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The role of hydride, martensite and atomic hydrogen in hydrogen-induced delayed fracture of TiNi alloy
Materials Science and Engineering A364 (2004) 333–338
The role of hydride, martensite and atomic hydrogen in hydrogen-induced delayed fracture of TiNi alloy J.Y. He, K.W. Gao, Y.J. Su, L.J. Qiao, W.Y. Chu∗ Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, China Received 14 May 2003; received in revised form 19 August 2003
Abstract The role of atomic hydrogen, hydrogen-induced martensite and hydride in hydrogen-induced delayed fracture of a TiNi shape memory alloy during charging under a sustained load has been investigated using a notched tensile specimen. The results show that hydrogen-induced delayed fracture of the TiNi alloy can occur, and the normalized threshold stress intensity factor, KIH /KIC , decrease linearly with the logarithm of the total hydrogen concentration CT , i.e., KIH /KIC = 2.01−0.25 ln CT . The content of the hydrides increase gradually, and then the intrinsic fracture toughness of the hydrogenated specimen decrease sharply during dynamic charging. The formation of hydrides is the main reason of hydrogen-induced delayed fracture of the TiNi alloy. The role of atomic hydrogen and hydrogen-induced martensite in hydrogen-induced delayed fracture is very small. © 2003 Elsevier B.V. All rights reserved. Keywords: TiNi shape memory alloy; Hydrogen-induced delayed fracture; Atomic hydrogen; Hydrogen-induced martensite; Hydride
1. Introduction NiTi alloys are widely used in industrial and medical devices because of their unique shape memory and/or superelastic effect, excellent ductility, good fatigue life, good corrosion resistance and biocompatibility [1,2]. The influence of hydrogen on the shape memory or/and superelastic properties [3,4] and the mechanical properties [5,6] of TiNi alloys has been reported. Recently, hydrogen-induced delayed fracture under constant load has been investigated using smooth tensile specimen [7]. The result showed that irreversible martensite transformation increased the susceptibility to hydrogen embrittlement, but the stability of the TiNi alloys to reversible martensite transformation also affected the susceptibility even when the applied stress was lower than the critical stress for martensite transformation [7]. The effect of hydrides and atomic hydrogen on the delayed fracture during dynamic charging, however, were not considered [7]. Hydrides, such as TiNiH [8,9], TiNiH1.4 [10] and the others [3], and hydrogen-induced martensite [4,11] can form during charging of TiNi alloys. On the other hand,
atomic hydrogen can cause hydrogen-induced delayed fracture under sustained load for steels and aluminium alloys [12–14]. Therefore, hydride, hydrogen-induced martensite and atomic hydrogen will appear simultaneously during dynamic charging of the TiNi alloy. What role does the hydride, hydrogen-induced martensite and atomic hydrogen play, respectively, in hydrogen-induced delayed fracture of TiNi shape memory alloy? For type 304 austenitic stainless steel, the main cause of hydrogen-induced delayed facture during charging under sustained load was due to atomic hydrogen, although hydrogen-induced martensites played also certain role [15]. For TiAl intermetallics, the relative difference between the fracture toughness, KIC , and the threshold stress intensity factor of hydrogen-induced fracture, KIH , was (1 − KIH /KIC ) × 100% = 50%, in which 30% was due to atomic hydrogen and 20% to hydride [16]. The objective of this work is to investigate quantitatively the role of hydride, hydrogen-induced martensite and atomic hydrogen in hydrogen-induced delayed fracture of the TiNi alloy.
2. Experimental procedure ∗
Corresponding author. Tel.: +86-1-6233-2345; fax: +86-1-6233-2345. E-mail address: [email protected] (W.Y. Chu). 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.062
A commercial TiNi alloy with a Ni/Ti ratio of 55/45 was used. Specimens with a gage sections of 0.5 mm × 3 mm
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J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
and 20 mm gage length were cut from a bulk, and an edge notch of 1.6 mm depth was cut from the edge to the center of the specimen with a root radius of 0.1 mm. Two loading holes in a diameter of 2 mm were drilled using the ultrasound method. The specimens were austensite at room temperature after annealing at 150 ◦ C for 0.5 h. The tensile load-displacement curve keeps a straight before fracture, thus, the fracture toughness, KIC , can be obtained through substituting the fracture load into the stress intensity factor of the single-edge notched specimen [13,16]. A set of specimens were loaded to selected KI levels and then charged with various current densities under the sustained load. The time to failure at different KI was recorded. The threshold stress intensity of hydrogen-induced delayed fracture (HIDF) was obtained basing upon the following equation [16]. KIH
KIy + KIn = 2
(1)
where KIy is the minimum KI at which HIDF occurred and KIn the maximum KI at which HIDF would not occur within the fixed time (e.g., 100 h). KIn must be smaller than KIy . To ensure the error of measured threshold value is less than 10 pct, (KIy − KIn ) ≤ 0.1(KIy + KIn ) must be satisfied, otherwise an additive specimen is loaded to (KIy + KIn )/2, and KIy or KIn will be decided once again. X-ray diffraction showed that there were hydrides and martensite after charging at 15 mA/cm2 for 24 h. After heating at 100 ◦ C, there were only hydrides, in which only one hydride, TiNiH, could be determined. The thermal difference analysis showed that the hydrides can decompose completely at 600 ◦ C. Hydrogen was collected when the small specimens precharged for 100 h with various current densities were put into a glass tube filled with silicon oil. The saturation amount of hydrogen evolved at 80 ◦ C is the diffusible hydrogen concentration C0 . After that, the small specimens were heated to 800 ◦ C and the amount of hydrogen evolved at 800 ◦ C because of decomposition of hydrides is the hydrogen concentration CH in the hydrides.
