Desorption of Ti600, TC21 and Ti40 Alloys

Rare Metal Materials and Engineering Volume 39, Issue 9, September 2010 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2010, 39(9): 1509−1512.

ARTICLE

Behavior of Hydrogen Absorption/Desorption of Ti600, TC21 and Ti40 Alloys Wang Xiaoli1,2, 1 3

Zhao Yongqing2,

Zeng Weidong1,

Northwestern Polytechnical University, Xi’an 710072, China;

2

Hou Hongliang3,

Wang Yaoqi3

Northwest Institute for Nonferrous Metal Research, Xi’an 710016, China;

Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China

Abstract: The behavior of hydrogen absorption/desorption of Ti600, TC21 and Ti40 alloys was studied. The results show that the initial temperatures of hydrogen absorption/desorption for Ti600, TC21 and Ti40 alloy are 573 and 578.5 °C, 580 and 628.1 °C, 515 and 540 °C, respectively. The behavior of hydrogen absorption/desorption of Ti600 alloy is similar to that of TC21 alloy. The ability of hydrogen absorption/desorption of Ti40 alloy is better than the others’. Key words: titanium alloys; hydrogen; initial temperature of hydrogen absorption/desorption

Titanium alloys are applied to aviation, space, energy etc due to their advantages, including good mechanical properties, thermal properties, excellent corrosion resistance and so on. However, the hot forming of titanium alloys restricts their application. The researchers focus their attention on processing ability improvement, such as near-β forging technology and β forging technology. Recently, an increased understanding of titanium metallurgy demonstrate that if used correctly, hydrogen as a temporary alloying element can benefit improvement of processing ability and microstructure/mechanical properties of titanium alloys[1-3]. Hydrogen as a temporary alloying element possesses high sorption capacity and diffusivity, which has a strong effect on phase transformation[1-5]. The thermohydrogen processing (THP) is based on the modifying effect of hydrogen as an alloying element on phase compositions, development of metastable phases, and kinetics of phase transformations in titanium alloys[3]. Firstly, hydrogen can be charged into titanium alloys to perform thermo-mechanical treatments and forming processes, and then be removed by vacuum annealing. In order to determine the hydrogen absorption/desorption temperature and to optimize the processing parameters of thermohydrogen processing, the behavior of hydrogen absorption/desorption of titanium alloys has been investigated. The

overall reaction of hydrogen absorption is composed of several steps[6-9], involving gas-phase mass transport of H2 molecules up to the surface of alloy, physisorption of H2 molecules and hydrogen dissociation and chemisorption on the surface, surface penetration by H atoms, transition of H atoms from their chemisorbed state into their soluted state, hydrogen diffusion in the α or β phase and motion of the α/β interface. Hydrogen desorption is generally divided into several steps consisting of decomposition of hydrides, hydrogen diffusion from α or β phase, interstitial sites and flaws to the surface, transition of H atoms from their soluted state into their chemisorbed state, and hydrogen atoms bonding into hydrogen molecules at the surface. In this work, the results on the behavior of hydrogen absorption/desorption in three types of titanium alloys were presented. According to the hydrogen absorption/desorption characteristics of these titanium alloys, the effects of different phases and the microstructure on the hydrogen absorption/desorption were investigated.

1 Experimental The specimens used in this work were flat sheet of near-α titanium alloy (Ti600), α+β titanium alloy (TC21) and β titanium alloy (Ti40) with dimensions of 100 mm×25 mm×2 mm.

Received date: September 17, 2009 Foundation item: National Defence “973” Project (2007CB613807) Corresponding author: Wang Xiaoli, Ph. D., Northwestern Polytechnical University, Xi’an 710072, Tel: 0086-29-88474096, E-mail: [email protected]; Zhao Yongqing, Ph. D., Professor, E-mail: [email protected] Copyright © 2010, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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2.1 Hydrogen absorption of titanium alloys Hydrogen absorption performances of titanium alloys are shown in Fig.1. The initial temperature of hydrogen absorption and the hydrogen pressure of saturation state for Ti600, TC21 and Ti40 alloys are 573, 580, 515 °C and 15.13, 15.5, 13.25 kPa, respectively. The initial temperature of hydrogen absorption and the hydrogen pressure of saturation state for Ti600 alloy are similar to those of TC21 alloy, which are higher than those of Ti40 alloy. After saturating, the hydrogen pressure begins to increase with increase of the temperature, and the final pressures are different among the three types of titanium alloys. The final pressure of Ti40 alloy is higher than those of Ti600 and TC21 alloys. From the initial temperature of hydrogen absorption to the temperature of hydrogen saturation, hydrogen absorption

