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The effect of surface oxidation behavior on the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy
Journal of Alloys and Compounds 647 (2015) 740e749
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effect of surface oxidation behavior on the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe titanium alloy Xiaohui Shi a, *, Weidong Zeng a, **, Qinyang Zhao b a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China School of Materials Science and Engineering, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 10 May 2015 Accepted 30 May 2015 Available online 10 June 2015
In this study, the effect of surface oxidation behavior on the fracture toughness of Tie5Ale5Moe5Ve1Cr e1Fe alloy suffered oxidation treatments at the temperatures of 500 C, 600 C, 700 C and 800 C is studied by means of OM, SEM and EDS analysis. For comparison, the fracture toughness of this alloy suffered vacuum annealing treatments at the temperatures of 550 C, 600 C, 650 C and 700 C are also investigated. The results show that at the heating temperatures below 600 C, the fracture toughness values of Tie5Ale5Moe5Ve1Cre1Fe alloy suffered vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the heating temperature of 700 C, the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy undergone oxidation treatment is almost 50 MPa m1/2 lower than that undergone vacuum annealing treatment. This is due to that at the oxidation temperature of 700 C, the oxide layer thickness is above 5 mm. In this situation, its influence on the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy cannot be ignored. The existence of oxide layer can decrease the total area of shear lips on the fracture surface, decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of KIC specimen. This can result in a big decrease of fracture resistance, which is detrimental to the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy. © 2015 Elsevier B.V. All rights reserved.
Keywords: Surface oxidation Tie5Ale5Moe5Ve1Cre1Fe Fracture toughness Vacuum annealing
1. Introduction Fracture toughness describes the ability of a material containing a crack to resist fracture. According to the damage tolerance design concept applied in aerospace industry, fracture toughness is an important index for material selection and component design to guarantee safe flight [1]. Titanium alloys are widely used in the field of aerospace due to high specific strength, good corrosion resistance and good high-temperature properties [2,3]. As an important structural material in aircrafts, high fracture toughness of titanium alloys have been pursued by material scientists for a long time. According to previous researches [4e7], the microstructure features of a titanium alloy can greatly influence its fracture toughness. For example, Bhattacharjee et al. [4] found that the fracture toughness of Tie10Ve2Fee3Al titanium alloy
* Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No.127 Youyi Xilu, Xi'an 710072, China. ** Corresponding author. E-mail addresses: [email protected] (X. Shi), [email protected] (W. Zeng). http://dx.doi.org/10.1016/j.jallcom.2015.05.211 0925-8388/© 2015 Elsevier B.V. All rights reserved.
approximately follows a Hall-Petch relationship with its b grain size. Greenfield et al. [5] found that the increasing grain boundary a thickness can obviously improve the fracture toughness of Tie5.25Ale5.5Ve0.9Fee0.5Cu alloy until a critical value of 5.5 mm is reached. It is due to that thicker grain boundary a can more effectively blunt the running crack tip, which can increase the fracture resistance. Cvijovi c-Alagic et al. [6] thought that the colony size and the aspect ratio of a phase are two main factors influencing the fracture toughness of Tie6Ale4V alloy with lamellar structure. They found that big colony size and high aspect ratio of a platelet are favorable to deflect the crack propagation path, which can effectively improve the fracture toughness. Richards [7] studied the fracture toughness of two a þ b titanium alloys containing a platelets in transformed b matrix. He found that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest a platelets. It is explained in terms of the distance between active centers of void nucleation, which is a function of the a platelet thickness and the platelet spacing. Based on above research results, engineers generally optimize the fracture toughness of titanium alloy by controlling its microstructure features. However, the microstructure features may
X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
be not the only factor influencing fracture toughness. Due to the high affinity for oxygen, exposure of titanium alloys to any oxygen containing atmosphere at elevated temperatures leads to formation of an hard and brittle oxide layer on the specimen surface, which can also exert big influence on mechanical properties [8e10]. Jia et al. [8] made a research on oxidation behavior and effect of oxidation on tensile properties of Ti60 alloy. They found that both the strength and ductility of the specimens with oxide scale decreased when compared with the specimens without oxide scale. Çelik et al. [10], however, found that the oxidation treatments brought about a remarkable increase in wear resistance of ultrafine-grained Ti because of the formation of adhered and hard TiO2 film developed on the surface. It can be found that the previous researches were mainly about the tensile properties and tribological properties of titanium alloys. Unfortunately, little work has been conducted concerning the effect of surface oxidation on the fracture toughness of titanium alloys. To date, it is still inevitable to ensure the titanium alloys being processed in the absence of oxygen containing atmosphere. Thus understanding the surface oxidation-fracture toughness interrelations of titanium alloys can be very meaningful. Ti-55511 (Tie5Ale5Moe5Ve1Cre1Fe) titanium alloy, which is a highly alloyed near-b titanium alloy [11], is designed based on considerations of damage tolerance targets. It has excellent combinations of strength, toughness, fatigue and crack growth resistance, and is especially suitable for making big critical aviation forgings [12]. In the previous reports, the research efforts were mainly focused on the application, microstructure and tensile properties of Ti-55511 alloy [13e15]. In order to basically understand the influence of surface oxidation on the fracture toughness of Ti-55511 alloy. In the present study, a tentative investigation on this issue has been conducted. Moreover, the influencing mechanisms of the surface oxidation on the fracture toughness of Ti55511 alloy are tried to be explained. 2. Experimental procedures 2.1. Materials
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Fig. 1. The typical microstructure of Ti-55511 titanium alloy after forging and double annealing treatment.
preparation. The first step is applied for releasing the residual stress in the forging. Moreover, some metastable b phase can be retained at this step. The second step, however, is a stabilization treatment, which can make a preparation for the following aging treatment. The final step, namely the aging step, is to prompt the precipitation process of the dispersive secondary a phase from the metastable b phase. After the forging and double annealing treatment, the obtained microstructure of Ti-55511 alloy is shown in Fig. 1. It can be found that the microstructure in Fig. 1 belongs to typical basketweave microstructure type. The original grain boundary a phase has been broken to some extent. Moreover, the thin a platelets generally in length of 5e15 mm and thickness of 1e2 mm weave with each other. The total volume fraction of a phase is about 40%. In addition, it can be found that a small amount of equiaxed a phase (approximately has a volume fraction of 3%) exists in the transformed matrix, which is due to the globularization behavior of a platelets when undergone slight deformation in the a þ b phase field. 2.2. Oxidation treatments and vacuum annealing treatments
The experimental Ti-55511 titanium alloy was supplied by Western Superconducting Technologies Co., Ltd., PR China. Table 1 shows its chemical composition. The b-transus temperature (Tb) of this alloy was identified as approximately 843 C via metallographic techniques. The detailed procedures are like following: (i) heating one specimen to 800 C and then held at this temperature for 4 h, followed by water quenching; (ii) observing the microstructure of this specimen using Olympus/PMG3 optical microscope, if there exists some retained a phase in microstructure, then increasing the heating temperature by 5 C; (iii) repeating step (ii) until no retained a phase can be observed in microstructure, then taking this temperature as T1. The average value of T1 and T1 5 C can be determined as the b-transus temperature of Ti-55511 titanium alloy. The as-received Ti-55511 alloy had been forged at Tb þ 15 C and then undergone double annealing treatment, which contains three steps: (i) heating to 820 C, holding for 2 h, and furnace cooling (FC) to 750 C; (ii) holding at 750 C for 2 h and air-cooled to room temperature; (iii) aging at 630 C for 4 h and air cooling [16]. Different steps have different meanings in microstructure
According to the research by Peng et al. [9], the surface oxidation is a very slow process at the temperature below 500 C, which generally has no influence on the tensile properties of Ti-55511 alloy. Thus the authors proposed that the surface oxidation may exert little influence on the fracture toughness of Ti-55511 alloy at the temperature below 500 C. In the present study, the four oxidation temperatures including 500 C, 600 C, 700 C and 800 C were selected, as listed in Table 2. All the oxidation treatments were conducted in the electric furnace (named N60/85HA) with FeeCreAl alloy resistance wire. In the electric furnace, the initial volume fraction of oxygen is about 21%. During the oxidation process, the Ti-55511 samples undergone heat preservation for 2 h at different temperatures, followed by furnace cooling. To reflect the effect of surface oxidation on the fracture toughness of Ti-55511 alloy, the vacuum annealing treatments were also conducted, which can be used for comparison. Four temperatures including
Table 2 The oxidation treatments and vacuum annealing treatments of Ti-55511 alloy. Vacuum annealing treatments
Table 1 The chemical composition of the Ti-55511 alloy (wt%). Ti
C
N
H
O
Al
Mo
V
Cr
Fe
Zr
Si
Bal.
