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Research on thermal stability of Ti40 alloy at 550 °C
Materials Science and Engineering A 477 (2008) 372–378
Research on thermal stability of Ti40 alloy at 550 ◦C S.W. Xin a,b,∗ , Y.Q. Zhao b , W.D. Zeng a , H. Wu b a
School of Materials, Northwestern Polytechnical University, Xi’an 710012, China Northwest Institute for Nonferrous Metal Research (NIN), Xi’an 710016, China
b
Received 4 February 2007; received in revised form 10 May 2007; accepted 12 June 2007
Abstract Microstructures and mechanical properties of Ti40 alloy thermal exposures at 550 ◦ C for different times were studied. The results show that there are only a few precipitates during thermal exposure, most of which distribute on grain boundaries and weakens its mechanical properties. By systematic analysis on its microstructures and mechanical properties, the thermal exposure at 550 ◦ C can be divided into three stages and the formation and growth of the precipitates were different at each stage. During the third stage, the main reason of un-matched properties between strength and hardness is due to the formation of continuous precipitated zones along grain boundaries. © 2007 Published by Elsevier B.V. Keywords: Ti40 alloy; Burn resistant titanium alloy; Thermal exposure; Mechanical properties
1. Introduction In the last decades, more and more attention was paid on Ti–V–Cr burn-resistant titanium alloys in different countries. For example Alloy C (Ti–35V–15Cr) was developed by Pratt and Whitney in USA [1,2]; Ti–25V–15Cr–2Al–0.2C was developed by IRC in UK [3,4] and Ti40(Ti–25V–15Cr–xSi) by Northwest Institute for Nonferrous Metal Research (NIN) in China [5,6]. The three alloys all have good burn resistance, and Alloy C has been used in F119 engine. In China, Ti40 alloy has been researched in NIN for many years and many achievements has been reached. By now, burn resistant mechanism [7,8], elevated temperature deformation mechanism [9], mechanical properties and microstructures under different conditions [10,11] have been systemically researched. It has been proved that Ti40 alloy can be used at 500 ◦ C for a long time. However, further use of burn resistant titanium alloy in aero-engines requires that Ti40 alloy can be applied to higher temperature, for example 550 ◦ C. Thus, the characteristics of microstructures and properties during thermal exposure at 550 ◦ C should be studied further, which will offer theoretical and practical instruction for Ti40 alloy used at higher temperature. It is true that there are some publications about thermal stability of Ti40 alloy [10–13] and other Ti–V–Cr alloys ∗
Corresponding author at: Northwest Institute for Nonferrous Metal Research (NIN), Xi’an 710016, China. Fax: +86 29 86231103. E-mail address: nwpu [email protected] (S.W. Xin). 0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.06.024
[14,15], and the publications about Ti40 alloy have pointed out that ␣ and Ti5 Si3 precipitations on grain boundaries degrade its thermal stability. However, there is little information about the alloy thermal exposure at 550 ◦ C and the results discussed in publications also cannot satisfy the instructional requirement of Ti40 alloy used at 550 ◦ C. At the same time, the research results in this paper will offer theoretical instruction to other  alloy used at higher temperature. 2. Experimental procedures Five hundred kilograms Ti40 ingot was melted by three melting by vacuum arc remelting (VAR) method at NIN in China. Its rings were forged by special process, which are mentioned elsewhere [16]. The composition is shown in Table 1. Samples of tensile and thermal exposure were cut from the rings. The size for tensile and heat treatment blank samples were 12 mm × 12 mm × 70 mm and 12 mm × 12 mm × 12 mm, respectively. Before thermal exposure, all specimens were annealed at 850 ◦ C for 1 h water quenching and followed by ageing at 550 ◦ C for 5 h, air-cooling (this heat treatment technology is recommended for Ti40 alloy at present in use). After that, all specimens were separately thermal exposed at 550 ◦ C for 5, 10, 20, 50, 100, 200 and 500 h, respectively, and then the specimens were machined into 5 mm in diameter standard tensile specimens for mechanical experiment with a gauge length of 25 mm. All tensile tests including forged condition (R), heat
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Table 1 Alloying compositions (wt%) Element
V
Cr
Si
Fe
N
C
O
H
Ti
Content (wt%)
25.77
14.70
0.26
0.09
0.01
0.01
0.07
0.001
Balance
