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Twins in cryomilled and spark plasma sintered Cu–Zn–Al
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Scripta Materialia 67 (2012) 245–248 www.elsevier.com/locate/scriptamat
Twins in cryomilled and spark plasma sintered Cu–Zn–Al Haiming Wen⇑ and Enrique J. Lavernia Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA Received 3 April 2012; revised 20 April 2012; accepted 24 April 2012 Available online 28 April 2012
Nanostructured Cu–30 wt.% Zn–0.8 wt.% Al alloy (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS). Cryomilling resulted in a high density of deformation twins with an average thickness as small as 4 nm. Following SPS, the bulk samples exhibited 10 vol.% of twins with an average twin thickness of 30 nm and unusual twin morphology, which are rationalized on the basis of grain boundary migration, twin boundary migration, recrystallization and detwinning during SPS. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Twinning; Grain boundary migration; Recrystallization; Twin boundary migration
Coherent twin boundaries (TBs) are effective barriers to dislocation motion and possess similar strengthening effects to those of grain boundaries (GBs); generally the strength increment due to TBs has a Hall–Petch type relationship with the TB spacing (twin thickness) [1,2]. Meanwhile, TBs can provide locations for dislocation accumulation and storage, thereby increasing strain hardening and accordingly ductility [1]. Monotonic increase in ductility with decreasing twin thickness from 100 to 4 nm was reported in a nanotwinned Cu [3]. In general, a high density of twins with nanoscale thicknesses is desired to simultaneously achieve high strength and considerable ductility. Nanoscale growth twins are introduced via physical [4,5] or chemical [1,3] processes. However, samples are typically limited to thin films or foils. Deformation twins can be generated using severe plastic deformation techniques, such as equal-channel angular pressing [6], high pressure torsion [7] and dynamic plastic deformation (DPD) [8,9], which still have the shortcoming of limited sample dimensions. Mechanical milling, including cryomilling, can also introduce deformation twins in powders [10]. Milled powders can then be consolidated into bulk samples with large dimensions [11]. However, the consolidation typically requires the application of elevated temperature and pressure, and is therefore a thermo-mechanical process (TMP). Hence, important and heretofore poorly understood questions that emerge are: how do deformation twins evolve during TMP and
⇑ Corresponding author; e-mail: [email protected]
what are the associated mechanisms? Can a nanotwinned structure be retained after TMP? In this work, a Cu–30 wt.% Zn–0.8 wt.% Al alloy (commercial designation brass 260) was selected for study, due to its technological importance, and fundamental relevance as a result of its low stacking fault energy (SFE). Brass 260 powders were cryomilled and then consolidated via spark plasma sintering (SPS). The twins in the cryomilled powders and sintered bulk samples were carefully investigated in an effort to provide insight into the mechanisms governing the evolution of twins during SPS, which is a TMP. Commercially pure 325 mesh brass 260 alloy powders (nominal composition Cu–30 Zn–0.8 Al, wt.%) were cryomilled for 12 h in liquid nitrogen with a ballto-powder ratio of 30:1, without a process control agent. Cryomilled brass powders were loaded into a graphite die of 20 mm in diameter and consolidated using a SPS-825S apparatus (SPS Syntex Inc.) at a sintering temperature of 800 °C for 5 min. A pressure of 100 MPa was applied at room temperature and maintained until the sintering was complete. Microstructures of cryomilled powders and SPS-consolidated bulk sample were studied by transmission electron microscopy (TEM) on a JEOL 2500SE microscope operating at 200 kV. TEM specimens were prepared by mechanical grinding and dimpling, followed by ion milling using a Gatan PIPS 691 instrument. There is high density of deformation twins in the cryomilled powders, as shown in the TEM images in Figure 1. Figure 1a indicates deformation twins in almost every grain. Note that twins may not be seen if a grain is not positioned in the right orientation.
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.04.024
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Figure 1. (a) TEM image of conventional magnification of cryomilled brass powders showing high density of deformation twins; (b) HRTEM image confirming the presence of twins; (c) statistical twin thickness distribution.