0.01
0.01
0.0015
0.066
5.2
5.0
4.9
0.96
0.91
<0.3
<0.1
550 600 650 700
C/4 C/4 C/4 C/4
h, h, h, h,
FC FC FC FC
Oxidation treatments 500 600 700 800
C/2 C/2 C/2 C/2
h, h, h, h,
FC FC FC FC
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550 C, 600 C, 650 C and 700 C were selected for vacuum annealing (see Table 2). To ensure the near-vacuum condition, the vacuum furnace named VPS50/10 was used, which can reach the final vacuum of 103 Pa. 2.3. Tests Cylindrical samples with 13.5 mm in diameter and 70 mm in length were electro-discharged machined from the vacuum annealed and the oxidized Ti-55511 alloy. Then the standard tensile specimens were machined from these cylindrical samples with a gauge length of 25 mm and a diameter of 5 mm. Three smooth tensile specimens were used to test tensile properties under each treatment at room temperature. The tensile properties of the asreceived Ti-55511 alloy were also measured. The standard CT (compact tension) specimen type with width/ thickness ratio of 2:1 were adopted for fracture toughness test, as shown in Fig. 2. Two specimens were adopted to evaluate the fracture toughness of Ti-55511 alloy under all conditions. The detailed KIC test procedures of vacuum annealed Ti-55511 alloy are as follows. Firstly, samples with the dimensions of 62.5 mm 60 mm 25 mm were machined from the vacuum annealed Ti-55511 alloy. Secondly, electro-discharged machined notches on the cuboid samples were prepared and the fatigue precracking of the notches were carried out using MTS810 fatigue testing machine. Finally, the prepared specimens were stretched to fracture on the condition that the specified increase rate of the pffiffiffiffiffi stress intensity factor is within the range 0.55 and 2:75 MPa m=s. During the testing process, the loadedisplacement curve will be recorded. After all, the apparent fracture toughness KQ can be calculated (see Eqs. (1) and (2)) if the fracture force FQ determined from the loadedisplacement curve and the average crack length a determined from fracture surface are available. However, KQ doesn't necessarily equals KIC only if the small scale yielding and plane strain conditions at the crack tip of test specimen are met. Thus in the present work, all the test specimens were machined thick enough and the fatigue precrack long enough to meet above two conditions.
. KQ ¼ FQ BW1=2 fða=WÞ
fða=WÞ ¼ ð2 þ a=WÞ
(1)
surfaces and crack propagation paths of all the KIC specimens were analyzed with a JSM-6390A scanning electron microscope (SEM). The thicknesses of the oxide layer at different oxidation temperatures were also observed with JSM-6390A. Moreover, to determine the chemical composition of the oxide layer, the EDS analyses were conducted.
3. Results and discussion 3.1. Microstructures and tensile properties Fig. 3 shows the microstructures of Ti-55511 alloy at different vacuum annealing temperatures. It can be found that the microstructures annealed at 550 C and 600 C are similar, which both contain long a platelets generally in length of 5e15 mm and thickness of 1e2 mm, as shown in Fig. 3a and b respectively. It can be found that the microstructures annealed at 550 C and 600 C show no big difference with the initial microstructure (Fig. 1). This is due to the limited phase transition driving force at relatively low annealing temperatures. For Ti-55511 alloy annealed at 650 C, however, the a platelets in microstructure (Fig. 3c) are obviously shorter and thinner when compared with Fig. 3a and b, which are within 10 mm in length and about 1 mm in thickness. Moreover, it can be found that the total volume fraction of a phase in Fig. 3c is obviously lower when compared with Fig. 3a and b, from about 40% to about 30%. As described above, Ti-55511 titanium alloy is a nearb titanium alloy [11], thus it is easy for the a phase to transform into b phase when heated at relatively high temperatures. At the annealing temperature of 650 C, the a phase becomes more unstable than that at 550 C and 600 C. Therefore, some a phase transformed into b phase, which leads to the decreased volume fraction of a phase. By increasing the annealing temperature to 700 C, the shape of the a platelets (Fig. 3d) changes little when compared with Fig. 3c. However, it can be observed that the total volume fraction of a phase slightly decreases, from about 30% to 25%. In conclusion, with increasing annealing temperature, the a platelets becomes shorter and the total volume fraction of a phase continuously decreases. Fig. 4 shows the microstructures of Ti-55511 alloy at different oxidation temperatures. It can be found that the microstructure features of Ti-55511 alloy oxidized at 500 C and 600 C are very similar to the microstructures vacuum annealed at 550 C and
0:866 þ 4:64ða=WÞ 13:32ða=WÞ2 þ 14:72ða=WÞ3 5:6ða=WÞ4 ð1 a=WÞ3=2
The KIC test procedures of Ti-55511 alloy under oxidation treatments are a little different with the vacuum annealed parts. The samples with the dimensions of 62.5 mm 60 mm 25 mm were oxidized in the electric furnace at different temperatures firstly. Then these samples gone through the same preparation (notching, fatigue precrack) and testing processes as described above. The fracture toughness of the as-received Ti-55511 alloy was also measured. In addition, one thing should be noted that all the properties used in present paper are the average values. The metallographic specimens were prepared by grinding, polishing and corroding and the microstructural observations were conducted with Olympus/PMG3 optical microscope. The fracture
(2)
600 C respectively. Moreover, the microstructure features of Ti55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C (see Fig. 3d). This can be attributed to their similar heating processes, which may lead to similar driving force of phase transformation. Fig. 4d shows the microstructure of Ti55511 alloy oxidized at 800 C, it can be found that the number of a platelets decreases obviously when compared with Fig. 4aec. Moreover, the a platelets in Fig. 4d are obviously shorter and thicker when compared with Fig. 4aec, which are within 5 mm in length and about 2 mm in thickness. It is well known that low aspect ratio a platelets have less total interfacial energy than that of big aspect ratio a platelets. Thus, the energy minimization provides the driving force for a platelet shortening and thickening, and the process is assisted by solute atom diffusion [17].
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Fig. 2. The schematic KIC specimen configurations (CT).
The tensile properties including yield strength (YS), ultimate tensile strength (UTS), elongation (EL) and reduction of area (RA) of Ti-55511 alloy at different vacuum annealing temperatures are listed in Table 3. It can be found that the tensile properties of Ti55511 alloy annealed at 550 C and 600 C are similar to the tensile properties of the as-received microstructure. They all get the yield strength of about 1100 MPa and the elongation of about 14%. Moreover, it should be noted that with increasing annealing temperature, the strengths of Ti-55511 alloy continuously decrease. At the annealing temperature of 700 C, the yield strength is only 940 MPa, which is 180 MPa lower than that at the annealing temperature of 550 C. The plasticity of Ti-55511 alloy, however, shows a reverse changing tendency to its strengths. The specimens annealed at 700 C get the highest plasticity among the four vacuum annealing treatments, which have the average elongation of 18.8% and the reduction of area of 65%. Generally speaking, the tensile properties of Ti-55511 alloy vacuum annealed below 600 C vary slightly. However, the tensile properties of Ti-55511 alloy have a big change when annealed above 650 C.
