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Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process
Accepted Manuscript Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process X.J. Jiang, H.T. Zhao, G. Yu, H.Y. Wu, C.L. Tan, X.Y. Zhang, M.Z. Ma, R.P. Liu PII:
S0925-8388(17)32580-X
DOI:
10.1016/j.jallcom.2017.07.204
Reference:
JALCOM 42620
To appear in:
Journal of Alloys and Compounds
Received Date: 12 May 2017 Revised Date:
19 July 2017
Accepted Date: 20 July 2017
Please cite this article as: X.J. Jiang, H.T. Zhao, G. Yu, H.Y. Wu, C.L. Tan, X.Y. Zhang, M.Z. Ma, R.P. Liu, Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.204. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process X.J. Jianga, b, H.T. Zhaob, G.Yub, H.Y. Wub, C.L. Tanc, X.Y. Zhanga, M.Z. Maa, R.P. Liua,* a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, China College of materials science and Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043,
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b
China c
Beijing Institute of Spacecraft System Engineering , Beijing 100094, China
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Abstract: The structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process are investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM)
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are used to analyze the phase transformation and microstructure evolution, meanwhile, mechanical properties are obtained by tensile test. According to the results, the kinetics parameters of β→α phase transformation can play a decisive role for the microstructure and mechanical properties of titanium (Ti) alloys, and the quantitative relationship between the kinetics parameters and the strength of Ti-41Zr-7.3Al alloy has been established. Based on diffusion controlled growth mechanism of lamellar
temperature
during
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α phase and Hall–Petch relationship, the strength of Ti-41Zr-7.3Al alloy relies on the onset transition cooling
T0,
and
cooling
rate
v
has
been
deduced
as
= 1102 + 1024[ − 767⁄ ]/ .
Key words: Titanium alloy; Microstructure evolution; mechanical properties
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*Corresponding author. address: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
e-mail: [email protected] (R.P. Liu) Tel: 0086-335-8074723
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Fax: 0086-335-8074545
ACCEPTED MANUSCRIPT 1.
Introduction Titanium (Ti) and its alloys exhibit an excellent combination of mechanical and physical
properties for key applications in aerospace, energy and chemical industry [1-3]. The mechanical properties of titanium alloys are important criteria for the material service capabilities both in
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aerospace and industrial applications. The achievement of remarkable mechanical properties of Ti alloys strongly depends on the process control of the β→α phase transformation [4-11]. The conditions of transformation, especially the temperature and the cooling rate, give rise to formation of lamellar α phase with different size [12-15]. According to the semi-empirical Hall–Petch relationship [16, 17], the
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thickness of lamellar α phase can significantly affect the strength of titanium alloys. So, in other words, the kinetics parameters of β→α phase transformation can play a decisive role for the strength of
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titanium alloys, and it is necessary to study the relationship between them.
Recently, the nuclear and growth mechanism of secondary α phase in Ti alloys has been studied by many researchers. Obasi [18] indicated that the phase transformation in Ti alloys during heating (α→β) and cooling (β→α) is governed by the so-called Burgers orientation relationship {0002}α || {110}β and <11-20>α || <111>β with six possible β-orientations during the α→β phase transformation
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and 12 possible α-orientations that can transform from a single parent β grain during β→α phase transformation. Thus, the basketweave morphology appears. In addition, Widmansta tten α (αWGB) appeared near the grain boundary. This phenomenon has been reported by Sun [19, 20], Bohemen [21],
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and Chen [7] in TA15 alloy, Ti-4.5Fe-6.8Mo-1.5Al alloy and Ti-5Al-5Mo-5V-3Cr-1Zr alloy, respectively. They highlighted that the αWGB nucleated through surface instability and that grain
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boundary α protuberance and αWGB growth started from a small protuberance and spread into a β grain with a sectorial morphology to become lamellar instead of spiculate or oblate cuboid in shape. Moreover, the thickness of lamellar α phase decreased with increasing cooling rate and decreasing heat treatment temperature. On the other hand, the studies by Guo [22] and Peng [23] showed that the thickness of lamellar α had a significant effect on the properties of the alloys tested, a low cooling rate leaded to a diffusion controlled growth of thick lamellar α phase and resulted in a lower tensile strength in TC4 alloy. The similar conclusion has also been reported by our research team in TiZrAlV and TiZrAl alloys [24-28]. In previous researches, the microstructure evolution during heat treatment and the effect of microstructures on mechanical properties have been studied systematically [22-25, 29-32]. However,
ACCEPTED MANUSCRIPT there is little quantitative formula can directly describe the relationship between the kinetics parameters of β→α phase transformation and the strength of titanium alloys. Xu [33] investigated the effects of processing parameters on globularization behavior and tensile properties of Ti-17 alloy, and clarified the quantitative relationship between the kinetics parameters of globularization and the strength during
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annealing. Liu [8, 31] also described the quantitative relationship between the kinetics parameters and mechanical behavior of Ti2448 alloy manufactured by electron beam melting and selective laser melting. In this paper, microstructure evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process is studied, and the quantitative relationship between the kinetics
2.
