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Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat treatment
Author’s Accepted Manuscript Improvement in fatigue strength of notched Ti-6Al4V alloy by short-time heat treatment Tatsuro Morita, Satoshi Tanaka, Susumu Ninomiya
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S0921-5093(16)30586-X http://dx.doi.org/10.1016/j.msea.2016.05.071 MSA33699
To appear in: Materials Science & Engineering A Received date: 26 March 2016 Revised date: 17 May 2016 Accepted date: 18 May 2016 Cite this article as: Tatsuro Morita, Satoshi Tanaka and Susumu Ninomiya, Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat t r e a t m e n t , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.05.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat treatment Tatsuro Moritaa*, Satoshi Tanakab1, Susumu Ninomiyac a
Faculty of Mechanical Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan b
Department of Mechanical and System Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto 606-8585, Japan c Japan Thermo-tech Co. Ltd., Kanagawa 230-0045, Japan *
Corresponding author. Tel.: +81 75 724 7326. fax: +81 75 724 7300. [email protected]
Abstract This study was conducted to improve the fatigue strength of notched Ti-6Al-4V alloy without a surface finish by short-time heat treatment. Specimens were heated at 1203 K and 1233 K for 60 s, and then quenched to create ST1203 material and ST1233 material, respectively. The microstructures of both heat-treated materials were composed of equiaxial grains and the prior phase. The fine acicular ’ phase was generated in the prior phase by quenching. The results of electron diffraction suggested that the metastable phase partially existed in the prior phase. The volume fractions of the prior phase and the ’ phase in this phase were higher in ST1233 material than in ST1203 material. The tensile strength was improved by the generation of the ’ phase; its improvement percentage was higher in ST1233 material than in ST1203 material. The fatigue strength was markedly improved by the heat treatment because crack propagation was strongly suppressed by the generation of the ’ phase and the strain-induced transformation of the metastable phase. The improvement percentage of the fatigue strength was higher in ST1233 material (40%) than in ST1203 material (20%). Keywords: short-time heat treatment, Ti-6Al-4V alloy, fatigue strength, notch, microstructure, mechanical properties
1
Present affiliation: Shimano Inc.
1
1.
Introduction
Ti-6Al-4V alloy is a typical titanium alloy with high specific strength and excellent corrosion resistance. This titanium alloy has been widely used in the aerospace industry, the medical implant industry, etc. The high performance and usefulness of Ti-6Al-4V alloy can be easily understood from its market share of more than 50% in the US titanium market [1]. The industrial application range of Ti-6Al-4V alloy will expand further if the strength can be improved by heat treatment and the cost-effectiveness heightened. From this viewpoint, we investigated the effect of short-time heat treatment on the mechanical properties and the fatigue strength of Ti-6Al-4V alloy [2-5]. The most effective heat treatment was composed of a first heat treatment at 1203 K for 60 s (quenching) and a second heat treatment at 853 K for 40 s (air cooling). This duplex heat treatment improved the tensile strength and the fatigue strength by 29% and 22%, respectively, with no reduction of ductility. In the above studies, smooth polished specimens were used to clarify the effect of the heat treatment without any influence of stress concentration or surface condition. However, actual mechanical parts generally possess stress risers such as notches and keyholes. Also, they are used with no surface finish after heat treatment. In the case of titanium, once fatigue cracks have generated at the surface, they continue to propagate without stopping until final fracture. Accordingly, the fatigue strength of titanium is markedly sensitive to factors that accelerate the generation of fatigue cracks such as stress concentration [6] and the formation of surface compounds [7-9]. According to our previous study [3], the first heat treatment (short-time solution treatment) in the above-mentioned duplex heat treatment generated a fine acicular ’ martensite phase and improved the tensile strength of Ti-6Al-4V alloy. At the same time, this heat treatment increased the ductility through the strain-induced transformation of the metastable phase. If the propagation of generated fatigue cracks is strongly suppressed by the increase of slip resistance and the occurrence of strain-induced transformation at the crack tips, improvement of the fatigue strength can be achieved by the heat treatment independently of stress concentration and surface condition. In this study, the effect of the short-time solution treatment was investigated to improve the fatigue strength of notched Ti-6Al-4V alloy (ELI) without a surface finish. Two suitable conditions for the heat treatment was selected in a pre-examination. The materials heat-treated under the selected conditions were subjected to detailed examinations. The same examinations were also performed on the untreated materials for comparison. 2