Hydrogen Pressure, P/kPa

25 580 ℃ 515 ℃ 573 ℃

Ti40 Ti600 TC21

20

15

585 ℃

600

700

800

Temperature/℃ Fig.1 Hydrogen absorption curves of titanium alloys

a

22 k=–0.0283 I

II

k= –0.09747

20 18

III

k=–0.20525

16 570

580

600

615

630 b

22 20 k= –0.08634

18 16 600

625

650

675

k= –0.07649

22 I

c

II

20 18

k= –0.24572

16

k=–0.07774 III

14 520

650 ℃ 684 ℃

500

Hydrogen Pressure/kPa

Results and Discussion

Hydrogen Pressure/kPa

2

curves are linear fitting, as shown in Fig.2. The slopes demonstrate the change rate of hydrogen pressure. The slope of Ti600 alloy has three segments. The values of slopes increase gradually, which are –0.0283, –0.09747 and –0.20525, respectively. The curve of change rate for Ti40 alloy also consists of three parts: I, P=61.92961-0.07649T (515-525 °C); II, P=150.88433–0.24572T (525-555 °C); Ⅲ, P=57.94122– 0.07774T (555-575 °C). The curve of change rate of TC21 alloy just has one segment and its value of slope is –0.08634. It is observed that the relationship of values for slopes follows k-(Ti40)>k-(Ti600)>k-(TC21) at middle temperature. Fig.3 shows reaction fraction of hydrogen absorption in titanium alloys at 700 °C. Fig.1 shows that the initial temperature of hydrogen absorption and the hydrogen pressure of saturation state for Ti40 alloy are the lowest in the three types of titanium alloys. Fig.3 shows that the reaction fraction α of hydrogen absorption follows the relationship α-(Ti40)>α-(Ti600)>α-(TC21). It means that the rate of hydrogen absorption of Ti40 alloy is the high-

Hydrogen Pressure/kPa

The specimens were mechanically polished to remove the surface oxide layer and ultrasonically cleaned with acetone in order to keep the specimens surface finish. A tubular thermohydrogen processing furnace was used to perform the hydrogen absorption/desorption kinetics. The specimens were heated to 450 °C in vacuum atmosphere, and a controlled amount of hydrogen with ultra-high purity was filled in the chamber. The change of hydrogen pressure in the furnace was recorded automatically. Before dehydrogenation experiments, the specimens were pre-charged with hydrogen at a constant temperature of 750 °C for 1 h and then air cooled to room temperature. The initial hydrogen content in specimens was about 1.1% (mass fraction). Hydrogenated specimens were introduced into a vessel made of stainless steel. Firstly, the cycles of evacuating and filling with hydrogen were repeated twice at room temperature. The system was evacuated to a pressure below 10-3 Pa and the vacuum system was shut down at 450 °C. By heating specimens to a given temperature, the dehydrogenation started and hydrogen pressure in the reaction vessel increased. According to the recorded hydrogen pressure with increase of the temperature, the initial dehydrogenation temperature could be obtained.

530

540

550

560

570

Temperature/℃ Fig.2 Change of hydrogen pressure with temperature for titanium alloys: (a) Ti600, (b) TC21, and (c) Ti40

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Wang Xiaoli et al. / Rare Metal Materials and Engineering, 2010, 39(9): 1509−1512 Ti600 700 ℃ TC21 Ti40

0.9

α

0.6

0.3

0.0

0

4

8

12

16

20

Time/min Fig.3 Reaction fraction of hydrogen absorption in titanium alloys at 700 °C

est among the three types of titanium alloys under the same condition. The reasons are as following. Firstly, the radius of hydrogen atom is 0.046 nm and it always exists in interstitial site. The distortion energy is the lowest when hydrogen as interstitial atom exists in interstitial site which has the biggest interstitial radius. Hence, hydrogen atom can be in tetrahedral interstitial site in bcc and octahedral interstitial site in hcp. Besides distortion energy, chemical bonds are formed between hydrogen and metal atom when hydrogen concentration exceeds the solubility of hydrogen in α-Ti or β-Ti, which can reduce the system energy. The result of the competition in the two kinds of energy can determine the existence form of hydrogen. The microstructure of Ti600 alloy is almost of α phase, while a little amount of β phases exists in TC21 alloy. As seen from the Ti-H phase diagram in Fig.4, the solubility of hydrogen in α phase is small. When hydrogen concentration exceeds the solubility, hydrides are formed. Therefore, the rate of hydrogen absorption in Ti600 alloy is higher than that of TC21 alloy. Secondly, the activation energy of hydrogen absorption in α-Ti[10] and β-Ti[11] are 51.8 and 27.6 kJ/mol respectively. The lower the activation energy is, the higher the absorption rate is. Therefore, the rate of hydrogen absorption in Ti40 alloy is the highest. Thirdly, as shown in Ti-H phase diagram, in the range of 500-700 °C, the maximum solubility of hydrogen in α-Ti is about 7at%, but β-Ti is capable of absorbing hydrogen up to