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parameters of β→α phase transformation and the strength of Ti-41Zr-7.3Al alloy is established. Experiment
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The starting material was mixed with sponge Ti (99.9 wt.%), sponge Zr (97 wt.%), and industrial-purity Al (99.5 wt.%) before being melted using cooled copper crucible electromagnetic induction melting. To ensure compositional homogeneity, the ingot was flipped and re-melted thrice. Differential scanning calorimetry (DSC) was used to determine the transition temperature at a cooling rate of 10 °C/min. According to the DSC result in Fig. 1, the ingot was homogenized at 1200 °C higher
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than the β transus temperature (760°C-900°C) of approximately 400 °C for 2 h and then quenching in water to room temperature. The composition of the ingot was Ti-41Zr-7.3Al in wt.%. The 12 mm thick plated ingots were heated to 800, 900, and 1000 °C for 1 h, then furnace cooled (FC 0.067 °C/s) and air
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cooled (AC 2 °C/s) to room temperature, respectively. The phases of the specimens were confirmed by conventional X-ray diffraction (XRD) with Cu Kα radiation (D/max-2500/PC). The microstructure of
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the heat treated plated samples was examined using optical microscopy (OM) and the transmission electron microscopy (TEM), which specimens were prepared via twin-jet electrochemical polishing in a solution containing 10% perchloric acid and 90% methanol at 14 V and −35 °C. The thickness of the
lamellar α phase used in this paper is indicated as an average value. Bone-shaped plate specimens with an original gauge length of 21 mm and a cross-sectional dimension of 3 mm×2 mm were prepared for the tensile tests. Uniaxial tensile tests were performed on an Instron 5982 testing machine at a strain rate of 5×10−4 s−1. 3.
Results and discussion The phase and microstructure of preprocessed ingots during homogenizing and quenching are
shown in Fig. 2. The XRD pattern in Fig. 2a shows that all diffraction peaks match with those of α''
ACCEPTED MANUSCRIPT martensitic phase, no α phase peaks and/or other phases exist. TEM image including SAED pattern in Fig. 2b shows the same result as XRD pattern, the acicular α'' martensitic phase can be observed clearly, and no residual lamellar α phase can be found. These results indicate that α phase transformed to β phase completely during homogenizing, and then because of the high content of Zr [34], acicular α''
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martensitic phase formed from β phase in the process of water quenching. When subsequent annealing temperature is higher than β transus temperature, α'' martensitic phase will transform to β phase again, and no α phase can be precipitated. The only way to obtain lamellar α phase is the furnace cooled and/or air cooled processes after subsequent annealing in this paper. So, the pre-existing microstructure
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after homogenizing and quenching has no effect on final structure after subsequent annealing and cooling.
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The XRD patterns of Ti-41Zr-7.3Al alloy after different heat treatment conditions are shown in Fig. 3. All diffraction peaks match with those of α phase, no β-phase peaks and/or other intermediate phases is observed in any of the XRD patterns. These results indicate that when annealing temperature is above the final temperature of β→α phase-transition, β→α phase transition will occur completely during the air-cooling and furnace-cooling processes after heat treatment at α+β region and β region, no
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β phase or martensitic phase will be preserved. Al is an α-stable alloying element no matter in Ti or Zr alloys, the addition of Al will improve the stability of α phase, and promote β→α phase transition during cooling process from high temperature. In Ti-41Zr-7.3Al alloys, β phase and martensitic phase
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can be preserved only by water quenching or rapid cooling process from α+β region or β region. So, only α phase exist during the air-cooling and furnace-cooling processes after heat treatment at α+β
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region and β region. And this phenomenon has been studied in our previous research [26, 28, 34]. Optical micrographs of Ti-41Zr-7.3Al alloy after furnace cooled to room temperature from 800,
900, and 1000 °C are illustrated in Fig. 4. All Microstructure mainly shows lamellar α phase. The basketweave morphology appears after annealing at 800 °C and 900 °C as shown in Fig. 4a and Fig. 4b, and the thicknesses of the lamellar α phase are approximately 3µm and 6µm for 800 °C and 900 °C, respectively. It is gradually increased with increased annealing temperature from 800 °C to 900 °C. However, when annealing temperature further increases to 1000 °C, as shown in Fig. 4c, the thicknesses of the lamellar α phase has no obvious change compared with 900 °C. This kind of phenomenon is mainly due to the onset transition temperature is almost 900 °C. When annealing temperature is above 900 °C, the additional cooling process has no effect on the growth of α phase, so
ACCEPTED MANUSCRIPT the thicknesses of the lamellar α phase has no obvious difference between 900 °C and 1000 °C. Furthermore, because of the rapid growth rate of Widmansta tten α phase, αWGB, in high temperature, and the growth of αWGB starts from very small nuclei and spread into β grains [19, 20]. αWGB can be observed clearly near the grain boundary after annealing at 1000 °C as shown in Fig. 4c. According to
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the diffusion controlled growth mechanism and the result about the instantaneous growth rate of plat new phase [25, 35], the increment thickness of the new α phase δb can be obtained by
=
0 −
−
/
(1)
where C0, Cα, and Cβ are the initial concentration values of the β phase, average concentration of new α
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phase, and concentration of the β phase near the interface after precipitation of the α phase, respectively, and can be regarded as constants, t is growth time, and D is diffusion coefficient. Here, cooling rate v is
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assumed to be unrelated to time, and the growth time of the new phase t can be substituted by (T0−T)/v, where T0 is the onset transition temperature during the cooling process. Thus, the δt can be substituted by -v-1δT. The expression (C0-Cα/Cβ−Cα) is marked as h. In combination with the study by Liang [25], the integral equation of the thicknesses of the lamellar α phase cooling from T0 to Tf can be expressed as (
)*+
, − -
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= 2 /! ℎ# $%& '
(2)
where b is the thicknesses of the lamellar α phase. T0 is the onset transition temperature during the cooling process. Tf is the temperature of final transformation or the temperature that is unable to induce
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grain growth in cooling. v is cooling rate. $%& '
(
)*+
,is diffusion coefficient, h is (C0-Cα)/(Cβ-Cα).
