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Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue
International Journal of Fatigue 27 (2005) 1530–1534 www.elsevier.com/locate/ijfatigue
Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue A. Evansa,b,*, S-B. Kima, J. Shackletona, G. Brunoa,b, M. Preussa, P.J. Withersa a
Manchester Materials Science Centre, University of Manchester, Grosvenor Street, Manchester M1 7HS, UK b Institut Laue-Langevin, 6, Rue Jules Horowitz, B.P.156, 38042 Grenoble Cedex 9, France Available online 31 August 2005
Abstract The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin2j technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes. q 2005 Elsevier Ltd. All rights reserved. Keywords: Shot peening; Residual stress relaxation; High temperature isothermal fatigue; Udimet 720Li
1. Introduction Shot peening is an established mechanical surface treatment used to enhance predominantly the fatigue performance and damage tolerance of treated components [1]. The process involves the bombardment of the treated surface with a stream of small spherical media called shot. The indentation caused by the impingement of the shot plastically deforms the surface of the material generating a work hardened surface layer. The misfit created by the plastically deformed region results in an in-plane compressive residual stress (RS) state generated in the near surface region (typically a few hundred micrometers in depth). The near surface compressive RS field reduces the effective applied stresses of the component during application, which results in delayed crack initiation and retarded early crack growth, extending component life [1]. Furthermore, the plastic deformation of the material increases the dislocation density in the near surface region, which is purported to
* Corresponding author. Address: Institut Laue-Langevin, 6, Rue Jules Horowitz, B.P.156, 38042 Grenoble Cedex 9, France. Tel.: C33 476 207941. E-mail address: [email protected] (A. Evans).
0142-1123/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2005.07.027
hinder dislocation movement, associated with crack initiation [2]. The compressive RS field is balanced by tensile RS beneath the compressive region. An understanding of the resistance to relaxation of the residual stress state is important for the structural integrity of treated components. Nickel base superalloys are technologically important materials for turbine engine applications due to their excellent mechanical and corrosion resistance properties at high temperatures [3,5]. Udimet 720Li is a polycrystalline nickel base superalloy, which is used primarily for turbine discs with a service temperature range in the region 650–700 8C [4]. Many studies of thermal relaxation of shot peening RS have found that a large proportion of the total relaxation occurs in the initial period of exposure, between 3 min and 1 h of exposure [5]. The process of thermal relaxation describes the reduction of the residual stress causing plastic misfit by diffusive or dislocational movement of atoms driven by the reduction of stored energy (both elastic strain energy and cold work). These mechanisms include the annihilation of metastable lattice defects, creep controlled dislocation rearrangement and recrystallisation at higher temperatures. The mechanical effects that control the relaxation under cyclic loading have been identified previously as: the initial magnitude and the gradient of residual stresses and the extent of local cold work, the fatigue stress parameters (amplitude, mean stress
A. Evans et al. / International Journal of Fatigue 27 (2005) 1530–1534
ratio and number of cycles), and the materials cyclic stress– strain behaviour [6]. The mechanical relaxation has been previously characterised by two stages, an initial decrease over the first few cycles, the magnitude and mechanism of which is dependent upon the load, and a subsequent slower (logarithmic) relaxation with increasing number of cycles. During the first cycle inhomogeneous yielding can act to reduce the plastic misfit and can result in the compressive RS at the surface reversing into tension [1]. An explanation of this overload relaxation, proposed by Holzapfel et al. [7] and by Zhuang et al. [6] uses the Bauschinger effect. They propose that the work-hardened surface will yield in compression on the compressive part of the initial cycle due to the lower compressive yield strength resulting from the in-plane tensile deformation during peening. This is aided by the compressive RS which is additional to any applied compressive stress. Subsequent relaxation has been suggested to stem from cyclic softening of the work hardened material with increased cycling [7]. The aim of this study is to determine the extent of the relaxation of the near surface stress following high temperature isothermal low cycle fatigue under overload strain controlled conditions. These were were chosen as being representative of those experienced in turbine discs as a result of a serious overload The relaxation is determined for three temperatures (350, 650 and 700 8C) and after 1 and 1000 cycles. To the authors knowledge this is the first systematic study of the kind in this alloy.