3. Results The fracture toughness of as-received TiNi alloy was measured, and the average value was KIC = 39.2 ± 2.8 MPa m1/2 . The diffusible hydrogen concentration C0 and hydrogen concentration in hydrides CH after charging at 15 mA/cm2 for different times are listed in the second and third row of Table 1, respectively. Suppose that all hydrides are TiNiH, the hydride content can be calculated based on the hydrogen concentration CH , and has been listed in the fourth row of Table 1. The fracture toughness of the specimens precharged for different times is designated as KIC (H), and the fracture toughness of the specimens precharged and annealed at
Table 1 Fracture toughness losses induced by hydride and martensite after charging at 15 mA/cm2 for different times t (h)
24
80
160
200
240
C0 (wppm) CH (wppm) WTiNiH (%) KIC (H) (MPa m1/2 ) ∗ (H) (MPa m1/2 ) KIC TiNiH KIC (MPa m1/2 ) M KIC (MPa m1/2 )
16.2 500 5.3 17.4 17.6 21.6
93.3 1800 19.2 11.8 12.9 26.3
231.3 3700 39.4 5.9 6.3 32.9
284.5 4200 44.8 1.9 2.7 36.5
307.8 4200 44.8 1.3 2.3 36.8
TiNiH /KIC (%) KIC M /K KIC IC (%)
0.2
1.1
0.4
0.8
1.1
55 0.5
67 3.0
84 0.8
93 2.0
94 2.5
∗ (H). K (H) and K ∗ (H) are listed in 100 ◦ C for 24 h as KIC IC IC the fifth and sixth row of Table 1, respectively. The loading rate of the fracture toughness test is too high to initiate hydrogen-induced cracks through diffusion and enrichment of atomic hydrogen. Therefore, the difference of the fracture toughness between the hydrogen-free and hydrogenated specimens, KIC −KIC (H), are only due to the martensite and hydrides, i.e., M TiNiH KIC − KIC (H) = KIC + KIC
(2)
M is the fracture toughness loss induced by where KIC TiNiH is that inhydrogen-induced martensite, and KIC duced by the hydrides. After annealing at 100 ◦ C for 24 h, martensite will decompose completely but hydrides do not. Thus, TiNiH ∗ KIC = KIC − KIC
(3)
Based on Eqs. (2) and (3), we can get M ∗ KIC = KIC (H) − KIC (H)
(4)
TiNi and K M measured after charging at The KIC IC 15 mA/cm2 for different times are listed in the seventh and eighth row of Table 1 respectively. The relative fracture TiNi toughness losses induced hydride and martensite, KIC M and KIC /KIC , are listed in the ninth and tenth row of M or Table 1 respectively. Table 1 shows that the KIC M /K KIC IC is very small and independent upon hydrogen concentration, which is about 0.7 MPa m1/2 or 1.7%; but TiNiH or K TiNiH /K is very large and increases with KIC IC IC increasing hydrogen concentration CH . The variation of time to delayed fracture during charging at sustained load with the normalized stress intensity factor, KI /KIC , is shown in Fig. 1. The normalized threshold stress intensity facture, KIH /KIC , and the threshold stress intensity factor, KIH , of hydrogen-induced fracture can be obtained based on Eq. (1), and are listed in the fourth and fifth row of Table 2, respectively. The variations of KIH /KIC with C0 and the total hydrogen concentration CT = C0 + CH are showed
J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
335
in Fig. 2(a) and (b), respectively. Basing on Fig. 2, we can obtain KIH /KIC = 1.32 − 0.26 ln C0
(5)
KIH /KIC = 2.01 − 0.25 ln CT
(6)
During dynamic charging, hydride content increases gradually, and then the intrinsic fracture toughness of hydrogenated specimen, KIC (H), decreases continuously (see Table 1). Hydrogen-induced martensite can also reduce the intrinsic fracture toughness. On the other hand, atomic hydrogen can induce delayed fracture after diffusion and enrichment. Therefore, the difference of (KIC − KIH ) during hydrogen-induced delayed fracture test should due to hydride, martensite and atomic hydrogen, i.e., TiNiH M H KIC = KIC − KIH = KIC + KIC + KIC
Fig. 1. KI /KIC vs. time to fracture during dynamic charging with various i.