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Combining the thicknesses of α phase are 3µm and 6µm for 800 °C and 900 °C, respectively, during the cooling rate of 0.067 °C/s in furnace cooling pattern, the value of Tf and 2ℎ# $%&./ ⁄ - 01 can be calculated as 767 °C and 0.135, respectively. The calculated value of Tf is consistent with the result of DSC (760 °C). On the other hand, the value of T0 cannot be less than Tf and more than 900°C (900 °C is the upper-limit temperature of β→α phase transition, according the result of DSC). So, the relation of the thickness of α phase based on the onset transition temperature during cooling T0 and cooling rate v of the Ti-41Zr-7.3Al alloy can be expressed as , = 0.135 − 767⁄
(3)
where the range of T0 is from 900 °C to 767 °C, and T0 is 900 °C while cooling from over that
ACCEPTED MANUSCRIPT temperature. The units are µm, °C, and °C/s for b, T0 and v in Eq. (3), respectively. The thicknesses of α phase in specimens air cooled from 800 °C and 900 °C, calculated by Eq. (3) in which 2 °C/s is used as air cooling rate, are 0.55µm and 1.1µm, respectively, which are accordant with the experimental values 0.46µm and 0.97µm, respectively, as shown in Fig. 5a and Fig. 5b. In addition, it is easier to form a
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greater degree of supercooling during air cooled process from higher annealing temperature. So, when annealing temperature increases to 1000 °C, as shown in Fig. 5c, the thicknesses of the lamellar α phase slightly increases to 1.08µm compared with 900 °C. Based on Eq. (3), the thickness of α phase is plotted as a function of the topographic parameters of annealing temperature and cooling rate (Fig. 6).
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According to the Fig. 6, the obvious change of the thickness of α phase is mainly concentrated in the region of cooling rate less than 2 °C/s.
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Fig. 7 and Table 1 demonstrate the mechanical properties of Ti-41Zr-7.3Al alloy heat treated under different conditions. The mechanical properties of Ti-41Zr-7.3Al alloy are evidently dependent on heat treatment conditions. After annealing, the tensile strength of samples increases from 1255 MPa to 1319 MPa with an decrease in the holding temperature from 900 °C to 800 °C with the cooling rate of 0.067 °C/s, and further increases to 1575 MPa as cooling rate increasing to 2 °C/s. In combination
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with the microstructure and the mechanical properties of Ti-41Zr-7.3Al alloy annealing at different conditions, it is easy to find that, with decreasing the thickness of the lamellar α phase, the tensile strength increases, whereas the elongation decreases. In addition, for annealing at 900 °C and 1000 °C,
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the thicknesses of the lamellar α phase and the tensile strength possess similar value during, respectively. However, the elongation of Ti-41Zr-7.3Al alloy annealing at 1000 °C is significantly
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lower than annealing at 900 °C. The reason for this phenomenon is caused by much more αWGB formed in Ti-41Zr-7.3Al alloy during higher annealing temperature. αWGB possesses equality strength but much lower elongation compared with basketweave microstructure. According to the semi-empirical Hall–Petch relationship, the thickness of lamellar α phase can
significantly affect the strength of titanium alloys, it can be expressed as = + 6 /2/!
(4)
Where σ is strength of alloy, σ0 and k are the natural lattice friction and a coefficient, respectively, depend only on alloy composition and crystal structures, b is the thickness of lamellar α phase. As shown in Fig. 8, in current alloys, σ and (b/2)-1/2 have relatively good linear relationship during different annealing processes.
ACCEPTED MANUSCRIPT To establish the quantitative relationship between the kinetics parameters of β→α phase transformation and the strength of Ti-41Zr-7.3Al alloy, b in Eq. (4) can be substituted by Eq. (3), and Eq. (4) can be expressed as = + 670.0675 − 767⁄8
/!