Additionally, some moire´ fringes are present in Figure 1a. However, multiple high-resolution TEM (HRTEM) images of lattice fringes indicated that most fringes in Figure 1a represent twins. Figure 1b is a representative HRTEM image which confirms the presence of twins. The TBs are not perfectly straight and are slightly curved due to the presence of dislocations at the TBs, which is an attribute of deformation twins [8,9]. Furthermore, the thicknesses of the twin/matrix lamellae (hereafter referred to as twin thickness) are measured based on numerous HRTEM images. Figure 1c presents the statistical twin thickness distribution, with a range of 1–10 nm and an average of 4 nm. Additionally, results from the grain size analyses (not shown here) indicate an average grain size of 26 nm [12]. The SFE of Cu–30 wt.% Zn is reported as 7 mJ m 2 [7] or 14 mJ m 2 [9], and Al is also an effective element to lower the SFE of Cu [6]. Therefore, the SFE of the brass 260 alloy with nominal composition of Cu– 30 wt.% Zn–0.8 wt.% Al is estimated to be less than 7 mJ m 2. The very low SFE facilitates the formation of a high density of deformation twins. Additionally, low temperature and high strain rate are also known to promote deformation twinning [10]. Cryomilling
involves liquid nitrogen temperature (77 K) and high strain rate [11], thereby contributing to the high density of deformation twins. The extremely small thickness for deformation twins (average 4 nm) has rarely been reported. The average twin thicknesses in Cu–32 wt.% Zn (SFE 14 mJ m 2) [9,13] and Cu–4.5 wt.% Al (SFE 12 mJ m 2) [14] processed by DPD at liquid nitrogen temperature are both 12 nm. Low SFE, high strain rate and low deformation temperature lead to smaller deformation twin thickness, because they effectively reduce the critical twin nucleus thickness and resist the propagation of twins [13,14]. The SFE of brass 260 alloy is lower than that of the two alloys indicated above. In addition, the temperature is strictly maintained at liquid nitrogen temperature during cryomilling of brass 260 powders; for DPD, the sample is cooled to liquid nitrogen temperature before each impact, while the temperature may rise during the dynamic deformation [8,9]. Furthermore, the very small grain size of the cryomilled brass 260 powders is likely to have contributed to the extremely small twin thickness, since the space in the grains is limited by the grain size. The twins in the sintered bulk samples were carefully studied. Figure 2a displays a TEM image of conventional magnification showing twins in some grains. As indicated by a representative HRTEM image in Figure 2b, the TBs are very straight and coherent, with few dislocations at the boundaries, which is in contrast with the TBs in the cryomilled powders. Statistical twin thickness distribution, Figure 2c, indicates that 94% of twin thicknesses are in the range of a few nanometers to 70 nm and the rest fall in the range of 80– 340 nm. The average of all the twin thicknesses is 30 nm, and that of the twin thicknesses below 70 nm is 22 nm. Additionally, the volume fraction of twin/matrix lamellae (hereafter referred to as twin volume fraction) is measured based on numerous TEM images. Twin volume fraction is defined as the area ratio of twin/matrix lamellae to the grains, and in practice the measurements were performed on grains on or near a major zone axis, which appear dark in the TEM images. Twin volume fraction is determined to be 10%. Note that the real twin volume fraction may be higher, considering that some twins may not be seen if the grains are not in the proper orientation. Moreover, the grain size analyses (not shown here) indicate that most grains are 6100 nm in size, while a few percent of large grains with size of 300–800 nm are also present, and the average size of all grains is 110 nm [12]. In view of the fact that SPS is a high temperature process and that brass 260 has a low SFE, annealing twins are likely to form during SPS [15]. Therefore, twins in the sintered bulk samples can be either deformation twins retained from cryomilled powders or annealing twins generated during SPS. The thickness of annealing twins is usually large compared to that of deformation twins, and TBs of annealing twins are perfectly coherent and straight with few dislocations [15]. These attributes may be used to distinguish annealing twins from deformation twins. However, original dislocations at the TBs of the deformation twins from the cryomilled powders are likely to get annihilated during the high temperature SPS process, and accordingly, the deformation TBs
H.M. Wen, E. J. Lavernia / Scripta Materialia 67 (2012) 245–248
Figure 2. (a) TEM image of conventional magnification of sintered bulk brass samples showing twins; (b) HRTEM image confirming the presence of twins; (c) statistical twin thickness distribution.
