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The enhancement of transformation induced plasticity effect on austenitic stainless steels by cyclic tensile loading and unloading
Materials Letters 65 (2011) 1545–1547
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The enhancement of transformation induced plasticity effect on austenitic stainless steels by cyclic tensile loading and unloading Yong Xu a, Shihong Zhang a,⁎, Hongwu Song a, Ming Cheng a, Haiqu Zhang b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Shenyang University, Shenyang 110044, China
a r t i c l e
i n f o
Article history: Received 7 December 2010 Accepted 21 February 2011 Available online 25 February 2011 Keywords: Austenitic steels Loading modes Phase transformation Mechanical properties X-ray techniques
a b s t r a c t The effect of loading modes of tensile deformation on the mechanical properties of a metastable austenite stainless steel has been investigated. The stress–strain curves, microstructures and fraction of the martensite are measured and analyzed separately. The results of tensile test indicate that a special loading mode referred as cyclic tensile loading and unloading can improve the strength and the formability of the specimens effectively. It is noted that the elongation and the ultimate strength are increased by 24.3% and 9.2% respectively at room temperature. Such enhancement of the transformation induced plasticity effect is mainly related to remarkable increase of the fraction of strain-induced martensite by the cyclic tensile loading and unloading. © 2011 Elsevier B.V. All rights reserved.
1. Introduction It has long been recognized that austenitic stainless steels are one of the attractive engineering materials, due to their high corrosion resistance and versatile mechanical properties. These metastable alloys belong to a kind of typical transformation induced plasticity (TRIP) steels [1,2] whose high strength and excellent ductility are expected for their strain-induced martensitic transformation at room temperature (RT) [3–5]. Consequently, they can usually be manufactured by cold working including cold rolling [6], drawing [7] and hydroforming [8]. A number of studies have been performed to understand the martensitic transformation behavior and TRIP effect in this kind of metastable materials. Manjanna [9] investigated the evolution of the fraction of martensite in 316LN austenitic stainless steel compressed at RT by using magnetic measurement. The nano-indentation and microstructural results were reported to provide micromechanical insight into the strain-induced phase transformation and deformation behavior of metastable austenite in TRIP steel [10]. Lebedev and Kosarchuk [11] conducted a series of experiments to assess the influence of both temperature and stress states on the kinetics of martensitic transformation. Nagayama et al. [12,13] and Tanaka et al. [14] investigated the TRIP phenomenon in some kind of maraging steel through thermomechanical loading process of martensitic transformation by partial and full unloading tests. The previous reports have just introduced some new findings or complicated
⁎ Corresponding author. Tel.: +86 24 83978266; fax: +86 24 23906831. E-mail address: [email protected] (S. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.066
experimental methods which still dissatisfy the industrial requirements. Further considerations thus should be put on the formability of the materials under common conditions. It is well known that in metastable stainless steels low stacking fault energy promotes austenitic phase transformation to martensitic phase [15]. However, there are few reports on the detail of promoting this phase transformation and enhancing the TRIP effect during deformation only by changing the loading mode at RT in this kind of metastable steels [16]. In this study, to identify the effect of the loading modes on the evolution of strain-induced martensitic transformation and TRIP effect, a loading mode referred as cyclic tensile loading and unloading (CTLU) is designed in tensile test on AISI 304 stainless steel at RT. As comparison, the conventional monotonic tensile loading (MTL) mode is also conducted. 2. Experimental The specimens used in the experiments were AISI 304 stainless steel tubes. The thickness of the tubes was 1 mm and the outer diameter was 20 mm. In order to investigate the effect of different loading modes on the TRIP effect of AISI 304 stainless steels, uniaxial tension was carried out on the MTS 5105 Servo Control Computer System Universal Testing Machine at RT (approximate 293 K in the experiment) which is between Ms and Md of AISI 304 stainless steels [4,17]. The length of the tensile specimens was 150 mm. The tensile strain was recorded over a 25 mm gage length with an extensometer. The loading methods applied in the experiment were MTL tension and CTLU tension. The CTLU tension was controlled by the applied stress which unloaded automatically when it approached the pre-set value and reloaded when it had unloaded to zero approximately. In other
