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Effect of surface treatments on fatigue life of Ti-6-22-22 alloy at room and high temperatures
Materials Science and Engineering A 383 (2004) 283–288
Effect of surface treatments on fatigue life of Ti-6-22-22 alloy at room and high temperatures Z.L. Yua,∗ , S.X. Lia , Y.Y. Liub , Q.Y. Zhangc , J.F. Leib , Z.X. Muc a c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110016, China b Titanium Alloy Laboratory, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110016, China The State Key Laboratory for Materials Modifications by Three Beams, Dalian University of Sciences and Technologies, Dalian 116024, China Received 2 March 2004
Abstract Shot-peening and ion implantation were adopted to treat the surface of machined specimens of Ti-6-22-22 alloy, then stress-controlled fatigue tests were performed at room temperature and 400 ◦ C. Experimental results indicate that the effects of shot-peening and ion implantation on the S–N curves are dependent on temperature. For Ti-6-22-22 alloy fatigued at room temperature, the effects of both, shot-peening and ion implantation of carbon ions on fatigue strength at high cyclic lives are slight. At 400 ◦ C, the fatigue strength at high cyclic lives is obviously increased for both, shot-peening and ion implantation of carbon ions in comparison with the untreated counterparts. © 2004 Published by Elsevier B.V. Keywords: Ti-6-22-22 alloy; Shot-peening; Ion implantation; S–N curves; Cracks initiation
1. Introduction Titanium alloys have high strength and low density, thus are widely used in aerospace industries [1]. The dynamic working conditions make materials suffer serious fatigue damage due to varying mechanical load and temperature; therefore, the surface quality has an important effect on fatigue life. As a result, sophisticated machining is required for sufficient surface quality. Shot-peening and ion implantation are the commonly used surface treatment technologies. Shot-peening raises the dislocation density in the surface layer by plastic deformation, producing compressive residual macro-stresses and changing the surface topography. At room temperature, the effect of residual stress has been shown to improve fatigue life [2–9]. Ion implantation strengthens the surface of materials. Though the implanted layer is very thin, it can improve fatigue life [10–16]. There are many studies about the effects ∗ Corresponding author. Tel.: +86 24 83978270; fax: +86 24 23971215. E-mail address: [email protected] (Z.L. Yu).
0921-5093/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.msea.2004.06.026
of shot-peening and ion implantation on fatigue life at room temperature, but studies of the effects on fatigue life at high temperature are rare. Ti-6-22-22 alloy was devised to be used at low to medium temperatures in aircraft, so it is meaningful to explore the fatigue performance of the alloy at higher temperatures. In practical use, especially during the aircraft taking-off to normal flying, the operating temperature extends from room temperature to about 400 ◦ C. The temperature and load variations in such applications will affect the treated surface, and in turn affect the fatigue life of specimens. This paper investigated the effects of shot-peening and ion implantation on fatigue lives of Ti-6-22-22 alloy at room temperature and 400 ◦ C.
2. Experiments The material used was two-phase ␣– Ti-6-22-22 alloy (nominal chemical composition: Ti–6Al–2Sn–2Zr–2Cr– 2Mo, in wt.%). Before fatigue tests, the alloy was heat treated as follows: 980 ◦ C/0.5 h/ac, 925 ◦ C/1 h/ac, 540 ◦ C/8 h/ac. The
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tigued fracture specimens were examined by scanning electron microscopy. 3. Results and discussion 3.1. Surface analysis after surface treatment
Fig. 1. The typical bi-lamellar microstructure of Ti-6-22-22 alloy.
