- Email: [email protected]
Grain size dependence of the serrated flow in a nickel based alloy
Materials Letters 150 (2015) 108–110
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Grain size dependence of the serrated flow in a nickel based alloy Zhigang Li a,b, Lanting Zhang a,b,c,n, Nairong Sun a,b, Liming Fu a,b, Aidang Shan a,b,c a
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China Shanghai Key Laboratory of High Temperature Materials and Precision Forming, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China c Gas Turbine Research Institute, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 October 2014 Accepted 3 March 2015 Available online 11 March 2015
In this paper, uniaxial tensile tests at a constant strain rate were performed on a nickel based alloy with different grain sizes, and the effect of grain size on serrated flow was studied. The results show that the obvious serration phenomena only occurs in the tensile curves for the large-grained samples ( 41.3 μm), and the critical plastic strain for the onset of serrations increases with the grain refinement. A large number of slip bands are observed in the deformed samples with relatively large grains rather than fine grains. The analysis reveals that the interaction between slip bands and boundaries is mainly responsible for the serration occurrence, and the mean serration amplitude is closely associated with the density of annealing twin boundary. & 2015 Elsevier B.V. All rights reserved.
Keywords: Serration Slip band Microstructure Mechanical properties Nickel based alloys
It is generally accepted that the tensile flow shows a serrated characteristic in substantial nickel based alloys within a temperature range under various strain rates conditions [1–5]. The evaluation of the phenomenon on the properties of materials has been performed in nickel based alloys [6,7]. Recently, there are considerable studies dealing with serrated flows. Most of the relevant works primarily study the effects of deformed temperature on serrations, and the strain rate dependence of serrated flows [2–5]. More recent works involve serrated flows in nickel, whereas they mainly study the relationships between strength and grain size, and the grain size is limited to nanoscale grain sizes (o100 nm) [8,9]. In other alloys, the limited relevant investigation has been performed in coarse-grained materials (4 40 μm) [10]. However, so far limited works have focused on the relationships between serrations and grain size in nickel based alloys. In this work, a study on serrated flows is being carried out on a nickel based alloy with cold rolling and subsequent annealing, and the grain size covers a relatively wide range. The present paper reports some of the results on the serrated flows as a function of grain size under a constant strain rate condition. An attempt has been made to identify the causes that result in the serrated flows on the nickel based alloy on the basis of the observations of deformed microstructures by scanning electron microscopy (SEM). The chemical composition of the studied material is: 46.8Ni– 17.8Cr–3.2Mo–5.8Nb–1.0Ti–0.5Al–0.03 C–Fe bal (wt%). A 15 mm n Corresponding author at: School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China. Tel.: þ 86 21 54747471; fax: þ 86 21 54745197. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.03.007 0167-577X/& 2015 Elsevier B.V. All rights reserved.
thick sheet of the studied alloy provided in a solution annealed condition (1040 1C for 1 h) was rolled at ambient temperature to a thickness of 1.5 mm. The rolled sheets were annealed at 1020 1C for various times from 2 to 180 min in a salt bath furnace to produce the samples within a grain size range from about 1 to 60 μm. The heat-treated specimens were polished, and then etched to show the annealed microstructures. The tensile tests were performed at the temperature of 200 1C under a constant strain rate of 1 10 3 s 1. The specimens were soaked for 15 min before tensile tests. Grain boundary character distribution and grain size distribution were characterized by electron backscattered diffraction (EBSD) which was described in detail in the previous paper [11]. Fig. 1 shows the grain boundary character distribution and grain size distribution in the processed samples with different grain sizes. All samples comprise of matrix phase (γ phase) and sparse primary carbides (not visible in the field of view), and the equiaxed grain structures exhibit unimodal distributions. There are plenty of annealing twin boundaries (TBs) in the large-grained samples (d41.3 μm), in contrast, a small number of TBs (drawn in red lines) are observed in the fine-grained sample (d 1.3 μm). It is found in Table 1 that the TBs fraction decreases monotonously with the grain refinement, whereas the TBs density defined as the length of TBs per unit area, firstly increases and then decreases with the grain refinement. In addition, the average number of neighboring grains gradually decreases with the increase of grain size. The true stress–true strain plot of annealed samples tested at 200 1C is shown in Fig. 2, which shows the evolution of flow stress in the samples with different grain sizes as a function of true strain. The
