C, static globularization also occurs, and the phenomenon of growing up is more obvious. At the same time, a part of primary a phase and martensite phase transform to b phase during heat preservation, then transforming to secondary lath a phase during cooling. As temperature is further raised to 1000 C, all of the primary a phase and martensite phase transform to b phase and then transforming to lath a phase. On the other hand, ultrafine duplex microstructure is obtained after a novel thermo-mechanical technique of plastic deformation and 600 C aging treatment (Fig. 5). Fig. 5a shows lath primary a phase and fine acicular martensite phase in solution treated specimen. And during deforming, lath primary a phase is broken up by low and high angle boundaries across a lath (Fig. 5b), which is the reason of distorted a phase in Fig. 4b. Then after 600 C aging, the static globularization of lath primary a phase results in the formation of equiaxial microstructure, and the decomposing of martensite phase results in the formation of fine acicular microstructure (Fig. 5c). Fig. 6 and Table 2 demonstrate the mechanical properties of TC4 alloy after different thermo-mechanical processes. The mechanical properties of TC4 alloy are evidently dependent on solidification conditions. Among the three common types of microstructure, equiaxial microstructure obtained by 800 C aging possesses best elongation of 17%, but the lowest strength of 958 MPa. On the contrary, lath microstructure possesses lowest elongation of 8%, but the best strength of 1180 MPa. In addition, coarse duplex microstructure possesses good comprehensive mechanical properties of strength (1070 MPa) and elongation (12%). On the other hand, the strength and the elongation of TC4 alloy with ultrafine duplex microstructure are all improved greatly after the processes of solid solution, deformation and aging at 600 C. Due to the ultrafine acicular and equiaxial microstructure, ultrahigh strength of 1350 MPa and excellent elongation of 11% are obtained. 4. Conclusions In conclusion, ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermomechanical technique of plastic deformation and 600 C aging treatment for solid solution treated TC4 alloy. The dual phase of a and a0 martensitic is obtained by quenching from aþb phase region. Then, after deforming and 600 C aging, the static globularization of lath primary a phase results in the formation of equiaxial microstructure and the decomposing of martensite phase results in the formation of fine acicular microstructure. Compared with lath microstructure, equiaxial microstructure and coarse duplex microstructure, the ultrafine duplex microstructure of TC4 alloy possesses ultrahigh strength of 1350 MPa and good elongation of 11%. Acknowledgements This work was supported by the SKPBRC (Grant no. 2013CB733000), the NSFC (Grant no. 51434008/51531005/ 51571174/51705345), the Natural Science Foundation for Distinguished Young Scholars of Hebei Province of China (Grant no. E2016203376), the Natural Science Foundation of Hebei Province of China (Grant No. E2017210050/E2017210054), the Natural Science Foundation of Hebei Provincial Department of Education (Grant No. QN2017133), and the Open Foundation of State Key Laboratory of Metastable Materials Science and Technology (Grant No. 201607).
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References [1] E.W. Collings, The Physical Metallurgy of Titanium Alloys, American Society of Metals, 1984. [2] R. Boyer, et al., Materials Property Handbook: Titanium Alloys, ASM International, 1994. [3] S.L. Semiatin, V. Seetharaman, I. Weiss, Advances in Science and Technology of Ti Alloy Processing, Metals and Materials Society, The Minerals, 1997. [4] J. Barnes, Superplastic forming 40 years and still growing, J. Mater. Eng. Perform. 16 (2007) 440e447. [5] D. Serra, Superplastic forming applications on aero engines. A review of ITP manufacturing processes, in: 6th EUROSPF Conference EuroSPF08, 2008. [6] J.L.W. Warwick, N.G. Jones, I. Bantounas, M. Preuss, D. Dye, In situ observation of texture and microstructure evolution during rolling and globularization of Ti-6Al-4V, Acta Mater. 61 (2013) 1603e1615. [7] S.L. Semiatin, V. Seetharaman, I. Weiss, Flow behavior and globularization kinetics during hot working of Ti-6Al-4V with a colony alpha microstructure, Mater. Sci. Eng. A 263 (1999) 257e271. [8] S. Roy, S. Suwas, Orientation dependent spheroidization response and macrozone formation during sub b-transus processing of Ti-6Al-4V alloy, Acta Mater. 