typical bi-lamellar microstructure was obtained as shown in Fig. 1. Fatigue specimens were mechanically machined with an hourglass gage section and threaded grip, the gage dimension was ∅ 4 mm × 6 mm. Shot-peening was carried out on an air-blast machine with S1 1 0 cast steel shot having a hardness of 50–60 HRC. The shot-peening pressure was 0.3 MPa and the final shot-peening intensity was 0.22 A. The shot-peened specimens were examined by hardness and metallography. The residual stress was measured by X-ray method, using a flat specimen peened at equal intensity. Carbon implantation was carried out at an accelerating voltage of 40 keV at room temperature. The incident irradiation dose was 1 × 1017 ions cm−2 . The maximum temperature was kept below 300 ◦ C during ion implantation. In order to guarantee homogenous implantation, the round specimens were rotated 60◦ after every ion implantation and were rotated a total of six times. Fatigue tests were performed using a Schenk 40KN servohydraulic test machine. The test temperatures were room temperature and 400 ◦ C. The cyclic stress waveform was sineshaped and the frequency was 30 Hz. The cyclic stress ratio R of minimum divided by maximum stress was 0.1. The fa-
After shot-peening, the deformation is more severe from the surface to a depth of about 50 m, in which the bi-lamellar microstructure has been curved (see Fig. 2(a)). The shear strain γ is estimated roughly according to the degree of curvature of the lamellai with depth below surface. The measurement of shear strain γ is schematically shown in Fig. 2(b), in which the prolonged line is drawn along a lath α, then the tangent is drawn at the curved position A of the lath α, the angle β between the prolonged line and the tangent is measured (see Fig. 2(b)), finally the shear strain γ is obtained by computing tgβ (γ = tgβ) and is shown in Fig. 3. For comparison, the data of measured microhardness of shot-peened specimens near the surface are also shown in this figure. The residual stress measured by X-ray is shown in Fig. 4. The largest microhardness is near the surface, and the shear strain and the microhardness decrease with the increase of the depth below surface. When the depth is up to 160 m, the microhardness reaches about 420, which is the microhardness of the undeformed matrix. The large compressive residual stress extends to a depth of about 150 m, and beyond that a smaller tension residual stress occurs for overall balance of the stresses. The distribution of implanted carbon below the surface was modeled by using a “trim-98” computer program [15], which estimated the average depth of projected range RP as ˚ and the standard deviation of projected range RP 808 A, ˚ The curve of carbon distribution was constructed as 308 A. by using calculated RP and RP according to the LSS theory [15], see Fig. 5. It is thought by experience that the calculated error on RP is not more than 15% and the error on RP is not more than 30%. The implanted carbon strengthens the alloy by solid solution strengthening, TiC pre-
Fig. 2. (a) The microstructure of the shot-peened specimen and (b) the schematic measurement of shear strain γ of a α-curved lath in the surface of the shot-peened specimen.
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Fig. 3. Strain and microhardnees profiles in the surface layer after shotpeening.
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Fig. 4. Distribution profile of residual stress in the surface layer after shotpeening.
cipitation strengthening, and radiation defect strengthening [15]. 3.2. Effect of shot-peening on S–N curves It is well-known that fatigue life is divided into two parts, crack initiation life and crack propagation life. Crack propagation life predominates when the cyclic stress is large but the crack initiation life predominates when the cyclic stress is low. The residual stress distribution was “S” shape after the titanium alloy was shot-peened (see Fig. 4). The residual stress is compressive near the surface, and a smaller residual tensile stress exists at subsurface to balance the stress state. It will be shown that because of the residual tensile stress, the fatigue cracks of shot-peened specimens generally nucleate beneath the surface, while the fatigue cracks of the untreated specimens nucleate at surface. At room temperature and large cyclic stress, the fatigue lives of shot-peened samples were greater than that of unpeened samples, as shown in Fig. 6(a). Because the fatigue
Fig. 5. Distribution profile of carbon in the surface layer.
cracks initiate at the beneath of the surface, for peened samples, the cracks are internal cracks. It is known that the crack growth driving force for internal cracks is slightly smaller than that for surface cracks. Furthermore, the internal cracks
Fig. 6. S–N curves of shot-peened and untreated specimens. (a) Room temperature; (b) 400 ◦ C.
