Microstructures of a burn resistant highly stabilized β-titanium alloy

Materials Science and Engineering A282 (2000) 153 – 157 www.elsevier.com/locate/msea

Microstructures of a burn resistant highly stabilized b-titanium alloy Y.Q. Zhao a,*, K.Y. Zhu a, H.L. Qu a, H. Wu a, L. Zhou a, Y.G. Zhou b, W.D. Zeng b, H.Q. Yu b a

Northwest Institute for Nonferrous Metal Research, PO Box 51, Xi’an, Shaanxi 710016, PR China b Northwest Polytechnical Uni6ersity, Xi’an PR China Received 8 September 1999; received in revised form 8 November 1999

Abstract Ti40 alloy (Ti–25V–15Cr–0.2Si) is a highly stabilized b-titanium alloy with good burn resistance. Its microstructures were examined and analyzed by OM, SEM, TEM, XRD and EDX. The results reveal that Ti40 is a single b-phase alloy. There are quad-point boundaries and a white zone around the grain boundaries in the as-cast structures, leading to difficulties in ingot breakdown. The recrystallization finishing temperature is 820°C, and the second recrystallization is at 1100°C. The subgrains emerge if solution treatment temperature is greater than 910°C and aging at 600°C, which leads to a good combination among tensile properties. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Burn resistant titanium alloys; Ti40 alloy; Microstructures

1. Introduction Research on b-titanium alloys can be dated back to 1950s from the studies of Ti-13-11-3 alloy. More than 20 kinds of b-alloys have been developed, and among them the famous b-alloys are Ti-15-3, b21s, Ti-10-2-3, SP700, bcez and so on. However, b-Ti alloys applied to practical uses are very limited. These alloys account for only 1% of the total Ti market [1]. But their excellent cold workability and good combinations of strength and fracture resistance drive the development of b-Ti alloys. For example, b21s and SP700 were designed in the early of 1990s and have been put into practical applications [2]. Alloy C (Ti – 35V – 15Cr) [3,4], developed in the USA in the early 1990s, is a highly stabilized b-Ti alloy with good burn resistance, which is to replace Ni base alloys and applied to modern advanced aero engines. It has been successfully used in the F119 engine that powers the F22 USAF jet fighter [5,6]. In China, burn resistant Ti-alloys have been researched by * Corresponding author. Tel.: +86-29-6231078; fax: + 86-296231103. E-mail address: [email protected] (Y.Q. Zhao)

the group of science and technology lead by the first author since 1993. Ti40 (Ti–25V–15Cr–0.2Si) [7,8], a burn resistant Ti alloy, was developed in 1996. The authors have systemically studied its burn resistant mechanism [9–12], elevated temperature deformation mechanism [12,13] and the burning behavior of conventional Ti alloys [11,12,14,15]. Ti–25V–15Cr alloy is a stable b-alloy [16], however, the microstructures of Ti40 alloy were not reported. Dr Y.G. Li et al. studied the structures of Ti–25V–15Cr–xAl and Ti–25V–15Cr– xAl–xC [17–19], which revealed that there were a and v phases precipitates leading to brittleness. The aim of this paper is to study the structures of Ti40 alloy. Its microstructures are varied and interesting images, and some ones are seldom seen in the conventional Ti alloys.

2. Experimental procedures A 5-kg Ti40 alloy ingot was used in this study. Its chemical composition was Ti–24.6V–14.6Cr–0.4Si. As ingot breakdown was very difficult [7,8,12,20], the ingot breakdown was conducted clad with a carbon steel.

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 7 6 1 - 3

154

Y.Q. Zhao et al. / Materials Science and Engineering A282 (2000) 153–157

Fig. 1. As-cast structures of Ti40 alloy (a) as-cast (b) solution treatment at 1050°C for 10 min.

Fig. 4. The grain boundaries after solution treated at 750°C, consisting of small grains. Table 1 EDX compositional data of Fig. 3f (wt.%) Fig. 2. X-ray diffraction spectrum of as-cast structure.

After different temperature solution treatment (300– 1100°C per 30 min W.Q.), solution, and aging at 600°C for 5 h, the microstructures were examined by optical micrograph (OM), JSM-5800 scanning electron microscopy (SEM), and H-600 transmission electron microscopy (TEM). Energy dispersive X-ray (EDX)

Element

Ti

V

Cr

Si

GB Matrix

90.61 60.69

6.76 24.54

2.39 15.18

0.19 0.42

analysis was carried out in the SEM. X-ray diffraction (XRD) was carried out on a Philips PW1700 diffractometer with CuKa as the radiation source.