Z. Li et al. / Materials Letters 150 (2015) 108–110
109
Fig. 1. Typical micrographs (left) and grain size distributions (right) for different grain size: (a) large-grained sample, and (b) fine-grained sample. Table 1 Parameters of grain boundary character distribution and serrated flow as a function of grain size. Grain size (μm)
TBs fraction (%)
TBs density (10 2 μm 1)
No. of neighboring grains
Type of serration
εc (%)
Serrated strain (%)
Δσavg (MPa)
1.3 4.4 17.1 30.4 58.7
o2 10.09 18.35 27.28 37.86
o1 6.02 8.16 6.79 5.53
7.2 6.3 6.1 5.7 5.5
– A A A A
– 23.5 23.2 22.1 20.6
– 3.5 7.7 15.7 20.0
– 5.1 12.2 7.6 3.2
obvious serrations of stress curves are observed in the large-grained samples, and the critical plastic strain εc for the onset of serrations gradually increases with the grain refinement. Moreover, the serrated strain defined as the true strain of serrated flow lasting, decreases with the grain refinement. In addition, the serrations are not found in the fine-grained sample. The serration is characterized by an abrupt rise followed by a drop to below the stress–strain curve, and thus the type of serration is identified as type A [10]. The mean serration amplitude, Δσavg, is determined by averaging the Δσ values beyond a ε of 23.5%, because 23.5% is the maximum critical strain εc observed on the true stress–true strain curve [12]. The value of mean serration amplitude, Δσavg, firstly increases and then decreases with the grain refinement, and it shows an increasing trend with the TBs density, as shown in Table 1. Typical microstructures of the deformed specimens are shown in Fig. 3. The SEM images show that the deformed microstructures display significant differences in the samples with different grain sizes. There are abundant traces of slip bands (SBs) inside grains in the largegrained sample as shown in Fig. 3(a). These SBs exhibit various morphological and distributed features. Some SBs are very coarse, showing extrusions or intrusions features, whereas some SBs are very fine and show straight line shapes. Some SBs exhibit a boundarycrossing distribution (Region A), and some distribute along grain boundaries (Region B). Moreover, some TBs are cut through by SBs, and lead the SBs to show planar slip features (Region C). However, in the fine-grained sample, no obvious SBs are observed, and the
Fig. 2. True stress–true strain curve for the samples with different grain sizes deformed at 200 1C under a constant rate of 1 10 3 s 1.
morphologies of TBs approximately keep the same features as that before plastic deformation, as shown in Fig. 3(b). The results indicate that there are striking differences in the uniformity of deformation between large-grained sample and fine-grained sample.
110
Z. Li et al. / Materials Letters 150 (2015) 108–110
Fig. 3. Deformed microstructures after tensile tests: (a) large-grained sample, and (b) fine-grained sample.
It is well known that, during plastic deformation of polycrystalline materials, the deformation of each individual grain should depend on the assistance of the slip systems that have the most favorable orientation in its neighboring grains [13]. In other words, for each individual grain, the more slip systems in its neighboring grains, the easier and more uniformly the individual grain deforms, and vice versa. It is found in Table 1 that the average number of neighboring grains decreases as the grain size increases. Thus, the SBs tend to appear in the large-grained sample rather than in the fine-grained sample during plastic deformation (Fig. 3(a)). In addition, in order to maintain the integrity of material during plastic deformation, these TBs are cut through by SBs in the large-grained sample (Fig. 3(a)). One well-known theory is that the serrated flow is a result of the interactions between moving dislocations and interstitial solute elements such as C, N atoms, as the deformation temperature is below about 700 K [5]. However, this theory cannot explain the phenomenon that the serrations are observed in the largegrained samples, but not observed in the fine-grained sample shown in Fig. 2. As shown in Fig. 3, during plastic deformation, the SBs are observed in the large-grained samples, but not found in the fine-grained sample. These results strongly suggest that there may be a link between the serrations and the SBs. As can be found in Fig. 2, the increase in flow stress with the grain refinement indicates that grain boundaries can effectively impede the movement of dislocations during plastic deformation at 200 1C. It is then conceivable that both the average velocity of moving dislocations and the density of mobile dislocations inside grains reduce, as the SBs move towards grain boundaries. According to the previous study [10], the decrease in both the density of mobile dislocations and the average velocity of moving dislocations can reduce the strain rate of samples during deformation, resulting in an abrupt rise in load. However, once the SBs transfer through grain boundaries into the neighboring grains (Region A in Fig. 3(a)), or new slip systems