The microstructures were investigated by optical observation, EBSD (electron back-scattered diffraction) analysis and TEM (transmission electron microscopy). The crystallographic phases were determined by electron diffraction and X-ray diffraction. The mechanical properties and the fatigue strength were investigated and the fracture surfaces were observed by SEM (scanning electron microscopy). The cross sections of fatigue specimens unbroken at 107 cycles were observed. Moreover, EBSD analysis was performed for a typical non-propagating crack found in a cross section of the heat-treated material. 2. Materials and experimental procedures 2.1 Materials and conditions of heat treatment Table 1 shows the chemical composition of the Ti-6Al-4V (ELI) alloy used in this study. The material was supplied as cold-rolled round bars and machined to the three specimen shapes shown in Fig. 1. The button-type specimens with the shape shown in Fig. 1(a) were used for hardness measurement and the investigation of microstructures. Figures 1(b) and (c) show the shapes of the tensile specimens and the fatigue specimens, respectively. The change of hardness due to the short-time solution treatment was examined to select suitable treatment conditions. The treatment temperatures were 1173 K, 1203 K and 1233 K and the holding times at each temperature were 30 s, 60 s and 120 s. According to the previous studies [2, 4], the tensile strength of Ti-6Al-4V alloy improved when the treatment temperature was 1173 K or higher. However, the ductility greatly reduced if the treatment temperature was higher than the β transformation point (1271 K). To improve the strength without a reduction in ductility, therefore, the treatment temperatures were selected in the range of 1173 K to 1271 K. After heating under these conditions, the specimens were quenched. The cross sections of the heat-treated specimens and the untreated specimen for comparison were polished to mirror surfaces with emery papers and alumina powders. The hardness measurement was performed five times using a micro-Vickers hardness tester under a test force of 2.94N (300 gf). Their averages were used as the data. Two suitable treatment conditions were selected based on the results of the hardness measurement. The selected treatment conditions were: 1203 K, 60 s and 1223 K, 60 s. Hereafter, the untreated material and the materials heat-treated at each temperature are called UN, ST1203 and ST1233 materials, respectively.
3
2.2 Experimental procedures The microstructures of UN, ST1203 and ST1233 materials were optically observed after etching with Kroll’s etchant. They were also investigated by EBSD analysis. From this analysis, IQ (image quality) maps, IPF (invers pole figure) maps and phase maps were obtained. Moreover, the microstructures of the heat-treated materials were observed in detail by TEM and electron diffraction was conducted. The samples for TEM were obtained from the cross sections of the button-type specimens, and then polished and ion-milled to form thin observation areas. X-ray diffraction was performed to determine crystallographic phases. The test conditions were: diffraction angle 2= 34-42 degs, angle division 0.02 deg, scan speed 0.02 deg/s. The tensile test was carried out under a force increase rate of 283 N/s at room temperature in air. Strain was measured by strain gauges bonded to the test sections. Three specimens of each material were tested and their averages were used as the data. The fracture surfaces were observed by SEM. The plane-bending fatigue test was conducted under a stress ratio R=-1 and cyclic speed of 25 Hz at room temperature in air. In this study, the fatigue strength was defined as the maximum stress amplitude in which the specimens were unbroken at 107 cycles. The fracture surfaces were observed by SEM. The cross sections of fatigue specimens unbroken at 107 cycles were observed by SEM. Moreover, EBSD analysis was performed for a typical non-propagating crack found on a cross section of ST1233 material. 3．Results and discussion 3.1 Relationship between treatment conditions and hardness Figure 2 shows the relationship between treatment time and hardness examined for each temperature. As shown in this figure, the increase range of hardness was small at 1173 K, but was significant at 1203 K and 1233 K. The most marked increase of hardness was achieved at 1233 K. The increase of hardness means an increase of slip resistance. In the usual case, the increase of slip resistance improves fatigue strength through the suppression of crack initiation and propagation. Accordingly, the suitable treatment conditions were selected based on the hardness. Firstly, since the increase range of hardness was small at 1173 K, this treatment temperature was excluded from the detailed examinations. The prior phase in the heat-treated materials was composed of the ’ phase and the metastable phase as explained in the next section. Although the increase in the 4