(5)
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Combining the strength of Ti-41Zr-7.3Al alloy are 1319 MPa and 1255 MPa for 800 °C and 900 °C, respectively, during the cooling rate of 0.067 °C/s in furnace cooling pattern, the value of σ0 and k can be calculated as 1102 and 206, respectively. Now the relation of the strength of Ti-41Zr-7.3Al alloy based on the onset transition temperature during cooling T0 and cooling rate v can
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be expressed as = 1102 + 1024[ − 767⁄ ]/
(6)
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The strength of Ti-41Zr-7.3Al alloy air cooled from 800 °C and 900 °C, calculated by Eq. (6) in which 2 °C/s is used as air cooling rate, are 1610MPa and 1460MPa, respectively, which are accordant with the experimental values 1575MPa and 1444MPa, respectively, as shown in Fig. 7 and Table 1. Based on Eq. (6), the strength of Ti-41Zr-7.3Al alloy is plotted as a function of the topographic parameters of annealing temperature and cooling rate (Fig. 9). According to the Fig. 9, it is easy to
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design the strength of Ti-41Zr-7.3Al alloy quantitatively by regulating and controlling heat treatment process, on the other hand, the range of strength can be characterized clearly in Ti-41Zr-7.3Al alloy during continuous cooling process. Conclusions
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4.
Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling
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process are investigated. The conclusions are summarized as follows: (1) The β→α phase transition will occur completely during the air-cooling and furnace-cooling processes after heat treatment at α+β region and β region, no β-phase will be preserved. The Microstructure of alloy mainly shows lamellar α phase, and the thickness of α phase gradually
increases with increased annealing temperature and decreased cooling rate. (2) The strength of alloy increases with decreased annealing temperature and increased cooling rate. The ultrahigh strength of 1575MPa is obtained under the heat treatment temperature of 800 °C and cooling rate of 2 °C/s. (3) The quantitative relationship between the kinetics parameters and the strength of
ACCEPTED MANUSCRIPT Ti-41Zr-7.3Al alloy has been established. Based on diffusion controlled growth mechanism of lamellar α phase and Hall–Petch relationship, the strength of Ti-41Zr-7.3Al alloy relies on the onset transition temperature during cooling T0, and cooling rate v has been deduced as = 1102 + 1024[ − 767⁄]/ .
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Acknowledgements This work was supported by the SKPBRC (Grant no. 2013CB733000), the NSFC (Grant no. 51434008/51531005/51571174/51602208/51502179), the
Natural
Science
Foundation
for
Distinguished Young Scholars of Hebei Province of China (Grant no. E2016203376), the Natural
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Science Foundation of Hebei Province of China (Grant No. E2017210050), the Natural Science Foundation of Hebei Provincial Department of Education (Grant No. QN2017133), and the Open
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Foundation of State Key Laboratory of Metastable Materials Science and Technology (Grant No. 201607). References
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ACCEPTED MANUSCRIPT [33] J.W. Xu, W.D. Zeng, Y.W. Zhao, Z.Q. Jia, X. Sun, Effect of globularization behavior of the lamellar alpha on tensile properties of Ti-17 alloy. J. Alloy. Compd. 673 (2016) 86-92. [34] X.J. Jiang, Y.K. Zhou, Z.H. Feng, C.Q. Xia, C.L. Tan, S.X. Liang, X.Y. Zhang, M.Z. Ma, R.P. Liu, Influence of Zr content on β-phase stability in α-type Ti-Al alloys. Mater. Sci. Eng. A 639
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in Metals and Alloys, Chapman and Hail, London, 1992, p. 281.
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Table captions Table 1 Mechanical properties of Ti-41Zr-7.3Al alloy during different annealing process.
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Table 1 annealing process
E (GPa)
σ0.2 (MPa)
σb (MPa)
ε (%)
800 °C FC
109
1150
1319
6.93
900 °C FC
111
1121
1255
1000 °C FC
109
1102
1250
800 °C AC
108
1490
1575
1310
1000 °C AC
1306
1444
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6.46
3.51
4.76
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110 109
1392
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900 °C AC
9.57
1.91
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Figure captions Fig. 1 DSC curve of Ti-41Zr-7.3Al alloy. Fig. 2 XRD pattern (a) and TEM microstructure (b) of pre-treated Ti-41Zr-7.3Al alloy.
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Fig. 3 XRD patterns of heat-treated Ti-41Zr-7.3Al alloys. Fig. 4 Optical micrographs of heat-treated Ti-41Zr-7.3Al alloy: (a) 800 °C FC, (b) 900 °C FC, (c) 1000 °C FC.
1000 °C AC.
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Fig. 5 TEM microstructures of heat-treated Ti-41Zr-7.3Al alloy: (a) 800 °C AC, (b) 900 °C AC, (c)
Fig. 6 Topographic map of the relationship between kinetics parameters and the thickness of α phase.
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Fig. 7 Stress-strain curves of heat-treated Ti-41Zr-7.3Al alloy. Fig. 8 σ value as a function of the thickness of lamellar α phase.