2. Experimental 2.1. Material and specimens The material used in this study was the Ni-base superalloy Udimet 720Li, the composition of which is given in Table 1. The cast and forged material was solution heat treated for 4 h at 1105 8C (below the g‘ solvus) and subsequently oil quenched. The two-step aging treatment consisted of 24 h at 650 8C followed by 16 h at 760 8C. Microstructural analysis revealed an average grain size of 8G1 mm, with no variation between the near surface and the parent material. The microstructure comprised intergranular primary g 0 (1–2 mm) and intragranular secondary and tertiary g 0 (150 and 10–20 nm, respectively). Six tensile/ fatigue specimens were machined from a BR710 HP2 Turbine disc. The gauge of each specimen was shot peened according to the conditions; shot specification 110H, Almen Intensity 6–8A, 200% coverage.
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2.1.1. Residual stress determination by X-ray diffraction Residual stress (RS) profiles were determined as a function of depth using the sin2j technique [8] on a laboratory X-ray diffractometer in conjunction with incremental electrolytic layer removal. The electrolytic removal was done on a 50!50 mm2 central window. By repeatedly removing w20 mm depth of material, followed by stress measurement by X-ray, depth profiles of the RS were obtained. An iron anode X-ray tube was used and measurements were made on the (311) Nickel diffraction peak, at a 2q angle of about 1288 with an irradiated area of approximately 1 mm2. Seven j offsets were measured between 0 and 408 for the two principal in-plane stress directions. A Gaussian function was used to fit the diffraction peaks. The peak position (2q) and the peak width (expressed as the standard deviation of the Gaussian distribution) were obtained for each measurement. Errors in stress were calculated by means of classical propagation formulae and proved to be of the order of G30 MPa. In our case, no grain size variation as a function of depth was observed so that the variation of the peak width gives a qualitative indication of the depth and magnitude of the cold work caused by shot peening. Since a quantitative measure of effective plastic work was not required there was no need for calibration measurements [9]. 2.1.2. High temperature isothermal fatigue The samples were exposed to strain controlled low cycle fatigue. The loading was performed by the IRC, Swansea, UK. Strain controlled fatigue was used to simulate the conditions of the shot peened fir tree notches, where the turbine blade connects to the turbine disc. Strain controlled conditions prevail due to the constraining effect of the surrounding mass of elastically deformed material. The strain range was controlled by contacting strain gauge bridge extensometers to the gauge section. Each fatigue cycle consisted of a ramp up to the maximum strain (1.2%) at a rate of 57!10K3 sK1, a 1 s dwell time and then a decrease to zero strain at a rate of 5!10K3 sK1, followed by a 1 s dwell at 0% strain, (RZmin strain/max strainZ0). Tests for 1 and 1000 cycles were carried out at 350, 650 and 700 8C. The total time that the samples spent at their respective testing temperatures including temperature stabilisation prior to testing was between 3 and 4 h. Cyclic hardening was observed after 1000 cycles at 350 8C, whereas the samples cyclic softened after 1000 cycles at 700 8C (Fig. 1).
3. Results and discussion 3.1. Cyclic stress–strain behaviour
Table 1 Chemical composition of the superalloy alloy 720Li Cr
Co
Mo
W
Al
Ti
Zr
B
C
Ni
16
14.7
3.0
1.25
2.5
5.0
0.03
0.015
0.010
Bal.