(7)
Table 2 Fracture toughness losses induced by hydride, martensite and atomic hydrogen during hydrogen-induced delayed fracture test i (mA/cm2 )
0.15
1.5
15
50
100
150
C0 (wppm) CH (wppm) KIH /KIC KIH (MPa m1/2 ) ∗ (H) (MPa m1/2 ) KIC
30 750 0.47 18.4 20.8
45 940 0.24 9.4 10.2
79 1710 0.11 4.3 8.2
120 2630 0.069 2.7 5.5
149 3100 0.058 2.3 4.3
181 3600 0.002 0.1 3.4
18.4
29.0
31.0
33.7
34.9
35.8
0.7
0.7
0.7
0.7
0.7
0.7
H (MPa m1/2 ) KIC
1.7
0.1
3.2
2.1
1.3
2.6
TiNiH KIC /KIC (%) M /K KIC IC (%) H /K KIC IC (%)
46.9 1.8
74.0 1.8
79.0 1.8
86.0 1.8
89.0 1.8
91.3 1.8
4.4 53.1
0.2 76.0
8.2 89.0
5.3 93.1
3.3 94.1
6.6 99.7
8.0
10.0
18.2
28.0
33.0
38.4
TiNiH KIC (MPa m1/2 ) M KIC (MPa m1/2 )
KIC /KIC (%) WTiNiH (%)
Fig. 2. KIH /KIC vs. difflisible hydrogen concentration C0 (a) and total hydrogen concentration CT (b).
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H is ascribed to atomic hydrogen. The fracwhere KIC ture toughness of the specimens precharged at various i ∗ (H), for 100 h and then annealed at 100 ◦ C for 24 h, KIC is measured and listed in the sixth row of Table 2. AcTiNiH can be calculated and is cording to Eq. (3), KIC listed in the seventh row of Table 2. Table 1 shows that M is independent upon hydrogen concentration. ThereKIC M = 0.7 MPa m1/2 for all fore, an average value of KIC hydrogen concentrations is used and listed in the eighth H = row of Table 2. Based on Eqs. (7) and (3), KIC H M KIC (H) − KIH − KIC , which can be calculated from the sixth, fifth and eighth rows of Table 2, and listed in the ninth row of Table 2. The relative fracture toughness losses are listed in the tenth to thirteenth row of Table 2, respectively. Table 2 indicates that the role of atomic hydrogen in hydrogen-induced delayed fracture is very small, and independent upon hydrogen concentration. Table 2 shows that the difference between the fracture toughness of hydrogen-free specimen and the threshold stress intensity factor of hydrogen-induced delayed fracture, (KIC −KIH ), is mainly due to the hydrides because the contribution of the hydride exceeds 90%. The variation of the relative fracture toughness losses with the total hydrogen concentration CT = C0 + CH or the hydride content is shown in Fig. 3. Fig. 3 indicates that the relative fracture toughness loss induced by the hydrides, and then the total fracture toughness loss during charging at sustained load increase with increasing hydride content or total hydrogen concentration. Fig. 1 is tF versus KI /KIC during dynamic charging at different i. For i = 100 mA/cm2 , time to delayed fracture corresponding to various KI /KIC is listed in the second row of Table 3. Multiplying the first row of Table 3 by KIC = 39.2 MPa m1/2 , the applied stress intensity factor during charging can be obtained, and is listed in the third row of Table 3. Since the specimen after charging for tF will fracture under the sustained KI , the KI listed in the third row of
Table 3 The fracture toughness losses after charging at sustained load for different time KI /KIC
0.24
0.21
0.19
0.16
0.062
tF or tH (h) H (H) (MPa m1/2 ) KI or KIC KIC (H) (MPa m1/2 )
10.5
52
85
121
214
9.4
8.2
7.5
6.3
14.9
9.5
9.0
6.8
2.7
5.5
1.3
1.5
0.5
0.3
H /K KIC IC (%)
23.6 14
29 3.3
29.5 3.8
31.7 1.3
35.8 0.8
TiNiH KIC /KIC (%) KIC /KIC (%)
60.2 76
74 78.6
75.3 80.9
80.9 83.9
91.3 93.9
H (MPa m1/2 ) KIC TiNiH KIC (MPa m1/2 )
2.4
Table 3 is just the intrinsic fracture toughness of the hydroH (H). In this case, the t is equal to genated specimen, KIC F the precharging time tH . Similar to Eq. (7), we can get H TiNiH M H (H) = KIC + KIC + KIC KIC = KIC − KIC
(8)
The fracture toughness of the specimens precharged at 100 mA/cm2 for tH , which is listed in the fourth row of Table 3 is designated as KIC (H). Substituting Eq. (2) into Eq. (7), we can obtain H H = KIC (H) − KIC (H) KIC
(9)
H subtracting the fourth and third row of Table 3, KIC can be calculated and is listed in the fifth row of Table 3. M = 0.7 MPa m1/2 Since KIC = 39.2 MPa m1/2 and KIC is independent upon hydrogen concentration (see Table 1), TiNiH can be calculated based on Eq. (2), i.e., K TiNiH = KIC IC M = (39.2 − 0.7) − K (H), and the KIC − KIC (H) − KIC IC result is listed in the sixth row of Table 3. The relative fracture toughness losses caused by atomic hydrogen and hydrides, and the total fracture toughness loss are listed in the seventh, eighth, and tenth row of Table 3, respectively. The variation of the relative fracture toughness losses with
100
∆ K IC/K IC ∆ K IC /K IC
100
∆ K IC
TiNiH
/K IC
∆ K IC/KIC ,%
∆ K IC/KIC ,%
80
∆ K IC
TiNiH
80
60
40
/K IC
60
40
20 20
∆ K IC /K IC H
∆ K IC /K IC H
0
0
0 0
500
1000
1500
2000
2500
3000
3500
4000
-4
CT£, 10 % Fig. 3. Total relative fracture toughness loss and that induced by hydride and atomic hydrogen vs. total hydrogen concentration.