amount of ’ phase increases the hardness, the amount of the metastable phase, which can induce strain transformation, decreases at the same time. As mentioned in the introduction, we expected that both phases would contribute to suppress the propagation of fatigue cracks: however, it was impossible to predict their suitable volume fraction before the fatigue test. Consequently, we selected two treatment temperatures, 1203 K and 1233 K, for the detailed examinations. Also, the treatment time of 60 s was chosen because the increase of hardness was saturated by this treatment time. 3.2 Microstructures Figure 3 shows the microstructures of UN, ST1203 and ST1233 materials observed by optical microscope and TEM. This figure includes the results of the electron diffraction and EBSD analysis (IQ, IPF and phase maps). Figure 4 shows the X-ray diffraction profiles. As can be understood from Fig. 3, the microstructure of UN material was composed of equiaxial grains (hcp) and the stable phase (bcc). In general, as temperature rises in the range below the transformation point (1271 K), the volume fraction of the phase increases. At the same time, since the concentration of vanadium, which is a stabilizer, decreases in this phase [10], the generation of the ’ phase by quenching is accelerated. In the case of short-time solution treatment, the amount of ’ phase generated increases with treatment temperature [2]. The above explanation was consistent with the microstructures of ST1203 material and ST1233 material. That is, the volume fraction of the prior phase was higher in ST1233 material than in ST1203 material (see IQ maps in Fig. 3). Here the prior phase made up all regions except the grains in the microstructures of the heat-treated materials. The fine acicular ’ phase was generated in the prior phase of both materials by quenching. The amount of the generated ’ phase was higher in ST1233 material than in ST1203 material. As shown in Fig. 4, the diffraction peaks of the phase disappeared in ST1203 material and ST1233 material. It is generally thought that all prior phase is transformed to the ’ phase by quenching [11]. However, since the lattice constants of the metastable phase are different from those of the stable phase, its existence cannot be confirmed by X-ray diffraction. According to the previous study [12], it is possible to confirm the existence of the metastable phase by electron diffraction, and in fact we obtained electron diffraction patterns corresponding to the phase from the prior phase in ST1203 and ST1233 materials (Fig. 3). The above results suggested that the metastable phase existed in the prior phase 5
of both heat-treated materials. The stabilizer (vanadium) could not uniformly redistribute in the heat treatments because the treatment time was very short (60 s). As a result, the metastable phase partially remained at positions having relatively high concentration of vanadium. In the range of this study, it was not possible to determine the positions of the metastable phase by EBSD analysis. However, there was a high possibility that the metastable phase existed in the black regions of the phase maps in which no phase was determined. The fraction of the black regions was higher in ST1203 material than in ST1233 material. These results suggested that the amount of the metastable phase was higher in ST1203 material than ST1233 material. 3.3 Mechanical properties Table 2 shows the mechanical properties and hardness of each material. Figure 5 shows the features of the tensile fracture surfaces. As shown in Table 2, the yield strength of ST1203 material was lower than the value of UN material because the strain-induced transformation of the metastable phase occurred from a low stress level [3, 13]. However, its occurrence improved the ductility (elongation, reduction of area). The fracture surface of ST1203 material showed a ductile feature with dimples, like that of UN material. After the strain-induced transformation proceeded sufficiently, the effect of the ’ phase appeared. As a result, the tensile strength and hardness of ST1203 material became higher than those of UN material. The yield strength of ST1233 material was almost the same as that of UN material because the effect of the ’ phase and the influence of the strain-induced transformation of the metastable phase were balanced. However, after the strain-induced transformation proceeded sufficiently, a marked effect of the ’ phase appeared. As a result, the tensile strength and hardness were significantly improved with no reduction in ductility. The fracture surface of ST1233 material showed a ductile feature like those of UN material and ST1203 material. 3.4 Fatigue strength Figure 6 shows the S-N curves of all materials. To aid comparison, the fatigue strength of each material obtained from Fig. 6 is shown in Table 2. Figure 7 shows the features of the fatigued fracture surfaces. This figure includes the features of the cross sections of the specimens unbroken at 107 cycles. As can be understood from Fig. 6, the heat treatments markedly improved the 6