become more coherent and straight. Additionally, TBs migrate when exposed to high temperature annealing [4], and similarly migration of the deformation TBs takes place during SPS, resulting in thickening of the twins. Consequently, in the present study, it is difficult to precisely distinguish annealing twins from deformation twins. Nevertheless, because TBs have low energy and accordingly the driving force for TB migration is low and the activation energy for TB migration is high, nanoscale twins have very high thermal stability [4,16]. After annealing at 800 °C for 1 h, the average thickness of growth twins in a sputtered Cu film changed from 4 nm to 20 nm [4]. Since the thicknesses of the original deformation twins in the cryomilled brass powders are unlikely to grow during SPS by TB migration from 1–10 nm to 80–340 nm, the thick twins in the sintered brass samples with discontinuous distribution of thickness in the range of 80–340 nm may be argued to be annealing twins. The twin volume fraction in the sintered brass samples is only 10%, while in the cryomilled powders, there is a high density of deformation twins in almost every grain, and twin/matrix lamellae constitute virtually the entire volume of the grains. The average twin thickness increases from 4 nm in the cryomilled powders to 30 nm in the sintered bulk sample. Both twin volume fraction
247
and twin thickness determine the twin density (average area of TBs per unit volume or average length of TBs per unit area), a comprehensive measure of probability of twinning. Although twin density was not quantitatively measured in this study, the drastic decrease in twin volume fraction and the notable increase in twin thickness indicate significant reduction in twin density in the sintered bulk sample as compared to that in the cryomilled powders. Since 100 MPa pressure was applied during SPS, SPS consolidation is TMP. Thermally activated processes active during SPS include GB migration to reduce GB energy, recrystallization to reduce stored strain energy, TB migration to reduce TB energy, and generation of annealing twins. During recrystallization, annealing twins can be also formed [17]. Meanwhile, two mechanically induced processes may occur during SPS, namely, detwinning (inverse process of twinning leading to annihilation of TBs) [5,18] and TB migration [19,20], which are deformation mechanisms known for metals possessing high density of nanoscale twins with extremely small thicknesses. Considering that 100 MPa is relatively low stress and that the temperature is high during SPS, the mechanically induced processes are less probable than the thermally activated processes. Sole TB migration, whether thermally activated or mechanically induced, will lead to an increase in average twin thickness, but would not cause a reduction in twin volume fraction. GB migration eliminates TBs in the grains that shrink, but may extend TBs in the grains that grow [4]. However, the TBs in the growing grains do not always get extended, and therefore GB migration tends to decrease the twin volume fraction. Recrystallization consumes deformed grains with a high density of deformation twins and generates recrystallized grains without twins or with annealing twins. In essence, recrystallization reduces twin volume fraction and increases twin thickness. Creation of annealing twins increases the twin volume fraction and leads to an increase in the total average twin thickness, since annealing twins are usually thick. Therefore, considering all the processes discussed, the increase in average twin thickness from 4 nm in the cryomilled powders to 30 nm in the sintered bulk samples is attributed to thermally activated TB migration of original deformation twins and generation of annealing twins. Mechanically induced TB migration may be also responsible for the increase in twin thickness, but it is less probable. The significant reduction in twin volume fraction from the cryomilled powders to sintered bulk samples is ascribed to GB migration, recrystallization and detwinning during SPS. Because of the relatively low stress applied, the probability for detwinning is lower than that for GB migration and recrystallization. Special twins with unique morphology are observed in the sintered bulk samples, as illustrated in Figure 3. The entire area of Figure 3a is in a large grain of 600 nm in size. A wide twin lamella is evident, with its two TBs depicted by curve A and line B. Note that there are several steps on TB A, and the lower left part of the boundary is significantly shifted with respect to the upper right portion. Therefore, the thickness of the lower left part of the twin lamella is much smaller than that of the upper right part. The steps on TB A can be
