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Grain boundary character distribution during the post-deformation recrystallization of Incoloy 800H at elevated temperature
Materials Letters 163 (2016) 24–27
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Grain boundary character distribution during the post-deformation recrystallization of Incoloy 800H at elevated temperature Yu Cao, Hongshuang Di n State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2015 Received in revised form 4 October 2015 Accepted 8 October 2015 Available online 13 October 2015
The present research studied the grain boundary character distribution in Incoloy 800 H corresponding to two different post-deformation recrystallization mechanisms through uniaxial hot compression test and electron backscattering diffraction analysis. The results showed that the recrystallized grain size following metadynamic and static recrystallization was modestly larger than that from static recrystallization. Besides, two different mechanisms were responsible for the generation of Σ3n boundaries. In the static recrystallization samples, ∑3 regeneration was more active than new twinning formation. However, the new twinning would become predominant when metadynamic and static recrystallization took effect simultaneously. & 2015 Elsevier B.V. All rights reserved.
Keywords: Incoloy 800 H Recrystallization Grain boundary junctions Grain boundary character distribution
1. Introduction Metallurgical events after hot deformation play a significant role in controlling the microstructure evolution to achieve the desirable mechanical properties of the final product [1]. These processes commonly include recovery, static recrystallization (SRX) and metadynamic recrystallization (MDRX). For the low and medium stacking fault energy (SFE) materials, the effect of recovery is relatively low and thus the recrystallization process will become predominant [2]. The general understanding is that both nucleation and growth of recrystallized grains occur during the intervals of hot deformation for SRX, whilst this restoration takes place during deformation for dynamic recrystallization (DRX). MDRX refers to the static growth of DRX nuclei after the interrupt of hot deformation [3]. During these post-deformation recrystallization stages, the continuous occurrence of twinningevents can be generated by consumption of stored strain energy and eventually contributes to the formation of highly twinned interconnecting Σ3n boundaries. Grain boundary engineering (GBE) through the control of grain boundary character distribution (GBCD) has been widely applied to achieving advanced structural and functional properties in high-performance polycrystalline materials [4–6]. It has been extensively accepted that the main mechanism for GBE depends on proliferation of twinning (a type of ∑3 boundary) in low SFE materials associated with interactions of other ∑3n (n r3) n
Corresponding author. E-mail address: [email protected] (H. Di).
http://dx.doi.org/10.1016/j.matlet.2015.10.034 0167-577X/& 2015 Elsevier B.V. All rights reserved.
boundaries. V. Randle [7] investigated the mechanisms of GBE and suggested that ∑3/∑(9 þ27) ratio can be effectively employed to determine the proliferation mechanism of ∑3n boundaries. Incoloy 800 H is an austenitic high-strength solid-solution Fe–Ni–Cr alloy. It was selected as a primary candidate for furnace components and equipments in petrochemical industry due to its high strength and corrosion resistance at elevated temperatures [8]. In the previous works, L. Tan et al. [9] explicated the GBE approach of Incoloy 800 H by means of thermomechanical processing and afterwards H. Akhiani et al. [8] also studied the effect of thermomechanical processing on GBCD in Incoloy 800 H with two different rolling modes followed by annealing. Therefore, it is important to realize the contribution of GBCD to the development of high-performance Incoloy 800 H through annealing after hot deformation. The aim of this paper is to investigate the effect of various hot deformation processing parameters (pre-strain and annealing time) on the GBCD during the post-deformation recrystallization.