1546
Y. Xu et al. / Materials Letters 65 (2011) 1545–1547
words, the applied stress was made intermittent. The uniaxial tensile tests were carried out at strain rate of 1 × 10− 3 s− 1 until fracture. Axovert 200 MAT optical microscope was used to observe the evolution of austenitic phase and strain-induced martensitic phase after tensile deformation. Austenitic phase and martensitic phase in the tensile specimens were identified and evaluated by X-ray diffraction (XRD). The XRD equipment used was a D/max 2500 type X-ray diffractometer with Cu Kα radiation at 50 kV and 300 mA. 3. Results and discussion Fig. 1 shows the stress–strain curves of MTL tension and CTLU tension. The main properties of specimens under two loading modes are summarized in Table 1. From the experimental results, it is apparent that the strength and the elongation of the specimens are significantly improved through CTLU tension. The fracture elongation and the ultimate strength are increased by 24.3% and 9.2% respectively under CTLU tension compared with the MTL tension. As can be seen in Fig. 1a and b, no obvious yield point appears under MTL tension. As to
Fig. 1. Comparison of the stress–strain curves under different tension modes: (a) engineering stress–engineering strain curves, (b) true stress–true strain curves and (c) strain hardening rate curves.
Table 1 The main properties of the tensile test under different loading modes. •
Loading modes
ε (s− 1)
σyield (MPa)
σb (MPa)
εfracture
MTL tension CTLU tension
1 × 10− 3 1 × 10− 3
275 270
650 710
0.70 0.87
the stress–strain curve under CTLU tension, there is no yield point that can be seen during the initial loading process before the first unloading. However, the stress–strain curve begins to show yield point during the second loading process (i.e. the first reloading process), and it is found that the change of curve at the yield point becomes more and more pronounced after each reloading. When the engineering strain surpasses 0.4 under CTLU tension, a number of hillshape fluctuation patterns and a followed stress platform occur in the flow curve as shown in Fig. 1a. It is also observed from Fig. 1c that the strain hardening rate value under CTLU mode is as much twice as that under MTL mode. Microstructure samples of the specimens before and after tension are prepared and observed by the optical microscope, in which the samples after tension undergo the same strain (εe = 0.7). As shown in
Fig. 2. Optical microstructure of AISI 304 stainless steel under different tension modes: (a) as-received condition, (b) under MTL tension and (c) under CTLU tension.
Y. Xu et al. / Materials Letters 65 (2011) 1545–1547
Fig. 3. X-ray diffraction patterns of the specimens under (a) as-received condition, (b) MTL tension and (c) CTLU tension.
Fig. 2a, the original microstructure of the specimens consists of equiaxed austenite grains with the mean grain size of 20 μm and a few annealing twins. As shown in Fig. 2b, after MTL tension, the austenite grains are stretched. A number of strain-induced martensitic laths could be found in the deformed austenite grains and on the boundaries while the matrix is still kept as austenite phase. However, more induced martensite is found after CTLU tension. Fig. 2c shows that numerous strain-induced martensite laths take place in the initial austenite grains, and the austenite boundaries are hardly observed after deformation under CTLU tension. To quantitative identify the fraction of strain-induced martensite in specimens under the two modes at the same strain, XRD analysis is conducted and the results are shown in Fig. 3. The diffraction range 2θ = 40–100° with the main peaks in the XRD spectra: γ(111), γ(200), γ(220), γ(311), α′(110), α′(200), α′(211). Direct comparison method [18,19] is utilized to identify the fraction of austenitic phase and martensitic phase of the specimens under different loading modes through the XRD spectra results. The volume fraction of austenite and martensite can be derived from numerous peaks with the following equation:
Vi =
1 n
n
∑j =1
Iγj Rγj
1 n
∑j =1
1 n
n
n
+ ∑j =1
Iij Rij Iαj Rαj
+
1 n
n
∑j =1
Iεj
ð1Þ
Rεj
Where Rij is the calculated theoretical intensity for (hkl) plane of iphase, Iij is the integrated intensity for (hkl) plane of i-phase and n is the number of (hkl) peaks for the i-phase. From the result of XRD, it can be found that there is little hcpmartensite (ε-phase). Therefore, equation can be simplified by Iεj = 0 as the specimens mainly consist of austenite (γ-phase) and bccmartensite (α′-phase) after tension. By the calculation, the fraction of strain-induced α′-martensite under MTL tension is 45% while the value is 74.8% under CTLU tension, meaning 1.5 times more than that under MTL tension.