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All the phenomena described above can be explained in the following. With increasing annealing temperature (especially above 650 C), the a platelets in microstructure tend to be shorter and the total volume fraction of a phase becomes lower. This results in the weakening of the interfacial strengthening effect [18]. Moreover, the increasing volume fraction of b phase (bcc structure) can improve the deformation ability of Ti-55511 alloy. At the annealing temperature below 600 C, the similar microstructure features of Ti-55511 alloy result in similar tensile properties. However, at the annealing temperatures above 650 C, the obvious decrease in length and volume fraction of a phase lead to a big change of tensile properties. The tensile properties of Ti-55511 alloy at different oxidation temperatures are also listed in Table 3. It can be found that the tensile properties of Ti-55511 alloy oxidized at 500 C and 600 C are very similar to that vacuum annealed at 550 C and 600 C respectively. Moreover, the tensile properties of Ti-55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C. This can be attributed to their similar microstructure features. When the oxidation temperature increases from 700 C to 800 C, the strengths of Ti-55511 alloy decrease slightly. However, its elongation undergoes a big loss, which drops from 16.2% to 8.8%. The oxidation layer on the surface of tensile specimens is responsible for it [9]. In conclusion, at the oxidation temperature below 700 C, the surface oxidation almost exerts no influence on the tensile properties of Ti-55511 alloy. 3.2. Fracture toughness The fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments are listed in Table 4. It can be found that with increasing annealing temperature, the fracture toughness of Ti-55511 alloy continuously increase. At the annealing temperpffiffiffiffiffi ature of 700 C, the fracture toughness can reach 120:8 MPa m, which is almost twice of that when annealed at 550 C (only pffiffiffiffiffi 65:2 MPa m). Generally speaking, the fracture toughness of Ti55511 alloy annealed below 600 C vary slightly, which are all pffiffiffiffiffi around 70 MPa m. However, a big increase of the fracture
Fig. 3. The microstructures of Ti-55511 alloy at different vacuum annealing temperatures: (a) 550 C/4 h, FC, (b) 600 C/4 h, FC, (c) 650 C/4 h, FC, (d) 700 C/4 h, FC.
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Fig. 4. The microstructures of Ti-55511 alloy at different oxidation temperatures: (a) 500 C/2 h, FC, (b) 600 C/2 h, FC, (c) 700 C/2 h, FC, (d) 800 C/2 h, FC.
Table 3 The tensile properties of Ti-55511 alloy under different conditions. UTS/MPa As-received 1135.0 Vacuum annealing treatments 550 C/4 h 1150.0 600 C/4 h 1116.7 650 C/4 h 1023.3 700 C/4 h 951.7 Oxidation treatments 500 C/2 h 1148.3 600 C/2 h 1126.7 700 C/2 h 957.7 800 C/2 h 941.0
YS/MPa
EL/%
RA/%
1090.0
14.0
57.0
1120.0 1093.3 996.7 940.0
13.0 15.0 17.8 18.8
49.7 56.7 60.3 65.0
1123.3 1103.3 953.3 937.5
14.5 13.8 16.2 8.8
59.6 55.8 64.2 43.0
toughness happens when annealed above 650 C. In order to explain above phenomena, the factors influencing the fracture toughness of materials must be clarified. Lütjering et al. [19] and Eylon et al. [20] thought that the fracture toughness is the result of two contributions: the intrinsic fracture resistance, i.e. the direct influence on the fracture properties of the material (ductility), and the extrinsic fracture toughness, i.e. the additional contribution of the crack front geometry (the tortuosity of the crack front). Based on above theory [19,20], big crack tip deformation zone as well as tortuous crack propagation path are both favorable to the fracture toughness. Fig. 5 shows the fracture surfaces of the KIC specimens at different vacuum annealing temperatures. It can be found that the fracture surfaces of Ti-55511 alloy annealed at 550 C and 600 C
are rough and characterized by big and deep ravines and secondary cracks, as shown in Fig. 5a and b respectively. As described in Refs. [21,22], long a platelets permit larger bifurcations of the main crack, enhance the crack deflection effect and thus increase crack path tortuosity. Based on above theory, the rough fracture surfaces in Fig. 5a and b can be easily explained. By contrast, the fracture surfaces of Ti-55511 alloy annealed at 650 C (Fig. 5c) and 700 C (Fig. 5d) are relatively flat, which are characterized by dense dimples. The widely distributed dimples on fracture surface indicate good deformation ability at the running crack tip during the fracture process. This can be attributed to the obviously lower yield strengths (namely higher plasticity) of Ti-55511 alloy annealed at 650 C and 700 C. According to previous research [23], the intrinsic fracture resistance plays the main role in deciding the fracture toughness of Ti-55511 alloy. Thus, the fracture toughness generally shows a reverse trend to the yield strength of a material, which has the correlation in the form of KIC f YS1 [24]. Based on above discussion, the fracture toughness shows a reverse trend to the yield strength in this work is not surprising at all. In addition, due to the sharp decrease of the yield strength for Ti-55511 alloy when annealed at 650 C, a big increase of the fracture toughness naturally happens. The fracture toughness values of Ti-55511 alloy undergone oxidation treatments are listed in Table 4. It can be found that at the oxidation temperatures below 700 C, the fracture toughness pffiffiffiffiffi values of this alloy stay around 70 MPa m. However, at the oxidation temperature of 800 C, the fracture toughness decreases pffiffiffiffiffi to merely 43:2 MPa m. In order to more clearly figure out the
Table 4 The fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments. pffiffiffiffiffi Vacuum annealing treatments KIC =MPa m Oxidation treatments 550 600 650 700
C/4 C/4 C/4 C/4
h h h h
65.2 73.5 105.1 120.8
500 600 700 800
C/2 C/2 C/2 C/2
h h h h
pffiffiffiffiffi KIC =MPa m 64.1 72.5 68.8 43.2
X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
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Fig. 5. The fracture surfaces of the KIC specimens at different vacuum annealing temperatures: (a) 550 C/4 h, FC, (b) 600 C/4 h, FC, (c) 650 C/4 h, FC, (d) 700 C/4 h, FC.
effect of surface oxidation behavior on the fracture toughness of Ti55511 alloy, the comparisons of the fracture toughness values between the specimens undergone vacuum annealing treatments and oxidation treatments are shown in Fig. 6. Generally speaking, at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the temperatures above 600 C, the fracture toughness variation between the two kinds of treatments becomes bigger and bigger with increasing heating temperature. For example, at 700 C, the fracture toughness of Ti-55511 alloy unpffiffiffiffiffi dergone oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment, which is a sharp decrease. In conclusion, the surface oxidation can lead to a big loss of the fracture toughness for Ti-55511 alloy at the oxidation temperatures above 600 C. This is extremely detrimental to the security of the
Fig. 6. The comparisons of the fracture toughness values between the specimens undergone vacuum annealing treatments and oxidation treatments.
bearing carriers. Till now, the reasons responsible for this phenomenon is still unclear. In the following text, above questions are tried to be answered.
3.3. The oxidation behavior and its influence on fracture toughness Understanding the surface oxidation behavior of Ti-55511 alloy is the key to answer the questions mentioned in previous section. Fig. 7 shows the oxide layers of Ti-55511 alloy at different oxidation temperatures. It can be found that at the oxidation temperature of 500 C (Fig. 7a), the oxide layer cannot be visually observed on the specimen surface. As the temperature increases, at 600 C, a thin oxide layer with the thickness of about 1 mm bond tightly with the substrate microstructure (beneath the oxide layer), as shown in Fig. 7b. At the oxidation temperature of 700 C, the oxide layer thickness increases to above 5 mm, as shown in Fig. 7c. When the oxidation temperature increases to 800 C, the oxide layer thickness can reach about 20 mm. Generally speaking, at relatively low temperatures (below 600 C), the formation and growth of oxide layer is very slow. When the oxidation temperature is above 700 C, the thickening process of oxide layer becomes faster and faster with increasing heating temperature. In order to investigate the phase composition in the oxide layer of Ti-55511 alloy, EDS analyses have been carried out. Representative EDS patterns of the oxidized specimens are shown in Fig. 8. It can be found that with increasing oxidation temperature, the oxygen content (atomic percent) of oxide layer continuously increases. At the oxidation temperatures of 600 C and 700 C, the oxygen contents in oxide layer don't vary a lot from each other, which are about 20 atom%. However, at the oxidation temperature of 800 C, the oxygen content in oxide layer sharply increases, which can reach 66.12 atom%. This indirectly proves the standpoint mentioned above, namely the oxidation process on the surface of Ti-55511 alloy becomes faster and faster with increasing heating temperature. In previous research, Peng et al. [9] had studied the phase component in the oxide layer of Ti-55511 alloy at different oxidation temperatures. They found that at the temperatures of
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Fig. 7. The oxide layer thicknesses of Ti-55511 alloy at different oxidation temperatures: (a) 500 C/2 h, FC, (b) 600 C/2 h, FC, (c) 700 C/2 h, FC, (d) 800 C/2 h, FC.