The Young’s modulus decreases slightly with increasing temperature, while the 0.2% yield strength is largely unaffected. It must be noted that since cycling is strain
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Fig. 1. Examples of stress–strain curves recorded after strain controlled fatigue with a maximum strain of 1.2%, RZ0 at 700 8C. Inserted is a schematic of the uniaxial fatigue specimens. All dimensions in mm.
controlled, at 0% strain a compressive stress of about K700 MPa is present (Fig. 1). At 650 and 700 8C, stable hysteresis loops eventually formed. Udimet 720Li has been known to exhibit cyclic softening under similar conditions [3]. 3.2. Changes in residual stress and peak width relaxation 3.2.1. As-peened state The residual stress profile in the as-peened condition was found to be isotropic in-plane. It attained a peak compression of around K1200 MPa located 50 mm below the surface (see Fig. 2). A peak tensile stress of w300 MPa was found 200 nm below the surface. The remaining depth of the sample is placed into slight residual tension to ensure stress balance [10]. The variation of the diffraction peak width across the as- peened sample (Fig. 2) suggests that the sample has been significantly plastically deformed to a depth of around 100 mm. 3.2.2. Relaxation after 1 cycle Fig. 2 shows that irrespective of the testing temperature, the initial cycle causes the maximum compressive residual stress (RS) parallel to the loading direction to relax by more than 50%. The surface residual stress changes sign from K550 MPa for the as-peened sample to C100 MPa for all three temperatures. While the depth of maximum compression appears to vary as a function of the testing temperature, in general the relaxation appears to be essentially independent of temperature. This suggests that the relaxation is strongly influenced by the plastic strain. The tensile plastic straining of the bulk of the sample as evidenced in Fig. 1, causes significant reduction of the misfit between the work-hardened shot peened near-surface region which is unlikely to deform in tension due to the original RS
and the previously undeformed bulk. By plastically straining the bulk in tension, the tensile RS are also reduced and redistributed to an extent that no tensile peak is observed within the first 300 mm. Relaxation of the compressive RS would also be expected to occur readily over the compressive half of the cycle (reverse straining). Over this regime the compressive RS and the applied stress encourage compressive yielding of the near surface region as the sample returns to 0% strain. This plastification is aided by the Bauschinger effect whereby the surface region has lower yield strength at the surface compared to the bulk
Fig. 2. Residual stress and integral width profiles after 1 cycle of 1.2% strain controlled high temperature isothermal fatigue of various temperatures.
A. Evans et al. / International Journal of Fatigue 27 (2005) 1530–1534
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reduction in width is observed compared to the as-peened condition, however at 700 8C there appears to be a reduction of the peak width occurring within 75 mm of the surface. This suggests that the reduction of cold work is thermally driven and occurs essentially independently of the changes in residual stress over this cycle which are predominantly mechanically driven. In this context it should be noted that the cold work is expected to be equivalent to a plastic strain between 20 and 40% [1] whereas the geometrically necessary plastic strain needed to cause the residual stress is only around 0.4% [12].
Fig. 3. Residual stress and integral width profiles after 1000 cycles of 1.2% strain controlled high temperature isothermal fatigue at various temperatures.
in compression, owing to the tensile work hardening generated by tensile stretching of the surface during peening. That these changes are primarily mechanically driven are corroborated by the observation that the RS changes in the transverse direction to the load are smaller [11]. After the initial cycle, the integral diffraction peak varies with temperature (Fig. 2). At 350 and 650 8C, little
3.2.3. Relaxation after 1000 cycles At 350 8C, the area of the stress–strain loop decreases with increasing number of cycles, while the slope increases. Therefore the material cyclically hardens. It is then not surprising that at 350 8C, further cycling beyond the initial cycle appears to cause very little further relaxation between 1 and 1000 cycles. Conversely, at higher temperatures, the area of the hysteresis loop does not decrease while the gradient decreases slightly, suggesting that the material does not strain harden above 650 8C. As a result, further RS relaxation after the initial cycle is apparent at high temperatures, Fig. 3. Most notably, upon further cycling, the surface RS returns to compression at 650 8C. This is even more marked at 700 8C. The peak compressive stress relaxes from around 500 MPa to between 200 and 300 MPa. The reduction in peak width within 75 mm depth is only significant at 700 8C (Fig. 3). The mechanism of this relaxation could be a combination of the thermally activated processes of annihilation and creep, in addition to cyclic deformation processes associated with cyclic softening. A comparison of the RS and peak width profiles following 1000 cycles with those after 3 h of solely thermal exposure at 700 8C is shown in Fig. 4. Despite the similar exposure times, thermal relaxation causes a reduction of RS of 500 MPa, whereas 1000 cycles of high temperature isothermal fatigue leads to a reduction of about 900 MPa. A similar difference can be observed at 350 8C. This is due to the significant plastic strain induced during the first fatigue cycle. Fatigue also causes greater reduction of the integral width relative to the solely thermal case, indicating that high temperature isothermal relaxation at 700 8C is perhaps driven by creep. This is not so apparent at 650 8C, where the average load or the exposure times may not be sufficient to activate creep mechanisms.