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are in a quasi-vacuum situation and may avoid the detrimental effects caused by environment such as hydrogen. The internal cracks are also partially embedded in the residual compressive stress region. All these factors are beneficial for the resistance of crack growth in the peened samples. However, at room temperature and low cyclic stress, the fatigue lives of peened samples are approached that of unpeened samples as shown in Fig. 6(a). In this case of low cyclic stress, the cycles for initiation of cracks on the surface for un-peened samples and at the subsurface for peened samples are apparently almost identical. Although the crack growth rate in the un-peened samples may be greater than that in the peened samples as mentioned above, the majority of cycles to fatigue failure was consumed in the crack initiation stage; therefore, the fatigue lives of peened samples is almost equal to that of un-peened samples. Fig. 6(b) shows the S–N curves of specimens at 400 ◦ C with and without shot-peening. Obviously different from S–N curves at room temperature, the curves cross at the maximum cyclic stress of 700 MPa or so. At 400 ◦ C and large cyclic stress, the fatigue lives of shotpeened samples are lower than that of un-peened samples as shown in Fig. 6(b). This could be related to surface hardening and ductility loss. From Fig. 3, we know that there is a hardened layer on the sample surface for peened samples. At 400 ◦ C, the beneficial compressive residual stress could be at least partially relaxed due to higher temperature and cyclic deformation; however, the severely deformed microstructures on the surface layer cannot be recovered completely. At 400 ◦ C, the yield strength of this alloy is about 750 MPa, while at room temperature the yield strength is 1050 MPa. At large cyclic stress, the maximum stress is larger than the yield stress of the alloy (see Fig. 6(b)). The hardened layer with higher strength and poor ductility cannot compensate for the plastic deformation of the underlying alloy. Therefore, cracks are easier to initiate at the surface of the peened samples and grow quickly due to release of residual compressive stress, giving lower fatigue lives in this situation.
At 400 ◦ C and low cyclic stress, the fatigue lives of peened samples are definitely greater than that of un-peened samples, as shown in Fig. 6(b). The reasons might be understood as follows. As mentioned above, at low cyclic stress, deformation of the hardened layer and underlying alloy are within elastic regime and the uncompensated deformation is not very large. Therefore, the crack initiation stage consumes many cycles. Fatigued at 400 ◦ C, the residual stress of titanium alloys partially relaxes, correspondingly the residual tensile stress at the subsurface will decrease. The initiation of microcracks is more difficult due to this residual tensile stress release. So, in this case, the surface higher dislocation density caused by peening can play a beneficial role on fatigue life [18,19]. In summary, at room temperature, the fatigue strengths for 5 × 106 cycle lives are the same for peened and un-peened specimens. On the other hand, at 400 ◦ C, the fatigue strength at 5 × 106 cycle lives for peened specimens is greater than that for un-peened specimens (about 600 MPa > 500 MPa). 3.3. Effect of ion-implantation on S–N curves Fig. 7 shows the S–N curves of implanted and unimplanted specimens. Carbon ion implantation has similar effects both at room temperature and 400 ◦ C, i.e., there is a crossover between the S–N curves of implanted and unimplanted specimens. However, the effect at 400 ◦ C is much more obvious. At room temperature and 400 ◦ C, when the cyclic stress is low, the fatigue life can be increased for implanted specimens. However, when the cyclic stress is large, fatigue life is decreased for implanted specimens. The fatigue strength of implanted specimens at 5 × 106 cycles is increased slightly at room temperature (Fig. 7(a)). However, at 400 ◦ C, the fatigue strength is increased substantially from 500 MPa of un-implanted specimens to 650 MPa of implanted specimens (Fig. 7(b)). Normally, the fatigue strength in the high life, low cyclic stress regime increases with an increase of strength of materi-
Fig. 7. S–N curves of ion-implanted and untreated specimens at (a) room temperature; (b) 400 ◦ C.
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als. Carbon ions were implanted into the surface, the strength of material surface increases, thus the fatigue strength of the materials increases both at room temperature and 400 ◦ C. Meanwhile, a residual compressive stress is produced due to carbon ion implantation, it also can improve fatigue life [17,20]. However, while the strength of the implanted surface increases, the ductility can decrease. For instance, Tjong and Zhu [21] found that there were brittle characteristics on surface cracks in experiments of Fe–24Cr–4Al alloy, which was nitrogen implanted. When the implanted specimens were
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given higher loads, ion implantation could accelerate the cracks initiating at machining scratches, while cracks of unimplanted specimens could be more easily blunted due to relatively good plasticity. This could explain why the fatigue life of implanted specimens decreased when the cyclic stress was large. Particularly, at 400 ◦ C and large cyclic stress, the fatigue lives of implanted samples are much lower than that of un-implanted samples as shown in Fig. 7(b). These ductility effects could be accentuated at 400 ◦ C. At 400 ◦ C, the yield strength of this alloy is about 750 MPa, while at room temperature the yield strength is 1050 MPa. At large cyclic
Fig. 8. SEM fractography of fatigued specimens. (a) Untreated specimen, fatigued at room temperature; (b) shot-peened specimen, fatigued at room temperature; (c) shot-peened specimen, fatigued at 400 ◦ C; (d) shot-peened specimen, fatigued at 400 ◦ C; (e) implanted specimen, fatigued at room temperature; (f) implanted specimen, fatigued at 400 ◦ C; σ max = 700 MPa.