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part. The complex and unique twin morphology illustrated in Figure 3 has not heretofore been reported in the literature. The specific mechanisms responsible for the formation of the twin morphology are presently unknown and require further study. The simultaneous occurrence of the multiple processes described above, i.e. GB migration, TB migration, detwinning and formation of annealing twins, results in complex GB–TB and TB–TB interactions, and may lead to the special twin morphology documented here. In summary, cryomilling induces in brass 260 powders high density of deformation twins with extremely small average thickness of 4 nm. The thermo-mechanical process of SPS results in a significant reduction in twin density and formation of unique twin morphology in the sintered bulk samples. The twin volume fraction decreases to 10%, which occurs as a result of GB migration, recrystallization and detwinning. The average twin thickness increases to 30 nm, which is attributed to TB migration and formation of annealing twins. Financial support from the Office of Naval Research (N00014-08-1-0405 & N00014-12-1-0237) is gratefully acknowledged.
Figure 3. TEM images of sintered bulk brass samples showing special twin morphology: (a) the whole area; (b) a magnified view of the yellow circle in (a) showing the intersection of two thin twin lamellae; (c) a magnified view of the turquoise ellipse in (a) showing steps on twin boundary A and intersection of several twins with twin boundary A. A, B, C, D, E, F, G denote twin boundaries. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
clearly seen in Figure 3c, a magnified view of the green ellipse in Figure 3a. Distinctively, there is a very thin twin lamella inside the wide twin lamella in Figure 3a, and it intersects with another twin lamella in the matrix with very similar thickness. The intersection of the two thin twin lamellae is magnified in Figure 3b, in which TBs C, D, E and F are depicted and twin relationships are labeled. Note that the part of TB A at the intersection is curved, and the twin relationship requirement is not met across this segment of the boundary, which may be composed of dislocations. Furthermore, the lower left part of TB A is shifted by 1 nm across the twin intersection with respect to the upper right part of the boundary. Another distinctive feature to be noted is that in the matrix below TB A there are several twins with their TBs intersecting TB A, Figure 3c. Additionally, there is also a step on TB G, and the lower part of the boundary is notably shifted with reference to the upper
[1] L. Lu, Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu, Science 304 (2004) 422. [2] Y.F. Shen, L. Lu, Q.H. Lu, Z.H. Jin, K. Lu, Scripta Mater. 52 (2005) 989. [3] L. Lu, X. Chen, X. Huang, K. Lu, Science 323 (2009) 607. [4] O. Anderoglu, A. Misra, H. Wang, X. Zhang, J. Appl. Phys. 103 (2008) 094322. [5] J. Wang, N. Li, O. Anderoglu, X. Zhang, A. Misra, J.Y. Huang, J.P. Hirth, Acta Mater. 58 (2010) 2262. [6] S. Qu, X.H. An, H.J. Yang, C.X. Huang, G. Yang, Q.S. Zang, Z.G. Wang, S.D. Wu, Z.F. Zhang, Acta Mater. 57 (2009) 1586. [7] L. Balogh, T. Ungar, Y. Zhao, Y.T. Zhu, Z. Horita, C. Xu, T.G. Langdon, Acta Mater. 56 (2008) 809. [8] W.S. Zhao, N.R. Tao, J.Y. Guo, Q.H. Lu, K. Lu, Scripta Mater. 