50
100
150
200
tF£, h Fig. 4. Relative fracture toughness loss induced by hydride and atomic hydrogen vs. time to delayed fracture during dynamic charging at constant KI .
J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
time to delayed fracture during charging at 100 mA/cm2 is shown in Fig. 4. Fig. 4 shows that the relative fracture toughness loss after charging for various times under a sustained load is ascribed mainly to the hydride, and the role of atomic hydrogen in hydrogen-induced delayed fracture of the NiTi alloy is very small.
4. Discussion A X80 pipeline steel loaded over yield was unloaded and charged with hydrogen. The yield strength of the charged specimen loaded in air was lower than the flow stress before unloading, and the difference was defined as the hydrogen-induced additive stress σ ad , which could be added to the external stress to enhance the plastic deformation [16]. Measurement of the X80 pipeline steel showed that the hydrogen-induced additive stress σ ad increased with increasing hydrogen concentration, e.g., σad = 8.7 MPa (corresponding C0 = 5.9 wppm), 21.8 MPa (C0 = 0.4 wppm), 30.5 MPa (C0 = 11.9 wppm) and 38.1 MPa (C0 = 13.7 wppm). For a TiNi alloy, the hydrogen-induced additive stress cannot be measured using the flow stress difference method because the yield strength is very close to the ultimate tensile strength. We believe, however, that there exists also the hydrogen-induced additive stress σ ad in the TiNi alloy, and the additive stress increases gradually during charging under sustained load because of hydrogen enrichment. Based on Eq. (2), the intrinsic fracture toughness of the hydrogenated specimen during the delayed fracture test is M + K TiNiH ). When the sum of the KIC (H) = KIC − (KIC IC applied stress intensity factor, KI , and the additive stress intensity factor induced by hydrogen-induced additive stress, KIC , which is KI + KIC , is equal to the intrinsic fracture toughness of the hydrogenated specimen, KIC (H) = TiNiH +K M ), the delayed fracture occurs during KIC −(KIC IC charging under the sustained KI . In this case, the applied KI is equal to the fracture toughness of the hydrogenated specimen after charging for tF , which is listed in the second row H (H), which is listed in the third row of Table 3, i.e., KI = KIC TiNiH +K M ) of Table 3. As a result, KIC (H)+KIC −(KIC IC or H TiNiH H KIC − KIC (H) = KI + KIC + KIC
(10)
H, comparing Eq. (10) to Eq. (8), we can get KI = KIC where KI is the additive stress intensity factor corresponding to the hydrogen-induced additive stress, which is dependent upon the diffusion and enrichment of atomic hydrogen. This means that the stress intensity factor corresponding to hydrogen-induced additive stress is equal to the fracture toughness loss induced by atomic hydrogen during charging under sustained load. Table 3 indicate that the hydride content increases, and then the intrinsic fracture toughness of the TiNiH +K M ) hydrogenated specimen KIC (H) = KIC −(KIC IC
337
decreases with increasing charging time or time to delayed fracture during charging under sustained load. For example, the hydride content after charging for 214 h is about 44.8% (see Table 1) and the intrinsic fracture toughness of the hydrogenated specimen is KIC (H) = 2.7 MPa m1/2 = 0.07 KIC . That is to say, the applied stress intensity factor necessary for hydrogen-induced delayed fracture during charging at 100 mA/cm2 is below 0.07 KIC . Therefore, the main reason of hydrogen-induced delayed fracture of the TiNi alloy during charging under sustained load is the sharp decrease of the intrinsic fracture toughness of the hydrogenated specimen caused by the hydrides. For non-hydrogen forming materials, such as steel [13], ceramics [16] et al., the hydrogen-induced delayed fracture is due to diffusion and enrichment of atomic hydrogen, and then the threshold stress intensity factor KIH decrease linearly with the diffusible (atomic) hydrogen concentration C0 , similar to Eq. (5). For the TiNi alloy, the hydrogen-induced delayed fracture is mainly ascribed to the hydrides instead of atomic hydrogen. Therefore, Eq. (5) is incorrect. Only the total hydrogen concentration can be used to describe the variation of KIH with hydrogen concentration.
5. Conclusions (1) Hydrogen-induced delayed fracture of a TiNi shape memory alloy can occur during charging under a sustained load, and the normalized threshold stress intensity fracture decreases linearly with the logarithm of the total hydrogen concentration CT , i.e. KIH /KIC = 2.01 − 0.25 ln CT . (2) The main reason of hydrogen-induced delayed fracture of the TiNi alloy is the gradually decrease of the intrinsic fracture toughness caused by the hydrides. (3) The role of the hydrogen-induced martensite and the atomic hydrogen in hydrogen-induced delayed fracture is small and independent basically upon hydrogen concentration.
Acknowledgements The project is support by the special funds for the major state basic research (G19990650).