fatigue strength. The improvement percentages of the fatigue strength of ST1203 material and ST1233 material were 20% and 40%, respectively (Table 2), and the improvement was much higher in ST1233 material than in ST1203 material. Elongated lines from the surface to the inside were observed on the fracture surface of UN material, as shown in Fig. 7. The existence of this line meant that two cracks initiated from each side of the notch root and propagated inside. In contrast, many lines were observed on the fracture surfaces of the heat-treated materials because many cracks initiated from the notch roots under higher stress amplitude and propagated inside. In UN material, no crack was found at the notch root on the cross section of the specimen unbroken at 107 cycles (Fig. 7). This result showed that the fatigue strength of UN material was determined by the initiation of cracks. In contrast, non-propagating cracks were observed on the cross sections of the heat-treated materials unbroken at 107 cycles. The existence of such cracks meant that the fatigue strengths of the heat-treated materials were determined by the propagation of cracks. In other words, the change of the microstructure suppressed fatigue crack propagation at a higher stress amplitude and improved the fatigue strength. Only a short, non-propagating crack was found in ST1203 material: however, multiple long cracks were observed in ST1233 material. The presence of such long non-propagating cracks suggested that fatigue crack propagation was more strongly suppressed in ST1233 material. Accordingly, a detailed EBSD analysis was conducted near the crack tip on the cross section of ST1233 material. Figure 8 shows the obtained IQ map and the phase map obtained on the cross section of ST1233 material. As shown in the figure, the crack propagation was arrested in the prior phase. This result suggested that the improvement in the fatigue strength was achieved through the following two possible mechanisms. As explained in section 3.2, the volume fraction of the prior phase was higher in ST1233 material than in ST1203 material. Also the amount of the generated ’ phase in the prior phase was higher in ST1233 material. Accordingly, there was high possibility that the crack propagation was suppressed by the abundant fine ’ phase generated in the prior phase because of the increase of slip resistance. As another possible cause, it was thought that the crack propagation was restricted by the strain-induced transformation of the metastable phase near the crack tip [11,13]. As mentioned in section 3.2, however, the amount of the metastable phase in ST1233 material was expected to be lower than that in ST1233 material. From this consideration, it can be said that the generation of the ’ phase was more effective in suppressing the 7
fatigue crack propagation than the existence of the metastable phase. 4. Conclusions 1. The volume fraction of the prior phase was higher in ST1233 material than in ST1203 material. Also, more ’ phase was generated in the prior phase of ST1233 material. The results of electron diffraction suggested that the metastable phase partially existed in the prior phase. 2. Although the yield strength of ST1203 material was lower than that of untreated material, its tensile strength and ductility were improved. The yield strength of ST1233 material was almost the same as that of the untreated material: however, the tensile strength was markedly improved with no reduction in ductility. 3. The improvement percentages of the fatigue strength of ST1203 and ST1233 materials were 20% and 40%, respectively. These improvements were achieved by the suppression of fatigue crack propagation. The crack propagation was mainly suppressed by the generation of fine ’ phase in the prior phase. Accordingly, the improvement percentage of the fatigue strength was much higher in ST1233 material than in ST1203 material. REFERENCES [1] G. Lutjering, J. C. Williams, Titanium, Springer-Verlag, Berlin, (2003) 7. [2] T. Morita, W. Niwayama, K. Kawasaki, T. Misaka, Trans. Jpn. Soc. Mech. Eng. 64 (1997) 2115-2120 (in Japanese). [3] T. Morita, K. Hatsuoka, T. Iizuka, K. Kawasaki, Mater. Trans. 46 (2005) 1681–1686. [4] T. Morita, T. Misaka, K. Kawasaki, T. Iizuka, J. Japan Inst. Met. Mater. 68 (2004) 862-867 (in Japanese). [5] T. Morita, K. Shinoda, K. Kawasaki, T. Misaka, J. Soc. Mater. Sci., Japan, 56 (2007) 345-351. [6] K. Takao, H. Nisitani, Proceedings of the 2nd International Conference of Fatigue, Fatigue Thresholds II, (1984) 827-834. [7] E. Mitchell, P. J. Brotherton, J. Inst. Met. 93 (1964–65) 381–386. [8] T. Morita, H. Takahashi, M. Shimizu, K. Kawasaki, Fat. Fract. Eng. Mater. Struct. 20 (1997) 85–92. [9] T. Morita, K. Asakura, C. Kagaya, Mate. Sci. Eng. A618 (2014) 438–446. [10] H. Sasano, S. Komori, K. Kimura, J. Jpn. Inst. Met. 38 (1974) 199–205 (in Japanese). [11] B. L. Averbach, M. F. Comerford, M. B. Beve, Trans. Metall. Soc. AIME 215 8
(1959) 682–685. [12] M. A. Imam, C. M. Gilmore, Met. Trans. 14A (1983) 233–240. [13] J. R. Kennedy, Mater. Sci. Eng. 57 (1983) 197–204.
Fig. 1 Shapes of specimens (mm): (a) button-type specimen; (b) tensile specimen (JIS Z 2241, No. 14A); (c) notched fatigue specimen (JIS Z 2274). Fig. 2 Relationship between treatment time and hardness. Fig. 3 Microstructures observed by optical microscope and TEM with the results of electron diffraction and EBSD analysis (IQ, IPF and phase maps). Fig. 4 X-ray diffraction profiles. Fig. 5 Features of tensile fracture surfaces. Fig. 6 S-N curves. Fig. 7 Features of fatigue fracture surfaces with those observed on the cross sections of the specimens unbroken at 107 cycles. Fig. 8 IQ map and phase map obtained on the cross section near the crack tip of ST1233 material unbroken at 107 cycles (a=440 MPa).
Table 1 Chemical compositions of Ti-6Al-4V (ELI) alloy (mass %). Al
V
Fe
O
C
N
H
Ti
6.41
3.90
0.170
0.006
0.006
0.003
0.001
Bal.