2. Experimental procedures A commercial Incoloy 800 H after hot rolling was used in this study. Cylindrical specimens, with a diameter of 10 mm and a height of 15 mm, were machined out of the hot rolled plate and hot deformation compression tests were performed on a computer-controlled, servo-hydraulic MMS-300 thermo-simulation machine. Considering the industrial operation for hot rolling of Incoloy 800 H (the rolling reduction ratio: 0.15–0.4 in true strain), three true strains (0.2, 0.3 and 0.4) were explored by high
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
temperature interrupted-compression tests at a strain rate of 1 s 1 and temperature of 1000 °C. The used pre-strains were associated with the initiation of dynamic recrystallization (DRX) (εc) and the peak strain (εp) as well. After unloading, the specimens were held in air at the deformation temperature for different times (10 s, 30 s, 60 s, and 400 s) to enable the proceeding of post-deformation recrystallization. The samples were immediately quenched ( o1 s) in water following deformation and annealing to preserve the high temperature microstructures. The approach to prepare samples for electron backscattering diffraction (EBSD) was described elsewhere [10]. In order to investigate the grain orientations and GBCD, EBSD data were collected using a TSL EBSD system interfaced to a FEI Quanta 600 scanning electron microscope (SEM). The Brandon criterion [11] was used to identify Σ3n boundaries and recrystallized grain size (Dr) was measured using the linear intercept method. In present work, the grain orientation spread (GOS) approach [10] was utilized to evaluate the recrystallized fraction (Fr) with a threshold value of 1°. Each statistical data were obtained from the evaluation of three distinct regions of the microstructure and the standard deviation was calculated as 72%.
3. Results and discussion Fig. 1(a) displayed the flow stress curve under the experimental deformation condition, indicating a peak stress (sp) corresponding to a peak strain (εp) of 0.41. It can be also observed that work hardening rate (θ ¼ ds/dε) decreased remarkably with sustained strain and attained to zero at εp. As indicated previously, the critical strain (εc) of DRX was calculated to be 0.27 [12]. Considering these characteristic points, three different true strains followed by annealing were explored to investigate various mechanisms of
25
Table 1 Variation of Dr and Fr under different deformation conditions. The background was highlighted for the completely recrystallized samples.
post-deformation recrystallization, namely, SRX at 0.2 and MDRXþSRX at 0.3/0.4. At the beginning of deformation (Fig. 1b), the original austenitic grains began to be elongated perpendicular to the compression axis and a large fraction of low angle boundaries were produced due to dislocation generation and dislocation boundary formation. Some bulges originated from the serration segments of grain boundaries and resulted in the nucleation of DRX grains. Specifically, most of the pre-existing twin boundaries lost their crystallographic characteristics resulting from grain rotation with concurrent deformation. As shown in Fig. 1c, some fine new DRX grains were detected at grain boundaries and particularly at triple junctions with the true strain increasing beyond εc. More new grains and incomplete necklace structures formed with deformation up to εp (Fig. 1d) and it is worth noting that few twin boundaries could be observed within new grains. Characterization of the as-deformed samples revealed that Fr and Dr increased modestly with increasing strain (see Table 1). Following annealing of the as-deformed samples at 1000 °C for various holding times, the image quality (IQ) maps from EBSD were presented in Fig. 2 and the ∑3n boundaries were highlighted to show the distribution features. Table 1 showed an evidence of static grain growth occurring in all of the samples after annealing
ε = 0.2
ε = 0.3
ε = 0.4
Fig. 1. Curve showing (a) true stress and working hardening rate versus true strain at 1000 °C and 1 s 1 (εc and εp were marked with arrows). (b)–(d) inverse pole figure (IPF) maps obtained from EBSD (step size ¼0.5 μm) for microstructures at different true strains. Gray and black lines represented low angle grain boundaries (misorientation angle: 2–15°) and high angle grain boundaries (misorientation angle: 4 15°). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
26
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
ε = 0.4
ε = 0.2
10μm 1
10μm
Σ9 2
Σ3
Σ27b Σ3
Σ3 Σ9 ε = 0.2
ε = 0.4
Fig. 2. Image quality (IQ) maps showing the grain structure and distribution of ∑3n (nr 3) boundaries following anneal at 1000 °C for (a) and (b) 30 s and (c) and (d) 400 s (step size ¼ 1 μm). ∑3: red, ∑9: yellow, ∑27a: blue, ∑27b: green. Two subsets were shown to demonstrate the interactions of Σ3n boundaries in Fig. 2(c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Length fractions of (a) ∑3, (b) ∑9, (c) ∑27 and (d) ∑3/(∑9þ ∑27) for samples at different true strains with various anneal times.