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stress, the maximum stress is larger than the yield stress of the alloy (see Fig. 7(b)). The implanted layer with high strength and poor ductility cannot compensate the plastic deformation of the matrix. The cracks are easier to initiate at the surface of the implanted samples and grow quickly due to partially release of residual stress at 400 ◦ C; therefore, lower fatigue lives result in this situation. 3.4. SEM fractography Fig. 8 shows the fractography of fatigued specimens observed by SEM. For the untreated specimens, the cracks entirely initiate at the surface at room temperature, basically caused by machined scratches (Fig. 8(a)). The cracks of peened specimens tested at room temperature mostly initiate subsurface, see Fig. 8(b). The depth of crack initiation sites below the surface was near 300 m, which corresponds to the maximum tensile stress at the subsurface. At 400 ◦ C, the depth of crack initiation was related to the amplitude of cyclic stress. When the cyclic stress is low, the cracks initiate subsurface (Fig. 8(c)), but when the cyclic stress is large, near 800 MPa, the cracks initiate at the surface (Fig. 8(d)). The cracks of implanted specimens fatigued at room temperature initiated at the surface (Fig. 8(e)), as for the untreated specimens. At 400 ◦ C, when the cyclic stress is greater than 750 MPa corresponding to the yield stress of the Ti alloy, fatigue cracks initiate at the surface, while below 750 MPa the cracks initiate subsurface (Fig. 8(f)). When cracks initiate subsurface for both, shot-peened and implanted specimens fatigued at 400 ◦ C, the effect of environment on fatigue life is obviated, increasing fatigue life. 4. Conclusions Based on the experiment results, some conclusions may be drawn: (1) For Ti-6-22-22 alloy fatigued at room temperature, the effects of both, shot-peening and ion implantation of carbon ions on fatigue strength at high cycles are very small. (2) At 400 ◦ C, the fatigue strength at high cyclic lives is obviously increased for both, shot-peening and ion implantation of carbon ions. However, when the cyclic stress is higher than the yield stress of the alloy, the fatigue lives of specimens treated by both methods can-
not be improved. This is because the treated surface layer with high strength but poor ductility cannot compensate for the plastic deformation of the underlying alloy.
Acknowledgement This work is financially support by Key Basic Research Project of China (G19990650).
References [1] X.D. Zhang, J.M.K. Wiezorek, W.A. Baeslack, D.J. Evans, H.L. Fraser, Acta Mater. 46 (1998) 4485. [2] B.R. Sridhar, K. Ramachandra, K.A. Padmanabhan, J Mater Sci. 31 (1996) 5953. [3] T. D¨orr, L. Wagner, in: Jack Champaigne (Ed.), Proceedings of the Sixth International Conference on Shot Peening, 1996, p. 223. [4] K. Mieczyslaw, in: J. Champaigne (Ed.), Proceedings of the Sixth International Conference on Shot Peening, 1996, p. 282. [5] L. Wagner, in: R.R. Boyer, D. Eylon, G. Lutjering (Eds.), Fatigue Behavior of Titanium Alloys, A Publication of the Minerals, Metal & Materials Society, 1998, p. 253. [6] A. Berg, A. Drechsler, L. Wagner, in: R.R. Boyer, D. Eylon, G. Lutjering (Eds.), Fatigue Behavior of Titanium Alloys, A Publication of the Minerals, Metal & Materials Society, 1998, p. 267. [7] U. Holzwarth, J. Kiese, L. Wagner, in: R.R. Boyer, D. Eylon, G. Lutjering (Eds.), Fatigue Behavior of Titanium Alloys, A Publication of the Minerals, Metal & Materials Society, 1998, p. 323. [8] A. Drechsler, T. Dorr, L. Wagner, Mater. Sci. Eng. A 243 (1998) 217. [9] L. Wagner, Mater. Sci. Eng. A 263 (1999) 210. [10] C.J. Mchargue, Int. Mater. Rev. 31 (1986) 49. [11] K.V. Jata, E.A. Starke, JOM-J MIN MET MAT S35 (1983) 23. [12] K.V. Jata, J. Han, E.A. Starke, Scripta Mater. 