Table 2 Mechanical properties, hardness and fatigue strength. Young’s modulus (GPa)
Yield strength( MPa)
Tensile strength( MPa)
Elongation (%)
Reduction of area (%)
Hardness (HV)
Fatigue strength (MPa)
UN
111
887
963
16
64
332
300
ST1203
98
756
993
21
69
372
360
ST1233
106
883
1072
15
66
411
420
9
10
11
12
13
14
15
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PII: DOI: Reference:
S0921-5093(16)30586-X http://dx.doi.org/10.1016/j.msea.2016.05.071 MSA33699
To appear in: Materials Science & Engineering A Received date: 26 March 2016 Revised date: 17 May 2016 Accepted date: 18 May 2016 Cite this article as: Tatsuro Morita, Satoshi Tanaka and Susumu Ninomiya, Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat t r e a t m e n t , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.05.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Improvement in fatigue strength of notched Ti-6Al-4V alloy by short-time heat treatment Tatsuro Moritaa*, Satoshi Tanakab1, Susumu Ninomiyac a
Faculty of Mechanical Engineering, Kyoto Institute of Technology, Kyoto 606-8585, Japan b
Department of Mechanical and System Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto 606-8585, Japan c Japan Thermo-tech Co. Ltd., Kanagawa 230-0045, Japan *
Corresponding author. Tel.: +81 75 724 7326. fax: +81 75 724 7300. [email protected]
Abstract This study was conducted to improve the fatigue strength of notched Ti-6Al-4V alloy without a surface finish by short-time heat treatment. Specimens were heated at 1203 K and 1233 K for 60 s, and then quenched to create ST1203 material and ST1233 material, respectively. The microstructures of both heat-treated materials were composed of equiaxial grains and the prior phase. The fine acicular ’ phase was generated in the prior phase by quenching. The results of electron diffraction suggested that the metastable phase partially existed in the prior phase. The volume fractions of the prior phase and the ’ phase in this phase were higher in ST1233 material than in ST1203 material. The tensile strength was improved by the generation of the ’ phase; its improvement percentage was higher in ST1233 material than in ST1203 material. The fatigue strength was markedly improved by the heat treatment because crack propagation was strongly suppressed by the generation of the ’ phase and the strain-induced transformation of the metastable phase. The improvement percentage of the fatigue strength was higher in ST1233 material (40%) than in ST1203 material (20%). Keywords: short-time heat treatment, Ti-6Al-4V alloy, fatigue strength, notch, microstructure, mechanical properties
1
Present affiliation: Shimano Inc.
1
1.
Introduction
Ti-6Al-4V alloy is a typical titanium alloy with high specific strength and excellent corrosion resistance. This titanium alloy has been widely used in the aerospace industry, the medical implant industry, etc. The high performance and usefulness of Ti-6Al-4V alloy can be easily understood from its market share of more than 50% in the US titanium market [1]. The industrial application range of Ti-6Al-4V alloy will expand further if the strength can be improved by heat treatment and the cost-effectiveness heightened. From this viewpoint, we investigated the effect of short-time heat treatment on the mechanical properties and the fatigue strength of Ti-6Al-4V alloy [2-5]. The most effective heat treatment was composed of a first heat treatment at 1203 K for 60 s (quenching) and a second heat treatment at 853 K for 40 s (air cooling). This duplex heat treatment improved the tensile strength and the fatigue strength by 29% and 22%, respectively, with no reduction of ductility. In the above studies, smooth polished specimens were used to clarify the effect of the heat treatment without any influence of stress concentration or surface condition. However, actual mechanical parts generally possess stress risers such as notches and keyholes. Also, they are used with no surface finish after heat treatment. In the case of titanium, once fatigue cracks have generated at the surface, they continue to propagate without stopping until final fracture. Accordingly, the fatigue strength of titanium is markedly sensitive to factors that accelerate the generation of fatigue cracks such as stress concentration [6] and the formation of surface compounds [7-9]. According to our previous study [3], the first heat treatment (short-time solution treatment) in the above-mentioned duplex heat treatment generated a fine acicular ’ martensite phase and improved the tensile strength of Ti-6Al-4V alloy. At the same time, this heat treatment increased the ductility through the strain-induced transformation of the metastable phase. If the propagation of generated fatigue cracks is strongly suppressed by the increase of slip resistance and the occurrence of strain-induced transformation at the crack tips, improvement of the fatigue strength can be achieved by the heat treatment independently of stress concentration and surface condition. In this study, the effect of the short-time solution treatment was investigated to improve the fatigue strength of notched Ti-6Al-4V alloy (ELI) without a surface finish. Two suitable conditions for the heat treatment was selected in a pre-examination. The materials heat-treated under the selected conditions were subjected to detailed examinations. The same examinations were also performed on the untreated materials for comparison. 2