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[6] K. Asaoka, K. Yokoyama, M. Nagumo, Metall. Mater. Trans. A. 33A (2002) 495. [7] K. Yokoyama, S. Watabe, K.Hamada.J. Sqkai, L. Asaoka, M. Nagumo, Mater Sci. Eng. A 341 (2003) 91. [8] H. Buchner, M.A. Gutjahr, K.D. Beccu, H. Saufferer, Z. Metall. 63 (1972) 497. [9] J.L. Soubeyroux, D. Fruchart, J. Alloys Comp. 196 (1993) 127. [10] M.Kh. Shorshorov, I.A. Stepanov, Yu.M. Flomenblit, V.V. Travkin, Phys. Met. Metall. 60 (1985) 109.
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The role of hydride, martensite and atomic hydrogen in hydrogen-induced delayed fracture of TiNi alloy J.Y. He, K.W. Gao, Y.J. Su, L.J. Qiao, W.Y. Chu∗ Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, China Received 14 May 2003; received in revised form 19 August 2003
Abstract The role of atomic hydrogen, hydrogen-induced martensite and hydride in hydrogen-induced delayed fracture of a TiNi shape memory alloy during charging under a sustained load has been investigated using a notched tensile specimen. The results show that hydrogen-induced delayed fracture of the TiNi alloy can occur, and the normalized threshold stress intensity factor, KIH /KIC , decrease linearly with the logarithm of the total hydrogen concentration CT , i.e., KIH /KIC = 2.01−0.25 ln CT . The content of the hydrides increase gradually, and then the intrinsic fracture toughness of the hydrogenated specimen decrease sharply during dynamic charging. The formation of hydrides is the main reason of hydrogen-induced delayed fracture of the TiNi alloy. The role of atomic hydrogen and hydrogen-induced martensite in hydrogen-induced delayed fracture is very small. © 2003 Elsevier B.V. All rights reserved. Keywords: TiNi shape memory alloy; Hydrogen-induced delayed fracture; Atomic hydrogen; Hydrogen-induced martensite; Hydride
1. Introduction NiTi alloys are widely used in industrial and medical devices because of their unique shape memory and/or superelastic effect, excellent ductility, good fatigue life, good corrosion resistance and biocompatibility [1,2]. The influence of hydrogen on the shape memory or/and superelastic properties [3,4] and the mechanical properties [5,6] of TiNi alloys has been reported. Recently, hydrogen-induced delayed fracture under constant load has been investigated using smooth tensile specimen [7]. The result showed that irreversible martensite transformation increased the susceptibility to hydrogen embrittlement, but the stability of the TiNi alloys to reversible martensite transformation also affected the susceptibility even when the applied stress was lower than the critical stress for martensite transformation [7]. The effect of hydrides and atomic hydrogen on the delayed fracture during dynamic charging, however, were not considered [7]. Hydrides, such as TiNiH [8,9], TiNiH1.4 [10] and the others [3], and hydrogen-induced martensite [4,11] can form during charging of TiNi alloys. On the other hand,
atomic hydrogen can cause hydrogen-induced delayed fracture under sustained load for steels and aluminium alloys [12–14]. Therefore, hydride, hydrogen-induced martensite and atomic hydrogen will appear simultaneously during dynamic charging of the TiNi alloy. What role does the hydride, hydrogen-induced martensite and atomic hydrogen play, respectively, in hydrogen-induced delayed fracture of TiNi shape memory alloy? For type 304 austenitic stainless steel, the main cause of hydrogen-induced delayed facture during charging under sustained load was due to atomic hydrogen, although hydrogen-induced martensites played also certain role [15]. For TiAl intermetallics, the relative difference between the fracture toughness, KIC , and the threshold stress intensity factor of hydrogen-induced fracture, KIH , was (1 − KIH /KIC ) × 100% = 50%, in which 30% was due to atomic hydrogen and 20% to hydride [16]. The objective of this work is to investigate quantitatively the role of hydride, hydrogen-induced martensite and atomic hydrogen in hydrogen-induced delayed fracture of the TiNi alloy.
2. Experimental procedure ∗
Corresponding author. Tel.: +86-1-6233-2345; fax: +86-1-6233-2345. E-mail address: [email protected] (W.Y. Chu). 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.062
A commercial TiNi alloy with a Ni/Ti ratio of 55/45 was used. Specimens with a gage sections of 0.5 mm × 3 mm
334
J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
and 20 mm gage length were cut from a bulk, and an edge notch of 1.6 mm depth was cut from the edge to the center of the specimen with a root radius of 0.1 mm. Two loading holes in a diameter of 2 mm were drilled using the ultrasound method. The specimens were austensite at room temperature after annealing at 150 ◦ C for 0.5 h. The tensile load-displacement curve keeps a straight before fracture, thus, the fracture toughness, KIC , can be obtained through substituting the fracture load into the stress intensity factor of the single-edge notched specimen [13,16]. A set of specimens were loaded to selected KI levels and then charged with various current densities under the sustained load. The time to failure at different KI was recorded. The threshold stress intensity of hydrogen-induced delayed fracture (HIDF) was obtained basing upon the following equation [16]. KIH
KIy + KIn = 2
(1)
where KIy is the minimum KI at which HIDF occurred and KIn the maximum KI at which HIDF would not occur within the fixed time (e.g., 100 h). KIn must be smaller than KIy . To ensure the error of measured threshold value is less than 10 pct, (KIy − KIn ) ≤ 0.1(KIy + KIn ) must be satisfied, otherwise an additive specimen is loaded to (KIy + KIn )/2, and KIy or KIn will be decided once again. X-ray diffraction showed that there were hydrides and martensite after charging at 15 mA/cm2 for 24 h. After heating at 100 ◦ C, there were only hydrides, in which only one hydride, TiNiH, could be determined. The thermal difference analysis showed that the hydrides can decompose completely at 600 ◦ C. Hydrogen was collected when the small specimens precharged for 100 h with various current densities were put into a glass tube filled with silicon oil. The saturation amount of hydrogen evolved at 80 ◦ C is the diffusible hydrogen concentration C0 . After that, the small specimens were heated to 800 ◦ C and the amount of hydrogen evolved at 800 ◦ C because of decomposition of hydrides is the hydrogen concentration CH in the hydrides.