53 (2005) 745. [9] G.H. Xiao, N.R. Tao, K. Lu, Mater. Sci. Eng. A 513–514 (2009) 13. [10] Y.T. Zhu, X.Z. Liao, X.L. Wu, Prog. Mater. Sci. 57 (2012) 1. [11] D.B. Witkin, E.J. Lavernia, Prog. Mater. Sci. 51 (2006) 1. [12] H.M. Wen, T.D. Topping, D. Isheim, D.N. Seidman, E.J. Lavernia, Acta Mater., submitted for publication. [13] G.H. Xiao, N.R. Tao, K. Lu, Scripta Mater. 59 (2008) 975. [14] Y. Zhang, N.R. Tao, K. Lu, Scripta Mater. 60 (2009) 211. [15] S. Mahajan, C.S. Pande, M.A. Imam, B.B. Rath, Acta Mater. 45 (1997) 2633. [16] C. Saldana, T.G. Murthy, M.R. Shankar, E.A. Stach, S. Chandrasekar, Appl. Phys. Lett. 94 (2009) 021910. [17] T. Baudin, A.L. Etter, R. Penelle, Mater. Charact. 58 (2007) 947. [18] H.M. Wen, Y.H. Zhao, Y. Li, O. Ertorer, K.M. Nesterov, R.K. Islamgaliev, R.Z. Valiev, E.J. Lavernia, Philos. Mag. 90 (2010) 4541. [19] Y.B. Wang, M.L. Sui, E. Ma, Philos. Mag. Lett. 87 (2007) 935. [20] X.Y. Li, Y.J. Wei, L. Lu, K. Lu, H.J. Gao, Nature 464 (2010) 877.
Scripta Materialia 67 (2012) 245–248 www.elsevier.com/locate/scriptamat
Twins in cryomilled and spark plasma sintered Cu–Zn–Al Haiming Wen⇑ and Enrique J. Lavernia Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, CA 95616, USA Received 3 April 2012; revised 20 April 2012; accepted 24 April 2012 Available online 28 April 2012
Nanostructured Cu–30 wt.% Zn–0.8 wt.% Al alloy (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS). Cryomilling resulted in a high density of deformation twins with an average thickness as small as 4 nm. Following SPS, the bulk samples exhibited 10 vol.% of twins with an average twin thickness of 30 nm and unusual twin morphology, which are rationalized on the basis of grain boundary migration, twin boundary migration, recrystallization and detwinning during SPS. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructure; Twinning; Grain boundary migration; Recrystallization; Twin boundary migration
Coherent twin boundaries (TBs) are effective barriers to dislocation motion and possess similar strengthening effects to those of grain boundaries (GBs); generally the strength increment due to TBs has a Hall–Petch type relationship with the TB spacing (twin thickness) [1,2]. Meanwhile, TBs can provide locations for dislocation accumulation and storage, thereby increasing strain hardening and accordingly ductility [1]. Monotonic increase in ductility with decreasing twin thickness from 100 to 4 nm was reported in a nanotwinned Cu [3]. In general, a high density of twins with nanoscale thicknesses is desired to simultaneously achieve high strength and considerable ductility. Nanoscale growth twins are introduced via physical [4,5] or chemical [1,3] processes. However, samples are typically limited to thin films or foils. Deformation twins can be generated using severe plastic deformation techniques, such as equal-channel angular pressing [6], high pressure torsion [7] and dynamic plastic deformation (DPD) [8,9], which still have the shortcoming of limited sample dimensions. Mechanical milling, including cryomilling, can also introduce deformation twins in powders [10]. Milled powders can then be consolidated into bulk samples with large dimensions [11]. However, the consolidation typically requires the application of elevated temperature and pressure, and is therefore a thermo-mechanical process (TMP). Hence, important and heretofore poorly understood questions that emerge are: how do deformation twins evolve during TMP and
⇑ Corresponding author; e-mail: [email protected]
what are the associated mechanisms? Can a nanotwinned structure be retained after TMP? In this work, a Cu–30 wt.% Zn–0.8 wt.% Al alloy (commercial designation brass 260) was selected for study, due to its technological importance, and fundamental relevance as a result of its low stacking fault energy (SFE). Brass 260 powders were cryomilled and then consolidated via spark plasma sintering (SPS). The twins in the cryomilled powders and sintered bulk samples were carefully investigated in an effort to provide insight into the mechanisms governing the evolution of twins during SPS, which is a TMP. Commercially pure 325 mesh brass 260 alloy powders (nominal composition Cu–30 Zn–0.8 Al, wt.