treatment condition (HT) and thermal exposure condition (TST) were carried out on an Instron testing machine with a strain rate 10−3 s−1 (the conditions for the samples were as follows: R stands for as-forged conditions, HT for as-annealed and asaged ones, TST for as-exposed ones.). Hardness experiments were carried out on an 450SVD Digimatic Vickers hardness tester. The loading weight is 10 kg and the duration was 30 s. The optical microstructure (OM), X-ray diffraction (XRD), transmission electronic microscope (TEM) microstructure and scanning electronic microscope (SEM) micrographs were separately carried out on OLMPUS PMG, PW1700 X-ray diffraction instrument, JEM-200CX transmission electron microscope and S-2700 scanning electronic microscope. The etchant consists of 10 vol.%HF, 30 vol.%HNO3 and 50 vol.%H2 O. 3. Results and discussion 3.1. Mechanical properties The mechanical properties for as-forged (R), as-annealed and aged (HT) and as-exposed (TST) at 550 ◦ C for different times are listed in Table 2. Fig. 1 shows the relationship between mechanical properties and thermal exposure times. It can be seen that strengthening effect can be got for as-annealed and as-aged samples (the region in the front of “Line a” in Fig. 1). During thermal exposure process, with increasing time, the ultimate tensile strength (UTS) slightly increased at first and then decreased. After thermal exposure for 50 h the UTS was irregularly jumping. The elongation (El) decreased and hardness (HV) slightly increased within all the exposure times. The change of El, HV and UTS before thermal exposure for 50 h is coinciding with the theory discussed in reference [17]. In that paper, the authors pointed out that the heat treatment of titanium alloy belongs to the third kind of heat treatment; the ageing treatment of titanium alloy is a precipitation strengthening process. When the time of thermal exposure is beyond 50 h, the reason of jumpi-
ness of UTS will be discussed below. As shown in Fig. 1, when the alloy exposed at 550 ◦ C for different times, the best time for the alloy with favourite mechanical properties should be no more than 10 h (between “Line a” and “Line b” in Fig. 1). If the duration time was longer than 50 h, the mechanical properties would deteriorate seriously, especially after 100 h, the specimens broke into many sections, and then the elongation and reduction in area for the alloy cannot be detected. 3.2. Microstructures 3.2.1. OM images Fig. 2 shows the OM microstructures under various conditions. Compared with Fig. 2a and b, it can be found that the microstructures of as-forged specimens and as-heat treated ones were almost the same. The grain boundaries for the latter were just smooth and sharp, which indicates that the annealing and ageing treatment for Ti40 alloy should only adjust the microstructures of as-forged specimens and cannot produce obvious strengthening effect. Compared with thermal exposed microstructures in different times (Fig. 2b–e), with the increasing exposure times there were increasing precipitates, especially the exposure time being over 50 h, the obvious changes were found in microstructures, the grain boundaries clearly broaden and precipitate-free zones were found adjacent to grain boundaries (as shown in Fig. 2d and e). The change of microstructure with increasing thermal exposure times are corresponding with the change of mechanical properties in Fig. 1. From references [11,12], it can be known that the precipitates are mainly ␣ and Ti5 Si3 . In order to confirm the content of the precipitations, XRD test was carried out. As shown in Fig. 3, after thermal exposure for 500 h, only matrix 
Table 2 Mechanical properties of Ti40 alloy after thermal exposure at different times Conditions
UTS (MPa)
Y.S. (MPa)
El (%)
R.A. (%)
HV
R HT 550 ◦ C/5 h 550 ◦ C/10 h 550 ◦ C/20 h 550 ◦ C/50 h 550 ◦ C/100 h 550 ◦ C/200 h 550 ◦ C/500 h
950 970 975 980 970 960 755 770 885
935 940 940 945 940 945 / / /
18.0 23.5 22.5 22.0 10.0 3.3 1.5 / /
54.0 42.0 33.5 30.5 13.0 3.2 2.0 / /
300.5 314.2 316.9 316.0 304.2 308.3 311.4 314.1 315.7
UTS, ultimate tensile strength; Y.S., yield strength; El, elongation; R.A., reduction of area; /, performance cannot be tested for complete brittle fracture.
Fig. 1. The relationship between mechanical properties (UTS, El and HV) and thermal exposure times for Ti40 alloy.
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Fig. 2. OM micrographs after different thermal exposure: (a) R; (b) HT; (c) (d) (e) are, respectively, thermal exposure at 550 ◦ C for 20, 50 and 500 h.
diffraction peak could be found in the XRD pattern, which indicates that very small fraction of ␣ and Ti5 Si3 phase were formed during thermal exposure process, and their fractions in  matrix was less than about 5% (wt%), and cannot be detected in XRD test. The changes of hardness and strength shown in Table 2 also show the same tendency. The hardness curve (HV) in Fig. 1 only increased a little with increasing thermal exposure times, and the
strengthening regions during thermal exposure (between “Line a” and “Line b” in Fig. 1) indicated that the strengthening effect was weak, which indicates that only a few  phases decomposed during thermal exposure and precipitation strength evenly could not be found. The results can be accepted because Ti40 alloy is a full- titanium alloy with higher Mo-equivalent (42) and lower phase transformation temperature, which leads to  microstruc-
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Fig. 3. XRD pattern of Ti40 alloy after thermal exposure at 550 ◦ C for 500 h.