3. Results The fracture toughness of as-received TiNi alloy was measured, and the average value was KIC = 39.2 ± 2.8 MPa m1/2 . The diffusible hydrogen concentration C0 and hydrogen concentration in hydrides CH after charging at 15 mA/cm2 for different times are listed in the second and third row of Table 1, respectively. Suppose that all hydrides are TiNiH, the hydride content can be calculated based on the hydrogen concentration CH , and has been listed in the fourth row of Table 1. The fracture toughness of the specimens precharged for different times is designated as KIC (H), and the fracture toughness of the specimens precharged and annealed at
Table 1 Fracture toughness losses induced by hydride and martensite after charging at 15 mA/cm2 for different times t (h)
24
80
160
200
240
C0 (wppm) CH (wppm) WTiNiH (%) KIC (H) (MPa m1/2 ) ∗ (H) (MPa m1/2 ) KIC TiNiH KIC (MPa m1/2 ) M KIC (MPa m1/2 )
16.2 500 5.3 17.4 17.6 21.6
93.3 1800 19.2 11.8 12.9 26.3
231.3 3700 39.4 5.9 6.3 32.9
284.5 4200 44.8 1.9 2.7 36.5
307.8 4200 44.8 1.3 2.3 36.8
TiNiH /KIC (%) KIC M /K KIC IC (%)
0.2
1.1
0.4
0.8
1.1
55 0.5
67 3.0
84 0.8
93 2.0
94 2.5
∗ (H). K (H) and K ∗ (H) are listed in 100 ◦ C for 24 h as KIC IC IC the fifth and sixth row of Table 1, respectively. The loading rate of the fracture toughness test is too high to initiate hydrogen-induced cracks through diffusion and enrichment of atomic hydrogen. Therefore, the difference of the fracture toughness between the hydrogen-free and hydrogenated specimens, KIC −KIC (H), are only due to the martensite and hydrides, i.e., M TiNiH KIC − KIC (H) = KIC + KIC
(2)
M is the fracture toughness loss induced by where KIC TiNiH is that inhydrogen-induced martensite, and KIC duced by the hydrides. After annealing at 100 ◦ C for 24 h, martensite will decompose completely but hydrides do not. Thus, TiNiH ∗ KIC = KIC − KIC
(3)
Based on Eqs. (2) and (3), we can get M ∗ KIC = KIC (H) − KIC (H)
(4)
TiNi and K M measured after charging at The KIC IC 15 mA/cm2 for different times are listed in the seventh and eighth row of Table 1 respectively. The relative fracture TiNi toughness losses induced hydride and martensite, KIC M and KIC /KIC , are listed in the ninth and tenth row of M or Table 1 respectively. Table 1 shows that the KIC M /K KIC IC is very small and independent upon hydrogen concentration, which is about 0.7 MPa m1/2 or 1.7%; but TiNiH or K TiNiH /K is very large and increases with KIC IC IC increasing hydrogen concentration CH . The variation of time to delayed fracture during charging at sustained load with the normalized stress intensity factor, KI /KIC , is shown in Fig. 1. The normalized threshold stress intensity facture, KIH /KIC , and the threshold stress intensity factor, KIH , of hydrogen-induced fracture can be obtained based on Eq. (1), and are listed in the fourth and fifth row of Table 2, respectively. The variations of KIH /KIC with C0 and the total hydrogen concentration CT = C0 + CH are showed
J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
335
in Fig. 2(a) and (b), respectively. Basing on Fig. 2, we can obtain KIH /KIC = 1.32 − 0.26 ln C0
(5)
KIH /KIC = 2.01 − 0.25 ln CT
(6)
During dynamic charging, hydride content increases gradually, and then the intrinsic fracture toughness of hydrogenated specimen, KIC (H), decreases continuously (see Table 1). Hydrogen-induced martensite can also reduce the intrinsic fracture toughness. On the other hand, atomic hydrogen can induce delayed fracture after diffusion and enrichment. Therefore, the difference of (KIC − KIH ) during hydrogen-induced delayed fracture test should due to hydride, martensite and atomic hydrogen, i.e., TiNiH M H KIC = KIC − KIH = KIC + KIC + KIC
Fig. 1. KI /KIC vs. time to fracture during dynamic charging with various i.