17 (1983) 479. [13] H. Schmidt, M. Soltani-Farshi, Mater. Sci. Eng. A 248 (1998) 73. [14] L. Fouilland-Paill´e, M. Gerland, P. Violan, Mater. Sci. Eng. A 201 (1995) 32. [15] R.G. Vardiman, R.A. Kant, J. Appl. Phys. 53 (1982) 690. [16] D.W. Zhang, Ms D thesis, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang, 1988 (in Chinese). [17] Y.H. Wang, Z.Q. Hu, Implantation and it’s Analysis, Aviation Industry Press, Beijing, 1992 (p. 160, in Chinese). [18] B.R. Sridhar, K. Ramachandra, K.A. Padmanabhan, J. Mater. Sci. 31 (1996) 4381. [19] M.C. Berger, J.K. Gregory, Mater. Sci. Eng. A 263 (1999) 200. [20] R. Hutchings, Mater. Sci. Eng. A 69 (1985) 129. [21] S.C. Tjong, S.M. Zhu, Scripta Mater. 32 (1995) 535.
Effect of surface treatments on fatigue life of Ti-6-22-22 alloy at room and high temperatures Z.L. Yua,∗ , S.X. Lia , Y.Y. Liub , Q.Y. Zhangc , J.F. Leib , Z.X. Muc a c
Shenyang National Laboratory for Materials Science, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110016, China b Titanium Alloy Laboratory, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110016, China The State Key Laboratory for Materials Modifications by Three Beams, Dalian University of Sciences and Technologies, Dalian 116024, China Received 2 March 2004
Abstract Shot-peening and ion implantation were adopted to treat the surface of machined specimens of Ti-6-22-22 alloy, then stress-controlled fatigue tests were performed at room temperature and 400 ◦ C. Experimental results indicate that the effects of shot-peening and ion implantation on the S–N curves are dependent on temperature. For Ti-6-22-22 alloy fatigued at room temperature, the effects of both, shot-peening and ion implantation of carbon ions on fatigue strength at high cyclic lives are slight. At 400 ◦ C, the fatigue strength at high cyclic lives is obviously increased for both, shot-peening and ion implantation of carbon ions in comparison with the untreated counterparts. © 2004 Published by Elsevier B.V. Keywords: Ti-6-22-22 alloy; Shot-peening; Ion implantation; S–N curves; Cracks initiation
1. Introduction Titanium alloys have high strength and low density, thus are widely used in aerospace industries [1]. The dynamic working conditions make materials suffer serious fatigue damage due to varying mechanical load and temperature; therefore, the surface quality has an important effect on fatigue life. As a result, sophisticated machining is required for sufficient surface quality. Shot-peening and ion implantation are the commonly used surface treatment technologies. Shot-peening raises the dislocation density in the surface layer by plastic deformation, producing compressive residual macro-stresses and changing the surface topography. At room temperature, the effect of residual stress has been shown to improve fatigue life [2–9]. Ion implantation strengthens the surface of materials. Though the implanted layer is very thin, it can improve fatigue life [10–16]. There are many studies about the effects ∗ Corresponding author. Tel.: +86 24 83978270; fax: +86 24 23971215. E-mail address: [email protected] (Z.L. Yu).
0921-5093/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.msea.2004.06.026
of shot-peening and ion implantation on fatigue life at room temperature, but studies of the effects on fatigue life at high temperature are rare. Ti-6-22-22 alloy was devised to be used at low to medium temperatures in aircraft, so it is meaningful to explore the fatigue performance of the alloy at higher temperatures. In practical use, especially during the aircraft taking-off to normal flying, the operating temperature extends from room temperature to about 400 ◦ C. The temperature and load variations in such applications will affect the treated surface, and in turn affect the fatigue life of specimens. This paper investigated the effects of shot-peening and ion implantation on fatigue lives of Ti-6-22-22 alloy at room temperature and 400 ◦ C.