ture with an extreme high stability. The microstructure keeps relative stability during thermal exposure which is an important characteristic of Ti40 alloy. It is well known that the strengthening effect of second-phase is dominated by three factors when the composition of secondphase has been known. The first one is the size of second-phase; the second one is the content of second-phase; the third one is the distributions of second-phase. In order to get improved comprehensive mechanical properties, a microstructure should be expected to get with fine, homogenous and a certain content second-phase after ageing treatment. In this experiment, the above results have proved that only a few second-phases can be got during thermal exposure and the deterioration of mechanical properties after thermal exposure for 50 h is mainly due to the distributions and size of precipitates. 3.2.2. SEM and TEM images Fig. 4 shows the SEM images of Ti40 alloy exposed at 550 ◦ C for 10, 50, and 100 h, respectively. It can be shown that the precipitates mainly distributed on grain boundaries, and there were precipitates-free zones adjacent to grain boundaries (it also can be shown in Fig. 2). The distributions of precipitates are also due to the stability of  phases, which leads to an increase of precipitated energy and the relative reduction of nucleation sites. Then more precipitates would nucleate on grain boundaries but not at vacancies or dislocations which plentifully exist inside grains. The formation of precipitates-free zones also proves that nucleation at vacancies is more difficult than at grain boundaries. In the initial stage of thermal exposure, there were a few ␣ and Ti5 Si3 phases nucleated on grain boundaries and few sites inside grains, which produced a certain strengthening effect and nearly had no effect on plasticity for the alloy, which corresponds with the region in between “Line a” and “Line b” in Fig. 1. With the increasing thermal exposure times, the precipitates on grain boundaries would grow up and spread along grain boundaries. Finally, continuous precipitated zones were formed along grain boundaries. As clearly shown in Fig. 5a and b there were continuous precipitated zones along grain boundaries for the alloy thermal exposed for 200 h, and precipitates had grown up to a
Fig. 4. SEM micrographs of Ti40 alloy after thermal exposure at 550 ◦ C for different times: (a) 10 h, (b) 50 h and (c) 100 h.
certain degree. After thermal exposure for 500 h (Fig. 5c), grainboundary precipitates kept growing up and in some sites there were diamond precipitates with 0.8 m edges. The above process of thermal exposure can be simulated by Fig. 6. The whole process can be divided into three stages. The first one is the initiation stage of precipitates, corresponds to the exposure time with no less than 10 h (the region between “Line a” and Line b in Fig. 1); the second one is growing up of the precip-
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Fig. 5. TEM bright-field images of precipitations at grain boundary after thermal exposure at 550 ◦ C for 200 h (a and b) and 500 h (c).
Fig. 6. Simulated diagrams of precipitating and growing processes of precipitation at grain boundaries.
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itates, corresponds to exposure time from 10 to 50 h (the region between “Line b” and “Line c” in Fig. 1); the third one is the formation of the precipitated zones, correspond to exposure time greater than 50 h (the region is after “Line c”). Understanding the precipitated characteristics for Ti40 alloy thermal exposure at 550 ◦ C for various times, it is easily comprehended that, after thermal exposure for 50 h, there is non-matched mechanical properties between strength (“UTS” curve in Fig. 1) and hardness (“HV” curve in Fig. 1). Because precipitated zones along grain boundaries produce a little effect on hardness but produces a stronger effect on tensile strength leading to unstable tensile strength (the jumpiness of “UTS” curve after thermal exposure for 50 h in Fig. 1). 3.2.3. SEM fractographic images Fig. 7 shows the tensile fractographs of Ti40 alloy thermal exposed at 550 ◦ C for 5, 20 and 200 h. It can be shown that cracks were originated from grain boundaries and the better plasticity was resulting from the dimples in transcrystalline fracture within grains, which reveals the precipitates at grain boundaries deteriorate thermal stability and precipitates within grains produce little effect on the plasticity. Compared with the fractograph of thermal exposure for 5 h (Fig. 7a), there were more intergranular fractures after thermal exposure for 20 h (Fig. 7b) which indicates the increasing precipitates on grain boundaries with increasing thermal exposure times. The fractograph after thermal exposure for 200 h (Fig. 7c) shown complete intergranular fracture, which indicates the continuous precipitated zones have been formed. 4. General discussion The above results suggest that Ti40 ring cannot be used at 550 ◦ C under present treat conditions. The continuous precipitated zones along grain boundaries are the main factor to restrict its use under high temperature. For the alloy used at higher temperature, two factors must be considered based on the forming mechanism of the precipitated zones. Firstly, it is the sites of precipitates; secondly, it is the amount of precipitates. For factor one, the above analysis has pointed out that the main reason of second-phases at grain boundaries is the compositions of Ti40 (high Mo-equivalent leads to a high precipitated energy of second-phases) and higher thermal exposure temperature (higher thermal exposure temperature leads to increasing required nucleation energy). It is obvious that the changes of the composition or the decrease of thermal exposure temperature are no any meaning. The composition of Ti40 was designed for getting good burn resistant property. The research of thermal stability must be on the base of present composition. And this paper aims at how to use Ti40 alloy at a higher temperature (550 ◦ C). Thus, the decrease of thermal exposure temperature is forbidden. Even so, the distributions of precipitates can be optimized by adjusting nucleation sites. Judging from above analysis, the quenching at high temperature cannot be accepted. Although the increases of vacancy density within grains by quenching at high temperature can lead to an increasing nucleation sites within grains, quenching at high temperature also leads to an
Fig. 7. SEM fractographs of Ti40 alloy after thermal exposure at 550 ◦ C for 5 h (a), 20 h (b) and 200 h (c).