4. Conclusions
Fig. 4. Residual stress and integral width profiles after 1000 cycles of 1.2% strain controlled high temperature isothermal fatigue at 700 8C compared to equivalent periods (3 h) of solely thermal exposure of 700 8C.
The resistance to relaxation of the residual stress state generated by shot peening of the turbine disc material, Udimet 720Li under high temperature isothermal strain controlled low cycle fatigue has been determined. Quite
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distinct behaviours have been observed after 1 and 1000 cycles; 1. A reduction of over 50% in the peak compressive RS takes place after only 1 cycle at all testing temperatures (350, 650 and 700 8C). The reduction is essentially independent of temperature and is due to the plastic deformation of the material, which reduces the misfit between the peened surface and the previously undeformed bulk. The peak width is only reduced significantly at 700 8C, suggesting a thermally activated mechanism of redistribution of metastable lattice defects generated by the peening. 2. After 1000 cycles, further RS relaxation is observed essentially only at 650 and 700 8C. The overall effect is to flatten the near surface RS profile. The near surface integral widths decreases further at 700 8C. The relaxation at high numbers of cycles appears to be controlled by temperature dependent mechanisms in contrast to the relaxation occurring after the first cycle. 3. The relaxation of RS is more pronounced under high temperature isothermal cycling than occurs under solely thermal exposure. The reduction of the integral width at 700 8C following 1000 cycles is more pronounced than for the solely thermal case.
Acknowledgements S.-B.Kim is grateful to Rolls-Royce for a Fellowship and for helpful discussions during his visit. PJW is grateful for funding from a Royal Society-Wolfson Merit Award. The help of Prof M. Bache at Swansea is acknowledged in making the stress strain data collected in Swansea available to us.
References [1] Preve`y P, Shepard M, Smith P. The effect of low plasticity burnishing (LPB) on the HCF performance and FOD resistance of Ti-6Al–4V. Proceedings 6th National turbine engine high cycle fatigue (HCF) conference, Jacksonville, FL, 5–8 March 2001. [2] Suresh S. Fatigue of materials. 2nd ed. Cambridge: Cambridge University Press; 1998. [3] Ren W, Nicholas T. The effects of low cycle fatigue and plastic deformation on the subsequent high cycle fatigue limit of nickel-base superalloy Udimet 720Li. Mater Sci Eng 2002;A332:238–48. [4] Helm D, Roder O. Influence of long term exposure in air on microstructure, surface stability and mechanical properties of UDIMET 720 Li. In: Pollock TM, Kissinger RD, Bowman RR, Green KA, McLean M, Olson S, Shirra JJ, editors. Superalloy 2000. Warrendale, PA: TMS; 2000. p. 487–93. [5] Cao W, Khadhraoui M, Brenier B, Guedou JY, Castex L. High temperature isothermal relaxation of residual stress in shot peened nickel base superalloy. Mater Sci Technol 1994;10:947–54. [6] Zhaung W, Halford G. Investigation of residual stress relaxation