Fig. 3. OM images of the forged samples and solution at different temperature for 30 min (a) as-forged; (b) 300°C (c) 750°C; (d) 820°C (e) 950°C; (f) 1100°C.

Y.Q. Zhao et al. / Materials Science and Engineering A282 (2000) 153–157

155

3. Results and discussions

3.1. As-cast structures Fig. 1 shows the as-cast structure and the structure after solution at 1050°C for 10 min. The as-cast structure consists of a single b-phase (Fig. 2) with subgrains. The grain boundaries (GB) are thin, and there is a

Fig. 8. X-ray diffraction spectrum of Fig. 7b.

Fig. 5. Non-continuous grain boundaries after solution at 1100°C for 30 min.

Fig. 9. SEM image of samples solution treated at 950°C for 30 min and aged at 600°C for 5 h. Table 3 EDX compositional date of the GB, the white zone in grains and matrix in Fig. 9 (wt.%)

Fig. 6. The network structure after solution at 1050°C for 1 h. Table 2 EDX compositional date of the network in Fig. 6 (wt.%) Position

Ti

V

Cr

Si

O

1 2 3

1.76 5.23 62.3

0.61 1.47 23.14

0.35 46.87 14.23

38.3 0.10 0.33

58.09 46.18 0.14

Fig. 7. OM images of samples solution treated at 860°C (a) and 950°C (b) for 30 min and aged at 600°C for 5 h.

Position

Ti

V

Cr

Si

GB Grain Matrix

60.12 59.15 61.47

25.77 26.70 25.06

13.67 13.80 13.02

0.44 0.35 0.45

white zone around the GB. After solution treatment, the grains become coarse because of grain growth at high temperature. Ti alloy grains are typically triplepoint GB, however, the as-cast structures displayed quad-points. These quad-points more easily induce stress concentration leading to cracks, which propagates along GB. On the other hand, the white zone around the GB indicates that there is micro-segregation of the compositions, which leads to difficulty of the coordination deformation between grains and GB. The cracks selectively propagate along GB because of the weakness of GB. The combined effects of the quadpoints and the white zone resulted in difficulty of the ingot breakdown.

Y.Q. Zhao et al. / Materials Science and Engineering A282 (2000) 153–157

156

3.2. Structures of solution treatment Fig. 3 shows the OM images of the forged samples and samples after solution treatment at different temperatures for 30 min. The grains are broken after forging. Solution temperatures below 780°C, the samples are the same types of structures: coarse b grains and wide GB. Detailed examinations of GB indicate that the GB consists of many small grains, as shown in Fig. 4. These small grains reveal that the formation of new grains from recrystallization starts from GB and grows towards the grain interior. The recrystallization finishes at 820°C (Fig. 3d). The second recrystallization take place at 1100°C, and its GB is very coarse, 40 mm in size. There are many subgrains within the coarse grains. The coarse GB is rich in Ti and poor in V, Cr and Si, as shown in Table 1. These results demonstrate that alloying elements reconstitute during the solution treatment: alloying elements such as V, Cr and Si diffuse into grains from GB leading to Ti enriching on GB. The higher the solution temperatures, the higher the Ti content in the GB. However, there are no a-phase precipitates even after solutioning at 1100°C. But the GB is not continuous (Fig. 5) and there are signs that a-phase precipitates lead to GB weakness. A very special structural image emerges when the Ti40 alloy is broken down at 950°C and solutionized at 1050°C for 1 h, as shown in Fig. 6. It resembles a

Fig. 10. TEM structure of samples solution treated at 950°C for 30 min and aged at 600°C for 5 h. Table 4 Tensile properties at RT of samples solution treated at different temperatures for 30 min and aged at 600°C for 5 h Solution temperature (°C)

UTS (MPa)

YS (MPa)

EL (%)

RA (%)

820 860 910

950 960 990

940 940 960

15 16 24

36 30 34

network crack, however, it is not so. This profile is found all over the sample surface, and is not confined to a local zone. The chemical compositions in the different positions of the network profile are different, as demonstrated in Table 2. Positions 1 and 2 are on the network. The former is SiO compound, and the latter is CrO compound. Position 3 is the matrix. The oxygen is introduced by the long time heating and forging at 950°C because the weight gains are 88 mg cm − 2 while Ti40 alloy oxidized at 900°C for 10 h. The oxygen causes the alloying elements to reconstitute quickly, and compounds of SiO and CrO are formed. Their growth is fast and vertical to each other. Why are there not compounds of TiO and VO, besides SiO and CrO compounds? The compounds formed from reactions of oxygen and vanadium are V2O5, firstly. The melting point of V2O5 is 675°C. It will volatilize while Ti40 is forged at 950°C and solutionized at 1050°C. Therefore there are not VO compound. The electronegativity from small to big is TiBSiBCr. So TiO compounds should be selectively formed. Why there are no TiO compounds is not clear. In order to get good combinations of mechanical properties, the solution treatment temperature should be in the range 820–950°C.