increasing amount of precipitates at grain boundaries. But also, the vacancies will diffuse from inside grains to grain boundaries during thermal exposure and which also maybe lead to an increase of precipitates at grain boundaries. Considering all factors, pre-ageing treatment at low temperature may be a feasible method. or  metastable phases will maybe precipitate for the alloy aged at lower temperature (for Ti40 alloy the temperature maybe range from 360 to 420 ◦ C). Because of the low diffusion coefficient and required nucleation energy, the metastable phases
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will dispersedly precipitate within grains, the sequent thermal exposure at high temperature will lead to ␣ phases nucleate at the sites of metastable phases or the metastable phases directly transform into ␣ phases, which will lead to a homogeneous distribution of ␣ phase within grains. It is prospective to improve the thermal stability by this way. To the second factor about the amount of precipitates, it can be considered in two ways. The first is to reduce the amount of precipitates in unit grain boundary by the ways of refining grains and increasing the amount of grain boundaries. The above analysis indicates that it cannot be achieved by heat treatment. The coarse equiaxed grains of as-forged samples indicate the alloy cannot recrystallize during heat treatment and there is nearly no difference in grains between as-forged samples and as-heat treated samples. The forged technology also cannot be adjusted for the poor workability of Ti40 alloy. Thus, the way of refining grains and increasing the amount of grain boundaries is unacceptable. The second way is to decrease the effect of harmful elements. In Ti40 alloy the element is oxygen element. Being harmful element it diffuses from inside grains to grain boundaries and improves the precipitation of ␣ phase during thermal exposure. Publications [15,18] have proved that in Ti–V–Cr alloy the addition of carbon element can effectively reduce the harmful effect of oxygen element and significantly improve the thermal stability. The authors think the addition of carbon element into Ti40 is an effective way to improve thermal stability at higher temperature. 5. Conclusions (1) There are only a few precipitates during thermal exposure, which mostly distribute on grain boundaries. With the increase of thermal exposure times the precipitates gradually evolve into continuous precipitated zones along grain boundaries. The distributing characteristic of precipitates is the main reason for Ti40 alloy to have a bad thermal stability at 550 ◦ C.
(2) The experimental results indicate that Ti40 ring cannot be used for 550 ◦ C under present treated conditions. The adjustment of thermal treatment conditions or carbon addition may be the feasible methods to increase its usage temperature. Acknowledgement The authors would like to acknowledge the financial support of Chinese National Key Project (MKPT-01-101) and 973 Project (51333). References [1] D.M. Berczik, US Patent 516,762, 1993-01-05. [2] G. Kneisel, Adv. Mater. Process. 9 (1993) 7. [3] Y.G. Li, X.D. Zhang, P.A. Blenkinsop, et al., in: P.A. Blenkinsop, W.J. Evens (Eds.), Proceedings of the 8th Word Conference on Titanium, The Institute of Materials, London, 1996, p. 2317. [4] X. Huang, L.M. Lei, F.S. Sun, et al., Rare Met. Mater. Eng. 33 (2004) 218. [5] Y.Q. Zhao, K.Y. Zhu, H. Wu, China Defense Patent. Application No. 97112302.0. [6] Y.Q. Zhao, L. Zhou, G.Z. Luo, Rare Met. Mater. Eng. (1996) 1. [7] Y.Q. Zhao, L. Zhou, J. Deng, Mater. Sci. Eng. A 267 (1999) 167. [8] Y.Q. Zhao, L. Zhou, J. Mater. Sci. Technol. (2001) 677. [9] Y.Q. Zhao, L. Zhou, Acta Metall. Sin. 1 (2001) 406. [10] Y.Q. Zhao, H.L. Qu, K.Y. Zhu, et al., J. Alloys Compd. 333 (2002) 165– 169. [11] K.Y. Zhu, Y.Q. Zhao, H.L. Qu, et al., J. Mater. Sci. 39 (2004) 2387–2394. [12] Y.Q. Zhao, H.L. Qu, K.Y. Zhu, et al., J. Mater. Sci. 38 (2003) 1579–1584. [13] Y.Q. Zhao, H.L. Qu, M.M. Wang, et al., J. Alloys Compd. 407 (2006) 118–224. [14] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, N.A. Walker, Acta Metall. Inc. 46 (1998) 5777–5794. [15] Y.G. Li, M.H. Loretto, D. Rugg, W. Voice, Acta Mater. 49 (2001) 3011–3017. [16] Y.Q. Zhao, K.Y. Zhu, H.L. Qu, et al., Mater. Sci. Eng. A 282 (2000) 153–157. [17] S.W. Xin, Zhao FY.Q., Heat Treat. Met. 31 (2006) 39–42. [18] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, D. Rugg, W. Voice, Acta Mater. 47 (1999) 2889–2905.