2. Experiments The material used was two-phase ␣– Ti-6-22-22 alloy (nominal chemical composition: Ti–6Al–2Sn–2Zr–2Cr– 2Mo, in wt.%). Before fatigue tests, the alloy was heat treated as follows: 980 ◦ C/0.5 h/ac, 925 ◦ C/1 h/ac, 540 ◦ C/8 h/ac. The
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tigued fracture specimens were examined by scanning electron microscopy. 3. Results and discussion 3.1. Surface analysis after surface treatment
Fig. 1. The typical bi-lamellar microstructure of Ti-6-22-22 alloy.
typical bi-lamellar microstructure was obtained as shown in Fig. 1. Fatigue specimens were mechanically machined with an hourglass gage section and threaded grip, the gage dimension was ∅ 4 mm × 6 mm. Shot-peening was carried out on an air-blast machine with S1 1 0 cast steel shot having a hardness of 50–60 HRC. The shot-peening pressure was 0.3 MPa and the final shot-peening intensity was 0.22 A. The shot-peened specimens were examined by hardness and metallography. The residual stress was measured by X-ray method, using a flat specimen peened at equal intensity. Carbon implantation was carried out at an accelerating voltage of 40 keV at room temperature. The incident irradiation dose was 1 × 1017 ions cm−2 . The maximum temperature was kept below 300 ◦ C during ion implantation. In order to guarantee homogenous implantation, the round specimens were rotated 60◦ after every ion implantation and were rotated a total of six times. Fatigue tests were performed using a Schenk 40KN servohydraulic test machine. The test temperatures were room temperature and 400 ◦ C. The cyclic stress waveform was sineshaped and the frequency was 30 Hz. The cyclic stress ratio R of minimum divided by maximum stress was 0.1. The fa-
After shot-peening, the deformation is more severe from the surface to a depth of about 50 m, in which the bi-lamellar microstructure has been curved (see Fig. 2(a)). The shear strain γ is estimated roughly according to the degree of curvature of the lamellai with depth below surface. The measurement of shear strain γ is schematically shown in Fig. 2(b), in which the prolonged line is drawn along a lath α, then the tangent is drawn at the curved position A of the lath α, the angle β between the prolonged line and the tangent is measured (see Fig. 2(b)), finally the shear strain γ is obtained by computing tgβ (γ = tgβ) and is shown in Fig. 3. For comparison, the data of measured microhardness of shot-peened specimens near the surface are also shown in this figure. The residual stress measured by X-ray is shown in Fig. 4. The largest microhardness is near the surface, and the shear strain and the microhardness decrease with the increase of the depth below surface. When the depth is up to 160 m, the microhardness reaches about 420, which is the microhardness of the undeformed matrix. The large compressive residual stress extends to a depth of about 150 m, and beyond that a smaller tension residual stress occurs for overall balance of the stresses. The distribution of implanted carbon below the surface was modeled by using a “trim-98” computer program [15], which estimated the average depth of projected range RP as ˚ and the standard deviation of projected range RP 808 A, ˚ The curve of carbon distribution was constructed as 308 A. by using calculated RP and RP according to the LSS theory [15], see Fig. 5. It is thought by experience that the calculated error on RP is not more than 15% and the error on RP is not more than 30%. The implanted carbon strengthens the alloy by solid solution strengthening, TiC pre-
Fig. 2. (a) The microstructure of the shot-peened specimen and (b) the schematic measurement of shear strain γ of a α-curved lath in the surface of the shot-peened specimen.
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Fig. 3. Strain and microhardnees profiles in the surface layer after shotpeening.