under cyclic load. Int J Fatigue 2001;S31–S37:23. [7] Holzapfel H, Schulze V, Vo¨hringer O, Macherauch E. Residual stress relaxation in an AISI 4140 steel due to quasistatic and cyclic loading at higher temperatures. Mater Sci Eng A 1998;248(1–2): 9–18. [8] Noyan IC, Cohen JB. Residual stress: measurement by diffraction and interpretation. New York, NY: Springer; 1987. [9] Prevey PS. The effect of cold work on the thermal stability of residual compression in surface enhanced IN718. Proceedings of the 20th ASM materials solutions conference & exposition, St. Louis, MO, 10–12 October 2000. [10] Ezeilo AN, Webster GA, Webster PJ, Webster PS. Comparison of shot peening residual stress distribution in a selection of materials. Proceedings of international conference of shot peening, Oxford, UK, 13–17 Sept. 1993. [11] Evans AD, Kim S-B, Bruno G, Preuss M, Shackleton J, Withers PJ, Residual stress relaxation in shot peened Udimet 720Li, submitted for publication. Metall Mater Trans A. [12] Satraki M, Evans AD, King A, Bruno G, Withers PJ, Eigenstrain generated by shot peening in Udimet 720Li inferred by means of finite element and analytical models. Proceeding of the 9th International Conference of Shot Peening, Paris, France, Sept. 2005, submitted for publication.
Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue A. Evansa,b,*, S-B. Kima, J. Shackletona, G. Brunoa,b, M. Preussa, P.J. Withersa a
Manchester Materials Science Centre, University of Manchester, Grosvenor Street, Manchester M1 7HS, UK b Institut Laue-Langevin, 6, Rue Jules Horowitz, B.P.156, 38042 Grenoble Cedex 9, France Available online 31 August 2005
Abstract The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin2j technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes. q 2005 Elsevier Ltd. All rights reserved. Keywords: Shot peening; Residual stress relaxation; High temperature isothermal fatigue; Udimet 720Li
1. Introduction Shot peening is an established mechanical surface treatment used to enhance predominantly the fatigue performance and damage tolerance of treated components [1]. The process involves the bombardment of the treated surface with a stream of small spherical media called shot. The indentation caused by the impingement of the shot plastically deforms the surface of the material generating a work hardened surface layer. The misfit created by the plastically deformed region results in an in-plane compressive residual stress (RS) state generated in the near surface region (typically a few hundred micrometers in depth). The near surface compressive RS field reduces the effective applied stresses of the component during application, which results in delayed crack initiation and retarded early crack growth, extending component life [1]. Furthermore, the plastic deformation of the material increases the dislocation density in the near surface region, which is purported to
* Corresponding author. Address: Institut Laue-Langevin, 6, Rue Jules Horowitz, B.P.156, 38042 Grenoble Cedex 9, France. Tel.: C33 476 207941. E-mail address: [email protected] (A. Evans).