Research on thermal stability of Ti40 alloy at 550 ◦C S.W. Xin a,b,∗ , Y.Q. Zhao b , W.D. Zeng a , H. Wu b a
School of Materials, Northwestern Polytechnical University, Xi’an 710012, China Northwest Institute for Nonferrous Metal Research (NIN), Xi’an 710016, China
b
Received 4 February 2007; received in revised form 10 May 2007; accepted 12 June 2007
Abstract Microstructures and mechanical properties of Ti40 alloy thermal exposures at 550 ◦ C for different times were studied. The results show that there are only a few precipitates during thermal exposure, most of which distribute on grain boundaries and weakens its mechanical properties. By systematic analysis on its microstructures and mechanical properties, the thermal exposure at 550 ◦ C can be divided into three stages and the formation and growth of the precipitates were different at each stage. During the third stage, the main reason of un-matched properties between strength and hardness is due to the formation of continuous precipitated zones along grain boundaries. © 2007 Published by Elsevier B.V. Keywords: Ti40 alloy; Burn resistant titanium alloy; Thermal exposure; Mechanical properties
1. Introduction In the last decades, more and more attention was paid on Ti–V–Cr burn-resistant titanium alloys in different countries. For example Alloy C (Ti–35V–15Cr) was developed by Pratt and Whitney in USA [1,2]; Ti–25V–15Cr–2Al–0.2C was developed by IRC in UK [3,4] and Ti40(Ti–25V–15Cr–xSi) by Northwest Institute for Nonferrous Metal Research (NIN) in China [5,6]. The three alloys all have good burn resistance, and Alloy C has been used in F119 engine. In China, Ti40 alloy has been researched in NIN for many years and many achievements has been reached. By now, burn resistant mechanism [7,8], elevated temperature deformation mechanism [9], mechanical properties and microstructures under different conditions [10,11] have been systemically researched. It has been proved that Ti40 alloy can be used at 500 ◦ C for a long time. However, further use of burn resistant titanium alloy in aero-engines requires that Ti40 alloy can be applied to higher temperature, for example 550 ◦ C. Thus, the characteristics of microstructures and properties during thermal exposure at 550 ◦ C should be studied further, which will offer theoretical and practical instruction for Ti40 alloy used at higher temperature. It is true that there are some publications about thermal stability of Ti40 alloy [10–13] and other Ti–V–Cr alloys ∗
Corresponding author at: Northwest Institute for Nonferrous Metal Research (NIN), Xi’an 710016, China. Fax: +86 29 86231103. E-mail address: nwpu [email protected] (S.W. Xin). 0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.06.024
[14,15], and the publications about Ti40 alloy have pointed out that ␣ and Ti5 Si3 precipitations on grain boundaries degrade its thermal stability. However, there is little information about the alloy thermal exposure at 550 ◦ C and the results discussed in publications also cannot satisfy the instructional requirement of Ti40 alloy used at 550 ◦ C. At the same time, the research results in this paper will offer theoretical instruction to other  alloy used at higher temperature. 2. Experimental procedures Five hundred kilograms Ti40 ingot was melted by three melting by vacuum arc remelting (VAR) method at NIN in China. Its rings were forged by special process, which are mentioned elsewhere [16]. The composition is shown in Table 1. Samples of tensile and thermal exposure were cut from the rings. The size for tensile and heat treatment blank samples were 12 mm × 12 mm × 70 mm and 12 mm × 12 mm × 12 mm, respectively. Before thermal exposure, all specimens were annealed at 850 ◦ C for 1 h water quenching and followed by ageing at 550 ◦ C for 5 h, air-cooling (this heat treatment technology is recommended for Ti40 alloy at present in use). After that, all specimens were separately thermal exposed at 550 ◦ C for 5, 10, 20, 50, 100, 200 and 500 h, respectively, and then the specimens were machined into 5 mm in diameter standard tensile specimens for mechanical experiment with a gauge length of 25 mm. All tensile tests including forged condition (R), heat
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Table 1 Alloying compositions (wt%) Element
V
Cr
Si
Fe
N
C
O
H
Ti
Content (wt%)
25.77
14.70
0.26
0.09
0.01
0.01
0.07
0.001
Balance