1547
Both the analysis of XRD spectra and microstructure morphology verify that the fraction of strain-induced martensite of the specimen under CTLU tension is more than that induced under MTL tension at the identical strain. The unloading process is the main reason for this result, which can be related to the effect of back stress evolution on the motion of dislocations and martensitic transformation under CTLU tension. During the unloading process, the pileup of dislocations fade out by reverse glide and the back stress is relaxed [20]. Such backflow then contributes to the growth of the preformed martensite. When the applied stress reloads, dislocations pile up again in other regions of the specimen where new martensite nucleates and the back stress re-forms gradually with the applied stress increasing. Therefore the fraction of the martensite has been greatly increased with the process of repeated unloading and reloading. Such mechanical behavior is relevant to the research and development of new plastic forming process (e.g. pulsating hydroforming) to improve the formability for metastable materials. In situ TEM observations of tensile deformation would be carried out in the further experiments to investigate the mechanism deeply. 4. Conclusions The CTLU tension has been provided by the present study to improve the strain hardening and the fraction of the strain-induced martensite which is 1.5 times more than that in MTL tension. Consequently the TRIP effect is enhanced and the formability is improved with the elongation increased by 24.3% by the special loading mode. Acknowledgements The authors would like to express thanks to Prof. J. D. Embury of McMaster University in Canada for the helpful discussion. References [1] Fischer FD, Reisner G, Werner E, Tanaka K, Cailletaud G, Antretter T. Int J Plast 2000;16:723–48. [2] Dan WJ, Li SH, Zhang WG, Lin ZQ. Mater Des 2008;29:604–12. [3] Hecker SS, Stout MG, Staudhammer KP, Smith JL. Metall Trans 1982;13A:619. [4] Rocha MR, Oliveira CA. Mater Sci Eng A 2009;517:281–5. [5] Bayerlein M, Christ HJ, Mughrabi H. Mater Sci Eng A 1989;A114:L11–6. [6] Gallée S, Pilvin P. J Mater Process Technol 2010;210:835–43. [7] Ahmetoglu M, Altan T. J Mater Process Technol 2000;98:25–33. [8] Raabe D. Acta Mater 1997;45:1137–51. [9] Manjanna J, Kobayashi S, Kamada Y, Takahashi S, Kikuchi H. J Mater Sci 2008;43: 2659–65. [10] Ahn TH, Oh CS, Kim DH. Scr Mater 2010;63:540–3. [11] Lebedev AA, Kosarchuk VV. Int J Plast 2000;16:749–67. [12] Nagayama K, Terasaki T, Tanaka K, Fischer FD, Antretter T, Cailletaud G, et al. Mater Sci Eng A 2001;308:25–37. [13] Nagayama K, Terasaki T, Goto S, Tanaka K, Fischer FD, Antretter T, et al. Mater Sci Eng A 2002;336:30–8. [14] Tanaka K, Terasaki T, Goto S, Antretter T, Fischer FD, Cailletaud G. Mater Sci Eng A 2003;341:189–96. [15] Talonen J, Hanninen H. Acta Mater 2007;55:6108–18. [16] Zhang SH, Yuan AY. Sci China E Tech Sci 2009;52:2263–8. [17] Peterson SF, Mataya MC, Matlock DK. JOM 1997:54–8. [18] De AK, Murdock DC, Mataya MC, Speer JG, Matlock DK. Scr Mater 2004;50:1445–9. [19] Hedstrom P, Lienert U, Almer J, Oden M. Scr Mater 2007;56:213–6. [20] Spencer K, Veron M, Zhang KY, Embury JD. Mater Sci Technol 2009;25:7–17.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
The enhancement of transformation induced plasticity effect on austenitic stainless steels by cyclic tensile loading and unloading Yong Xu a, Shihong Zhang a,⁎, Hongwu Song a, Ming Cheng a, Haiqu Zhang b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Shenyang University, Shenyang 110044, China
a r t i c l e
i n f o
Article history: Received 7 December 2010 Accepted 21 February 2011 Available online 25 February 2011 Keywords: Austenitic steels Loading modes Phase transformation Mechanical properties X-ray techniques
a b s t r a c t The effect of loading modes of tensile deformation on the mechanical properties of a metastable austenite stainless steel has been investigated. The stress–strain curves, microstructures and fraction of the martensite are measured and analyzed separately. The results of tensile test indicate that a special loading mode referred as cyclic tensile loading and unloading can improve the strength and the formability of the specimens effectively. It is noted that the elongation and the ultimate strength are increased by 24.3% and 9.2% respectively at room temperature. Such enhancement of the transformation induced plasticity effect is mainly related to remarkable increase of the fraction of strain-induced martensite by the cyclic tensile loading and unloading. © 2011 Elsevier B.V. All rights reserved.