Fig. 8. The representative EDS patterns at the oxide layer of Ti-55511 alloy: (a) 600 C/2 h, FC, (b) 700 C/2 h, FC, (c) 800 C/2 h, FC.
600 C and 700 C, the main oxides are TiO and Ti2O3. At the temperature of 800 C, the main oxide is TiO2 only. As described above, the fracture toughness is the result of two contributions: the intrinsic part, i.e. the crack tip deformation
ability, and the extrinsic part, i.e. the tortuosity of the crack front. At the oxidation temperature below 600 C, the oxide layer thicknesses are generally lower than 1 mm. Thus the influence of the oxide layer on the fracture toughness of Ti-55511 alloy under these
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Fig. 9. The schematic diagram of the fractured KIC specimen with surface oxide layer.
Fig. 10. The macro fracture surfaces of Ti-55511 alloy undergone different heat treatments at 700 C: (a) vacuum annealing treatment, 700 C/4 h, FC, (b) oxidation treatment, 700 C/2 h, FC.
two oxidation treatments can be almost ignored. The specimens which were vacuum annealed and oxidized below 600 C generally have the similar microstructure features and tensile properties. This makes them obtain similar crack front tortuosity (extrinsic fracture resistance) and similar crack tip deformation ability (intrinsic fracture resistance). Thus it's understandable that at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments generally show no difference. At the oxidation temperatures of 700 C and 800 C, the oxide layer thickness is above 5 mm, and even bigger than 20 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy cannot be ignored. Take 700 C for example, the microstructure features and tensile properties of Ti-55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C. However, the fracture toughness of Ti-55511 alloy unpffiffiffiffiffi dergone oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment. Thus it is exactly the existence of the surface oxide layer (see Fig. 9) causing low fracture resistance of Ti-55511 alloy. The authors proposed that three
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reasons are responsible for the big loss of fracture toughness caused by surface oxidation. Firstly, as we know, during the static loading process in the fracture toughness test, the material at the side surface of KIC specimen is in plane stress condition. With increasing distance from the surface, the stress triaxiality of the interior material increases continuously [25] and finally the plane strain condition is reached. High stress triaxiality can cause rapid damage growth [26], which can offer limited fracture resistance. However, due to the low stress triaxiality of the surface and near-surface material, the fracture at these places always happens in the form of shear cracking. The shear fracture involves a big extent of plastic deformation, which can offer considerable fracture resistance. On the macro fracture surface of KIC specimen, the shear fracture shows up as the ‘shear lips’. The bigger the total shear lip area is, the higher the fracture resistance it can offer. Fig. 10 shows the macro fracture surfaces for Ti-55511 alloy undergone vacuum annealing and oxidation treatments at 700 C. It can be observed that the shear lips of the fractured KIC specimen with oxide layer (Fig. 10b) are obviously smaller than that without oxide layer (Fig. 10a). This is due to that the interior material near the brittle oxide layer, which obtains high plasticity, cannot realize large-scale shear fracture due to its relatively high stress triaxiality and the limitation of the outside oxide layer. Therefore, the existence of oxide layer decreases the total area of shear lips on the fracture surface. As a result, the fracture resistance offered by shear fracture decreases. Secondly, the existence of oxide layer can greatly change the crack propagation tortuosity. Fig. 11 compares the crack propagation paths of Ti-55511 alloy undergone vacuum annealing treatment and oxidation treatment at 700 C. It can be found that the crack propagation path of Ti-55511 alloy without oxide layer (Fig. a and b) is obviously more tortuous than that with oxide layer (Fig. c and d). This is due to that the brittle oxide layer on specimen surface tends to fracture at the direction perpendicular to the loading axis. However, the surface material of the vacuum annealed KIC specimen obtains good plasticity. During the fracture toughness test, surface material is in plane stress condition, which makes it fracture generally at an angle of approximately 45 with the loading direction. Finally, the zigzag crack propagation path forms. As described in previous text, tortuous crack propagation path can increase the extrinsic fracture resistance. Thus, the existence of oxide layer decreases the crack propagation tortuosity. As a result, the extrinsic fracture resistance decreases. Thirdly, by comparing Fig. 11b and d, it can be found that long and deep secondary cracks distributed on the side surface of the vacuum annealed KIC specimen. These secondary cracks can spread over 600 mm away from the crack propagation path. However, the secondary cracks on the side surface of the oxidized KIC specimen are obviously shorter, shallower and narrowly distributed when compared with the vacuum annealed KIC specimen. This is due to that the existence of hard and brittle oxide layer limited the size of crack tip plastic zone on the side surface of KIC specimen. As we know, the secondary cracks appear only if certain energy has been absorbed by material. Thus the formation of secondary cracks is favorable to resist the fracture process of KIC specimen. The existence of oxide layer can limit the formation of secondary cracks on the side surface of KIC specimen. As a result, the fracture resistance offered by secondary cracks decreases. In conclusion, one lesson should be learned that the surface oxidation of Ti-55511 titanium alloy can greatly affect its safe applications. Thus during its thermomechanical treatments, the oxygen content in atmosphere should be strictly controlled.
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Fig. 11. The crack propagation paths of Ti-55511 alloy: (a), (b) vacuum annealing treatment, 700 C/4 h, FC; (c), (d) oxidation treatment, 700 C/2 h, FC.
4. Conclusions (1) With increasing annealing temperature, the fracture toughness of Ti-55511 alloy continuously increase. At the annealing temperature of 700 C, the fracture toughness can reach pffiffiffiffiffi 120:8 MPa m, which is almost twice of that when annealed pffiffiffiffiffi at 550 C (only 65:2 MPa m). Generally speaking, the fracture toughness of Ti-55511 alloy annealed below 600 C vary pffiffiffiffiffi slightly, which are all around 70 MPa m. However, a big increase of the fracture toughness happens when annealed above 650 C. This is due to the sharp decrease of the yield strength for Ti-55511 alloy when annealed at 650 C, which can greatly increase the intrinsic fracture resistance. (2) At the oxidation temperature of 500 C, the oxide layer cannot be visually observed on the specimen surface. As the temperature increases, at 600 C, a thin oxide layer with the thickness of about 1 mm bond tightly with the substrate microstructure (beneath the oxide layer). At the oxidation temperature of 700 C, the oxide layer thickness increases to above 5 mm. When the oxidation temperature increases to 800 C, the oxide layer thickness can reach about 20 mm. Generally speaking, at relatively low temperatures (below 600 C), the formation and growth of oxide layer is very slow. When the oxidation temperature is above 700 C, the thickening process of oxide layer becomes faster and faster with increasing heating temperature. (3) Generally speaking, at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the temperatures above 600 C, the fracture toughness variation between the two kinds of treatments becomes bigger and bigger with increasing heating temperature. For example, at 700 C, the fracture toughness of Ti-55511 alloy undergone pffiffiffiffiffi oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment, which is a sharp decrease. Above phenomena can be explained in the
following. At the oxidation temperatures below 600 C, the oxide layer thicknesses are generally lower than 1 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy can be almost ignored. At the oxidation temperatures of 700 C, however, the oxide layer thickness is above 5 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy cannot be ignored. (4) The existence of oxide layer can decrease the total area of shear lips on the fracture surface, decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of KIC specimen. This can result in a big decrease of fracture resistance, which is detrimental to the fracture toughness of Ti-55511 alloy.
Acknowledgments The authors thank the financial supports from the National Natural Science Foundation of China (Grant No. 51075333) and the Program for New Century Excellent Talents in University (Grant No. NCE-07-0696).