treatment condition (HT) and thermal exposure condition (TST) were carried out on an Instron testing machine with a strain rate 10−3 s−1 (the conditions for the samples were as follows: R stands for as-forged conditions, HT for as-annealed and asaged ones, TST for as-exposed ones.). Hardness experiments were carried out on an 450SVD Digimatic Vickers hardness tester. The loading weight is 10 kg and the duration was 30 s. The optical microstructure (OM), X-ray diffraction (XRD), transmission electronic microscope (TEM) microstructure and scanning electronic microscope (SEM) micrographs were separately carried out on OLMPUS PMG, PW1700 X-ray diffraction instrument, JEM-200CX transmission electron microscope and S-2700 scanning electronic microscope. The etchant consists of 10 vol.%HF, 30 vol.%HNO3 and 50 vol.%H2 O. 3. Results and discussion 3.1. Mechanical properties The mechanical properties for as-forged (R), as-annealed and aged (HT) and as-exposed (TST) at 550 ◦ C for different times are listed in Table 2. Fig. 1 shows the relationship between mechanical properties and thermal exposure times. It can be seen that strengthening effect can be got for as-annealed and as-aged samples (the region in the front of “Line a” in Fig. 1). During thermal exposure process, with increasing time, the ultimate tensile strength (UTS) slightly increased at first and then decreased. After thermal exposure for 50 h the UTS was irregularly jumping. The elongation (El) decreased and hardness (HV) slightly increased within all the exposure times. The change of El, HV and UTS before thermal exposure for 50 h is coinciding with the theory discussed in reference [17]. In that paper, the authors pointed out that the heat treatment of titanium alloy belongs to the third kind of heat treatment; the ageing treatment of titanium alloy is a precipitation strengthening process. When the time of thermal exposure is beyond 50 h, the reason of jumpi-
ness of UTS will be discussed below. As shown in Fig. 1, when the alloy exposed at 550 ◦ C for different times, the best time for the alloy with favourite mechanical properties should be no more than 10 h (between “Line a” and “Line b” in Fig. 1). If the duration time was longer than 50 h, the mechanical properties would deteriorate seriously, especially after 100 h, the specimens broke into many sections, and then the elongation and reduction in area for the alloy cannot be detected. 3.2. Microstructures 3.2.1. OM images Fig. 2 shows the OM microstructures under various conditions. Compared with Fig. 2a and b, it can be found that the microstructures of as-forged specimens and as-heat treated ones were almost the same. The grain boundaries for the latter were just smooth and sharp, which indicates that the annealing and ageing treatment for Ti40 alloy should only adjust the microstructures of as-forged specimens and cannot produce obvious strengthening effect. Compared with thermal exposed microstructures in different times (Fig. 2b–e), with the increasing exposure times there were increasing precipitates, especially the exposure time being over 50 h, the obvious changes were found in microstructures, the grain boundaries clearly broaden and precipitate-free zones were found adjacent to grain boundaries (as shown in Fig. 2d and e). The change of microstructure with increasing thermal exposure times are corresponding with the change of mechanical properties in Fig. 1. From references [11,12], it can be known that the precipitates are mainly ␣ and Ti5 Si3 . In order to confirm the content of the precipitations, XRD test was carried out. As shown in Fig. 3, after thermal exposure for 500 h, only matrix 
Table 2 Mechanical properties of Ti40 alloy after thermal exposure at different times Conditions
UTS (MPa)
Y.S. (MPa)
El (%)
R.A. (%)
HV
R HT 550 ◦ C/5 h 550 ◦ C/10 h 550 ◦ C/20 h 550 ◦ C/50 h 550 ◦ C/100 h 550 ◦ C/200 h 550 ◦ C/500 h
950 970 975 980 970 960 755 770 885
935 940 940 945 940 945 / / /
18.0 23.5 22.5 22.0 10.0 3.3 1.5 / /
54.0 42.0 33.5 30.5 13.0 3.2 2.0 / /
300.5 314.2 316.9 316.0 304.2 308.3 311.4 314.1 315.7
UTS, ultimate tensile strength; Y.S., yield strength; El, elongation; R.A., reduction of area; /, performance cannot be tested for complete brittle fracture.
Fig. 1. The relationship between mechanical properties (UTS, El and HV) and thermal exposure times for Ti40 alloy.
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Fig. 2. OM micrographs after different thermal exposure: (a) R; (b) HT; (c) (d) (e) are, respectively, thermal exposure at 550 ◦ C for 20, 50 and 500 h.
diffraction peak could be found in the XRD pattern, which indicates that very small fraction of ␣ and Ti5 Si3 phase were formed during thermal exposure process, and their fractions in  matrix was less than about 5% (wt%), and cannot be detected in XRD test. The changes of hardness and strength shown in Table 2 also show the same tendency. The hardness curve (HV) in Fig. 1 only increased a little with increasing thermal exposure times, and the
strengthening regions during thermal exposure (between “Line a” and “Line b” in Fig. 1) indicated that the strengthening effect was weak, which indicates that only a few  phases decomposed during thermal exposure and precipitation strength evenly could not be found. The results can be accepted because Ti40 alloy is a full- titanium alloy with higher Mo-equivalent (42) and lower phase transformation temperature, which leads to  microstruc-
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Fig. 3. XRD pattern of Ti40 alloy after thermal exposure at 550 ◦ C for 500 h.