are activated in the neighboring grains (Region B in Fig. 3(a)), the density of mobile dislocations and the average velocity of moving dislocations increase suddenly, leading to an abrupt increase in the strain rate of sample. Correspondingly, the stress drops to below the general level of the true stress–true strain curve. In addition, the TBs are believed to be valid barriers which hinder dislocations sliding during plastic deformation even at high temperature due to the different crystallographic orientations on each side of TBs [14]. Therefore, TBs can intensify the stress concentration, possibly resulting in an increase in the serration amplitude, as SBs move towards TBs and cut through TBs (Region C in Fig. 3(a)). However, in the fine-grained sample, it may be difficult to occur for the local stress concentration during
plastic deformation due to the relatively uniform deformation (Fig. 3 (b)) as a result of the increasing number of neighboring grains (Table 1). Therefore, the serrations, showing the locking type A features, occur in the large-grained samples, but not in the finegrained sample during plastic deformation. On the basis of the present study, it may be concluded that, as the grain size increases, the decreasing number of neighboring grains and the changing TBs density induce the interactions between SBs and boundaries to occur in the large-grained samples, resulting in the occurrence of serrations during plastic deformation. The TBs can exacerbate the serrations, and the mean serration amplitude shows an increasing tendency with the TBs density.
Acknowledgments The financial support from the Science and Technology Committee of Shanghai Municipality (Grant nos.12JC1405000 and 14521100600) are gratefully acknowledged. References [1] Xu YJ, Qi DQ, Du K, Cui CY, Ye HQ. Stacking fault effects on dynamic strain aging in a Ni–Co-based superalloy. Scr Mater 2014;87:37–40. [2] Hrutkay K, Kaoumi D. Tensile deformation behavior of a nickel based superalloy at different temperatures. Mater Sci Eng: A 2014;599:196–203. [3] Cui CY, Gu YF, Yuan Y, Harada H. Dynamic strain aging in a new Ni–Co base superalloy. Scr Mater 2011;64:502–5. [4] Gopinath K, Gogia AK, Kamat SV, Ramamurty U. Dynamic strain ageing in Nibase superalloy 720Li. Acta Mater 2009;57:1243–53. [5] Hale C, Rollings W, Weaver M. Activation energy calculations for discontinuous yielding in inconel 718SPF. Mater Sci Eng: A 2001;300:153–64. [6] Max B, Viguier B, Andrieu E, Cloue JM. A re-examination of the Portevin-Le Chatelier Effect in alloy 718 in connection with oxidation-assisted intergranular cracking. Metall Mater Trans A 2014;45:5431–41. [7] Garat V, Cloue J-M, Poquillon D, Andrieu E. Influence of Portevin–Le Chatelier effect on rupture mode of alloy 718 specimens. J Nucl Mater 2008;375:95–101. [8] Seo BB, Jahed Z, Burek MJ, Tsui TY. Influence of grain size on the strength size dependence exhibited by sub-micron scale nickel structures with complex cross-sectional geometries. Mater Sci Eng: A 2014;596:275–84. [9] Cao R, Deng C. The ultra-small strongest grain size in nanocrystalline Ni nanowires. Scr Mater 2015;94:9–12. [10] Rodriguez P. Serrated plastic flow. Bull Mater Sci 1984;6:653–63. [11] Li Z, Zhang L, Sun N, Sun Y, Shan A. Effects of prior deformation and annealing process on microstructure and annealing twin density in a nickel based alloy. Mater Charact 2014;95:299–306. [12] Gopinath K, Gogia A, Kamat S, Ramamurty U. Dynamic strain ageing in Ni-base superalloy 720Li. Acta Mater 2009;57:1243–53. [13] Callister WD, Rethwisch DG. Materials science and engineering: an introduction. New York: Wiley; 2007. [14] Yuan Y, Gu YF, Osada T, Zhong ZH, Yokokawa T, Harada H. A new method to strengthen turbine disc superalloys at service temperatures. Scr Mater 2012;66:884–9.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Grain size dependence of the serrated flow in a nickel based alloy Zhigang Li a,b, Lanting Zhang a,b,c,n, Nairong Sun a,b, Liming Fu a,b, Aidang Shan a,b,c a
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China Shanghai Key Laboratory of High Temperature Materials and Precision Forming, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China c Gas Turbine Research Institute, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 29 October 2014 Accepted 3 March 2015 Available online 11 March 2015
In this paper, uniaxial tensile tests at a constant strain rate were performed on a nickel based alloy with different grain sizes, and the effect of grain size on serrated flow was studied. The results show that the obvious serration phenomena only occurs in the tensile curves for the large-grained samples ( 41.3 μm), and the critical plastic strain for the onset of serrations increases with the grain refinement. A large number of slip bands are observed in the deformed samples with relatively large grains rather than fine grains. The analysis reveals that the interaction between slip bands and boundaries is mainly responsible for the serration occurrence, and the mean serration amplitude is closely associated with the density of annealing twin boundary. & 2015 Elsevier B.V. All rights reserved.