Fig.9 Topographic map of the relationship between kinetics parameters and the strength of
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Ti-41Zr-7.3Al alloy.
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ACCEPTED MANUSCRIPT The growth kinetics of α phase in Ti-41Zr-7.3Al alloy is investigated.
2.
The ultrahigh strength of 1575MPa is obtained after 800 °C annealing and air cooling process.
3.
The quantitative relationship between kinetics parameters and strength has been established.
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1.
S0925-8388(17)32580-X
DOI:
10.1016/j.jallcom.2017.07.204
Reference:
JALCOM 42620
To appear in:
Journal of Alloys and Compounds
Received Date: 12 May 2017 Revised Date:
19 July 2017
Accepted Date: 20 July 2017
Please cite this article as: X.J. Jiang, H.T. Zhao, G. Yu, H.Y. Wu, C.L. Tan, X.Y. Zhang, M.Z. Ma, R.P. Liu, Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.204. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process X.J. Jianga, b, H.T. Zhaob, G.Yub, H.Y. Wub, C.L. Tanc, X.Y. Zhanga, M.Z. Maa, R.P. Liua,* a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University,
Qinhuangdao 066004, China College of materials science and Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043,
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b
China c
Beijing Institute of Spacecraft System Engineering , Beijing 100094, China
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Abstract: The structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process are investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM)
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are used to analyze the phase transformation and microstructure evolution, meanwhile, mechanical properties are obtained by tensile test. According to the results, the kinetics parameters of β→α phase transformation can play a decisive role for the microstructure and mechanical properties of titanium (Ti) alloys, and the quantitative relationship between the kinetics parameters and the strength of Ti-41Zr-7.3Al alloy has been established. Based on diffusion controlled growth mechanism of lamellar
temperature
during
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α phase and Hall–Petch relationship, the strength of Ti-41Zr-7.3Al alloy relies on the onset transition cooling
T0,
and
cooling
rate
v
has
been
deduced
as
= 1102 + 1024[ − 767⁄ ]/ .
Key words: Titanium alloy; Microstructure evolution; mechanical properties
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*Corresponding author. address: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
e-mail: [email protected] (R.P. Liu) Tel: 0086-335-8074723
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Fax: 0086-335-8074545
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Introduction Titanium (Ti) and its alloys exhibit an excellent combination of mechanical and physical
properties for key applications in aerospace, energy and chemical industry [1-3]. The mechanical properties of titanium alloys are important criteria for the material service capabilities both in
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aerospace and industrial applications. The achievement of remarkable mechanical properties of Ti alloys strongly depends on the process control of the β→α phase transformation [4-11]. The conditions of transformation, especially the temperature and the cooling rate, give rise to formation of lamellar α phase with different size [12-15]. According to the semi-empirical Hall–Petch relationship [16, 17], the
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thickness of lamellar α phase can significantly affect the strength of titanium alloys. So, in other words, the kinetics parameters of β→α phase transformation can play a decisive role for the strength of
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titanium alloys, and it is necessary to study the relationship between them.
Recently, the nuclear and growth mechanism of secondary α phase in Ti alloys has been studied by many researchers. Obasi [18] indicated that the phase transformation in Ti alloys during heating (α→β) and cooling (β→α) is governed by the so-called Burgers orientation relationship {0002}α || {110}β and <11-20>α || <111>β with six possible β-orientations during the α→β phase transformation
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and 12 possible α-orientations that can transform from a single parent β grain during β→α phase transformation. Thus, the basketweave morphology appears. In addition, Widmansta tten α (αWGB) appeared near the grain boundary. This phenomenon has been reported by Sun [19, 20], Bohemen [21],
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and Chen [7] in TA15 alloy, Ti-4.5Fe-6.8Mo-1.5Al alloy and Ti-5Al-5Mo-5V-3Cr-1Zr alloy, respectively. They highlighted that the αWGB nucleated through surface instability and that grain
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boundary α protuberance and αWGB growth started from a small protuberance and spread into a β grain with a sectorial morphology to become lamellar instead of spiculate or oblate cuboid in shape. Moreover, the thickness of lamellar α phase decreased with increasing cooling rate and decreasing heat treatment temperature. On the other hand, the studies by Guo [22] and Peng [23] showed that the thickness of lamellar α had a significant effect on the properties of the alloys tested, a low cooling rate leaded to a diffusion controlled growth of thick lamellar α phase and resulted in a lower tensile strength in TC4 alloy. The similar conclusion has also been reported by our research team in TiZrAlV and TiZrAl alloys [24-28]. In previous researches, the microstructure evolution during heat treatment and the effect of microstructures on mechanical properties have been studied systematically [22-25, 29-32]. However,
ACCEPTED MANUSCRIPT there is little quantitative formula can directly describe the relationship between the kinetics parameters of β→α phase transformation and the strength of titanium alloys. Xu [33] investigated the effects of processing parameters on globularization behavior and tensile properties of Ti-17 alloy, and clarified the quantitative relationship between the kinetics parameters of globularization and the strength during
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annealing. Liu [8, 31] also described the quantitative relationship between the kinetics parameters and mechanical behavior of Ti2448 alloy manufactured by electron beam melting and selective laser melting. In this paper, microstructure evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling process is studied, and the quantitative relationship between the kinetics
2.