as Dr range from 20 to 56 μm at the strain of 0.2, 12 to 60 μm at 0.3 and 18 to 76 μm at 0.4. Besides, recrystallized fraction was observed to distinctly increase with prolonged holding time. It also increased as a function of the pre-strain because higher stored
energy can enhance the driving force for grain boundary mobility and subsequent recrystallization. Note that the completion times of recrystallization were different for the three testing pre-strains: 400 s for 0.2, 60 s for 0.3 and 30 s for 0.4 (highlighted with red in
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
Table 1). It can also be conjectured from this observation that Dr increased modestly at higher pre-strain for a given holding time. This means that the Dr following MDRX þSRX is significantly larger than that measured in the SRX condition, which is due to the fact that MDRX grains nucleate dynamically during deformation and they will continue to grow without any incubation time compared with SRX grains at the post-deformation stage. As shown in Fig. 2, dramatical differences in the microstructure were detected depending on the pre-strain and holding time. Fig. 2a exhibited a partially recrystallized (SRX) microstructure of the deformed sample with a pre-strain of 0.2 after annealing for 30 s. Straining the sample to 0.4 was observed to accelerate the rate of recrystallization during annealing and a fully recrystallized (MDRX þSRX) microstructure with more uniform grains structure was observed in Fig. 2b. With the holding time increasing to 400 s, as shown in Figs. 2c and d, the recrystallization process was totally accomplished even at a pre-strain of 0.2 associated with larger grain size. Concurrent with the annealing, the migration of grain boundaries can result in some specific misorientations (e.g. coincident site lattice) to minimize the stored energy. Thus, an overall increase in the number of ∑3n (especially ∑3) boundaries is observable. In contrast, ∑9 and ∑27 boundaries, the intensities of which are considerably lower in comparison with ∑3 boundary, mostly have short segments particularly when generated through the interactions of ∑3n boundaries. The GBCD statistics shown in this work were given by the length fractions, which were quantified and presented in Fig. 3 to reveal their evolution trend during the post-deformation recrystallization. It can be inferred from Figs. 3a–c that the length ratio of ∑3n boundaries gradually grew with increase of holding time and pre-strain in the partially recrystallized samples due to the relatively fine size and short of ∑3n boundaries in the newly recrystallized grains. As the recrystallized grains still continued the growth after a full recrystallization, the grain size increased and annealing twins formed as well. The larger grain size contributed to a significant reduction in the overall length of random grain boundaries and led to the increase in the ∑3 length fraction. Meanwhile, the number of ∑9 and ∑27 boundaries decreased constantly as annealing progressed. There are two main mechanisms contributing to the increase in ∑3n boundaries, which are termed as “∑3 regeneration mechanism” and “new twinning mechanism”. After analyzing data collected under considerable experimental conditions, it was discovered that the ∑3/∑(9 þ 27) ratio is lower in ∑3 regeneration microstructure than that in the new twinning microstructure [7]. It is also suggested that these two mechanisms can take effect concurrently for twin-related GBE but one of them generally predominate [8]. The twinning mechanisms could be derived from Fig. 3d. The results showed ∑3/∑(9 þ27) ratio increased slightly in the as-deformed samples with the progressive deformation, which indicated that the main twinning mechanism at the initial stage of DRX was likely to be new twinning. Moreover, the variation tendencies of ∑3/∑(9 þ27) ratio with holding time were different in SRX and MDRXþ SRX samples. For the former, the ratio peak value was obtained at 30 s followed by the declining ∑3/∑(9 þ27) ratio with a higher interaction of ∑3n boundaries. This definitely signifies the predominance of Σ3 regeneration mechanism and such
27
∑3 boundaries were incorporated into the grain boundary network and characterized by a convoluted morphology. For the latter, after the completion of recrystallization, the ratios increased continuously during the growth of recrystallized grains with increasing holding time since the new twinning mechanism became dominant and caused the formation of mainly long and straight pairs of Σ3n boundaries (see Fig. 2d). Furthermore, two subsets of the IQ maps were depicted in Fig. 2c in order to analyze grain boundary connectivity and ∑3n boundaries morphology. Several examples of triple junction interactions (∑3 þ∑9-∑27 and ∑3 þ∑3-∑9) were illustrated, which produced an increase in the number of ∑9 and ∑27 boundaries. Some convoluted ∑3n boundaries were incorporated to the random boundaries, the morphology of which conformed to the mechanism predicted by the ∑3/∑(9 þ27) ratio. This configuration can break up the interconnectivity of the random grain boundary network, which is perceived to improve the properties of metallic materials as the role of GBE [4–7].