The microstructures were investigated by optical observation, EBSD (electron back-scattered diffraction) analysis and TEM (transmission electron microscopy). The crystallographic phases were determined by electron diffraction and X-ray diffraction. The mechanical properties and the fatigue strength were investigated and the fracture surfaces were observed by SEM (scanning electron microscopy). The cross sections of fatigue specimens unbroken at 107 cycles were observed. Moreover, EBSD analysis was performed for a typical non-propagating crack found in a cross section of the heat-treated material. 2. Materials and experimental procedures 2.1 Materials and conditions of heat treatment Table 1 shows the chemical composition of the Ti-6Al-4V (ELI) alloy used in this study. The material was supplied as cold-rolled round bars and machined to the three specimen shapes shown in Fig. 1. The button-type specimens with the shape shown in Fig. 1(a) were used for hardness measurement and the investigation of microstructures. Figures 1(b) and (c) show the shapes of the tensile specimens and the fatigue specimens, respectively. The change of hardness due to the short-time solution treatment was examined to select suitable treatment conditions. The treatment temperatures were 1173 K, 1203 K and 1233 K and the holding times at each temperature were 30 s, 60 s and 120 s. According to the previous studies [2, 4], the tensile strength of Ti-6Al-4V alloy improved when the treatment temperature was 1173 K or higher. However, the ductility greatly reduced if the treatment temperature was higher than the β transformation point (1271 K). To improve the strength without a reduction in ductility, therefore, the treatment temperatures were selected in the range of 1173 K to 1271 K. After heating under these conditions, the specimens were quenched. The cross sections of the heat-treated specimens and the untreated specimen for comparison were polished to mirror surfaces with emery papers and alumina powders. The hardness measurement was performed five times using a micro-Vickers hardness tester under a test force of 2.94N (300 gf). Their averages were used as the data. Two suitable treatment conditions were selected based on the results of the hardness measurement. The selected treatment conditions were: 1203 K, 60 s and 1223 K, 60 s. Hereafter, the untreated material and the materials heat-treated at each temperature are called UN, ST1203 and ST1233 materials, respectively.
3
2.2 Experimental procedures The microstructures of UN, ST1203 and ST1233 materials were optically observed after etching with Kroll’s etchant. They were also investigated by EBSD analysis. From this analysis, IQ (image quality) maps, IPF (invers pole figure) maps and phase maps were obtained. Moreover, the microstructures of the heat-treated materials were observed in detail by TEM and electron diffraction was conducted. The samples for TEM were obtained from the cross sections of the button-type specimens, and then polished and ion-milled to form thin observation areas. X-ray diffraction was performed to determine crystallographic phases. The test conditions were: diffraction angle 2= 34-42 degs, angle division 0.02 deg, scan speed 0.02 deg/s. The tensile test was carried out under a force increase rate of 283 N/s at room temperature in air. Strain was measured by strain gauges bonded to the test sections. Three specimens of each material were tested and their averages were used as the data. The fracture surfaces were observed by SEM. The plane-bending fatigue test was conducted under a stress ratio R=-1 and cyclic speed of 25 Hz at room temperature in air. In this study, the fatigue strength was defined as the maximum stress amplitude in which the specimens were unbroken at 107 cycles. The fracture surfaces were observed by SEM. The cross sections of fatigue specimens unbroken at 107 cycles were observed by SEM. Moreover, EBSD analysis was performed for a typical non-propagating crack found on a cross section of ST1233 material. 3．Results and discussion 3.1 Relationship between treatment conditions and hardness Figure 2 shows the relationship between treatment time and hardness examined for each temperature. As shown in this figure, the increase range of hardness was small at 1173 K, but was significant at 1203 K and 1233 K. The most marked increase of hardness was achieved at 1233 K. The increase of hardness means an increase of slip resistance. In the usual case, the increase of slip resistance improves fatigue strength through the suppression of crack initiation and propagation. Accordingly, the suitable treatment conditions were selected based on the hardness. Firstly, since the increase range of hardness was small at 1173 K, this treatment temperature was excluded from the detailed examinations. The prior phase in the heat-treated materials was composed of the ’ phase and the metastable phase as explained in the next section. Although the increase in the 4