Keywords: Serration Slip band Microstructure Mechanical properties Nickel based alloys
It is generally accepted that the tensile flow shows a serrated characteristic in substantial nickel based alloys within a temperature range under various strain rates conditions [1–5]. The evaluation of the phenomenon on the properties of materials has been performed in nickel based alloys [6,7]. Recently, there are considerable studies dealing with serrated flows. Most of the relevant works primarily study the effects of deformed temperature on serrations, and the strain rate dependence of serrated flows [2–5]. More recent works involve serrated flows in nickel, whereas they mainly study the relationships between strength and grain size, and the grain size is limited to nanoscale grain sizes (o100 nm) [8,9]. In other alloys, the limited relevant investigation has been performed in coarse-grained materials (4 40 μm) [10]. However, so far limited works have focused on the relationships between serrations and grain size in nickel based alloys. In this work, a study on serrated flows is being carried out on a nickel based alloy with cold rolling and subsequent annealing, and the grain size covers a relatively wide range. The present paper reports some of the results on the serrated flows as a function of grain size under a constant strain rate condition. An attempt has been made to identify the causes that result in the serrated flows on the nickel based alloy on the basis of the observations of deformed microstructures by scanning electron microscopy (SEM). The chemical composition of the studied material is: 46.8Ni– 17.8Cr–3.2Mo–5.8Nb–1.0Ti–0.5Al–0.03 C–Fe bal (wt%). A 15 mm n Corresponding author at: School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China. Tel.: þ 86 21 54747471; fax: þ 86 21 54745197. E-mail address: [email protected] (L. Zhang).
http://dx.doi.org/10.1016/j.matlet.2015.03.007 0167-577X/& 2015 Elsevier B.V. All rights reserved.
thick sheet of the studied alloy provided in a solution annealed condition (1040 1C for 1 h) was rolled at ambient temperature to a thickness of 1.5 mm. The rolled sheets were annealed at 1020 1C for various times from 2 to 180 min in a salt bath furnace to produce the samples within a grain size range from about 1 to 60 μm. The heat-treated specimens were polished, and then etched to show the annealed microstructures. The tensile tests were performed at the temperature of 200 1C under a constant strain rate of 1 10 3 s 1. The specimens were soaked for 15 min before tensile tests. Grain boundary character distribution and grain size distribution were characterized by electron backscattered diffraction (EBSD) which was described in detail in the previous paper [11]. Fig. 1 shows the grain boundary character distribution and grain size distribution in the processed samples with different grain sizes. All samples comprise of matrix phase (γ phase) and sparse primary carbides (not visible in the field of view), and the equiaxed grain structures exhibit unimodal distributions. There are plenty of annealing twin boundaries (TBs) in the large-grained samples (d41.3 μm), in contrast, a small number of TBs (drawn in red lines) are observed in the fine-grained sample (d 1.3 μm). It is found in Table 1 that the TBs fraction decreases monotonously with the grain refinement, whereas the TBs density defined as the length of TBs per unit area, firstly increases and then decreases with the grain refinement. In addition, the average number of neighboring grains gradually decreases with the increase of grain size. The true stress–true strain plot of annealed samples tested at 200 1C is shown in Fig. 2, which shows the evolution of flow stress in the samples with different grain sizes as a function of true strain. The
Z. Li et al. / Materials Letters 150 (2015) 108–110
109
Fig. 1. Typical micrographs (left) and grain size distributions (right) for different grain size: (a) large-grained sample, and (b) fine-grained sample. Table 1 Parameters of grain boundary character distribution and serrated flow as a function of grain size. Grain size (μm)
TBs fraction (%)
TBs density (10 2 μm 1)
No. of neighboring grains
Type of serration
εc (%)