4. Conclusions In summary, the effects of pre-strain and annealing time on grain boundary character distribution in Incoloy 800 H were studied during the post-deformation recrystallization at 1000 °C. The results revealed that variation of pre-strain resulted in different types of post-deformation recrystallization. During the annealing process, Fr increased with sustained pre-strain at a fixed holding time and the Dr following MDRX þSRX was larger than that in SRX conditions. The increase in ∑3n boundaries could be attributed to two main mechanisms. For SRX samples, ∑3 regeneration was the main mechanism, while new twinning was predominant for the MDRXþSRX samples.
Acknowledgments This work was jointly supported by National Program on Key Basic Research Project (No. 2011CB606306-2) and China Scholarship Council for the award of a traveling fellowship (No. 201406080064).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
H. Beladi, P. Cizek, P.D. Hodgson, Acta Mater. 59 (2011) 1482–1492. A.S. Taylor, P.D. Hodgson, Mater. Sci. Eng. A 529 (2011) 164–169. G. Kugler, R. Turk, Acta Mater. 52 (2004) 4659–4668. T.G. Liu, S. Xia, H. Li, B.X. Zhou, Q. Bai, Mater. Lett. 133 (2014) 97–100. Y. Zhou, K.T. Aust, U. Erb, G. Palumbo, Scr. Mater. 45 (2001) 49–54. V. Randle, Acta Mater. 52 (2004) 4067–4081. V. Randle, G. Owen, Acta Mater. 54 (2006) 1777–1783. H. Akhiani, M. Nezakat, M. Sanayei, J. Szpunar, Mater. Sci. Eng. A 626 (2015) 51–60. L. Tan, T.R. Allen, Metall. Mater. Trans. A 36 (2005) 1921–1925. Y. Cao, H.S. Di, J.Q. Zhang, J.C. Zhang, T.J. Ma, Misra RDK, Mater. Sci. Eng. A 585 (2013) 71–85. D.G. Brandon, Acta Metall. 14 (1966) 1479–1484. Y. Cao, H.S. Di, J.C. Zhang, J.Q. Zhang, T.J. Ma, Acta Metall. Sin. 48 (2012) 1175–1185.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Grain boundary character distribution during the post-deformation recrystallization of Incoloy 800H at elevated temperature Yu Cao, Hongshuang Di n State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 September 2015 Received in revised form 4 October 2015 Accepted 8 October 2015 Available online 13 October 2015
The present research studied the grain boundary character distribution in Incoloy 800 H corresponding to two different post-deformation recrystallization mechanisms through uniaxial hot compression test and electron backscattering diffraction analysis. The results showed that the recrystallized grain size following metadynamic and static recrystallization was modestly larger than that from static recrystallization. Besides, two different mechanisms were responsible for the generation of Σ3n boundaries. In the static recrystallization samples, ∑3 regeneration was more active than new twinning formation. However, the new twinning would become predominant when metadynamic and static recrystallization took effect simultaneously. & 2015 Elsevier B.V. All rights reserved.
Keywords: Incoloy 800 H Recrystallization Grain boundary junctions Grain boundary character distribution
1. Introduction Metallurgical events after hot deformation play a significant role in controlling the microstructure evolution to achieve the desirable mechanical properties of the final product [1]. These processes commonly include recovery, static recrystallization (SRX) and metadynamic recrystallization (MDRX). For the low and medium stacking fault energy (SFE) materials, the effect of recovery is relatively low and thus the recrystallization process will become predominant [2]. The general understanding is that both nucleation and growth of recrystallized grains occur during the intervals of hot deformation for SRX, whilst this restoration takes place during deformation for dynamic recrystallization (DRX). MDRX refers to the static growth of DRX nuclei after the interrupt of hot deformation [3]. During these post-deformation recrystallization stages, the continuous occurrence of twinningevents can be generated by consumption of stored strain energy and eventually contributes to the formation of highly twinned interconnecting Σ3n boundaries. Grain boundary engineering (GBE) through the control of grain boundary character distribution (GBCD) has been widely applied to achieving advanced structural and functional properties in high-performance polycrystalline materials [4–6]. It has been extensively accepted that the main mechanism for GBE depends on proliferation of twinning (a type of ∑3 boundary) in low SFE materials associated with interactions of other ∑3n (n r3) n
Corresponding author. E-mail address: [email protected] (H. Di).