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Fig. 4. Distribution profile of residual stress in the surface layer after shotpeening.
cipitation strengthening, and radiation defect strengthening [15]. 3.2. Effect of shot-peening on S–N curves It is well-known that fatigue life is divided into two parts, crack initiation life and crack propagation life. Crack propagation life predominates when the cyclic stress is large but the crack initiation life predominates when the cyclic stress is low. The residual stress distribution was “S” shape after the titanium alloy was shot-peened (see Fig. 4). The residual stress is compressive near the surface, and a smaller residual tensile stress exists at subsurface to balance the stress state. It will be shown that because of the residual tensile stress, the fatigue cracks of shot-peened specimens generally nucleate beneath the surface, while the fatigue cracks of the untreated specimens nucleate at surface. At room temperature and large cyclic stress, the fatigue lives of shot-peened samples were greater than that of unpeened samples, as shown in Fig. 6(a). Because the fatigue
Fig. 5. Distribution profile of carbon in the surface layer.
cracks initiate at the beneath of the surface, for peened samples, the cracks are internal cracks. It is known that the crack growth driving force for internal cracks is slightly smaller than that for surface cracks. Furthermore, the internal cracks
Fig. 6. S–N curves of shot-peened and untreated specimens. (a) Room temperature; (b) 400 ◦ C.
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are in a quasi-vacuum situation and may avoid the detrimental effects caused by environment such as hydrogen. The internal cracks are also partially embedded in the residual compressive stress region. All these factors are beneficial for the resistance of crack growth in the peened samples. However, at room temperature and low cyclic stress, the fatigue lives of peened samples are approached that of unpeened samples as shown in Fig. 6(a). In this case of low cyclic stress, the cycles for initiation of cracks on the surface for un-peened samples and at the subsurface for peened samples are apparently almost identical. Although the crack growth rate in the un-peened samples may be greater than that in the peened samples as mentioned above, the majority of cycles to fatigue failure was consumed in the crack initiation stage; therefore, the fatigue lives of peened samples is almost equal to that of un-peened samples. Fig. 6(b) shows the S–N curves of specimens at 400 ◦ C with and without shot-peening. Obviously different from S–N curves at room temperature, the curves cross at the maximum cyclic stress of 700 MPa or so. At 400 ◦ C and large cyclic stress, the fatigue lives of shotpeened samples are lower than that of un-peened samples as shown in Fig. 6(b). This could be related to surface hardening and ductility loss. From Fig. 3, we know that there is a hardened layer on the sample surface for peened samples. At 400 ◦ C, the beneficial compressive residual stress could be at least partially relaxed due to higher temperature and cyclic deformation; however, the severely deformed microstructures on the surface layer cannot be recovered completely. At 400 ◦ C, the yield strength of this alloy is about 750 MPa, while at room temperature the yield strength is 1050 MPa. At large cyclic stress, the maximum stress is larger than the yield stress of the alloy (see Fig. 6(b)). The hardened layer with higher strength and poor ductility cannot compensate for the plastic deformation of the underlying alloy. Therefore, cracks are easier to initiate at the surface of the peened samples and grow quickly due to release of residual compressive stress, giving lower fatigue lives in this situation.
At 400 ◦ C and low cyclic stress, the fatigue lives of peened samples are definitely greater than that of un-peened samples, as shown in Fig. 6(b). The reasons might be understood as follows. As mentioned above, at low cyclic stress, deformation of the hardened layer and underlying alloy are within elastic regime and the uncompensated deformation is not very large. Therefore, the crack initiation stage consumes many cycles. Fatigued at 400 ◦ C, the residual stress of titanium alloys partially relaxes, correspondingly the residual tensile stress at the subsurface will decrease. The initiation of microcracks is more difficult due to this residual tensile stress release. So, in this case, the surface higher dislocation density caused by peening can play a beneficial role on fatigue life [18,19]. In summary, at room temperature, the fatigue strengths for 5 × 106 cycle lives are the same for peened and un-peened specimens. On the other hand, at 400 ◦ C, the fatigue strength at 5 × 106 cycle lives for peened specimens is greater than that for un-peened specimens (about 600 MPa > 500 MPa). 3.3. Effect of ion-implantation on S–N curves Fig. 7 shows the S–N curves of implanted and unimplanted specimens. Carbon ion implantation has similar effects both at room temperature and 400 ◦ C, i.e., there is a crossover between the S–N curves of implanted and unimplanted specimens. However, the effect at 400 ◦ C is much more obvious. At room temperature and 400 ◦ C, when the cyclic stress is low, the fatigue life can be increased for implanted specimens. However, when the cyclic stress is large, fatigue life is decreased for implanted specimens. The fatigue strength of implanted specimens at 5 × 106 cycles is increased slightly at room temperature (Fig. 7(a)). However, at 400 ◦ C, the fatigue strength is increased substantially from 500 MPa of un-implanted specimens to 650 MPa of implanted specimens (Fig. 7(b)). Normally, the fatigue strength in the high life, low cyclic stress regime increases with an increase of strength of materi-
Fig. 7. S–N curves of ion-implanted and untreated specimens at (a) room temperature; (b) 400 ◦ C.