Serrated strain (%)
Δσavg (MPa)
1.3 4.4 17.1 30.4 58.7
o2 10.09 18.35 27.28 37.86
o1 6.02 8.16 6.79 5.53
7.2 6.3 6.1 5.7 5.5
– A A A A
– 23.5 23.2 22.1 20.6
– 3.5 7.7 15.7 20.0
– 5.1 12.2 7.6 3.2
obvious serrations of stress curves are observed in the large-grained samples, and the critical plastic strain εc for the onset of serrations gradually increases with the grain refinement. Moreover, the serrated strain defined as the true strain of serrated flow lasting, decreases with the grain refinement. In addition, the serrations are not found in the fine-grained sample. The serration is characterized by an abrupt rise followed by a drop to below the stress–strain curve, and thus the type of serration is identified as type A [10]. The mean serration amplitude, Δσavg, is determined by averaging the Δσ values beyond a ε of 23.5%, because 23.5% is the maximum critical strain εc observed on the true stress–true strain curve [12]. The value of mean serration amplitude, Δσavg, firstly increases and then decreases with the grain refinement, and it shows an increasing trend with the TBs density, as shown in Table 1. Typical microstructures of the deformed specimens are shown in Fig. 3. The SEM images show that the deformed microstructures display significant differences in the samples with different grain sizes. There are abundant traces of slip bands (SBs) inside grains in the largegrained sample as shown in Fig. 3(a). These SBs exhibit various morphological and distributed features. Some SBs are very coarse, showing extrusions or intrusions features, whereas some SBs are very fine and show straight line shapes. Some SBs exhibit a boundarycrossing distribution (Region A), and some distribute along grain boundaries (Region B). Moreover, some TBs are cut through by SBs, and lead the SBs to show planar slip features (Region C). However, in the fine-grained sample, no obvious SBs are observed, and the
Fig. 2. True stress–true strain curve for the samples with different grain sizes deformed at 200 1C under a constant rate of 1 10 3 s 1.
morphologies of TBs approximately keep the same features as that before plastic deformation, as shown in Fig. 3(b). The results indicate that there are striking differences in the uniformity of deformation between large-grained sample and fine-grained sample.
110
Z. Li et al. / Materials Letters 150 (2015) 108–110
Fig. 3. Deformed microstructures after tensile tests: (a) large-grained sample, and (b) fine-grained sample.
It is well known that, during plastic deformation of polycrystalline materials, the deformation of each individual grain should depend on the assistance of the slip systems that have the most favorable orientation in its neighboring grains [13]. In other words, for each individual grain, the more slip systems in its neighboring grains, the easier and more uniformly the individual grain deforms, and vice versa. It is found in Table 1 that the average number of neighboring grains decreases as the grain size increases. Thus, the SBs tend to appear in the large-grained sample rather than in the fine-grained sample during plastic deformation (Fig. 3(a)). In addition, in order to maintain the integrity of material during plastic deformation, these TBs are cut through by SBs in the large-grained sample (Fig. 3(a)). One well-known theory is that the serrated flow is a result of the interactions between moving dislocations and interstitial solute elements such as C, N atoms, as the deformation temperature is below about 700 K [5]. However, this theory cannot explain the phenomenon that the serrations are observed in the largegrained samples, but not observed in the fine-grained sample shown in Fig. 2. As shown in Fig. 3, during plastic deformation, the SBs are observed in the large-grained samples, but not found in the fine-grained sample. These results strongly suggest that there may be a link between the serrations and the SBs. As can be found in Fig. 2, the increase in flow stress with the grain refinement indicates that grain boundaries can effectively impede the movement of dislocations during plastic deformation at 200 1C. It is then conceivable that both the average velocity of moving dislocations and the density of mobile dislocations