http://dx.doi.org/10.1016/j.matlet.2015.10.034 0167-577X/& 2015 Elsevier B.V. All rights reserved.
boundaries. V. Randle [7] investigated the mechanisms of GBE and suggested that ∑3/∑(9 þ27) ratio can be effectively employed to determine the proliferation mechanism of ∑3n boundaries. Incoloy 800 H is an austenitic high-strength solid-solution Fe–Ni–Cr alloy. It was selected as a primary candidate for furnace components and equipments in petrochemical industry due to its high strength and corrosion resistance at elevated temperatures [8]. In the previous works, L. Tan et al. [9] explicated the GBE approach of Incoloy 800 H by means of thermomechanical processing and afterwards H. Akhiani et al. [8] also studied the effect of thermomechanical processing on GBCD in Incoloy 800 H with two different rolling modes followed by annealing. Therefore, it is important to realize the contribution of GBCD to the development of high-performance Incoloy 800 H through annealing after hot deformation. The aim of this paper is to investigate the effect of various hot deformation processing parameters (pre-strain and annealing time) on the GBCD during the post-deformation recrystallization.
2. Experimental procedures A commercial Incoloy 800 H after hot rolling was used in this study. Cylindrical specimens, with a diameter of 10 mm and a height of 15 mm, were machined out of the hot rolled plate and hot deformation compression tests were performed on a computer-controlled, servo-hydraulic MMS-300 thermo-simulation machine. Considering the industrial operation for hot rolling of Incoloy 800 H (the rolling reduction ratio: 0.15–0.4 in true strain), three true strains (0.2, 0.3 and 0.4) were explored by high
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
temperature interrupted-compression tests at a strain rate of 1 s 1 and temperature of 1000 °C. The used pre-strains were associated with the initiation of dynamic recrystallization (DRX) (εc) and the peak strain (εp) as well. After unloading, the specimens were held in air at the deformation temperature for different times (10 s, 30 s, 60 s, and 400 s) to enable the proceeding of post-deformation recrystallization. The samples were immediately quenched ( o1 s) in water following deformation and annealing to preserve the high temperature microstructures. The approach to prepare samples for electron backscattering diffraction (EBSD) was described elsewhere [10]. In order to investigate the grain orientations and GBCD, EBSD data were collected using a TSL EBSD system interfaced to a FEI Quanta 600 scanning electron microscope (SEM). The Brandon criterion [11] was used to identify Σ3n boundaries and recrystallized grain size (Dr) was measured using the linear intercept method. In present work, the grain orientation spread (GOS) approach [10] was utilized to evaluate the recrystallized fraction (Fr) with a threshold value of 1°. Each statistical data were obtained from the evaluation of three distinct regions of the microstructure and the standard deviation was calculated as 72%.
3. Results and discussion Fig. 1(a) displayed the flow stress curve under the experimental deformation condition, indicating a peak stress (sp) corresponding to a peak strain (εp) of 0.41. It can be also observed that work hardening rate (θ ¼ ds/dε) decreased remarkably with sustained strain and attained to zero at εp. As indicated previously, the critical strain (εc) of DRX was calculated to be 0.27 [12]. Considering these characteristic points, three different true strains followed by annealing were explored to investigate various mechanisms of
25
Table 1 Variation of Dr and Fr under different deformation conditions. The background was highlighted for the completely recrystallized samples.