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[20] D. Eylon, J.A. Hall, C.M. Pierce, D.L. Ruckle, Metall. Trans. A 7 (1976) 1817e1826. [21] V. Sinha, W.O. Soboyejo, Mater. Sci. Eng. A 319e321 (2001) 607e612. [22] A. Ghosh, S. Sivaprasad, A. Bhattacharjee, et al., Mater. Sci. Eng. A 568 (2013) 61e67. [23] X.H. Shi, W.D. Zeng, C.L. Shi, et al., J. Alloys. Compd. 632 (2015) 748e755. [24] J.P. Hirth, F.H. Froes, Metall. Trans. A 8 (1977) 1165e1176. [25] A. AsserineLebert, J. Besson, A.F. Gourgues, Mater. Sci. Eng. A 395 (2005) 186e194. [26] F. Rivalin, J. Besson, M. Di Fant, A. Pineau, Eng. Fract. Mech. 68 (2000) 347e364.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The effect of surface oxidation behavior on the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe titanium alloy Xiaohui Shi a, *, Weidong Zeng a, **, Qinyang Zhao b a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, China School of Materials Science and Engineering, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2015 Received in revised form 10 May 2015 Accepted 30 May 2015 Available online 10 June 2015
In this study, the effect of surface oxidation behavior on the fracture toughness of Tie5Ale5Moe5Ve1Cr e1Fe alloy suffered oxidation treatments at the temperatures of 500 C, 600 C, 700 C and 800 C is studied by means of OM, SEM and EDS analysis. For comparison, the fracture toughness of this alloy suffered vacuum annealing treatments at the temperatures of 550 C, 600 C, 650 C and 700 C are also investigated. The results show that at the heating temperatures below 600 C, the fracture toughness values of Tie5Ale5Moe5Ve1Cre1Fe alloy suffered vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the heating temperature of 700 C, the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy undergone oxidation treatment is almost 50 MPa m1/2 lower than that undergone vacuum annealing treatment. This is due to that at the oxidation temperature of 700 C, the oxide layer thickness is above 5 mm. In this situation, its influence on the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy cannot be ignored. The existence of oxide layer can decrease the total area of shear lips on the fracture surface, decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of KIC specimen. This can result in a big decrease of fracture resistance, which is detrimental to the fracture toughness of Tie5Ale5Moe5Ve1Cre1Fe alloy. © 2015 Elsevier B.V. All rights reserved.
Keywords: Surface oxidation Tie5Ale5Moe5Ve1Cre1Fe Fracture toughness Vacuum annealing
1. Introduction Fracture toughness describes the ability of a material containing a crack to resist fracture. According to the damage tolerance design concept applied in aerospace industry, fracture toughness is an important index for material selection and component design to guarantee safe flight [1]. Titanium alloys are widely used in the field of aerospace due to high specific strength, good corrosion resistance and good high-temperature properties [2,3]. As an important structural material in aircrafts, high fracture toughness of titanium alloys have been pursued by material scientists for a long time. According to previous researches [4e7], the microstructure features of a titanium alloy can greatly influence its fracture toughness. For example, Bhattacharjee et al. [4] found that the fracture toughness of Tie10Ve2Fee3Al titanium alloy
* Corresponding author. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No.127 Youyi Xilu, Xi'an 710072, China. ** Corresponding author. E-mail addresses: [email protected] (X. Shi), [email protected] (W. Zeng). http://dx.doi.org/10.1016/j.jallcom.2015.05.211 0925-8388/© 2015 Elsevier B.V. All rights reserved.
approximately follows a Hall-Petch relationship with its b grain size. Greenfield et al. [5] found that the increasing grain boundary a thickness can obviously improve the fracture toughness of Tie5.25Ale5.5Ve0.9Fee0.5Cu alloy until a critical value of 5.5 mm is reached. It is due to that thicker grain boundary a can more effectively blunt the running crack tip, which can increase the fracture resistance. Cvijovi c-Alagic et al. [6] thought that the colony size and the aspect ratio of a phase are two main factors influencing the fracture toughness of Tie6Ale4V alloy with lamellar structure. They found that big colony size and high aspect ratio of a platelet are favorable to deflect the crack propagation path, which can effectively improve the fracture toughness. Richards [7] studied the fracture toughness of two a þ b titanium alloys containing a platelets in transformed b matrix. He found that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest a platelets. It is explained in terms of the distance between active centers of void nucleation, which is a function of the a platelet thickness and the platelet spacing. Based on above research results, engineers generally optimize the fracture toughness of titanium alloy by controlling its microstructure features. However, the microstructure features may
X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
be not the only factor influencing fracture toughness. Due to the high affinity for oxygen, exposure of titanium alloys to any oxygen containing atmosphere at elevated temperatures leads to formation of an hard and brittle oxide layer on the specimen surface, which can also exert big influence on mechanical properties [8e10]. Jia et al. [8] made a research on oxidation behavior and effect of oxidation on tensile properties of Ti60 alloy. They found that both the strength and ductility of the specimens with oxide scale decreased when compared with the specimens without oxide scale. Çelik et al. [10], however, found that the oxidation treatments brought about a remarkable increase in wear resistance of ultrafine-grained Ti because of the formation of adhered and hard TiO2 film developed on the surface. It can be found that the previous researches were mainly about the tensile properties and tribological properties of titanium alloys. Unfortunately, little work has been conducted concerning the effect of surface oxidation on the fracture toughness of titanium alloys. To date, it is still inevitable to ensure the titanium alloys being processed in the absence of oxygen containing atmosphere. Thus understanding the surface oxidation-fracture toughness interrelations of titanium alloys can be very meaningful. Ti-55511 (Tie5Ale5Moe5Ve1Cre1Fe) titanium alloy, which is a highly alloyed near-b titanium alloy [11], is designed based on considerations of damage tolerance targets. It has excellent combinations of strength, toughness, fatigue and crack growth resistance, and is especially suitable for making big critical aviation forgings [12]. In the previous reports, the research efforts were mainly focused on the application, microstructure and tensile properties of Ti-55511 alloy [13e15]. In order to basically understand the influence of surface oxidation on the fracture toughness of Ti-55511 alloy. In the present study, a tentative investigation on this issue has been conducted. Moreover, the influencing mechanisms of the surface oxidation on the fracture toughness of Ti55511 alloy are tried to be explained. 2. Experimental procedures 2.1. Materials
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Fig. 1. The typical microstructure of Ti-55511 titanium alloy after forging and double annealing treatment.
preparation. The first step is applied for releasing the residual stress in the forging. Moreover, some metastable b phase can be retained at this step. The second step, however, is a stabilization treatment, which can make a preparation for the following aging treatment. The final step, namely the aging step, is to prompt the precipitation process of the dispersive secondary a phase from the metastable b phase. After the forging and double annealing treatment, the obtained microstructure of Ti-55511 alloy is shown in Fig. 1. It can be found that the microstructure in Fig. 1 belongs to typical basketweave microstructure type. The original grain boundary a phase has been broken to some extent. Moreover, the thin a platelets generally in length of 5e15 mm and thickness of 1e2 mm weave with each other. The total volume fraction of a phase is about 40%. In addition, it can be found that a small amount of equiaxed a phase (approximately has a volume fraction of 3%) exists in the transformed matrix, which is due to the globularization behavior of a platelets when undergone slight deformation in the a þ b phase field. 2.2. Oxidation treatments and vacuum annealing treatments
The experimental Ti-55511 titanium alloy was supplied by Western Superconducting Technologies Co., Ltd., PR China. Table 1 shows its chemical composition. The b-transus temperature (Tb) of this alloy was identified as approximately 843 C via metallographic techniques. The detailed procedures are like following: (i) heating one specimen to 800 C and then held at this temperature for 4 h, followed by water quenching; (ii) observing the microstructure of this specimen using Olympus/PMG3 optical microscope, if there exists some retained a phase in microstructure, then increasing the heating temperature by 5 C; (iii) repeating step (ii) until no retained a phase can be observed in microstructure, then taking this temperature as T1. The average value of T1 and T1 5 C can be determined as the b-transus temperature of Ti-55511 titanium alloy. The as-received Ti-55511 alloy had been forged at Tb þ 15 C and then undergone double annealing treatment, which contains three steps: (i) heating to 820 C, holding for 2 h, and furnace cooling (FC) to 750 C; (ii) holding at 750 C for 2 h and air-cooled to room temperature; (iii) aging at 630 C for 4 h and air cooling [16]. Different steps have different meanings in microstructure
According to the research by Peng et al. [9], the surface oxidation is a very slow process at the temperature below 500 C, which generally has no influence on the tensile properties of Ti-55511 alloy. Thus the authors proposed that the surface oxidation may exert little influence on the fracture toughness of Ti-55511 alloy at the temperature below 500 C. In the present study, the four oxidation temperatures including 500 C, 600 C, 700 C and 800 C were selected, as listed in Table 2. All the oxidation treatments were conducted in the electric furnace (named N60/85HA) with FeeCreAl alloy resistance wire. In the electric furnace, the initial volume fraction of oxygen is about 21%. During the oxidation process, the Ti-55511 samples undergone heat preservation for 2 h at different temperatures, followed by furnace cooling. To reflect the effect of surface oxidation on the fracture toughness of Ti-55511 alloy, the vacuum annealing treatments were also conducted, which can be used for comparison. Four temperatures including
Table 2 The oxidation treatments and vacuum annealing treatments of Ti-55511 alloy. Vacuum annealing treatments
Table 1 The chemical composition of the Ti-55511 alloy (wt%). Ti
C
N
H
O
Al
Mo
V
Cr
Fe
Zr
Si
Bal.