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parameters of β→α phase transformation and the strength of Ti-41Zr-7.3Al alloy is established. Experiment
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The starting material was mixed with sponge Ti (99.9 wt.%), sponge Zr (97 wt.%), and industrial-purity Al (99.5 wt.%) before being melted using cooled copper crucible electromagnetic induction melting. To ensure compositional homogeneity, the ingot was flipped and re-melted thrice. Differential scanning calorimetry (DSC) was used to determine the transition temperature at a cooling rate of 10 °C/min. According to the DSC result in Fig. 1, the ingot was homogenized at 1200 °C higher
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than the β transus temperature (760°C-900°C) of approximately 400 °C for 2 h and then quenching in water to room temperature. The composition of the ingot was Ti-41Zr-7.3Al in wt.%. The 12 mm thick plated ingots were heated to 800, 900, and 1000 °C for 1 h, then furnace cooled (FC 0.067 °C/s) and air
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cooled (AC 2 °C/s) to room temperature, respectively. The phases of the specimens were confirmed by conventional X-ray diffraction (XRD) with Cu Kα radiation (D/max-2500/PC). The microstructure of
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the heat treated plated samples was examined using optical microscopy (OM) and the transmission electron microscopy (TEM), which specimens were prepared via twin-jet electrochemical polishing in a solution containing 10% perchloric acid and 90% methanol at 14 V and −35 °C. The thickness of the
lamellar α phase used in this paper is indicated as an average value. Bone-shaped plate specimens with an original gauge length of 21 mm and a cross-sectional dimension of 3 mm×2 mm were prepared for the tensile tests. Uniaxial tensile tests were performed on an Instron 5982 testing machine at a strain rate of 5×10−4 s−1. 3.
Results and discussion The phase and microstructure of preprocessed ingots during homogenizing and quenching are
shown in Fig. 2. The XRD pattern in Fig. 2a shows that all diffraction peaks match with those of α''
ACCEPTED MANUSCRIPT martensitic phase, no α phase peaks and/or other phases exist. TEM image including SAED pattern in Fig. 2b shows the same result as XRD pattern, the acicular α'' martensitic phase can be observed clearly, and no residual lamellar α phase can be found. These results indicate that α phase transformed to β phase completely during homogenizing, and then because of the high content of Zr [34], acicular α''
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martensitic phase formed from β phase in the process of water quenching. When subsequent annealing temperature is higher than β transus temperature, α'' martensitic phase will transform to β phase again, and no α phase can be precipitated. The only way to obtain lamellar α phase is the furnace cooled and/or air cooled processes after subsequent annealing in this paper. So, the pre-existing microstructure
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after homogenizing and quenching has no effect on final structure after subsequent annealing and cooling.
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The XRD patterns of Ti-41Zr-7.3Al alloy after different heat treatment conditions are shown in Fig. 3. All diffraction peaks match with those of α phase, no β-phase peaks and/or other intermediate phases is observed in any of the XRD patterns. These results indicate that when annealing temperature is above the final temperature of β→α phase-transition, β→α phase transition will occur completely during the air-cooling and furnace-cooling processes after heat treatment at α+β region and β region, no
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β phase or martensitic phase will be preserved. Al is an α-stable alloying element no matter in Ti or Zr alloys, the addition of Al will improve the stability of α phase, and promote β→α phase transition during cooling process from high temperature. In Ti-41Zr-7.3Al alloys, β phase and martensitic phase
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can be preserved only by water quenching or rapid cooling process from α+β region or β region. So, only α phase exist during the air-cooling and furnace-cooling processes after heat treatment at α+β
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region and β region. And this phenomenon has been studied in our previous research [26, 28, 34]. Optical micrographs of Ti-41Zr-7.3Al alloy after furnace cooled to room temperature from 800,
900, and 1000 °C are illustrated in Fig. 4. All Microstructure mainly shows lamellar α phase. The basketweave morphology appears after annealing at 800 °C and 900 °C as shown in Fig. 4a and Fig. 4b, and the thicknesses of the lamellar α phase are approximately 3µm and 6µm for 800 °C and 900 °C, respectively. It is gradually increased with increased annealing temperature from 800 °C to 900 °C. However, when annealing temperature further increases to 1000 °C, as shown in Fig. 4c, the thicknesses of the lamellar α phase has no obvious change compared with 900 °C. This kind of phenomenon is mainly due to the onset transition temperature is almost 900 °C. When annealing temperature is above 900 °C, the additional cooling process has no effect on the growth of α phase, so
ACCEPTED MANUSCRIPT the thicknesses of the lamellar α phase has no obvious difference between 900 °C and 1000 °C. Furthermore, because of the rapid growth rate of Widmansta tten α phase, αWGB, in high temperature, and the growth of αWGB starts from very small nuclei and spread into β grains [19, 20]. αWGB can be observed clearly near the grain boundary after annealing at 1000 °C as shown in Fig. 4c. According to
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the diffusion controlled growth mechanism and the result about the instantaneous growth rate of plat new phase [25, 35], the increment thickness of the new α phase δb can be obtained by
=
0 −
−
/
(1)
where C0, Cα, and Cβ are the initial concentration values of the β phase, average concentration of new α
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phase, and concentration of the β phase near the interface after precipitation of the α phase, respectively, and can be regarded as constants, t is growth time, and D is diffusion coefficient. Here, cooling rate v is
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assumed to be unrelated to time, and the growth time of the new phase t can be substituted by (T0−T)/v, where T0 is the onset transition temperature during the cooling process. Thus, the δt can be substituted by -v-1δT. The expression (C0-Cα/Cβ−Cα) is marked as h. In combination with the study by Liang [25], the integral equation of the thicknesses of the lamellar α phase cooling from T0 to Tf can be expressed as (
)*+
, − -
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= 2 /! ℎ# $%& '
(2)
where b is the thicknesses of the lamellar α phase. T0 is the onset transition temperature during the cooling process. Tf is the temperature of final transformation or the temperature that is unable to induce
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grain growth in cooling. v is cooling rate. $%& '
(
)*+
,is diffusion coefficient, h is (C0-Cα)/(Cβ-Cα).