ture with an extreme high stability. The microstructure keeps relative stability during thermal exposure which is an important characteristic of Ti40 alloy. It is well known that the strengthening effect of second-phase is dominated by three factors when the composition of secondphase has been known. The first one is the size of second-phase; the second one is the content of second-phase; the third one is the distributions of second-phase. In order to get improved comprehensive mechanical properties, a microstructure should be expected to get with fine, homogenous and a certain content second-phase after ageing treatment. In this experiment, the above results have proved that only a few second-phases can be got during thermal exposure and the deterioration of mechanical properties after thermal exposure for 50 h is mainly due to the distributions and size of precipitates. 3.2.2. SEM and TEM images Fig. 4 shows the SEM images of Ti40 alloy exposed at 550 ◦ C for 10, 50, and 100 h, respectively. It can be shown that the precipitates mainly distributed on grain boundaries, and there were precipitates-free zones adjacent to grain boundaries (it also can be shown in Fig. 2). The distributions of precipitates are also due to the stability of  phases, which leads to an increase of precipitated energy and the relative reduction of nucleation sites. Then more precipitates would nucleate on grain boundaries but not at vacancies or dislocations which plentifully exist inside grains. The formation of precipitates-free zones also proves that nucleation at vacancies is more difficult than at grain boundaries. In the initial stage of thermal exposure, there were a few ␣ and Ti5 Si3 phases nucleated on grain boundaries and few sites inside grains, which produced a certain strengthening effect and nearly had no effect on plasticity for the alloy, which corresponds with the region in between “Line a” and “Line b” in Fig. 1. With the increasing thermal exposure times, the precipitates on grain boundaries would grow up and spread along grain boundaries. Finally, continuous precipitated zones were formed along grain boundaries. As clearly shown in Fig. 5a and b there were continuous precipitated zones along grain boundaries for the alloy thermal exposed for 200 h, and precipitates had grown up to a
Fig. 4. SEM micrographs of Ti40 alloy after thermal exposure at 550 ◦ C for different times: (a) 10 h, (b) 50 h and (c) 100 h.
certain degree. After thermal exposure for 500 h (Fig. 5c), grainboundary precipitates kept growing up and in some sites there were diamond precipitates with 0.8 m edges. The above process of thermal exposure can be simulated by Fig. 6. The whole process can be divided into three stages. The first one is the initiation stage of precipitates, corresponds to the exposure time with no less than 10 h (the region between “Line a” and Line b in Fig. 1); the second one is growing up of the precip-
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Fig. 5. TEM bright-field images of precipitations at grain boundary after thermal exposure at 550 ◦ C for 200 h (a and b) and 500 h (c).
Fig. 6. Simulated diagrams of precipitating and growing processes of precipitation at grain boundaries.
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itates, corresponds to exposure time from 10 to 50 h (the region between “Line b” and “Line c” in Fig. 1); the third one is the formation of the precipitated zones, correspond to exposure time greater than 50 h (the region is after “Line c”). Understanding the precipitated characteristics for Ti40 alloy thermal exposure at 550 ◦ C for various times, it is easily comprehended that, after thermal exposure for 50 h, there is non-matched mechanical properties between strength (“UTS” curve in Fig. 1) and hardness (“HV” curve in Fig. 1). Because precipitated zones along grain boundaries produce a little effect on hardness but produces a stronger effect on tensile strength leading to unstable tensile strength (the jumpiness of “UTS” curve after thermal exposure for 50 h in Fig. 1). 3.2.3. SEM fractographic images Fig. 7 shows the tensile fractographs of Ti40 alloy thermal exposed at 550 ◦ C for 5, 20 and 200 h. It can be shown that cracks were originated from grain boundaries and the better plasticity was resulting from the dimples in transcrystalline fracture within grains, which reveals the precipitates at grain boundaries deteriorate thermal stability and precipitates within grains produce little effect on the plasticity. Compared with the fractograph of thermal exposure for 5 h (Fig. 7a), there were more intergranular fractures after thermal exposure for 20 h (Fig. 7b) which indicates the increasing precipitates on grain boundaries with increasing thermal exposure times. The fractograph after thermal exposure for 200 h (Fig. 7c) shown complete intergranular fracture, which indicates the continuous precipitated zones have been formed. 4. General discussion The above results suggest that Ti40 ring cannot be used at 550 ◦ C under present treat conditions. The continuous precipitated zones along grain boundaries are the main factor to restrict its use under high temperature. For the alloy used at higher temperature, two factors must be considered based on the forming mechanism of the precipitated zones. Firstly, it is the sites of precipitates; secondly, it is the amount of precipitates. For factor one, the above analysis has pointed out that the main reason of second-phases at grain boundaries is the compositions of Ti40 (high Mo-equivalent leads to a high precipitated energy of second-phases) and higher thermal exposure temperature (higher thermal exposure temperature leads to increasing required nucleation energy). It is obvious that the changes of the composition or the decrease of thermal exposure temperature are no any meaning. The composition of Ti40 was designed for getting good burn resistant property. The research of thermal stability must be on the base of present composition. And this paper aims at how to use Ti40 alloy at a higher temperature (550 ◦ C). Thus, the decrease of thermal exposure temperature is forbidden. Even so, the distributions of precipitates can be optimized by adjusting nucleation sites. Judging from above analysis, the quenching at high temperature cannot be accepted. Although the increases of vacancy density within grains by quenching at high temperature can lead to an increasing nucleation sites within grains, quenching at high temperature also leads to an
Fig. 7. SEM fractographs of Ti40 alloy after thermal exposure at 550 ◦ C for 5 h (a), 20 h (b) and 200 h (c).