0.01
0.01
0.0015
0.066
5.2
5.0
4.9
0.96
0.91
<0.3
<0.1
550 600 650 700
C/4 C/4 C/4 C/4
h, h, h, h,
FC FC FC FC
Oxidation treatments 500 600 700 800
C/2 C/2 C/2 C/2
h, h, h, h,
FC FC FC FC
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550 C, 600 C, 650 C and 700 C were selected for vacuum annealing (see Table 2). To ensure the near-vacuum condition, the vacuum furnace named VPS50/10 was used, which can reach the final vacuum of 103 Pa. 2.3. Tests Cylindrical samples with 13.5 mm in diameter and 70 mm in length were electro-discharged machined from the vacuum annealed and the oxidized Ti-55511 alloy. Then the standard tensile specimens were machined from these cylindrical samples with a gauge length of 25 mm and a diameter of 5 mm. Three smooth tensile specimens were used to test tensile properties under each treatment at room temperature. The tensile properties of the asreceived Ti-55511 alloy were also measured. The standard CT (compact tension) specimen type with width/ thickness ratio of 2:1 were adopted for fracture toughness test, as shown in Fig. 2. Two specimens were adopted to evaluate the fracture toughness of Ti-55511 alloy under all conditions. The detailed KIC test procedures of vacuum annealed Ti-55511 alloy are as follows. Firstly, samples with the dimensions of 62.5 mm 60 mm 25 mm were machined from the vacuum annealed Ti-55511 alloy. Secondly, electro-discharged machined notches on the cuboid samples were prepared and the fatigue precracking of the notches were carried out using MTS810 fatigue testing machine. Finally, the prepared specimens were stretched to fracture on the condition that the specified increase rate of the pffiffiffiffiffi stress intensity factor is within the range 0.55 and 2:75 MPa m=s. During the testing process, the loadedisplacement curve will be recorded. After all, the apparent fracture toughness KQ can be calculated (see Eqs. (1) and (2)) if the fracture force FQ determined from the loadedisplacement curve and the average crack length a determined from fracture surface are available. However, KQ doesn't necessarily equals KIC only if the small scale yielding and plane strain conditions at the crack tip of test specimen are met. Thus in the present work, all the test specimens were machined thick enough and the fatigue precrack long enough to meet above two conditions.
. KQ ¼ FQ BW1=2 fða=WÞ
fða=WÞ ¼ ð2 þ a=WÞ
(1)
surfaces and crack propagation paths of all the KIC specimens were analyzed with a JSM-6390A scanning electron microscope (SEM). The thicknesses of the oxide layer at different oxidation temperatures were also observed with JSM-6390A. Moreover, to determine the chemical composition of the oxide layer, the EDS analyses were conducted.
3. Results and discussion 3.1. Microstructures and tensile properties Fig. 3 shows the microstructures of Ti-55511 alloy at different vacuum annealing temperatures. It can be found that the microstructures annealed at 550 C and 600 C are similar, which both contain long a platelets generally in length of 5e15 mm and thickness of 1e2 mm, as shown in Fig. 3a and b respectively. It can be found that the microstructures annealed at 550 C and 600 C show no big difference with the initial microstructure (Fig. 1). This is due to the limited phase transition driving force at relatively low annealing temperatures. For Ti-55511 alloy annealed at 650 C, however, the a platelets in microstructure (Fig. 3c) are obviously shorter and thinner when compared with Fig. 3a and b, which are within 10 mm in length and about 1 mm in thickness. Moreover, it can be found that the total volume fraction of a phase in Fig. 3c is obviously lower when compared with Fig. 3a and b, from about 40% to about 30%. As described above, Ti-55511 titanium alloy is a nearb titanium alloy [11], thus it is easy for the a phase to transform into b phase when heated at relatively high temperatures. At the annealing temperature of 650 C, the a phase becomes more unstable than that at 550 C and 600 C. Therefore, some a phase transformed into b phase, which leads to the decreased volume fraction of a phase. By increasing the annealing temperature to 700 C, the shape of the a platelets (Fig. 3d) changes little when compared with Fig. 3c. However, it can be observed that the total volume fraction of a phase slightly decreases, from about 30% to 25%. In conclusion, with increasing annealing temperature, the a platelets becomes shorter and the total volume fraction of a phase continuously decreases. Fig. 4 shows the microstructures of Ti-55511 alloy at different oxidation temperatures. It can be found that the microstructure features of Ti-55511 alloy oxidized at 500 C and 600 C are very similar to the microstructures vacuum annealed at 550 C and
0:866 þ 4:64ða=WÞ 13:32ða=WÞ2 þ 14:72ða=WÞ3 5:6ða=WÞ4 ð1 a=WÞ3=2
The KIC test procedures of Ti-55511 alloy under oxidation treatments are a little different with the vacuum annealed parts. The samples with the dimensions of 62.5 mm 60 mm 25 mm were oxidized in the electric furnace at different temperatures firstly. Then these samples gone through the same preparation (notching, fatigue precrack) and testing processes as described above. The fracture toughness of the as-received Ti-55511 alloy was also measured. In addition, one thing should be noted that all the properties used in present paper are the average values. The metallographic specimens were prepared by grinding, polishing and corroding and the microstructural observations were conducted with Olympus/PMG3 optical microscope. The fracture
(2)
600 C respectively. Moreover, the microstructure features of Ti55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C (see Fig. 3d). This can be attributed to their similar heating processes, which may lead to similar driving force of phase transformation. Fig. 4d shows the microstructure of Ti55511 alloy oxidized at 800 C, it can be found that the number of a platelets decreases obviously when compared with Fig. 4aec. Moreover, the a platelets in Fig. 4d are obviously shorter and thicker when compared with Fig. 4aec, which are within 5 mm in length and about 2 mm in thickness. It is well known that low aspect ratio a platelets have less total interfacial energy than that of big aspect ratio a platelets. Thus, the energy minimization provides the driving force for a platelet shortening and thickening, and the process is assisted by solute atom diffusion [17].
X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
Fig. 2. The schematic KIC specimen configurations (CT).
The tensile properties including yield strength (YS), ultimate tensile strength (UTS), elongation (EL) and reduction of area (RA) of Ti-55511 alloy at different vacuum annealing temperatures are listed in Table 3. It can be found that the tensile properties of Ti55511 alloy annealed at 550 C and 600 C are similar to the tensile properties of the as-received microstructure. They all get the yield strength of about 1100 MPa and the elongation of about 14%. Moreover, it should be noted that with increasing annealing temperature, the strengths of Ti-55511 alloy continuously decrease. At the annealing temperature of 700 C, the yield strength is only 940 MPa, which is 180 MPa lower than that at the annealing temperature of 550 C. The plasticity of Ti-55511 alloy, however, shows a reverse changing tendency to its strengths. The specimens annealed at 700 C get the highest plasticity among the four vacuum annealing treatments, which have the average elongation of 18.8% and the reduction of area of 65%. Generally speaking, the tensile properties of Ti-55511 alloy vacuum annealed below 600 C vary slightly. However, the tensile properties of Ti-55511 alloy have a big change when annealed above 650 C.