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Combining the thicknesses of α phase are 3µm and 6µm for 800 °C and 900 °C, respectively, during the cooling rate of 0.067 °C/s in furnace cooling pattern, the value of Tf and 2ℎ# $%&./ ⁄ - 01 can be calculated as 767 °C and 0.135, respectively. The calculated value of Tf is consistent with the result of DSC (760 °C). On the other hand, the value of T0 cannot be less than Tf and more than 900°C (900 °C is the upper-limit temperature of β→α phase transition, according the result of DSC). So, the relation of the thickness of α phase based on the onset transition temperature during cooling T0 and cooling rate v of the Ti-41Zr-7.3Al alloy can be expressed as , = 0.135 − 767⁄
(3)
where the range of T0 is from 900 °C to 767 °C, and T0 is 900 °C while cooling from over that
ACCEPTED MANUSCRIPT temperature. The units are µm, °C, and °C/s for b, T0 and v in Eq. (3), respectively. The thicknesses of α phase in specimens air cooled from 800 °C and 900 °C, calculated by Eq. (3) in which 2 °C/s is used as air cooling rate, are 0.55µm and 1.1µm, respectively, which are accordant with the experimental values 0.46µm and 0.97µm, respectively, as shown in Fig. 5a and Fig. 5b. In addition, it is easier to form a
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greater degree of supercooling during air cooled process from higher annealing temperature. So, when annealing temperature increases to 1000 °C, as shown in Fig. 5c, the thicknesses of the lamellar α phase slightly increases to 1.08µm compared with 900 °C. Based on Eq. (3), the thickness of α phase is plotted as a function of the topographic parameters of annealing temperature and cooling rate (Fig. 6).
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According to the Fig. 6, the obvious change of the thickness of α phase is mainly concentrated in the region of cooling rate less than 2 °C/s.
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Fig. 7 and Table 1 demonstrate the mechanical properties of Ti-41Zr-7.3Al alloy heat treated under different conditions. The mechanical properties of Ti-41Zr-7.3Al alloy are evidently dependent on heat treatment conditions. After annealing, the tensile strength of samples increases from 1255 MPa to 1319 MPa with an decrease in the holding temperature from 900 °C to 800 °C with the cooling rate of 0.067 °C/s, and further increases to 1575 MPa as cooling rate increasing to 2 °C/s. In combination
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with the microstructure and the mechanical properties of Ti-41Zr-7.3Al alloy annealing at different conditions, it is easy to find that, with decreasing the thickness of the lamellar α phase, the tensile strength increases, whereas the elongation decreases. In addition, for annealing at 900 °C and 1000 °C,
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the thicknesses of the lamellar α phase and the tensile strength possess similar value during, respectively. However, the elongation of Ti-41Zr-7.3Al alloy annealing at 1000 °C is significantly
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lower than annealing at 900 °C. The reason for this phenomenon is caused by much more αWGB formed in Ti-41Zr-7.3Al alloy during higher annealing temperature. αWGB possesses equality strength but much lower elongation compared with basketweave microstructure. According to the semi-empirical Hall–Petch relationship, the thickness of lamellar α phase can
significantly affect the strength of titanium alloys, it can be expressed as = + 6 /2/!
(4)
Where σ is strength of alloy, σ0 and k are the natural lattice friction and a coefficient, respectively, depend only on alloy composition and crystal structures, b is the thickness of lamellar α phase. As shown in Fig. 8, in current alloys, σ and (b/2)-1/2 have relatively good linear relationship during different annealing processes.
ACCEPTED MANUSCRIPT To establish the quantitative relationship between the kinetics parameters of β→α phase transformation and the strength of Ti-41Zr-7.3Al alloy, b in Eq. (4) can be substituted by Eq. (3), and Eq. (4) can be expressed as = + 670.0675 − 767⁄8
/!