0142-1123/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2005.07.027
hinder dislocation movement, associated with crack initiation [2]. The compressive RS field is balanced by tensile RS beneath the compressive region. An understanding of the resistance to relaxation of the residual stress state is important for the structural integrity of treated components. Nickel base superalloys are technologically important materials for turbine engine applications due to their excellent mechanical and corrosion resistance properties at high temperatures [3,5]. Udimet 720Li is a polycrystalline nickel base superalloy, which is used primarily for turbine discs with a service temperature range in the region 650–700 8C [4]. Many studies of thermal relaxation of shot peening RS have found that a large proportion of the total relaxation occurs in the initial period of exposure, between 3 min and 1 h of exposure [5]. The process of thermal relaxation describes the reduction of the residual stress causing plastic misfit by diffusive or dislocational movement of atoms driven by the reduction of stored energy (both elastic strain energy and cold work). These mechanisms include the annihilation of metastable lattice defects, creep controlled dislocation rearrangement and recrystallisation at higher temperatures. The mechanical effects that control the relaxation under cyclic loading have been identified previously as: the initial magnitude and the gradient of residual stresses and the extent of local cold work, the fatigue stress parameters (amplitude, mean stress
A. Evans et al. / International Journal of Fatigue 27 (2005) 1530–1534
ratio and number of cycles), and the materials cyclic stress– strain behaviour [6]. The mechanical relaxation has been previously characterised by two stages, an initial decrease over the first few cycles, the magnitude and mechanism of which is dependent upon the load, and a subsequent slower (logarithmic) relaxation with increasing number of cycles. During the first cycle inhomogeneous yielding can act to reduce the plastic misfit and can result in the compressive RS at the surface reversing into tension [1]. An explanation of this overload relaxation, proposed by Holzapfel et al. [7] and by Zhuang et al. [6] uses the Bauschinger effect. They propose that the work-hardened surface will yield in compression on the compressive part of the initial cycle due to the lower compressive yield strength resulting from the in-plane tensile deformation during peening. This is aided by the compressive RS which is additional to any applied compressive stress. Subsequent relaxation has been suggested to stem from cyclic softening of the work hardened material with increased cycling [7]. The aim of this study is to determine the extent of the relaxation of the near surface stress following high temperature isothermal low cycle fatigue under overload strain controlled conditions. These were were chosen as being representative of those experienced in turbine discs as a result of a serious overload The relaxation is determined for three temperatures (350, 650 and 700 8C) and after 1 and 1000 cycles. To the authors knowledge this is the first systematic study of the kind in this alloy.
2. Experimental 2.1. Material and specimens The material used in this study was the Ni-base superalloy Udimet 720Li, the composition of which is given in Table 1. The cast and forged material was solution heat treated for 4 h at 1105 8C (below the g‘ solvus) and subsequently oil quenched. The two-step aging treatment consisted of 24 h at 650 8C followed by 16 h at 760 8C. Microstructural analysis revealed an average grain size of 8G1 mm, with no variation between the near surface and the parent material. The microstructure comprised intergranular primary g 0 (1–2 mm) and intragranular secondary and tertiary g 0 (150 and 10–20 nm, respectively). Six tensile/ fatigue specimens were machined from a BR710 HP2 Turbine disc. The gauge of each specimen was shot peened according to the conditions; shot specification 110H, Almen Intensity 6–8A, 200% coverage.
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2.1.1. Residual stress determination by X-ray diffraction Residual stress (RS) profiles were determined as a function of depth using the sin2j technique [8] on a laboratory X-ray diffractometer in conjunction with incremental electrolytic layer removal. The electrolytic removal was done on a 50!50 mm2 central window. By repeatedly removing w20 mm depth of material, followed by stress measurement by X-ray, depth profiles of the RS were obtained. An iron anode X-ray tube was used and measurements were made on the (311) Nickel diffraction peak, at a 2q angle of about 1288 with an irradiated area of approximately 1 mm2. Seven j offsets were measured between 0 and 408 for the two principal in-plane stress directions. A Gaussian function was used to fit the diffraction peaks. The peak position (2q) and the peak width (expressed as the standard deviation of the Gaussian distribution) were obtained for each measurement. Errors in stress were calculated by means of classical propagation formulae and proved to be of the order of G30 MPa. In our case, no grain size variation as a function of depth was observed so that the variation of the peak width gives a qualitative indication of the depth and magnitude of the cold work caused by shot peening. Since a quantitative measure of effective plastic work was not required there was no need for calibration measurements [9]. 2.1.2. High temperature isothermal fatigue The samples were exposed to strain controlled low cycle fatigue. The loading was performed by the IRC, Swansea, UK. Strain controlled fatigue was used to simulate the conditions of the shot peened fir tree notches, where the turbine blade connects to the turbine disc. Strain controlled conditions prevail due to the constraining effect of the surrounding mass of elastically deformed material. The strain range was controlled by contacting strain gauge bridge extensometers to the gauge section. Each fatigue cycle consisted of a ramp up to the maximum strain (1.2%) at a rate of 57!10K3 sK1, a 1 s dwell time and then a decrease to zero strain at a rate of 5!10K3 sK1, followed by a 1 s dwell at 0% strain, (RZmin strain/max strainZ0). Tests for 1 and 1000 cycles were carried out at 350, 650 and 700 8C. The total time that the samples spent at their respective testing temperatures including temperature stabilisation prior to testing was between 3 and 4 h. Cyclic hardening was observed after 1000 cycles at 350 8C, whereas the samples cyclic softened after 1000 cycles at 700 8C (Fig. 1).