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All the phenomena described above can be explained in the following. With increasing annealing temperature (especially above 650 C), the a platelets in microstructure tend to be shorter and the total volume fraction of a phase becomes lower. This results in the weakening of the interfacial strengthening effect [18]. Moreover, the increasing volume fraction of b phase (bcc structure) can improve the deformation ability of Ti-55511 alloy. At the annealing temperature below 600 C, the similar microstructure features of Ti-55511 alloy result in similar tensile properties. However, at the annealing temperatures above 650 C, the obvious decrease in length and volume fraction of a phase lead to a big change of tensile properties. The tensile properties of Ti-55511 alloy at different oxidation temperatures are also listed in Table 3. It can be found that the tensile properties of Ti-55511 alloy oxidized at 500 C and 600 C are very similar to that vacuum annealed at 550 C and 600 C respectively. Moreover, the tensile properties of Ti-55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C. This can be attributed to their similar microstructure features. When the oxidation temperature increases from 700 C to 800 C, the strengths of Ti-55511 alloy decrease slightly. However, its elongation undergoes a big loss, which drops from 16.2% to 8.8%. The oxidation layer on the surface of tensile specimens is responsible for it [9]. In conclusion, at the oxidation temperature below 700 C, the surface oxidation almost exerts no influence on the tensile properties of Ti-55511 alloy. 3.2. Fracture toughness The fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments are listed in Table 4. It can be found that with increasing annealing temperature, the fracture toughness of Ti-55511 alloy continuously increase. At the annealing temperpffiffiffiffiffi ature of 700 C, the fracture toughness can reach 120:8 MPa m, which is almost twice of that when annealed at 550 C (only pffiffiffiffiffi 65:2 MPa m). Generally speaking, the fracture toughness of Ti55511 alloy annealed below 600 C vary slightly, which are all pffiffiffiffiffi around 70 MPa m. However, a big increase of the fracture
Fig. 3. The microstructures of Ti-55511 alloy at different vacuum annealing temperatures: (a) 550 C/4 h, FC, (b) 600 C/4 h, FC, (c) 650 C/4 h, FC, (d) 700 C/4 h, FC.
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X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
Fig. 4. The microstructures of Ti-55511 alloy at different oxidation temperatures: (a) 500 C/2 h, FC, (b) 600 C/2 h, FC, (c) 700 C/2 h, FC, (d) 800 C/2 h, FC.
Table 3 The tensile properties of Ti-55511 alloy under different conditions. UTS/MPa As-received 1135.0 Vacuum annealing treatments 550 C/4 h 1150.0 600 C/4 h 1116.7 650 C/4 h 1023.3 700 C/4 h 951.7 Oxidation treatments 500 C/2 h 1148.3 600 C/2 h 1126.7 700 C/2 h 957.7 800 C/2 h 941.0
YS/MPa
EL/%
RA/%
1090.0
14.0
57.0
1120.0 1093.3 996.7 940.0
13.0 15.0 17.8 18.8
49.7 56.7 60.3 65.0
1123.3 1103.3 953.3 937.5
14.5 13.8 16.2 8.8
59.6 55.8 64.2 43.0
toughness happens when annealed above 650 C. In order to explain above phenomena, the factors influencing the fracture toughness of materials must be clarified. Lütjering et al. [19] and Eylon et al. [20] thought that the fracture toughness is the result of two contributions: the intrinsic fracture resistance, i.e. the direct influence on the fracture properties of the material (ductility), and the extrinsic fracture toughness, i.e. the additional contribution of the crack front geometry (the tortuosity of the crack front). Based on above theory [19,20], big crack tip deformation zone as well as tortuous crack propagation path are both favorable to the fracture toughness. Fig. 5 shows the fracture surfaces of the KIC specimens at different vacuum annealing temperatures. It can be found that the fracture surfaces of Ti-55511 alloy annealed at 550 C and 600 C
are rough and characterized by big and deep ravines and secondary cracks, as shown in Fig. 5a and b respectively. As described in Refs. [21,22], long a platelets permit larger bifurcations of the main crack, enhance the crack deflection effect and thus increase crack path tortuosity. Based on above theory, the rough fracture surfaces in Fig. 5a and b can be easily explained. By contrast, the fracture surfaces of Ti-55511 alloy annealed at 650 C (Fig. 5c) and 700 C (Fig. 5d) are relatively flat, which are characterized by dense dimples. The widely distributed dimples on fracture surface indicate good deformation ability at the running crack tip during the fracture process. This can be attributed to the obviously lower yield strengths (namely higher plasticity) of Ti-55511 alloy annealed at 650 C and 700 C. According to previous research [23], the intrinsic fracture resistance plays the main role in deciding the fracture toughness of Ti-55511 alloy. Thus, the fracture toughness generally shows a reverse trend to the yield strength of a material, which has the correlation in the form of KIC f YS1 [24]. Based on above discussion, the fracture toughness shows a reverse trend to the yield strength in this work is not surprising at all. In addition, due to the sharp decrease of the yield strength for Ti-55511 alloy when annealed at 650 C, a big increase of the fracture toughness naturally happens. The fracture toughness values of Ti-55511 alloy undergone oxidation treatments are listed in Table 4. It can be found that at the oxidation temperatures below 700 C, the fracture toughness pffiffiffiffiffi values of this alloy stay around 70 MPa m. However, at the oxidation temperature of 800 C, the fracture toughness decreases pffiffiffiffiffi to merely 43:2 MPa m. In order to more clearly figure out the
Table 4 The fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments. pffiffiffiffiffi Vacuum annealing treatments KIC =MPa m Oxidation treatments 550 600 650 700
C/4 C/4 C/4 C/4
h h h h
65.2 73.5 105.1 120.8
500 600 700 800
C/2 C/2 C/2 C/2
h h h h
pffiffiffiffiffi KIC =MPa m 64.1 72.5 68.8 43.2
X. Shi et al. / Journal of Alloys and Compounds 647 (2015) 740e749
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Fig. 5. The fracture surfaces of the KIC specimens at different vacuum annealing temperatures: (a) 550 C/4 h, FC, (b) 600 C/4 h, FC, (c) 650 C/4 h, FC, (d) 700 C/4 h, FC.
effect of surface oxidation behavior on the fracture toughness of Ti55511 alloy, the comparisons of the fracture toughness values between the specimens undergone vacuum annealing treatments and oxidation treatments are shown in Fig. 6. Generally speaking, at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the temperatures above 600 C, the fracture toughness variation between the two kinds of treatments becomes bigger and bigger with increasing heating temperature. For example, at 700 C, the fracture toughness of Ti-55511 alloy unpffiffiffiffiffi dergone oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment, which is a sharp decrease. In conclusion, the surface oxidation can lead to a big loss of the fracture toughness for Ti-55511 alloy at the oxidation temperatures above 600 C. This is extremely detrimental to the security of the
Fig. 6. The comparisons of the fracture toughness values between the specimens undergone vacuum annealing treatments and oxidation treatments.
bearing carriers. Till now, the reasons responsible for this phenomenon is still unclear. In the following text, above questions are tried to be answered.
3.3. The oxidation behavior and its influence on fracture toughness Understanding the surface oxidation behavior of Ti-55511 alloy is the key to answer the questions mentioned in previous section. Fig. 7 shows the oxide layers of Ti-55511 alloy at different oxidation temperatures. It can be found that at the oxidation temperature of 500 C (Fig. 7a), the oxide layer cannot be visually observed on the specimen surface. As the temperature increases, at 600 C, a thin oxide layer with the thickness of about 1 mm bond tightly with the substrate microstructure (beneath the oxide layer), as shown in Fig. 7b. At the oxidation temperature of 700 C, the oxide layer thickness increases to above 5 mm, as shown in Fig. 7c. When the oxidation temperature increases to 800 C, the oxide layer thickness can reach about 20 mm. Generally speaking, at relatively low temperatures (below 600 C), the formation and growth of oxide layer is very slow. When the oxidation temperature is above 700 C, the thickening process of oxide layer becomes faster and faster with increasing heating temperature. In order to investigate the phase composition in the oxide layer of Ti-55511 alloy, EDS analyses have been carried out. Representative EDS patterns of the oxidized specimens are shown in Fig. 8. It can be found that with increasing oxidation temperature, the oxygen content (atomic percent) of oxide layer continuously increases. At the oxidation temperatures of 600 C and 700 C, the oxygen contents in oxide layer don't vary a lot from each other, which are about 20 atom%. However, at the oxidation temperature of 800 C, the oxygen content in oxide layer sharply increases, which can reach 66.12 atom%. This indirectly proves the standpoint mentioned above, namely the oxidation process on the surface of Ti-55511 alloy becomes faster and faster with increasing heating temperature. In previous research, Peng et al. [9] had studied the phase component in the oxide layer of Ti-55511 alloy at different oxidation temperatures. They found that at the temperatures of
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Fig. 7. The oxide layer thicknesses of Ti-55511 alloy at different oxidation temperatures: (a) 500 C/2 h, FC, (b) 600 C/2 h, FC, (c) 700 C/2 h, FC, (d) 800 C/2 h, FC.