(7)
Table 2 Fracture toughness losses induced by hydride, martensite and atomic hydrogen during hydrogen-induced delayed fracture test i (mA/cm2 )
0.15
1.5
15
50
100
150
C0 (wppm) CH (wppm) KIH /KIC KIH (MPa m1/2 ) ∗ (H) (MPa m1/2 ) KIC
30 750 0.47 18.4 20.8
45 940 0.24 9.4 10.2
79 1710 0.11 4.3 8.2
120 2630 0.069 2.7 5.5
149 3100 0.058 2.3 4.3
181 3600 0.002 0.1 3.4
18.4
29.0
31.0
33.7
34.9
35.8
0.7
0.7
0.7
0.7
0.7
0.7
H (MPa m1/2 ) KIC
1.7
0.1
3.2
2.1
1.3
2.6
TiNiH KIC /KIC (%) M /K KIC IC (%) H /K KIC IC (%)
46.9 1.8
74.0 1.8
79.0 1.8
86.0 1.8
89.0 1.8
91.3 1.8
4.4 53.1
0.2 76.0
8.2 89.0
5.3 93.1
3.3 94.1
6.6 99.7
8.0
10.0
18.2
28.0
33.0
38.4
TiNiH KIC (MPa m1/2 ) M KIC (MPa m1/2 )
KIC /KIC (%) WTiNiH (%)
Fig. 2. KIH /KIC vs. difflisible hydrogen concentration C0 (a) and total hydrogen concentration CT (b).
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H is ascribed to atomic hydrogen. The fracwhere KIC ture toughness of the specimens precharged at various i ∗ (H), for 100 h and then annealed at 100 ◦ C for 24 h, KIC is measured and listed in the sixth row of Table 2. AcTiNiH can be calculated and is cording to Eq. (3), KIC listed in the seventh row of Table 2. Table 1 shows that M is independent upon hydrogen concentration. ThereKIC M = 0.7 MPa m1/2 for all fore, an average value of KIC hydrogen concentrations is used and listed in the eighth H = row of Table 2. Based on Eqs. (7) and (3), KIC H M KIC (H) − KIH − KIC , which can be calculated from the sixth, fifth and eighth rows of Table 2, and listed in the ninth row of Table 2. The relative fracture toughness losses are listed in the tenth to thirteenth row of Table 2, respectively. Table 2 indicates that the role of atomic hydrogen in hydrogen-induced delayed fracture is very small, and independent upon hydrogen concentration. Table 2 shows that the difference between the fracture toughness of hydrogen-free specimen and the threshold stress intensity factor of hydrogen-induced delayed fracture, (KIC −KIH ), is mainly due to the hydrides because the contribution of the hydride exceeds 90%. The variation of the relative fracture toughness losses with the total hydrogen concentration CT = C0 + CH or the hydride content is shown in Fig. 3. Fig. 3 indicates that the relative fracture toughness loss induced by the hydrides, and then the total fracture toughness loss during charging at sustained load increase with increasing hydride content or total hydrogen concentration. Fig. 1 is tF versus KI /KIC during dynamic charging at different i. For i = 100 mA/cm2 , time to delayed fracture corresponding to various KI /KIC is listed in the second row of Table 3. Multiplying the first row of Table 3 by KIC = 39.2 MPa m1/2 , the applied stress intensity factor during charging can be obtained, and is listed in the third row of Table 3. Since the specimen after charging for tF will fracture under the sustained KI , the KI listed in the third row of
Table 3 The fracture toughness losses after charging at sustained load for different time KI /KIC
0.24
0.21
0.19
0.16
0.062
tF or tH (h) H (H) (MPa m1/2 ) KI or KIC KIC (H) (MPa m1/2 )
10.5
52
85
121
214
9.4
8.2
7.5
6.3
14.9
9.5
9.0
6.8
2.7
5.5
1.3
1.5
0.5
0.3
H /K KIC IC (%)
23.6 14
29 3.3
29.5 3.8
31.7 1.3
35.8 0.8
TiNiH KIC /KIC (%) KIC /KIC (%)
60.2 76
74 78.6
75.3 80.9
80.9 83.9
91.3 93.9
H (MPa m1/2 ) KIC TiNiH KIC (MPa m1/2 )
2.4
Table 3 is just the intrinsic fracture toughness of the hydroH (H). In this case, the t is equal to genated specimen, KIC F the precharging time tH . Similar to Eq. (7), we can get H TiNiH M H (H) = KIC + KIC + KIC KIC = KIC − KIC
(8)
The fracture toughness of the specimens precharged at 100 mA/cm2 for tH , which is listed in the fourth row of Table 3 is designated as KIC (H). Substituting Eq. (2) into Eq. (7), we can obtain H H = KIC (H) − KIC (H) KIC
(9)
H subtracting the fourth and third row of Table 3, KIC can be calculated and is listed in the fifth row of Table 3. M = 0.7 MPa m1/2 Since KIC = 39.2 MPa m1/2 and KIC is independent upon hydrogen concentration (see Table 1), TiNiH can be calculated based on Eq. (2), i.e., K TiNiH = KIC IC M = (39.2 − 0.7) − K (H), and the KIC − KIC (H) − KIC IC result is listed in the sixth row of Table 3. The relative fracture toughness losses caused by atomic hydrogen and hydrides, and the total fracture toughness loss are listed in the seventh, eighth, and tenth row of Table 3, respectively. The variation of the relative fracture toughness losses with
100
∆ K IC/K IC ∆ K IC /K IC
100
∆ K IC
TiNiH
/K IC
∆ K IC/KIC ,%
∆ K IC/KIC ,%
80
∆ K IC
TiNiH
80
60
40
/K IC
60
40
20 20
∆ K IC /K IC H
∆ K IC /K IC H
0
0
0 0
500
1000
1500
2000
2500
3000
3500
4000
-4
CT£, 10 % Fig. 3. Total relative fracture toughness loss and that induced by hydride and atomic hydrogen vs. total hydrogen concentration.