(5)
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Combining the strength of Ti-41Zr-7.3Al alloy are 1319 MPa and 1255 MPa for 800 °C and 900 °C, respectively, during the cooling rate of 0.067 °C/s in furnace cooling pattern, the value of σ0 and k can be calculated as 1102 and 206, respectively. Now the relation of the strength of Ti-41Zr-7.3Al alloy based on the onset transition temperature during cooling T0 and cooling rate v can
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be expressed as = 1102 + 1024[ − 767⁄ ]/
(6)
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The strength of Ti-41Zr-7.3Al alloy air cooled from 800 °C and 900 °C, calculated by Eq. (6) in which 2 °C/s is used as air cooling rate, are 1610MPa and 1460MPa, respectively, which are accordant with the experimental values 1575MPa and 1444MPa, respectively, as shown in Fig. 7 and Table 1. Based on Eq. (6), the strength of Ti-41Zr-7.3Al alloy is plotted as a function of the topographic parameters of annealing temperature and cooling rate (Fig. 9). According to the Fig. 9, it is easy to
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design the strength of Ti-41Zr-7.3Al alloy quantitatively by regulating and controlling heat treatment process, on the other hand, the range of strength can be characterized clearly in Ti-41Zr-7.3Al alloy during continuous cooling process. Conclusions
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4.
Structural evolution and mechanical properties of Ti-41Zr-7.3Al alloy during continuous cooling
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process are investigated. The conclusions are summarized as follows: (1) The β→α phase transition will occur completely during the air-cooling and furnace-cooling processes after heat treatment at α+β region and β region, no β-phase will be preserved. The Microstructure of alloy mainly shows lamellar α phase, and the thickness of α phase gradually
increases with increased annealing temperature and decreased cooling rate. (2) The strength of alloy increases with decreased annealing temperature and increased cooling rate. The ultrahigh strength of 1575MPa is obtained under the heat treatment temperature of 800 °C and cooling rate of 2 °C/s. (3) The quantitative relationship between the kinetics parameters and the strength of
ACCEPTED MANUSCRIPT Ti-41Zr-7.3Al alloy has been established. Based on diffusion controlled growth mechanism of lamellar α phase and Hall–Petch relationship, the strength of Ti-41Zr-7.3Al alloy relies on the onset transition temperature during cooling T0, and cooling rate v has been deduced as = 1102 + 1024[ − 767⁄]/ .
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Acknowledgements This work was supported by the SKPBRC (Grant no. 2013CB733000), the NSFC (Grant no. 51434008/51531005/51571174/51602208/51502179), the
Natural
Science
Foundation
for
Distinguished Young Scholars of Hebei Province of China (Grant no. E2016203376), the Natural
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Science Foundation of Hebei Province of China (Grant No. E2017210050), the Natural Science Foundation of Hebei Provincial Department of Education (Grant No. QN2017133), and the Open
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Foundation of State Key Laboratory of Metastable Materials Science and Technology (Grant No. 201607). References
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(2015) 407-411. [35] D.A. Porter, K.E. Easterling, Diffusional Transformations in Solids [M]. Phase Transformations
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in Metals and Alloys, Chapman and Hail, London, 1992, p. 281.
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Table captions Table 1 Mechanical properties of Ti-41Zr-7.3Al alloy during different annealing process.
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Table 1 annealing process
E (GPa)
σ0.2 (MPa)
σb (MPa)
ε (%)
800 °C FC
109
1150
1319
6.93
900 °C FC
111
1121
1255
1000 °C FC
109
1102
1250
800 °C AC
108
1490
1575
1310
1000 °C AC
1306
1444
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6.46
3.51
4.76
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110 109
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Figure captions Fig. 1 DSC curve of Ti-41Zr-7.3Al alloy. Fig. 2 XRD pattern (a) and TEM microstructure (b) of pre-treated Ti-41Zr-7.3Al alloy.
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Fig. 3 XRD patterns of heat-treated Ti-41Zr-7.3Al alloys. Fig. 4 Optical micrographs of heat-treated Ti-41Zr-7.3Al alloy: (a) 800 °C FC, (b) 900 °C FC, (c) 1000 °C FC.
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Fig. 5 TEM microstructures of heat-treated Ti-41Zr-7.3Al alloy: (a) 800 °C AC, (b) 900 °C AC, (c)
Fig. 6 Topographic map of the relationship between kinetics parameters and the thickness of α phase.
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Fig. 7 Stress-strain curves of heat-treated Ti-41Zr-7.3Al alloy. Fig. 8 σ value as a function of the thickness of lamellar α phase.
Fig.9 Topographic map of the relationship between kinetics parameters and the strength of
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ACCEPTED MANUSCRIPT The growth kinetics of α phase in Ti-41Zr-7.3Al alloy is investigated.
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The ultrahigh strength of 1575MPa is obtained after 800 °C annealing and air cooling process.
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The quantitative relationship between kinetics parameters and strength has been established.
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