Fig. 8. The representative EDS patterns at the oxide layer of Ti-55511 alloy: (a) 600 C/2 h, FC, (b) 700 C/2 h, FC, (c) 800 C/2 h, FC.
600 C and 700 C, the main oxides are TiO and Ti2O3. At the temperature of 800 C, the main oxide is TiO2 only. As described above, the fracture toughness is the result of two contributions: the intrinsic part, i.e. the crack tip deformation
ability, and the extrinsic part, i.e. the tortuosity of the crack front. At the oxidation temperature below 600 C, the oxide layer thicknesses are generally lower than 1 mm. Thus the influence of the oxide layer on the fracture toughness of Ti-55511 alloy under these
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Fig. 9. The schematic diagram of the fractured KIC specimen with surface oxide layer.
Fig. 10. The macro fracture surfaces of Ti-55511 alloy undergone different heat treatments at 700 C: (a) vacuum annealing treatment, 700 C/4 h, FC, (b) oxidation treatment, 700 C/2 h, FC.
two oxidation treatments can be almost ignored. The specimens which were vacuum annealed and oxidized below 600 C generally have the similar microstructure features and tensile properties. This makes them obtain similar crack front tortuosity (extrinsic fracture resistance) and similar crack tip deformation ability (intrinsic fracture resistance). Thus it's understandable that at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments generally show no difference. At the oxidation temperatures of 700 C and 800 C, the oxide layer thickness is above 5 mm, and even bigger than 20 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy cannot be ignored. Take 700 C for example, the microstructure features and tensile properties of Ti-55511 alloy oxidized at 700 C are almost the same with that vacuum annealed at 700 C. However, the fracture toughness of Ti-55511 alloy unpffiffiffiffiffi dergone oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment. Thus it is exactly the existence of the surface oxide layer (see Fig. 9) causing low fracture resistance of Ti-55511 alloy. The authors proposed that three
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reasons are responsible for the big loss of fracture toughness caused by surface oxidation. Firstly, as we know, during the static loading process in the fracture toughness test, the material at the side surface of KIC specimen is in plane stress condition. With increasing distance from the surface, the stress triaxiality of the interior material increases continuously [25] and finally the plane strain condition is reached. High stress triaxiality can cause rapid damage growth [26], which can offer limited fracture resistance. However, due to the low stress triaxiality of the surface and near-surface material, the fracture at these places always happens in the form of shear cracking. The shear fracture involves a big extent of plastic deformation, which can offer considerable fracture resistance. On the macro fracture surface of KIC specimen, the shear fracture shows up as the ‘shear lips’. The bigger the total shear lip area is, the higher the fracture resistance it can offer. Fig. 10 shows the macro fracture surfaces for Ti-55511 alloy undergone vacuum annealing and oxidation treatments at 700 C. It can be observed that the shear lips of the fractured KIC specimen with oxide layer (Fig. 10b) are obviously smaller than that without oxide layer (Fig. 10a). This is due to that the interior material near the brittle oxide layer, which obtains high plasticity, cannot realize large-scale shear fracture due to its relatively high stress triaxiality and the limitation of the outside oxide layer. Therefore, the existence of oxide layer decreases the total area of shear lips on the fracture surface. As a result, the fracture resistance offered by shear fracture decreases. Secondly, the existence of oxide layer can greatly change the crack propagation tortuosity. Fig. 11 compares the crack propagation paths of Ti-55511 alloy undergone vacuum annealing treatment and oxidation treatment at 700 C. It can be found that the crack propagation path of Ti-55511 alloy without oxide layer (Fig. a and b) is obviously more tortuous than that with oxide layer (Fig. c and d). This is due to that the brittle oxide layer on specimen surface tends to fracture at the direction perpendicular to the loading axis. However, the surface material of the vacuum annealed KIC specimen obtains good plasticity. During the fracture toughness test, surface material is in plane stress condition, which makes it fracture generally at an angle of approximately 45 with the loading direction. Finally, the zigzag crack propagation path forms. As described in previous text, tortuous crack propagation path can increase the extrinsic fracture resistance. Thus, the existence of oxide layer decreases the crack propagation tortuosity. As a result, the extrinsic fracture resistance decreases. Thirdly, by comparing Fig. 11b and d, it can be found that long and deep secondary cracks distributed on the side surface of the vacuum annealed KIC specimen. These secondary cracks can spread over 600 mm away from the crack propagation path. However, the secondary cracks on the side surface of the oxidized KIC specimen are obviously shorter, shallower and narrowly distributed when compared with the vacuum annealed KIC specimen. This is due to that the existence of hard and brittle oxide layer limited the size of crack tip plastic zone on the side surface of KIC specimen. As we know, the secondary cracks appear only if certain energy has been absorbed by material. Thus the formation of secondary cracks is favorable to resist the fracture process of KIC specimen. The existence of oxide layer can limit the formation of secondary cracks on the side surface of KIC specimen. As a result, the fracture resistance offered by secondary cracks decreases. In conclusion, one lesson should be learned that the surface oxidation of Ti-55511 titanium alloy can greatly affect its safe applications. Thus during its thermomechanical treatments, the oxygen content in atmosphere should be strictly controlled.
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Fig. 11. The crack propagation paths of Ti-55511 alloy: (a), (b) vacuum annealing treatment, 700 C/4 h, FC; (c), (d) oxidation treatment, 700 C/2 h, FC.
4. Conclusions (1) With increasing annealing temperature, the fracture toughness of Ti-55511 alloy continuously increase. At the annealing temperature of 700 C, the fracture toughness can reach pffiffiffiffiffi 120:8 MPa m, which is almost twice of that when annealed pffiffiffiffiffi at 550 C (only 65:2 MPa m). Generally speaking, the fracture toughness of Ti-55511 alloy annealed below 600 C vary pffiffiffiffiffi slightly, which are all around 70 MPa m. However, a big increase of the fracture toughness happens when annealed above 650 C. This is due to the sharp decrease of the yield strength for Ti-55511 alloy when annealed at 650 C, which can greatly increase the intrinsic fracture resistance. (2) At the oxidation temperature of 500 C, the oxide layer cannot be visually observed on the specimen surface. As the temperature increases, at 600 C, a thin oxide layer with the thickness of about 1 mm bond tightly with the substrate microstructure (beneath the oxide layer). At the oxidation temperature of 700 C, the oxide layer thickness increases to above 5 mm. When the oxidation temperature increases to 800 C, the oxide layer thickness can reach about 20 mm. Generally speaking, at relatively low temperatures (below 600 C), the formation and growth of oxide layer is very slow. When the oxidation temperature is above 700 C, the thickening process of oxide layer becomes faster and faster with increasing heating temperature. (3) Generally speaking, at the heating temperatures below 600 C, the fracture toughness values of Ti-55511 alloy undergone vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the temperatures above 600 C, the fracture toughness variation between the two kinds of treatments becomes bigger and bigger with increasing heating temperature. For example, at 700 C, the fracture toughness of Ti-55511 alloy undergone pffiffiffiffiffi oxidation treatment is almost 50 MPa m lower than that undergone vacuum annealing treatment, which is a sharp decrease. Above phenomena can be explained in the
following. At the oxidation temperatures below 600 C, the oxide layer thicknesses are generally lower than 1 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy can be almost ignored. At the oxidation temperatures of 700 C, however, the oxide layer thickness is above 5 mm. In this situation, the influence of the oxide layer on the fracture toughness of Ti-55511 alloy cannot be ignored. (4) The existence of oxide layer can decrease the total area of shear lips on the fracture surface, decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of KIC specimen. This can result in a big decrease of fracture resistance, which is detrimental to the fracture toughness of Ti-55511 alloy.
Acknowledgments The authors thank the financial supports from the National Natural Science Foundation of China (Grant No. 51075333) and the Program for New Century Excellent Talents in University (Grant No. NCE-07-0696).
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