3. Results and discussion 3.1. Cyclic stress–strain behaviour
Table 1 Chemical composition of the superalloy alloy 720Li Cr
Co
Mo
W
Al
Ti
Zr
B
C
Ni
16
14.7
3.0
1.25
2.5
5.0
0.03
0.015
0.010
Bal.
The Young’s modulus decreases slightly with increasing temperature, while the 0.2% yield strength is largely unaffected. It must be noted that since cycling is strain
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Fig. 1. Examples of stress–strain curves recorded after strain controlled fatigue with a maximum strain of 1.2%, RZ0 at 700 8C. Inserted is a schematic of the uniaxial fatigue specimens. All dimensions in mm.
controlled, at 0% strain a compressive stress of about K700 MPa is present (Fig. 1). At 650 and 700 8C, stable hysteresis loops eventually formed. Udimet 720Li has been known to exhibit cyclic softening under similar conditions [3]. 3.2. Changes in residual stress and peak width relaxation 3.2.1. As-peened state The residual stress profile in the as-peened condition was found to be isotropic in-plane. It attained a peak compression of around K1200 MPa located 50 mm below the surface (see Fig. 2). A peak tensile stress of w300 MPa was found 200 nm below the surface. The remaining depth of the sample is placed into slight residual tension to ensure stress balance [10]. The variation of the diffraction peak width across the as- peened sample (Fig. 2) suggests that the sample has been significantly plastically deformed to a depth of around 100 mm. 3.2.2. Relaxation after 1 cycle Fig. 2 shows that irrespective of the testing temperature, the initial cycle causes the maximum compressive residual stress (RS) parallel to the loading direction to relax by more than 50%. The surface residual stress changes sign from K550 MPa for the as-peened sample to C100 MPa for all three temperatures. While the depth of maximum compression appears to vary as a function of the testing temperature, in general the relaxation appears to be essentially independent of temperature. This suggests that the relaxation is strongly influenced by the plastic strain. The tensile plastic straining of the bulk of the sample as evidenced in Fig. 1, causes significant reduction of the misfit between the work-hardened shot peened near-surface region which is unlikely to deform in tension due to the original RS
and the previously undeformed bulk. By plastically straining the bulk in tension, the tensile RS are also reduced and redistributed to an extent that no tensile peak is observed within the first 300 mm. Relaxation of the compressive RS would also be expected to occur readily over the compressive half of the cycle (reverse straining). Over this regime the compressive RS and the applied stress encourage compressive yielding of the near surface region as the sample returns to 0% strain. This plastification is aided by the Bauschinger effect whereby the surface region has lower yield strength at the surface compared to the bulk
Fig. 2. Residual stress and integral width profiles after 1 cycle of 1.2% strain controlled high temperature isothermal fatigue of various temperatures.
A. Evans et al. / International Journal of Fatigue 27 (2005) 1530–1534
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reduction in width is observed compared to the as-peened condition, however at 700 8C there appears to be a reduction of the peak width occurring within 75 mm of the surface. This suggests that the reduction of cold work is thermally driven and occurs essentially independently of the changes in residual stress over this cycle which are predominantly mechanically driven. In this context it should be noted that the cold work is expected to be equivalent to a plastic strain between 20 and 40% [1] whereas the geometrically necessary plastic strain needed to cause the residual stress is only around 0.4% [12].