50
100
150
200
tF£, h Fig. 4. Relative fracture toughness loss induced by hydride and atomic hydrogen vs. time to delayed fracture during dynamic charging at constant KI .
J.Y. He et al. / Materials Science and Engineering A364 (2004) 333–338
time to delayed fracture during charging at 100 mA/cm2 is shown in Fig. 4. Fig. 4 shows that the relative fracture toughness loss after charging for various times under a sustained load is ascribed mainly to the hydride, and the role of atomic hydrogen in hydrogen-induced delayed fracture of the NiTi alloy is very small.
4. Discussion A X80 pipeline steel loaded over yield was unloaded and charged with hydrogen. The yield strength of the charged specimen loaded in air was lower than the flow stress before unloading, and the difference was defined as the hydrogen-induced additive stress σ ad , which could be added to the external stress to enhance the plastic deformation [16]. Measurement of the X80 pipeline steel showed that the hydrogen-induced additive stress σ ad increased with increasing hydrogen concentration, e.g., σad = 8.7 MPa (corresponding C0 = 5.9 wppm), 21.8 MPa (C0 = 0.4 wppm), 30.5 MPa (C0 = 11.9 wppm) and 38.1 MPa (C0 = 13.7 wppm). For a TiNi alloy, the hydrogen-induced additive stress cannot be measured using the flow stress difference method because the yield strength is very close to the ultimate tensile strength. We believe, however, that there exists also the hydrogen-induced additive stress σ ad in the TiNi alloy, and the additive stress increases gradually during charging under sustained load because of hydrogen enrichment. Based on Eq. (2), the intrinsic fracture toughness of the hydrogenated specimen during the delayed fracture test is M + K TiNiH ). When the sum of the KIC (H) = KIC − (KIC IC applied stress intensity factor, KI , and the additive stress intensity factor induced by hydrogen-induced additive stress, KIC , which is KI + KIC , is equal to the intrinsic fracture toughness of the hydrogenated specimen, KIC (H) = TiNiH +K M ), the delayed fracture occurs during KIC −(KIC IC charging under the sustained KI . In this case, the applied KI is equal to the fracture toughness of the hydrogenated specimen after charging for tF , which is listed in the second row H (H), which is listed in the third row of Table 3, i.e., KI = KIC TiNiH +K M ) of Table 3. As a result, KIC (H)+KIC −(KIC IC or H TiNiH H KIC − KIC (H) = KI + KIC + KIC
(10)
H, comparing Eq. (10) to Eq. (8), we can get KI = KIC where KI is the additive stress intensity factor corresponding to the hydrogen-induced additive stress, which is dependent upon the diffusion and enrichment of atomic hydrogen. This means that the stress intensity factor corresponding to hydrogen-induced additive stress is equal to the fracture toughness loss induced by atomic hydrogen during charging under sustained load. Table 3 indicate that the hydride content increases, and then the intrinsic fracture toughness of the TiNiH +K M ) hydrogenated specimen KIC (H) = KIC −(KIC IC
337
decreases with increasing charging time or time to delayed fracture during charging under sustained load. For example, the hydride content after charging for 214 h is about 44.8% (see Table 1) and the intrinsic fracture toughness of the hydrogenated specimen is KIC (H) = 2.7 MPa m1/2 = 0.07 KIC . That is to say, the applied stress intensity factor necessary for hydrogen-induced delayed fracture during charging at 100 mA/cm2 is below 0.07 KIC . Therefore, the main reason of hydrogen-induced delayed fracture of the TiNi alloy during charging under sustained load is the sharp decrease of the intrinsic fracture toughness of the hydrogenated specimen caused by the hydrides. For non-hydrogen forming materials, such as steel [13], ceramics [16] et al., the hydrogen-induced delayed fracture is due to diffusion and enrichment of atomic hydrogen, and then the threshold stress intensity factor KIH decrease linearly with the diffusible (atomic) hydrogen concentration C0 , similar to Eq. (5). For the TiNi alloy, the hydrogen-induced delayed fracture is mainly ascribed to the hydrides instead of atomic hydrogen. Therefore, Eq. (5) is incorrect. Only the total hydrogen concentration can be used to describe the variation of KIH with hydrogen concentration.
5. Conclusions (1) Hydrogen-induced delayed fracture of a TiNi shape memory alloy can occur during charging under a sustained load, and the normalized threshold stress intensity fracture decreases linearly with the logarithm of the total hydrogen concentration CT , i.e. KIH /KIC = 2.01 − 0.25 ln CT . (2) The main reason of hydrogen-induced delayed fracture of the TiNi alloy is the gradually decrease of the intrinsic fracture toughness caused by the hydrides. (3) The role of the hydrogen-induced martensite and the atomic hydrogen in hydrogen-induced delayed fracture is small and independent basically upon hydrogen concentration.
Acknowledgements The project is support by the special funds for the major state basic research (G19990650).
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