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als. Carbon ions were implanted into the surface, the strength of material surface increases, thus the fatigue strength of the materials increases both at room temperature and 400 ◦ C. Meanwhile, a residual compressive stress is produced due to carbon ion implantation, it also can improve fatigue life [17,20]. However, while the strength of the implanted surface increases, the ductility can decrease. For instance, Tjong and Zhu [21] found that there were brittle characteristics on surface cracks in experiments of Fe–24Cr–4Al alloy, which was nitrogen implanted. When the implanted specimens were
287
given higher loads, ion implantation could accelerate the cracks initiating at machining scratches, while cracks of unimplanted specimens could be more easily blunted due to relatively good plasticity. This could explain why the fatigue life of implanted specimens decreased when the cyclic stress was large. Particularly, at 400 ◦ C and large cyclic stress, the fatigue lives of implanted samples are much lower than that of un-implanted samples as shown in Fig. 7(b). These ductility effects could be accentuated at 400 ◦ C. At 400 ◦ C, the yield strength of this alloy is about 750 MPa, while at room temperature the yield strength is 1050 MPa. At large cyclic
Fig. 8. SEM fractography of fatigued specimens. (a) Untreated specimen, fatigued at room temperature; (b) shot-peened specimen, fatigued at room temperature; (c) shot-peened specimen, fatigued at 400 ◦ C; (d) shot-peened specimen, fatigued at 400 ◦ C; (e) implanted specimen, fatigued at room temperature; (f) implanted specimen, fatigued at 400 ◦ C; σ max = 700 MPa.
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stress, the maximum stress is larger than the yield stress of the alloy (see Fig. 7(b)). The implanted layer with high strength and poor ductility cannot compensate the plastic deformation of the matrix. The cracks are easier to initiate at the surface of the implanted samples and grow quickly due to partially release of residual stress at 400 ◦ C; therefore, lower fatigue lives result in this situation. 3.4. SEM fractography Fig. 8 shows the fractography of fatigued specimens observed by SEM. For the untreated specimens, the cracks entirely initiate at the surface at room temperature, basically caused by machined scratches (Fig. 8(a)). The cracks of peened specimens tested at room temperature mostly initiate subsurface, see Fig. 8(b). The depth of crack initiation sites below the surface was near 300 m, which corresponds to the maximum tensile stress at the subsurface. At 400 ◦ C, the depth of crack initiation was related to the amplitude of cyclic stress. When the cyclic stress is low, the cracks initiate subsurface (Fig. 8(c)), but when the cyclic stress is large, near 800 MPa, the cracks initiate at the surface (Fig. 8(d)). The cracks of implanted specimens fatigued at room temperature initiated at the surface (Fig. 8(e)), as for the untreated specimens. At 400 ◦ C, when the cyclic stress is greater than 750 MPa corresponding to the yield stress of the Ti alloy, fatigue cracks initiate at the surface, while below 750 MPa the cracks initiate subsurface (Fig. 8(f)). When cracks initiate subsurface for both, shot-peened and implanted specimens fatigued at 400 ◦ C, the effect of environment on fatigue life is obviated, increasing fatigue life. 4. Conclusions Based on the experiment results, some conclusions may be drawn: (1) For Ti-6-22-22 alloy fatigued at room temperature, the effects of both, shot-peening and ion implantation of carbon ions on fatigue strength at high cycles are very small. (2) At 400 ◦ C, the fatigue strength at high cyclic lives is obviously increased for both, shot-peening and ion implantation of carbon ions. However, when the cyclic stress is higher than the yield stress of the alloy, the fatigue lives of specimens treated by both methods can-
not be improved. This is because the treated surface layer with high strength but poor ductility cannot compensate for the plastic deformation of the underlying alloy.
Acknowledgement This work is financially support by Key Basic Research Project of China (G19990650).
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