1. Introduction It has long been recognized that austenitic stainless steels are one of the attractive engineering materials, due to their high corrosion resistance and versatile mechanical properties. These metastable alloys belong to a kind of typical transformation induced plasticity (TRIP) steels [1,2] whose high strength and excellent ductility are expected for their strain-induced martensitic transformation at room temperature (RT) [3–5]. Consequently, they can usually be manufactured by cold working including cold rolling [6], drawing [7] and hydroforming [8]. A number of studies have been performed to understand the martensitic transformation behavior and TRIP effect in this kind of metastable materials. Manjanna [9] investigated the evolution of the fraction of martensite in 316LN austenitic stainless steel compressed at RT by using magnetic measurement. The nano-indentation and microstructural results were reported to provide micromechanical insight into the strain-induced phase transformation and deformation behavior of metastable austenite in TRIP steel [10]. Lebedev and Kosarchuk [11] conducted a series of experiments to assess the influence of both temperature and stress states on the kinetics of martensitic transformation. Nagayama et al. [12,13] and Tanaka et al. [14] investigated the TRIP phenomenon in some kind of maraging steel through thermomechanical loading process of martensitic transformation by partial and full unloading tests. The previous reports have just introduced some new findings or complicated
⁎ Corresponding author. Tel.: +86 24 83978266; fax: +86 24 23906831. E-mail address: [email protected] (S. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.02.066
experimental methods which still dissatisfy the industrial requirements. Further considerations thus should be put on the formability of the materials under common conditions. It is well known that in metastable stainless steels low stacking fault energy promotes austenitic phase transformation to martensitic phase [15]. However, there are few reports on the detail of promoting this phase transformation and enhancing the TRIP effect during deformation only by changing the loading mode at RT in this kind of metastable steels [16]. In this study, to identify the effect of the loading modes on the evolution of strain-induced martensitic transformation and TRIP effect, a loading mode referred as cyclic tensile loading and unloading (CTLU) is designed in tensile test on AISI 304 stainless steel at RT. As comparison, the conventional monotonic tensile loading (MTL) mode is also conducted. 2. Experimental The specimens used in the experiments were AISI 304 stainless steel tubes. The thickness of the tubes was 1 mm and the outer diameter was 20 mm. In order to investigate the effect of different loading modes on the TRIP effect of AISI 304 stainless steels, uniaxial tension was carried out on the MTS 5105 Servo Control Computer System Universal Testing Machine at RT (approximate 293 K in the experiment) which is between Ms and Md of AISI 304 stainless steels [4,17]. The length of the tensile specimens was 150 mm. The tensile strain was recorded over a 25 mm gage length with an extensometer. The loading methods applied in the experiment were MTL tension and CTLU tension. The CTLU tension was controlled by the applied stress which unloaded automatically when it approached the pre-set value and reloaded when it had unloaded to zero approximately. In other
1546
Y. Xu et al. / Materials Letters 65 (2011) 1545–1547