3.3. Structures of solution treatment and aging Fig. 7 shows the OM images of samples solution treated at 860 and 950°C for 30 min and aged at 600°C for 5 h. They are equiaxed single b-phase structures (Fig. 8). The examinations of selected area diffraction (SAD) further reveal this result. The lattice parameter of the b-phase is a=3.039 nm calculated from Fig. 8. The small particles in Fig. 7b are subgrains. It can be seen from the SEM images (Fig. 9) that there are lots of small particles in the grains and on the GB. However, their compositions are almost the same as the base (as shown in Table 3), which indicates that these particles are not precipitates but are subgrains. It can be seen more clearly in the TEM images (Fig. 10). That is to say, with increasing of solution temperatures, the grains become coarse and the subgrains emerge. The appearance of subgrains leads to good combinations of tensile plasticity and strength, as shown in Table 4. The subgrains are similar to that of the alloy, but have finer grains. Tensile deformation begins firstly from individual subgrains by slip. The slip from one subgrain to another close one is easier than that from one grain to another close one. With increasing of deformation amount, the slip occupies more and more subgrains, and gradually grows to the grains. Therefore, the deformation and growth of cavity are retarded, and big deformation can be gotten before fracture. This is to get the aims of strengthening and toughness together by the fine grains.

Y.Q. Zhao et al. / Materials Science and Engineering A282 (2000) 153–157

4. Conclusions (1) There are quad-point boundaries and white zone around the grain boundaries in the as-cast structures of the Ti40 (Ti–25V – 15Cr – 0.2Si) alloy, leading to difficulties in ingot breakdown; (2) The recrystallization finishing temperature is 820°C, and the second recrystallization is at 1100°C; (3) There are subgrains if solution treatment temperature is greater than 910°C and aged at 600°C, which leads to a good combination of tensile properties.

References [1] P.J. Bania, in: D. Eylon, R.R. Boyer, D.A. Koss (Eds.), Beta Titanium Alloys in the 90s, TMS Publications, Warrendale, PA, 1993, p. 3. [2] R. Boyer, G. Welsch, E.W. Collings (Eds.), Materials Properties Handbook — Titanium Alloys, The Materials Information Society, USA, 1994, pp. 685–930. [3] UK Patent Application, 2 238057, 1987. [4] K. Giary, Adv. Mat. Proc. 9 (1993) 7. [5] S.R. Seagle, Mat. Sci. Eng. A 213 (1996) 1. [6] R.R. Boyer, Mat. Sci. Eng. A 213 (1996) 103.

.

157

[7] Y.Q. Zhao, K.Y. Zhu, M.X. Zhao, Chinese Patent Application 97 112303.9, 1997. [8] Y.Q. Zhao, Research on Burn Resistant Titanium Alloys — Final Research Report, Northwest Institute for Nonferrous Metal Research, 1996. [9] Y.Q. Zhao, L. Zhou, J. Deng, Rare Metal. Mat. Eng. 2 (1999) 77. [10] Y.Q. Zhao, L. Zhou, J. Deng, Mat. Sci. Eng. A 267 (1999) 167. [11] Y.Q. Zhao, L. Zhou, J. Deng, in: Proc. 9th World Conference on Titanium, 2000, Russian (in press). [12] Y.Q. Zhao, Doctoral Thesis, Northeastern University, 1998. [13] Y.Q. Zhao, L. Zhou, J. Deng, in: Proc. 9th World Conference on Titanium, 2000, Russian (in press). [14] Y.Q. Zhao, L. Zhou, J. Deng, J. Alloys Compound. 284 (1999) 190. [15] Y.Q. Zhao, L. Zhou, J. Deng, in: L. Zhou, D. Eylon (Eds.), Proc. Xi’an International Titanium Conference, Xi’an, 1999, p. 696. [16] Y.Q. Zhao, Chin. J. Nonferrous Metals 3 (1998) 463. [17] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, Mat. Sci. Technol. 14 (1998) 732. [18] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, Acta. Mater 16 (1998) 5777. [19] Y.G. Li, P.A. Blenkinsop, M.H. Loretto, in: Proc. 9th World Conference on Titanium, 2000, Russian (in press). [20] P.A. Russo, S.R. Seagle, in: P.A. Blenkinsop, W.J. Evans, H.M. Flower (Eds.), Titanium;95: Science and Technology, UK, p. 841