post-deformation recrystallization, namely, SRX at 0.2 and MDRXþSRX at 0.3/0.4. At the beginning of deformation (Fig. 1b), the original austenitic grains began to be elongated perpendicular to the compression axis and a large fraction of low angle boundaries were produced due to dislocation generation and dislocation boundary formation. Some bulges originated from the serration segments of grain boundaries and resulted in the nucleation of DRX grains. Specifically, most of the pre-existing twin boundaries lost their crystallographic characteristics resulting from grain rotation with concurrent deformation. As shown in Fig. 1c, some fine new DRX grains were detected at grain boundaries and particularly at triple junctions with the true strain increasing beyond εc. More new grains and incomplete necklace structures formed with deformation up to εp (Fig. 1d) and it is worth noting that few twin boundaries could be observed within new grains. Characterization of the as-deformed samples revealed that Fr and Dr increased modestly with increasing strain (see Table 1). Following annealing of the as-deformed samples at 1000 °C for various holding times, the image quality (IQ) maps from EBSD were presented in Fig. 2 and the ∑3n boundaries were highlighted to show the distribution features. Table 1 showed an evidence of static grain growth occurring in all of the samples after annealing
ε = 0.2
ε = 0.3
ε = 0.4
Fig. 1. Curve showing (a) true stress and working hardening rate versus true strain at 1000 °C and 1 s 1 (εc and εp were marked with arrows). (b)–(d) inverse pole figure (IPF) maps obtained from EBSD (step size ¼0.5 μm) for microstructures at different true strains. Gray and black lines represented low angle grain boundaries (misorientation angle: 2–15°) and high angle grain boundaries (misorientation angle: 4 15°). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
26
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
ε = 0.4
ε = 0.2
10μm 1
10μm
Σ9 2
Σ3
Σ27b Σ3
Σ3 Σ9 ε = 0.2
ε = 0.4
Fig. 2. Image quality (IQ) maps showing the grain structure and distribution of ∑3n (nr 3) boundaries following anneal at 1000 °C for (a) and (b) 30 s and (c) and (d) 400 s (step size ¼ 1 μm). ∑3: red, ∑9: yellow, ∑27a: blue, ∑27b: green. Two subsets were shown to demonstrate the interactions of Σ3n boundaries in Fig. 2(c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Length fractions of (a) ∑3, (b) ∑9, (c) ∑27 and (d) ∑3/(∑9þ ∑27) for samples at different true strains with various anneal times.
as Dr range from 20 to 56 μm at the strain of 0.2, 12 to 60 μm at 0.3 and 18 to 76 μm at 0.4. Besides, recrystallized fraction was observed to distinctly increase with prolonged holding time. It also increased as a function of the pre-strain because higher stored
energy can enhance the driving force for grain boundary mobility and subsequent recrystallization. Note that the completion times of recrystallization were different for the three testing pre-strains: 400 s for 0.2, 60 s for 0.3 and 30 s for 0.4 (highlighted with red in
Y. Cao, H. Di / Materials Letters 163 (2016) 24–27
Table 1). It can also be conjectured from this observation that Dr increased modestly at higher pre-strain for a given holding time. This means that the Dr following MDRX þSRX is significantly larger than that measured in the SRX condition, which is due to the fact that MDRX grains nucleate dynamically during deformation and they will continue to grow without any incubation time compared with SRX grains at the post-deformation stage. As shown in Fig. 2, dramatical differences in the microstructure were detected depending on the pre-strain and holding time. Fig. 2a exhibited a partially recrystallized (SRX) microstructure of the deformed sample with a pre-strain of 0.2 after annealing for 30 s. Straining the sample to 0.4 was observed to accelerate the rate of recrystallization during annealing and a fully recrystallized (MDRX þSRX) microstructure with more uniform grains structure was observed in Fig. 2b. With the holding time increasing to 400 s, as shown in Figs. 2c and d, the recrystallization process was totally accomplished even at a pre-strain of 0.2 associated with larger grain size. Concurrent with the annealing, the migration of grain boundaries can result in some specific misorientations (e.g. coincident site lattice) to minimize the stored energy. Thus, an overall increase in the number of ∑3n (especially ∑3) boundaries is observable. In contrast, ∑9 and ∑27 boundaries, the intensities of which are considerably lower in comparison with ∑3 boundary, mostly have short segments particularly when generated through the interactions of ∑3n boundaries. The GBCD statistics shown in this work were given by the length