134 (2017) 283e301. [9] X.N. Peng, H.Z. Guo, Z.F. Shi, C. Qin, Z.L. Zhao, Z.K. Yao, Study on the hot deformation behavior of TC4-DT alloy with equiaxed aþb starting structure based on processing map, Mater. Sci. Eng. A 605 (2014) 80e88. [10] Y.G. Liu, M.Q. Li, Deformation mechanisms of nanocrystalline alpha titanium in Ti-6Al-4V, Mater. Lett. 185 (2016) 488e490. [11] Y.C. Wang, T.G. Langdon, Influence of phase volume fractions on the processing of a Tie6Ale4Valloy by high-pressure torsion, Mater. Sci. Eng. A 559 (2013) 861e867. [12] M.H. Cai, C.Y. Lee, Y.K. Lee, Effect of grain size on tensile properties of finegrained metastable b titanium alloys fabricated by stress-induced martensite and its reverse transformations, Scr. Mater. 66 (2012) 606e609. [13] A. Zafari, X.S. Wei, W. Xua, K. Xia, Formation of nanocrystalline b structure in metastable beta Ti alloy during high pressure torsion: the role played by stress induced martensitic transformation, Acta Mater. 97 (2015) 146e155. [14] Y. Estrin, A. Vinogradov, Extreme grain refinement by severe plastic deformation: a wealth of challenging science, Acta Mater. 61 (2013) 782e817. [15] C. Xu, S. Schroeder, P.B. Berbon, T.G. Langdon, Principles of ECAP-Conform as a continuous process for achieving grain refinement: application to an aluminum alloy, Acta Mater. 58 (2010) 1379e1386. [16] I.P. Semenova, G.I. Raab, V.V. Polyakova, N.F. Izmailova, S.P. Pavlinich, R.Z. Valiev, Ultrafine-grained Ti-6Al-4V-alloy used for production of complexshaped articles with enhanced service properties, Rev. Adv. Mater. Sci. 31 (2) (2012) 179e184. [17] T.C. Lowe, Y.T. Zhu, S.L. Semiatin, D.R. Berg, Overview and outlook for Materials processed by severe plastic deformation, in: T.C. Lowe, R.Z. Valiev (Eds.), Investigations and Applications of Severe Plastic Deformation, Springer, Dordrecht, 2000, pp. 347e356. [18] S. Zherebtsov, M. Murzinova, G. Salishchev, S.L. Semiatin, Spheroidization of the lamellar microstructure in Ti-6Al-4V alloy during warm deformation and annealing, Acta Mater. 59 (2011) 4138e4150. [19] Y.G. Liu, M.Q. Li, Nanocrystallization mechanism of beta phase in Ti-6Al-4V subjected to severe plastic deformation, Mater. Sci. Eng. A 669 (2016) 7e13. [20] S.V. Zherebtsov, G.A. Salishchev, R.M. Galeyev, O.R. Valiakhmetov, S.Yu. Mironov, S.L. Semiatin, Production of submicrocrystalline structure in large-scale Tie6Ale4V billet by warm severe deformation processing, Scr. Mater. 51 (2004) 1147e1151. [21] J.Z. Dong, F.G. Li, C.P. Wang, Micromechanical behavior study of a phase with different morphologies of Tie6Ale4V alloy by microindentation, Mater. Sci. Eng. A 580 (2013) 105e113. [22] P. Guo, Y.Q. Zhao, W.D. Zeng, Q. Hong, The effect of microstructure on the mechanical properties of TC4-DT titanium alloys, Mater. Sci. Eng. A 563 (2013) 106e111. [23] G.Q. Wu, C.L. Shi, W. Sha, A.X. Sha, H.R. Jiang, Effect of microstructure on the fatigue properties of Tie6Ale4V titanium alloys, Mater. Des. 46 (2013) 668e674. [24] I. Weiss, F.H. Froes, D. Eylon, G.E. Welsch, Modification of alpha morphology in Ti-6Al-4V by thermomechanical processing, Metall. Trans. A 17 (1986) 1935e1947. [25] G.C. Obasi, S. Birosca, D.G. Leo Prakash, J. Quinta da Fonseca, M. Preuss, The influence of rolling temperature on texture evolution and variant selection during a/b/a phase transformation in Ti-6Al-4V, Acta Mater. 60 (2012) 6013e6024. [26] Z.C. Sun, S.S. Guo, H. Yang, Nucleation and growth mechanism of a-lamellae of Ti alloy TA15 cooling from an aþb phase field, Acta Mater. 61 (2013) 2057e2064.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Ultrafine duplex microstructure and excellent mechanical properties of TC4 alloy via a novel thermo-mechanical treatment X.J. Jiang a, b, G.Y. Chen a, X.L. Men a, X.L. Dong a, R.H. Han a, X.Y. Zhang b, R.P. Liu b, * a Hebei Provincial Key Laboratory of Traffic Engineering Materials, School of Materials Science and Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 April 2018 Received in revised form 11 June 2018 Accepted 11 July 2018
Combined with the decomposition of martensitic phase and the globularization of close-packed hexagonal a phase, TC4 alloy with ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermo-mechanical technique of plastic deformation and aging treatment. The dual phase of a and martensitic is obtained by quenching from aþb phase region. Then, the ultrafine equiaxial microstructure is come from the globularization of deformed a phase, and the ultrafine acicular microstructure forms by the decomposition of deformed martensitic phase during aging treatment. Compared with lath microstructure, equiaxial microstructure and coarse duplex microstructure, the ultrafine duplex microstructure of TC4 alloy possesses ultrahigh strength of 1350 MPa and good elongation of 11%. © 2018 Elsevier B.V. All rights reserved.