increasing amount of precipitates at grain boundaries. But also, the vacancies will diffuse from inside grains to grain boundaries during thermal exposure and which also maybe lead to an increase of precipitates at grain boundaries. Considering all factors, pre-ageing treatment at low temperature may be a feasible method. or  metastable phases will maybe precipitate for the alloy aged at lower temperature (for Ti40 alloy the temperature maybe range from 360 to 420 ◦ C). Because of the low diffusion coefficient and required nucleation energy, the metastable phases
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will dispersedly precipitate within grains, the sequent thermal exposure at high temperature will lead to ␣ phases nucleate at the sites of metastable phases or the metastable phases directly transform into ␣ phases, which will lead to a homogeneous distribution of ␣ phase within grains. It is prospective to improve the thermal stability by this way. To the second factor about the amount of precipitates, it can be considered in two ways. The first is to reduce the amount of precipitates in unit grain boundary by the ways of refining grains and increasing the amount of grain boundaries. The above analysis indicates that it cannot be achieved by heat treatment. The coarse equiaxed grains of as-forged samples indicate the alloy cannot recrystallize during heat treatment and there is nearly no difference in grains between as-forged samples and as-heat treated samples. The forged technology also cannot be adjusted for the poor workability of Ti40 alloy. Thus, the way of refining grains and increasing the amount of grain boundaries is unacceptable. The second way is to decrease the effect of harmful elements. In Ti40 alloy the element is oxygen element. Being harmful element it diffuses from inside grains to grain boundaries and improves the precipitation of ␣ phase during thermal exposure. Publications [15,18] have proved that in Ti–V–Cr alloy the addition of carbon element can effectively reduce the harmful effect of oxygen element and significantly improve the thermal stability. The authors think the addition of carbon element into Ti40 is an effective way to improve thermal stability at higher temperature. 5. Conclusions (1) There are only a few precipitates during thermal exposure, which mostly distribute on grain boundaries. With the increase of thermal exposure times the precipitates gradually evolve into continuous precipitated zones along grain boundaries. The distributing characteristic of precipitates is the main reason for Ti40 alloy to have a bad thermal stability at 550 ◦ C.
(2) The experimental results indicate that Ti40 ring cannot be used for 550 ◦ C under present treated conditions. The adjustment of thermal treatment conditions or carbon addition may be the feasible methods to increase its usage temperature. Acknowledgement The authors would like to acknowledge the financial support of Chinese National Key Project (MKPT-01-101) and 973 Project (51333). References [1] D.M. Berczik, US Patent 516,762, 1993-01-05. [2] G. Kneisel, Adv. Mater. Process. 9 (1993) 7. [3] Y.G. Li, X.D. Zhang, P.A. Blenkinsop, et al., in: P.A. Blenkinsop, W.J. Evens (Eds.), Proceedings of the 8th Word Conference on Titanium, The Institute of Materials, London, 1996, p. 2317. [4] X. Huang, L.M. Lei, F.S. Sun, et al., Rare Met. Mater. Eng. 33 (2004) 218. [5] Y.Q. Zhao, K.Y. Zhu, H. Wu, China Defense Patent. Application No. 97112302.0. [6] Y.Q. Zhao, L. Zhou, G.Z. Luo, Rare Met. Mater. Eng. (1996) 1. [7] Y.Q. Zhao, L. Zhou, J. Deng, Mater. Sci. Eng. A 267 (1999) 167. [8] Y.Q. Zhao, L. Zhou, J. Mater. Sci. Technol. (2001) 677. [9] Y.Q. Zhao, L. Zhou, Acta Metall. Sin. 1 (2001) 406. [10] Y.Q. Zhao, H.L. Qu, K.Y. Zhu, et al., J. Alloys Compd. 333 (2002) 165– 169. [11] K.Y. Zhu, Y.Q. Zhao, H.L. Qu, et al., J. Mater. Sci. 39 (2004) 2387–2394. [12] Y.Q. Zhao, H.L. Qu, K.Y. Zhu, et al., J. Mater. Sci. 38 (2003) 1579–1584. [13] Y.Q. Zhao, H.L. Qu, M.M. Wang, et al., J. Alloys Compd. 407 (2006) 118–224. [14] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, N.A. Walker, Acta Metall. Inc. 46 (1998) 5777–5794. [15] Y.G. Li, M.H. Loretto, D. Rugg, W. Voice, Acta Mater. 49 (2001) 3011–3017. [16] Y.Q. Zhao, K.Y. Zhu, H.L. Qu, et al., Mater. Sci. Eng. A 282 (2000) 153–157. [17] S.W. Xin, Zhao FY.Q., Heat Treat. Met. 31 (2006) 39–42. [18] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, D. Rugg, W. Voice, Acta Mater. 47 (1999) 2889–2905.