%) were cryomilled for 12 h in liquid nitrogen with a ballto-powder ratio of 30:1, without a process control agent. Cryomilled brass powders were loaded into a graphite die of 20 mm in diameter and consolidated using a SPS-825S apparatus (SPS Syntex Inc.) at a sintering temperature of 800 °C for 5 min. A pressure of 100 MPa was applied at room temperature and maintained until the sintering was complete. Microstructures of cryomilled powders and SPS-consolidated bulk sample were studied by transmission electron microscopy (TEM) on a JEOL 2500SE microscope operating at 200 kV. TEM specimens were prepared by mechanical grinding and dimpling, followed by ion milling using a Gatan PIPS 691 instrument. There is high density of deformation twins in the cryomilled powders, as shown in the TEM images in Figure 1. Figure 1a indicates deformation twins in almost every grain. Note that twins may not be seen if a grain is not positioned in the right orientation.
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.04.024
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Figure 1. (a) TEM image of conventional magnification of cryomilled brass powders showing high density of deformation twins; (b) HRTEM image confirming the presence of twins; (c) statistical twin thickness distribution.
Additionally, some moire´ fringes are present in Figure 1a. However, multiple high-resolution TEM (HRTEM) images of lattice fringes indicated that most fringes in Figure 1a represent twins. Figure 1b is a representative HRTEM image which confirms the presence of twins. The TBs are not perfectly straight and are slightly curved due to the presence of dislocations at the TBs, which is an attribute of deformation twins [8,9]. Furthermore, the thicknesses of the twin/matrix lamellae (hereafter referred to as twin thickness) are measured based on numerous HRTEM images. Figure 1c presents the statistical twin thickness distribution, with a range of 1–10 nm and an average of 4 nm. Additionally, results from the grain size analyses (not shown here) indicate an average grain size of 26 nm [12]. The SFE of Cu–30 wt.% Zn is reported as 7 mJ m 2 [7] or 14 mJ m 2 [9], and Al is also an effective element to lower the SFE of Cu [6]. Therefore, the SFE of the brass 260 alloy with nominal composition of Cu– 30 wt.% Zn–0.8 wt.% Al is estimated to be less than 7 mJ m 2. The very low SFE facilitates the formation of a high density of deformation twins. Additionally, low temperature and high strain rate are also known to promote deformation twinning [10]. Cryomilling
involves liquid nitrogen temperature (77 K) and high strain rate [11], thereby contributing to the high density of deformation twins. The extremely small thickness for deformation twins (average 4 nm) has rarely been reported. The average twin thicknesses in Cu–32 wt.% Zn (SFE 14 mJ m 2) [9,13] and Cu–4.5 wt.% Al (SFE 12 mJ m 2) [14] processed by DPD at liquid nitrogen temperature are both 12 nm. Low SFE, high strain rate and low deformation temperature lead to smaller deformation twin thickness, because they effectively reduce the critical twin nucleus thickness and resist the propagation of twins [13,14]. The SFE of brass 260 alloy is lower than that of the two alloys indicated above. In addition, the temperature is strictly maintained at liquid nitrogen temperature during cryomilling of brass 260 powders; for DPD, the sample is cooled to liquid nitrogen temperature before each impact, while the temperature may rise during the dynamic deformation [8,9]. Furthermore, the very small grain size of the cryomilled brass 260 powders is likely to have contributed to the extremely small twin thickness, since the space in the grains is limited by the grain size. The twins in the sintered bulk samples were carefully studied. Figure 2a displays a TEM image of conventional magnification showing twins in some grains. As indicated by a representative HRTEM image in Figure 2b, the TBs are very straight and coherent, with few dislocations at the boundaries, which is in contrast with the TBs in the cryomilled powders. Statistical twin thickness distribution, Figure 2c, indicates that 94% of twin thicknesses are in the range of a few nanometers to 70 nm and the rest fall in the range