Keywords: Titanium alloy Ti-6Al-4V Mechanical properties Microstructure
1. Introduction TC4 is an aþb titanium alloy, which exhibits an excellent combination of mechanical and physical properties such as good formability, low density, high specific strength, excellent corrosion resistance and high temperature strength retention for key applications in aerospace and medical apparatus, it also contributes to 80e90% of titanium usage on airframes including the fuselage, nacelles, landing gear, wings and empennage [1e5]. The initial lath microstructure associated with as-cast ingots may promote resistance to fatigue crack growth and high-temperature creep, but suffers from significant decreases in fatigue crack initiation resistance and plasticity compared to the more desirable equiaxial primary a (ap) microstructures [6]. Normally, to obtain equiaxial microstructure in Ti-6Al-4V alloy, a series of hot working and heat treatment steps are involved, and these thermo-mechanical treatments will inevitably lead to microstructure coarsening. Although the plasticity can be improved, the strength will decline [6e9]. According to the previous researches, severe plastic deformation (SPD) methods such as equal angular channel pressing (ECAP) and high pressure torsion (HPT) are efficient techniques to obtain
* Corresponding author. E-mail address: [email protected] (R.P. Liu). https://doi.org/10.1016/j.jallcom.2018.07.141 0925-8388/© 2018 Elsevier B.V. All rights reserved.
ultrafine nanoscale microstructure and excellent comprehensive mechanical properties [10e14]. However, SPD methods have many disadvantages, such as crack, heterogeneous microstructure and limited specimen size. In addition, the coarse grain TC4 alloy has limited room deformation ability. So the SPD methods are not suitable for industrial production. On the other hand, the formation of a UFG structure in TC4 alloy is possible at elevated temperatures. The transition from laboratory SPD methods to industrial technologies is connected with the development of new more efficient SPD modifications, a combination of SPD methods with conventional deformation and thermo-mechanical treatment (TMT) [15e17]. And the proposed approach to TMT of titanium alloys is well studied to obtain ultrafine equiaxial microstructure in TC4 alloy [18e20]. Duplex microstructure, which is widely used in titanium alloy, possesses both the good plasticity of equiaxial microstructure and high strength of lath microstructure [21e23]. Normally, to obtain duplex microstructure is similar to equiaxial microstructure, a series of thermo-mechanical treatments are needed. According to Weiss et al. [24], ap lath break-up during the deformation process of Ti alloy can be explained by two mechanisms. One is that during hot deformation, low and high angle boundaries are both formed across ap lath. Another possible mechanism of a lath break-up is that the localized shear and rotation of a lath can occur during hot deformation. To summarize, the equiaxial ap is formed as a result of
618
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
deformation and recovery/recrystallization of initially course lathe. On the other hand, the secondary as lath is inherited from the transformation of the b/martensite matrix during annealing/cooling, which has been reported to follow the Burgers relationship, {0002}ajj{110}b and 〈11e20〉ajj〈111〉b with 12 possible a-orientations [25]. In these processes, the size of secondary as lath can be controlled by annealing temperature and cooling rate, however, the size of equiaxial ap is easy to grow up and coarsen. In this paper, TC4 alloy with ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermo-mechanical technique of plastic deformation and aging treatment. The dual phase of a and martensitic is obtained by quenching from aþb phase region. Then, the ultrafine equiaxial microstructure is come from the globularization of deformed a phase, and the ultrafine acicular microstructure forms by the decomposition of deformed martensitic phase during aging treatment. In addition, the excellent mechanical properties are obtained. Fig. 1. DSC curve of TC4 alloy.