fractions, which were quantified and presented in Fig. 3 to reveal their evolution trend during the post-deformation recrystallization. It can be inferred from Figs. 3a–c that the length ratio of ∑3n boundaries gradually grew with increase of holding time and pre-strain in the partially recrystallized samples due to the relatively fine size and short of ∑3n boundaries in the newly recrystallized grains. As the recrystallized grains still continued the growth after a full recrystallization, the grain size increased and annealing twins formed as well. The larger grain size contributed to a significant reduction in the overall length of random grain boundaries and led to the increase in the ∑3 length fraction. Meanwhile, the number of ∑9 and ∑27 boundaries decreased constantly as annealing progressed. There are two main mechanisms contributing to the increase in ∑3n boundaries, which are termed as “∑3 regeneration mechanism” and “new twinning mechanism”. After analyzing data collected under considerable experimental conditions, it was discovered that the ∑3/∑(9 þ 27) ratio is lower in ∑3 regeneration microstructure than that in the new twinning microstructure [7]. It is also suggested that these two mechanisms can take effect concurrently for twin-related GBE but one of them generally predominate [8]. The twinning mechanisms could be derived from Fig. 3d. The results showed ∑3/∑(9 þ27) ratio increased slightly in the as-deformed samples with the progressive deformation, which indicated that the main twinning mechanism at the initial stage of DRX was likely to be new twinning. Moreover, the variation tendencies of ∑3/∑(9 þ27) ratio with holding time were different in SRX and MDRXþ SRX samples. For the former, the ratio peak value was obtained at 30 s followed by the declining ∑3/∑(9 þ27) ratio with a higher interaction of ∑3n boundaries. This definitely signifies the predominance of Σ3 regeneration mechanism and such
27
∑3 boundaries were incorporated into the grain boundary network and characterized by a convoluted morphology. For the latter, after the completion of recrystallization, the ratios increased continuously during the growth of recrystallized grains with increasing holding time since the new twinning mechanism became dominant and caused the formation of mainly long and straight pairs of Σ3n boundaries (see Fig. 2d). Furthermore, two subsets of the IQ maps were depicted in Fig. 2c in order to analyze grain boundary connectivity and ∑3n boundaries morphology. Several examples of triple junction interactions (∑3 þ∑9-∑27 and ∑3 þ∑3-∑9) were illustrated, which produced an increase in the number of ∑9 and ∑27 boundaries. Some convoluted ∑3n boundaries were incorporated to the random boundaries, the morphology of which conformed to the mechanism predicted by the ∑3/∑(9 þ27) ratio. This configuration can break up the interconnectivity of the random grain boundary network, which is perceived to improve the properties of metallic materials as the role of GBE [4–7].
4. Conclusions In summary, the effects of pre-strain and annealing time on grain boundary character distribution in Incoloy 800 H were studied during the post-deformation recrystallization at 1000 °C. The results revealed that variation of pre-strain resulted in different types of post-deformation recrystallization. During the annealing process, Fr increased with sustained pre-strain at a fixed holding time and the Dr following MDRX þSRX was larger than that in SRX conditions. The increase in ∑3n boundaries could be attributed to two main mechanisms. For SRX samples, ∑3 regeneration was the main mechanism, while new twinning was predominant for the MDRXþSRX samples.
Acknowledgments This work was jointly supported by National Program on Key Basic Research Project (No. 2011CB606306-2) and China Scholarship Council for the award of a traveling fellowship (No. 201406080064).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
H. Beladi, P. Cizek, P.D. Hodgson, Acta Mater. 59 (2011) 1482–1492. A.S. Taylor, P.D. Hodgson, Mater. Sci. Eng. A 529 (2011) 164–169. G. Kugler, R. Turk, Acta Mater. 52 (2004) 4659–4668. T.G. Liu, S. Xia, H. Li, B.X. Zhou, Q. Bai, Mater. Lett. 133 (2014) 97–100. Y. Zhou, K.T. Aust, U. Erb, G. Palumbo, Scr. Mater. 45 (2001) 49–54. V. Randle, Acta Mater. 52 (2004) 4067–4081. V. Randle, G. Owen, Acta Mater. 54 (2006) 1777–1783. H. Akhiani, M. Nezakat, M. Sanayei, J. Szpunar, Mater. Sci. Eng. A 626 (2015) 51–60. L. Tan, T.R. Allen, Metall. Mater. Trans. A 36 (2005) 1921–1925. Y. Cao, H.S. Di, J.Q. Zhang, J.C. Zhang, T.J. Ma, Misra RDK, Mater. Sci. Eng. A 585 (2013) 71–85. D.G. Brandon, Acta Metall. 14 (1966) 1479–1484. Y. Cao, H.S. Di, J.C. Zhang, J.Q. Zhang, T.J. Ma, Acta Metall. Sin. 48 (2012) 1175–1185.