amount of ’ phase increases the hardness, the amount of the metastable phase, which can induce strain transformation, decreases at the same time. As mentioned in the introduction, we expected that both phases would contribute to suppress the propagation of fatigue cracks: however, it was impossible to predict their suitable volume fraction before the fatigue test. Consequently, we selected two treatment temperatures, 1203 K and 1233 K, for the detailed examinations. Also, the treatment time of 60 s was chosen because the increase of hardness was saturated by this treatment time. 3.2 Microstructures Figure 3 shows the microstructures of UN, ST1203 and ST1233 materials observed by optical microscope and TEM. This figure includes the results of the electron diffraction and EBSD analysis (IQ, IPF and phase maps). Figure 4 shows the X-ray diffraction profiles. As can be understood from Fig. 3, the microstructure of UN material was composed of equiaxial grains (hcp) and the stable phase (bcc). In general, as temperature rises in the range below the transformation point (1271 K), the volume fraction of the phase increases. At the same time, since the concentration of vanadium, which is a stabilizer, decreases in this phase [10], the generation of the ’ phase by quenching is accelerated. In the case of short-time solution treatment, the amount of ’ phase generated increases with treatment temperature [2]. The above explanation was consistent with the microstructures of ST1203 material and ST1233 material. That is, the volume fraction of the prior phase was higher in ST1233 material than in ST1203 material (see IQ maps in Fig. 3). Here the prior phase made up all regions except the grains in the microstructures of the heat-treated materials. The fine acicular ’ phase was generated in the prior phase of both materials by quenching. The amount of the generated ’ phase was higher in ST1233 material than in ST1203 material. As shown in Fig. 4, the diffraction peaks of the phase disappeared in ST1203 material and ST1233 material. It is generally thought that all prior phase is transformed to the ’ phase by quenching [11]. However, since the lattice constants of the metastable phase are different from those of the stable phase, its existence cannot be confirmed by X-ray diffraction. According to the previous study [12], it is possible to confirm the existence of the metastable phase by electron diffraction, and in fact we obtained electron diffraction patterns corresponding to the phase from the prior phase in ST1203 and ST1233 materials (Fig. 3). The above results suggested that the metastable phase existed in the prior phase 5
of both heat-treated materials. The stabilizer (vanadium) could not uniformly redistribute in the heat treatments because the treatment time was very short (60 s). As a result, the metastable phase partially remained at positions having relatively high concentration of vanadium. In the range of this study, it was not possible to determine the positions of the metastable phase by EBSD analysis. However, there was a high possibility that the metastable phase existed in the black regions of the phase maps in which no phase was determined. The fraction of the black regions was higher in ST1203 material than in ST1233 material. These results suggested that the amount of the metastable phase was higher in ST1203 material than ST1233 material. 3.3 Mechanical properties Table 2 shows the mechanical properties and hardness of each material. Figure 5 shows the features of the tensile fracture surfaces. As shown in Table 2, the yield strength of ST1203 material was lower than the value of UN material because the strain-induced transformation of the metastable phase occurred from a low stress level [3, 13]. However, its occurrence improved the ductility (elongation, reduction of area). The fracture surface of ST1203 material showed a ductile feature with dimples, like that of UN material. After the strain-induced transformation proceeded sufficiently, the effect of the ’ phase appeared. As a result, the tensile strength and hardness of ST1203 material became higher than those of UN material. The yield strength of ST1233 material was almost the same as that of UN material because the effect of the ’ phase and the influence of the strain-induced transformation of the metastable phase were balanced. However, after the strain-induced transformation proceeded sufficiently, a marked effect of the ’ phase appeared. As a result, the tensile strength and hardness were significantly improved with no reduction in ductility. The fracture surface of ST1233 material showed a ductile feature like those of UN material and ST1203 material. 3.4 Fatigue strength Figure 6 shows the S-N curves of all materials. To aid comparison, the fatigue strength of each material obtained from Fig. 6 is shown in Table 2. Figure 7 shows the features of the fatigued fracture surfaces. This figure includes the features of the cross sections of the specimens unbroken at 107 cycles. As can be understood from Fig. 6, the heat treatments markedly improved the 6