2. Experiment For the present experiment, the TC4 alloy ingots (the chemical composition is shown in Table 1) used in the present study was provided by BAOTi Group, China. Specimens with sizes approaching 10 mm 10 mm 50 mm were machined. Differential scanning calorimetry (DSC) was used to determine the transition temperature at a cooling rate of 10 C/min. According to the result of DSC, as shown in Fig. 1, the button-shaped specimens with an average of 12 mm thickness were cut and homogenised at 1200 C for 2 h, and then followed by solution treatment at 950 C for 2 h and subsequent quenching in water. Then, the specimens were cold rolled to obtain the strain of 40%, and subsequent aging and annealing treatments were carried out at 600 C, 800 C, 950 C, 1000 C for 2 h, separately, as shown in the flow chart of Fig. 2. The following test samples were all taken from the rolled ingots along the rolling direction (RD). The phases of the specimens were confirmed by conventional X-ray diffraction (XRD) with Cu Ka radiation (D/max2500/PC), and the quantitative ratio of a and b phases in each state were determined by using Jade5 software. The microstructure of the heat treated plated samples was examined using optical microscopy (OM) and transmission electron microscopy (TEM), the TEM specimens (3 specimens were collected from the edge to the center in each state) were prepared via twin-jet electrochemical polishing in a solution containing 10% perchloric acid and 90% methanol at 14 V and 35 C. The selected OM and TEM photos in this paper are all representatives of uniform microstructures. Boneshaped plate specimens with an original gauge length of 21 mm and a cross-sectional dimension of 2 mm 3 mm were prepared for the tensile tests. Uniaxial tensile tests were performed on an Instron 5982 testing machine at a strain rate of 5 104 s1, and 5 samples of each state were tested in tension (the measurement error is within 2%).
Fig. 2. Process flow chart.
3. Results and discussion The XRD patterns of TC4 alloy after thermo-mechanical treatment are shown in Fig. 3. It is clear that the phases are similar, the XRD patterns are mainly a/a0 and b phases peaks, no intermetallic phase and/or other martensitic phase peaks is observed. However, Fig. 3. The XRD patterns of TC4 alloy after different thermo-mechanical processes. Table 1 Chemical composition of TC4 alloy.
As-received
Ti (wt%)
Al (wt%)
V (wt%)
O (wt%)
N (wt%)
Bal.
5.8
4.2
0.13
0.007
the proportion of phases is different. The b phase is nearly 47% in solution treated specimen, and others are a0 martensite phase and a phase, because there is no difference in the crystal structure of a0 martensite phase and a phase, which are all close-packed
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
619
Fig. 4. Optical micrographs of TC4 alloy: (a) solid solution treatment, (b) deforming, (c) 800 C aging, (d) 950 C annealing, (e) 1000 C annealing.
hexagonal structure, so a0 martensite phase and a phase are uniformly marked as a phase in XRD patterns. After 600 C aging, a0 martensite phase decompose into a phase and b phase, then b phase increases to 50%. When aging temperature increases to 800 C, a part of b phase transforms to a phase, so b phase decreases to 43%. As heat treatment (annealing) temperature is further improved to the phase transition temperature, more b phase
transforms to a phase in the cooling process, and 40% and 34% b phase are preserved during 950 C and 1000 C, respectively. The microstructure of the heat treated plated samples are shown in Figs. 4 and 5. The lath primary a phase is observed in solution treated specimen as shown in Fig. 4a, this kind of a phase is directly transformed from b phase during heat preservation in aþb two phases region. Obasi [25] indicated that the b/a phase
620
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
Fig. 5. TEM microstructure of TC4 alloy: (a) solid solution treatment, (b) deforming, (c) 600 C aging.