words, the applied stress was made intermittent. The uniaxial tensile tests were carried out at strain rate of 1 × 10− 3 s− 1 until fracture. Axovert 200 MAT optical microscope was used to observe the evolution of austenitic phase and strain-induced martensitic phase after tensile deformation. Austenitic phase and martensitic phase in the tensile specimens were identified and evaluated by X-ray diffraction (XRD). The XRD equipment used was a D/max 2500 type X-ray diffractometer with Cu Kα radiation at 50 kV and 300 mA. 3. Results and discussion Fig. 1 shows the stress–strain curves of MTL tension and CTLU tension. The main properties of specimens under two loading modes are summarized in Table 1. From the experimental results, it is apparent that the strength and the elongation of the specimens are significantly improved through CTLU tension. The fracture elongation and the ultimate strength are increased by 24.3% and 9.2% respectively under CTLU tension compared with the MTL tension. As can be seen in Fig. 1a and b, no obvious yield point appears under MTL tension. As to
Fig. 1. Comparison of the stress–strain curves under different tension modes: (a) engineering stress–engineering strain curves, (b) true stress–true strain curves and (c) strain hardening rate curves.
Table 1 The main properties of the tensile test under different loading modes. •
Loading modes
ε (s− 1)
σyield (MPa)
σb (MPa)
εfracture
MTL tension CTLU tension
1 × 10− 3 1 × 10− 3
275 270
650 710
0.70 0.87
the stress–strain curve under CTLU tension, there is no yield point that can be seen during the initial loading process before the first unloading. However, the stress–strain curve begins to show yield point during the second loading process (i.e. the first reloading process), and it is found that the change of curve at the yield point becomes more and more pronounced after each reloading. When the engineering strain surpasses 0.4 under CTLU tension, a number of hillshape fluctuation patterns and a followed stress platform occur in the flow curve as shown in Fig. 1a. It is also observed from Fig. 1c that the strain hardening rate value under CTLU mode is as much twice as that under MTL mode. Microstructure samples of the specimens before and after tension are prepared and observed by the optical microscope, in which the samples after tension undergo the same strain (εe = 0.7). As shown in
Fig. 2. Optical microstructure of AISI 304 stainless steel under different tension modes: (a) as-received condition, (b) under MTL tension and (c) under CTLU tension.
Y. Xu et al. / Materials Letters 65 (2011) 1545–1547
Fig. 3. X-ray diffraction patterns of the specimens under (a) as-received condition, (b) MTL tension and (c) CTLU tension.
Fig. 2a, the original microstructure of the specimens consists of equiaxed austenite grains with the mean grain size of 20 μm and a few annealing twins. As shown in Fig. 2b, after MTL tension, the austenite grains are stretched. A number of strain-induced martensitic laths could be found in the deformed austenite grains and on the boundaries while the matrix is still kept as austenite phase. However, more induced martensite is found after CTLU tension. Fig. 2c shows that numerous strain-induced martensite laths take place in the initial austenite grains, and the austenite boundaries are hardly observed after deformation under CTLU tension. To quantitative identify the fraction of strain-induced martensite in specimens under the two modes at the same strain, XRD analysis is conducted and the results are shown in Fig. 3. The diffraction range 2θ = 40–100° with the main peaks in the XRD spectra: γ(111), γ(200), γ(220), γ(311), α′(110), α′(200), α′(211). Direct comparison method [18,19] is utilized to identify the fraction of austenitic phase and martensitic phase of the specimens under different loading modes through the XRD spectra results. The volume fraction of austenite and martensite can be derived from numerous peaks with the following equation:
Vi =
1 n
n
∑j =1
Iγj Rγj
1 n
∑j =1
1 n
n
n
+ ∑j =1
Iij Rij Iαj Rαj
+
1 n
n
∑j =1
Iεj
ð1Þ
Rεj
Where Rij is the calculated theoretical intensity for (hkl) plane of iphase, Iij is the integrated intensity for (hkl) plane of i-phase and n is the number of (hkl) peaks for the i-phase. From the result of XRD, it can be found that there is little hcpmartensite (ε-phase). Therefore, equation can be simplified by Iεj = 0 as the specimens mainly consist of austenite (γ-phase) and bccmartensite (α′-phase) after tension. By the calculation, the fraction of strain-induced α′-martensite under MTL tension is 45% while the value is 74.8% under CTLU tension, meaning 1.5 times more than that under MTL tension.