Fig. 3. Residual stress and integral width profiles after 1000 cycles of 1.2% strain controlled high temperature isothermal fatigue at various temperatures.
in compression, owing to the tensile work hardening generated by tensile stretching of the surface during peening. That these changes are primarily mechanically driven are corroborated by the observation that the RS changes in the transverse direction to the load are smaller [11]. After the initial cycle, the integral diffraction peak varies with temperature (Fig. 2). At 350 and 650 8C, little
3.2.3. Relaxation after 1000 cycles At 350 8C, the area of the stress–strain loop decreases with increasing number of cycles, while the slope increases. Therefore the material cyclically hardens. It is then not surprising that at 350 8C, further cycling beyond the initial cycle appears to cause very little further relaxation between 1 and 1000 cycles. Conversely, at higher temperatures, the area of the hysteresis loop does not decrease while the gradient decreases slightly, suggesting that the material does not strain harden above 650 8C. As a result, further RS relaxation after the initial cycle is apparent at high temperatures, Fig. 3. Most notably, upon further cycling, the surface RS returns to compression at 650 8C. This is even more marked at 700 8C. The peak compressive stress relaxes from around 500 MPa to between 200 and 300 MPa. The reduction in peak width within 75 mm depth is only significant at 700 8C (Fig. 3). The mechanism of this relaxation could be a combination of the thermally activated processes of annihilation and creep, in addition to cyclic deformation processes associated with cyclic softening. A comparison of the RS and peak width profiles following 1000 cycles with those after 3 h of solely thermal exposure at 700 8C is shown in Fig. 4. Despite the similar exposure times, thermal relaxation causes a reduction of RS of 500 MPa, whereas 1000 cycles of high temperature isothermal fatigue leads to a reduction of about 900 MPa. A similar difference can be observed at 350 8C. This is due to the significant plastic strain induced during the first fatigue cycle. Fatigue also causes greater reduction of the integral width relative to the solely thermal case, indicating that high temperature isothermal relaxation at 700 8C is perhaps driven by creep. This is not so apparent at 650 8C, where the average load or the exposure times may not be sufficient to activate creep mechanisms.
4. Conclusions
Fig. 4. Residual stress and integral width profiles after 1000 cycles of 1.2% strain controlled high temperature isothermal fatigue at 700 8C compared to equivalent periods (3 h) of solely thermal exposure of 700 8C.
The resistance to relaxation of the residual stress state generated by shot peening of the turbine disc material, Udimet 720Li under high temperature isothermal strain controlled low cycle fatigue has been determined. Quite
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distinct behaviours have been observed after 1 and 1000 cycles; 1. A reduction of over 50% in the peak compressive RS takes place after only 1 cycle at all testing temperatures (350, 650 and 700 8C). The reduction is essentially independent of temperature and is due to the plastic deformation of the material, which reduces the misfit between the peened surface and the previously undeformed bulk. The peak width is only reduced significantly at 700 8C, suggesting a thermally activated mechanism of redistribution of metastable lattice defects generated by the peening. 2. After 1000 cycles, further RS relaxation is observed essentially only at 650 and 700 8C. The overall effect is to flatten the near surface RS profile. The near surface integral widths decreases further at 700 8C. The relaxation at high numbers of cycles appears to be controlled by temperature dependent mechanisms in contrast to the relaxation occurring after the first cycle. 3. The relaxation of RS is more pronounced under high temperature isothermal cycling than occurs under solely thermal exposure. The reduction of the integral width at 700 8C following 1000 cycles is more pronounced than for the solely thermal case.
Acknowledgements S.-B.Kim is grateful to Rolls-Royce for a Fellowship and for helpful discussions during his visit. PJW is grateful for funding from a Royal Society-Wolfson Merit Award. The help of Prof M. Bache at Swansea is acknowledged in making the stress strain data collected in Swansea available to us.
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