Fig. 6. Stress-strain curves of TC4 alloy after different thermo-mechanical processes.
transformation in Ti alloys is governed by the so-called Burgers orientation relationship {0002}a jj {110}b and 〈11e20〉a jj 〈111〉b with 12 possible a-orientations that can transform from a single parent b grain. So the basketweave morphology would appear. In €tten a (aWGB) was also appeared near the grain addition, Widmansta boundary. This phenomenon has been reported by Sun [26] in TA15 alloy, he pointed out that the aWGB nucleated through surface instability and the protuberance of grain boundary a and the growth of aWGB started from a small protuberance and spread into a b grain with a sectorial morphology, to become lath instead of spiculate or oblate cuboid in shape. After deforming, the lath a phase is distorted (Fig. 4b). According to Weiss et al. [24], there are two mechanisms for a lath break-up during the deformation process of titanium alloy. One is that both low and high angle boundaries across a lath are formed, another is that the localized shear and rotation of a lath occurs. Then, equiaxial microstructure, coarse duplex microstructure and lath microstructure form after subsequent heat treatment at 800 C, 950 C, 1000 C for 2 h, separately, as shown in Fig. 4c to Fig. e. The equiaxial microstructure is come from static globularization and growth of distorted
Table 2 Mechanical properties of TC4 alloy after different thermo-mechanical processes. Heat treatment temperature
Microstructure
Yield strength/MPa
Tensile strength/MPa
Elongation/%
600 800 950 1000
ultrafine duplex equiaxial coarse duplex lath
1256 900 1009 1103
1350 958 1070 1180
11 17 12 8
X.J. Jiang et al. / Journal of Alloys and Compounds 767 (2018) 617e621
primary a phase at 800 C aging. When heat treatment temperature is up to 950 C, static globularization also occurs, and the phenomenon of growing up is more obvious. At the same time, a part of primary a phase and martensite phase transform to b phase during heat preservation, then transforming to secondary lath a phase during cooling. As temperature is further raised to 1000 C, all of the primary a phase and martensite phase transform to b phase and then transforming to lath a phase. On the other hand, ultrafine duplex microstructure is obtained after a novel thermo-mechanical technique of plastic deformation and 600 C aging treatment (Fig. 5). Fig. 5a shows lath primary a phase and fine acicular martensite phase in solution treated specimen. And during deforming, lath primary a phase is broken up by low and high angle boundaries across a lath (Fig. 5b), which is the reason of distorted a phase in Fig. 4b. Then after 600 C aging, the static globularization of lath primary a phase results in the formation of equiaxial microstructure, and the decomposing of martensite phase results in the formation of fine acicular microstructure (Fig. 5c). Fig. 6 and Table 2 demonstrate the mechanical properties of TC4 alloy after different thermo-mechanical processes. The mechanical properties of TC4 alloy are evidently dependent on solidification conditions. Among the three common types of microstructure, equiaxial microstructure obtained by 800 C aging possesses best elongation of 17%, but the lowest strength of 958 MPa. On the contrary, lath microstructure possesses lowest elongation of 8%, but the best strength of 1180 MPa. In addition, coarse duplex microstructure possesses good comprehensive mechanical properties of strength (1070 MPa) and elongation (12%). On the other hand, the strength and the elongation of TC4 alloy with ultrafine duplex microstructure are all improved greatly after the processes of solid solution, deformation and aging at 600 C. Due to the ultrafine acicular and equiaxial microstructure, ultrahigh strength of 1350 MPa and excellent elongation of 11% are obtained. 4. Conclusions In conclusion, ultrafine duplex microstructure and excellent mechanical properties is prepared by using a novel thermomechanical technique of plastic deformation and 600 C aging treatment for solid solution treated TC4 alloy. The dual phase of a and a0 martensitic is obtained by quenching from aþb phase region. Then, after deforming and 600 C aging, the static globularization of lath primary a phase results in the formation of equiaxial microstructure and the decomposing of martensite phase results in the formation of fine acicular microstructure. Compared with lath microstructure, equiaxial microstructure and coarse duplex microstructure, the ultrafine duplex microstructure of TC4 alloy possesses ultrahigh strength of 1350 MPa and good elongation of 11%. Acknowledgements This work was supported by the SKPBRC (Grant no. 2013CB733000), the NSFC (Grant no. 51434008/51531005/ 51571174/51705345), the Natural Science Foundation for Distinguished Young Scholars of Hebei Province of China (Grant no. E2016203376), the Natural Science Foundation of Hebei Province of China (Grant No. E2017210050/E2017210054), the Natural Science Foundation of Hebei Provincial Department of Education (Grant No. QN2017133), and the Open Foundation of State Key Laboratory of Metastable Materials Science and Technology (Grant No. 201607).
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