1547
Both the analysis of XRD spectra and microstructure morphology verify that the fraction of strain-induced martensite of the specimen under CTLU tension is more than that induced under MTL tension at the identical strain. The unloading process is the main reason for this result, which can be related to the effect of back stress evolution on the motion of dislocations and martensitic transformation under CTLU tension. During the unloading process, the pileup of dislocations fade out by reverse glide and the back stress is relaxed [20]. Such backflow then contributes to the growth of the preformed martensite. When the applied stress reloads, dislocations pile up again in other regions of the specimen where new martensite nucleates and the back stress re-forms gradually with the applied stress increasing. Therefore the fraction of the martensite has been greatly increased with the process of repeated unloading and reloading. Such mechanical behavior is relevant to the research and development of new plastic forming process (e.g. pulsating hydroforming) to improve the formability for metastable materials. In situ TEM observations of tensile deformation would be carried out in the further experiments to investigate the mechanism deeply. 4. Conclusions The CTLU tension has been provided by the present study to improve the strain hardening and the fraction of the strain-induced martensite which is 1.5 times more than that in MTL tension. Consequently the TRIP effect is enhanced and the formability is improved with the elongation increased by 24.3% by the special loading mode. Acknowledgements The authors would like to express thanks to Prof. J. D. Embury of McMaster University in Canada for the helpful discussion. References [1] Fischer FD, Reisner G, Werner E, Tanaka K, Cailletaud G, Antretter T. Int J Plast 2000;16:723–48. [2] Dan WJ, Li SH, Zhang WG, Lin ZQ. Mater Des 2008;29:604–12. [3] Hecker SS, Stout MG, Staudhammer KP, Smith JL. Metall Trans 1982;13A:619. [4] Rocha MR, Oliveira CA. Mater Sci Eng A 2009;517:281–5. [5] Bayerlein M, Christ HJ, Mughrabi H. Mater Sci Eng A 1989;A114:L11–6. [6] Gallée S, Pilvin P. J Mater Process Technol 2010;210:835–43. [7] Ahmetoglu M, Altan T. J Mater Process Technol 2000;98:25–33. [8] Raabe D. Acta Mater 1997;45:1137–51. [9] Manjanna J, Kobayashi S, Kamada Y, Takahashi S, Kikuchi H. J Mater Sci 2008;43: 2659–65. [10] Ahn TH, Oh CS, Kim DH. Scr Mater 2010;63:540–3. [11] Lebedev AA, Kosarchuk VV. Int J Plast 2000;16:749–67. [12] Nagayama K, Terasaki T, Tanaka K, Fischer FD, Antretter T, Cailletaud G, et al. Mater Sci Eng A 2001;308:25–37. [13] Nagayama K, Terasaki T, Goto S, Tanaka K, Fischer FD, Antretter T, et al. Mater Sci Eng A 2002;336:30–8. [14] Tanaka K, Terasaki T, Goto S, Antretter T, Fischer FD, Cailletaud G. Mater Sci Eng A 2003;341:189–96. [15] Talonen J, Hanninen H. Acta Mater 2007;55:6108–18. [16] Zhang SH, Yuan AY. Sci China E Tech Sci 2009;52:2263–8. [17] Peterson SF, Mataya MC, Matlock DK. JOM 1997:54–8. [18] De AK, Murdock DC, Mataya MC, Speer JG, Matlock DK. Scr Mater 2004;50:1445–9. [19] Hedstrom P, Lienert U, Almer J, Oden M. Scr Mater 2007;56:213–6. [20] Spencer K, Veron M, Zhang KY, Embury JD. Mater Sci Technol 2009;25:7–17.