inside grains reduce, as the SBs move towards grain boundaries. According to the previous study [10], the decrease in both the density of mobile dislocations and the average velocity of moving dislocations can reduce the strain rate of samples during deformation, resulting in an abrupt rise in load. However, once the SBs transfer through grain boundaries into the neighboring grains (Region A in Fig. 3(a)), or new slip systems are activated in the neighboring grains (Region B in Fig. 3(a)), the density of mobile dislocations and the average velocity of moving dislocations increase suddenly, leading to an abrupt increase in the strain rate of sample. Correspondingly, the stress drops to below the general level of the true stress–true strain curve. In addition, the TBs are believed to be valid barriers which hinder dislocations sliding during plastic deformation even at high temperature due to the different crystallographic orientations on each side of TBs [14]. Therefore, TBs can intensify the stress concentration, possibly resulting in an increase in the serration amplitude, as SBs move towards TBs and cut through TBs (Region C in Fig. 3(a)). However, in the fine-grained sample, it may be difficult to occur for the local stress concentration during
plastic deformation due to the relatively uniform deformation (Fig. 3 (b)) as a result of the increasing number of neighboring grains (Table 1). Therefore, the serrations, showing the locking type A features, occur in the large-grained samples, but not in the finegrained sample during plastic deformation. On the basis of the present study, it may be concluded that, as the grain size increases, the decreasing number of neighboring grains and the changing TBs density induce the interactions between SBs and boundaries to occur in the large-grained samples, resulting in the occurrence of serrations during plastic deformation. The TBs can exacerbate the serrations, and the mean serration amplitude shows an increasing tendency with the TBs density.
Acknowledgments The financial support from the Science and Technology Committee of Shanghai Municipality (Grant nos.12JC1405000 and 14521100600) are gratefully acknowledged. References [1] Xu YJ, Qi DQ, Du K, Cui CY, Ye HQ. Stacking fault effects on dynamic strain aging in a Ni–Co-based superalloy. Scr Mater 2014;87:37–40. [2] Hrutkay K, Kaoumi D. Tensile deformation behavior of a nickel based superalloy at different temperatures. Mater Sci Eng: A 2014;599:196–203. [3] Cui CY, Gu YF, Yuan Y, Harada H. Dynamic strain aging in a new Ni–Co base superalloy. Scr Mater 2011;64:502–5. [4] Gopinath K, Gogia AK, Kamat SV, Ramamurty U. Dynamic strain ageing in Nibase superalloy 720Li. Acta Mater 2009;57:1243–53. [5] Hale C, Rollings W, Weaver M. Activation energy calculations for discontinuous yielding in inconel 718SPF. Mater Sci Eng: A 2001;300:153–64. [6] Max B, Viguier B, Andrieu E, Cloue JM. A re-examination of the Portevin-Le Chatelier Effect in alloy 718 in connection with oxidation-assisted intergranular cracking. Metall Mater Trans A 2014;45:5431–41. [7] Garat V, Cloue J-M, Poquillon D, Andrieu E. Influence of Portevin–Le Chatelier effect on rupture mode of alloy 718 specimens. J Nucl Mater 2008;375:95–101. [8] Seo BB, Jahed Z, Burek MJ, Tsui TY. Influence of grain size on the strength size dependence exhibited by sub-micron scale nickel structures with complex cross-sectional geometries. Mater Sci Eng: A 2014;596:275–84. [9] Cao R, Deng C. The ultra-small strongest grain size in nanocrystalline Ni nanowires. Scr Mater 2015;94:9–12. [10] Rodriguez P. Serrated plastic flow. Bull Mater Sci 1984;6:653–63. [11] Li Z, Zhang L, Sun N, Sun Y, Shan A. Effects of prior deformation and annealing process on microstructure and annealing twin density in a nickel based alloy. Mater Charact 2014;95:299–306. [12] Gopinath K, Gogia A, Kamat S, Ramamurty U. Dynamic strain ageing in Ni-base superalloy 720Li. Acta Mater 2009;57:1243–53. [13] Callister WD, Rethwisch DG. Materials science and engineering: an introduction. New York: Wiley; 2007. [14] Yuan Y, Gu YF, Osada T, Zhong ZH, Yokokawa T, Harada H. A new method to strengthen turbine disc superalloys at service temperatures. Scr Mater 2012;66:884–9.