fatigue strength. The improvement percentages of the fatigue strength of ST1203 material and ST1233 material were 20% and 40%, respectively (Table 2), and the improvement was much higher in ST1233 material than in ST1203 material. Elongated lines from the surface to the inside were observed on the fracture surface of UN material, as shown in Fig. 7. The existence of this line meant that two cracks initiated from each side of the notch root and propagated inside. In contrast, many lines were observed on the fracture surfaces of the heat-treated materials because many cracks initiated from the notch roots under higher stress amplitude and propagated inside. In UN material, no crack was found at the notch root on the cross section of the specimen unbroken at 107 cycles (Fig. 7). This result showed that the fatigue strength of UN material was determined by the initiation of cracks. In contrast, non-propagating cracks were observed on the cross sections of the heat-treated materials unbroken at 107 cycles. The existence of such cracks meant that the fatigue strengths of the heat-treated materials were determined by the propagation of cracks. In other words, the change of the microstructure suppressed fatigue crack propagation at a higher stress amplitude and improved the fatigue strength. Only a short, non-propagating crack was found in ST1203 material: however, multiple long cracks were observed in ST1233 material. The presence of such long non-propagating cracks suggested that fatigue crack propagation was more strongly suppressed in ST1233 material. Accordingly, a detailed EBSD analysis was conducted near the crack tip on the cross section of ST1233 material. Figure 8 shows the obtained IQ map and the phase map obtained on the cross section of ST1233 material. As shown in the figure, the crack propagation was arrested in the prior phase. This result suggested that the improvement in the fatigue strength was achieved through the following two possible mechanisms. As explained in section 3.2, the volume fraction of the prior phase was higher in ST1233 material than in ST1203 material. Also the amount of the generated ’ phase in the prior phase was higher in ST1233 material. Accordingly, there was high possibility that the crack propagation was suppressed by the abundant fine ’ phase generated in the prior phase because of the increase of slip resistance. As another possible cause, it was thought that the crack propagation was restricted by the strain-induced transformation of the metastable phase near the crack tip [11,13]. As mentioned in section 3.2, however, the amount of the metastable phase in ST1233 material was expected to be lower than that in ST1233 material. From this consideration, it can be said that the generation of the ’ phase was more effective in suppressing the 7
fatigue crack propagation than the existence of the metastable phase. 4. Conclusions 1. The volume fraction of the prior phase was higher in ST1233 material than in ST1203 material. Also, more ’ phase was generated in the prior phase of ST1233 material. The results of electron diffraction suggested that the metastable phase partially existed in the prior phase. 2. Although the yield strength of ST1203 material was lower than that of untreated material, its tensile strength and ductility were improved. The yield strength of ST1233 material was almost the same as that of the untreated material: however, the tensile strength was markedly improved with no reduction in ductility. 3. The improvement percentages of the fatigue strength of ST1203 and ST1233 materials were 20% and 40%, respectively. These improvements were achieved by the suppression of fatigue crack propagation. The crack propagation was mainly suppressed by the generation of fine ’ phase in the prior phase. Accordingly, the improvement percentage of the fatigue strength was much higher in ST1233 material than in ST1203 material. REFERENCES [1] G. Lutjering, J. C. Williams, Titanium, Springer-Verlag, Berlin, (2003) 7. [2] T. Morita, W. Niwayama, K. Kawasaki, T. Misaka, Trans. Jpn. Soc. Mech. Eng. 64 (1997) 2115-2120 (in Japanese). [3] T. Morita, K. Hatsuoka, T. Iizuka, K. Kawasaki, Mater. Trans. 46 (2005) 1681–1686. [4] T. Morita, T. Misaka, K. Kawasaki, T. Iizuka, J. Japan Inst. Met. Mater. 68 (2004) 862-867 (in Japanese). [5] T. Morita, K. Shinoda, K. Kawasaki, T. Misaka, J. Soc. Mater. Sci., Japan, 56 (2007) 345-351. [6] K. Takao, H. Nisitani, Proceedings of the 2nd International Conference of Fatigue, Fatigue Thresholds II, (1984) 827-834. [7] E. Mitchell, P. J. Brotherton, J. Inst. Met. 93 (1964–65) 381–386. [8] T. Morita, H. Takahashi, M. Shimizu, K. Kawasaki, Fat. Fract. Eng. Mater. Struct. 20 (1997) 85–92. [9] T. Morita, K. Asakura, C. Kagaya, Mate. Sci. Eng. A618 (2014) 438–446. [10] H. Sasano, S. Komori, K. Kimura, J. Jpn. Inst. Met. 38 (1974) 199–205 (in Japanese). [11] B. L. Averbach, M. F. Comerford, M. B. Beve, Trans. Metall. Soc. AIME 215 8
(1959) 682–685. [12] M. A. Imam, C. M. Gilmore, Met. Trans. 14A (1983) 233–240. [13] J. R. Kennedy, Mater. Sci. Eng. 57 (1983) 197–204.
Fig. 1 Shapes of specimens (mm): (a) button-type specimen; (b) tensile specimen (JIS Z 2241, No. 14A); (c) notched fatigue specimen (JIS Z 2274). Fig. 2 Relationship between treatment time and hardness. Fig. 3 Microstructures observed by optical microscope and TEM with the results of electron diffraction and EBSD analysis (IQ, IPF and phase maps). Fig. 4 X-ray diffraction profiles. Fig. 5 Features of tensile fracture surfaces. Fig. 6 S-N curves. Fig. 7 Features of fatigue fracture surfaces with those observed on the cross sections of the specimens unbroken at 107 cycles. Fig. 8 IQ map and phase map obtained on the cross section near the crack tip of ST1233 material unbroken at 107 cycles (a=440 MPa).
Table 1 Chemical compositions of Ti-6Al-4V (ELI) alloy (mass %). Al
V
Fe
O
C
N
H
Ti
6.41
3.90
0.170
0.006
0.006
0.003
0.001
Bal.
Table 2 Mechanical properties, hardness and fatigue strength. Young’s modulus (GPa)
Yield strength( MPa)
Tensile strength( MPa)
Elongation (%)
Reduction of area (%)
Hardness (HV)
Fatigue strength (MPa)
UN
111
887
963
16
64
332
300
ST1203
98
756
993
21
69
372
360
ST1233
106
883
1072
15
66
411
420
9
10
11
12
13
14
15