Hydrogen Pressure, P/kPa

Temperature/℃

900 β

700

α+β

500

β+δ

α

300 100 0

300 ℃ 6.72

10

39 51.2

α+δ

20

30

40

50

H/at% Fig.4 Ti-H phase diagram[3]

50 at% at 600 °C[1-3]. The β-Ti alloy can absorb hydrogen at a low temperature, so the initial temperature of hydrogen absorption of Ti40 alloy is low. Finally, crystal structure of titanium alloys can affect the hydrogen diffusion. The diffusion activation energy is low when the density is small. The density of bcc structure is 0.68, which is smaller than 0.74 of hcp structure. Therefore, the diffusion activation energy of hydrogen atom in bcc structure is lower than that in hcp structure. As seen from the Ti-H phase diagram, the solubility of hydrogen in β phase is higher than that in α phase at a high temperature, so hydrogen concentration gradient for Ti40 alloy is higher than that for Ti600 alloy and TC21 alloy. Therefore, hydrogen atom can diffuse quickly in Ti40 alloy. 2.2 Hydrogen desorption of titanium alloys The hydrogen desorption curves of titanium alloys are shown in Fig.5. The hydrogen desorption curve of Ti40 alloy is different from the others. In the initial stage of hydrogen desorption, the pressure of hydrogen desorption for Ti600 alloy and TC21 alloy increases slowly, but when the temperature reaches a certain degree, such as 578.5 °C for Ti600 and 628.1 °C for TC21 alloy, the hydrogen pressure increases sharply and then continues increases with a certain rate. However, there exists no sharp rise of hydrogen pressure for Ti40 alloy and the slope of hydrogen desorption curve increases gradually. The reason for the above mentioned phenomena would be the solubility limit of α phase and β phase and the existence form of hydrogen in titanium alloys. There are three existence forms of hydrogen in titanium alloys, e.g. hydrogen molecules in flaws, hydrogen atoms as interstitial atom in tetrahedral or octahedral interstitial sites and hydrides. For near-α alloy Ti600, α phase can only absorb a little hydrogen and then titanium hydride will be formed under high hydrogen pressure ambient. With increasing of the temperature and the activity of titanium alloy surface, solution hydrogen atoms and hydrogen molecules in flaws will be released. The desorption curve of TC21 alloy is similar to that of Ti600 alloy, but its transition temperature is higher than that of Ti600 alloy. The reason would be that TC21 alloy contains a small amount of β phase which can absorb more hydrogen atoms. The microstructure

δ

60

40

Ti40

30

Ti600 TC21

20 10

578.5 ℃ 540 ℃

0 500

628.1 ℃

600

700

800

Temperature/℃ Fig.5 Hydrogen desorption curves of titanium alloys

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of Ti40 alloy is β phase, which can absorb a large amount of hydrogen atoms at high temperature. Therefore, the rate of hydrogen desorption for Ti40 alloy increases with increasing of the temperature.

3 Conclusions

Energy, 1999, 24(6): 565 2 Eliaz, N.; Eliezer, D.; Olson, D. L. Materials Science and Engineering, 2000, A289: 41 3 Froes, F. H.; Senkov, O. N.; Qazi, J. I. Mater. Rev., 2004, 49: 227 4 Han, Mingchen. Aerospace Materials & Technology, 1999, 1: 23 5 Hou, Hongling; Li, Zhiqiang. The Chinese Journal of Nonferrous Metals, 2003, 13(3): 533

1) The initial hydrogen absorption/desorption temperature and the hydrogen pressure in saturation state for Ti600, TC21 and Ti40 alloys are 573 and 578.5 °C, 580 and 628.1 °C, 515 and 540 °C, and 15.13, 15.5, 13.25 kPa, respectively. 2) The reaction fraction α of hydrogen absorption follows the relationship α-(Ti40)>α-(Ti600)>α-(TC21). It means that the ability of hydrogen absorption/desorption in Ti40 alloy is better than the others. These results are beneficial to improving the processing parameters of thermohydrogen process.

10 Parazoglou, T. P.; Hepworth, M. T. Transactions of the Metallur-

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