of 80– 340 nm. The average of all the twin thicknesses is 30 nm, and that of the twin thicknesses below 70 nm is 22 nm. Additionally, the volume fraction of twin/matrix lamellae (hereafter referred to as twin volume fraction) is measured based on numerous TEM images. Twin volume fraction is defined as the area ratio of twin/matrix lamellae to the grains, and in practice the measurements were performed on grains on or near a major zone axis, which appear dark in the TEM images. Twin volume fraction is determined to be 10%. Note that the real twin volume fraction may be higher, considering that some twins may not be seen if the grains are not in the proper orientation. Moreover, the grain size analyses (not shown here) indicate that most grains are 6100 nm in size, while a few percent of large grains with size of 300–800 nm are also present, and the average size of all grains is 110 nm [12]. In view of the fact that SPS is a high temperature process and that brass 260 has a low SFE, annealing twins are likely to form during SPS [15]. Therefore, twins in the sintered bulk samples can be either deformation twins retained from cryomilled powders or annealing twins generated during SPS. The thickness of annealing twins is usually large compared to that of deformation twins, and TBs of annealing twins are perfectly coherent and straight with few dislocations [15]. These attributes may be used to distinguish annealing twins from deformation twins. However, original dislocations at the TBs of the deformation twins from the cryomilled powders are likely to get annihilated during the high temperature SPS process, and accordingly, the deformation TBs
H.M. Wen, E. J. Lavernia / Scripta Materialia 67 (2012) 245–248
Figure 2. (a) TEM image of conventional magnification of sintered bulk brass samples showing twins; (b) HRTEM image confirming the presence of twins; (c) statistical twin thickness distribution.
become more coherent and straight. Additionally, TBs migrate when exposed to high temperature annealing [4], and similarly migration of the deformation TBs takes place during SPS, resulting in thickening of the twins. Consequently, in the present study, it is difficult to precisely distinguish annealing twins from deformation twins. Nevertheless, because TBs have low energy and accordingly the driving force for TB migration is low and the activation energy for TB migration is high, nanoscale twins have very high thermal stability [4,16]. After annealing at 800 °C for 1 h, the average thickness of growth twins in a sputtered Cu film changed from 4 nm to 20 nm [4]. Since the thicknesses of the original deformation twins in the cryomilled brass powders are unlikely to grow during SPS by TB migration from 1–10 nm to 80–340 nm, the thick twins in the sintered brass samples with discontinuous distribution of thickness in the range of 80–340 nm may be argued to be annealing twins. The twin volume fraction in the sintered brass samples is only 10%, while in the cryomilled powders, there is a high density of deformation twins in almost every grain, and twin/matrix lamellae constitute virtually the entire volume of the grains. The average twin thickness increases from 4 nm in the cryomilled powders to 30 nm in the sintered bulk sample. Both twin volume fraction
247
and twin thickness determine the twin density (average area of TBs per unit volume or average length of TBs per unit area), a comprehensive measure of probability of twinning. Although twin density was not quantitatively measured in this study, the drastic decrease in twin volume fraction and the notable increase in twin thickness indicate significant reduction in twin density in the sintered bulk sample as compared to that in the cryomilled powders. Since 100 MPa pressure was applied during SPS, SPS consolidation is TMP. Thermally activated processes active during SPS include GB migration to reduce GB energy, recrystallization to reduce stored strain energy, TB migration to reduce TB energy, and generation of annealing twins. During recrystallization, annealing twins can be also formed [17]. Meanwhile, two mechanically induced processes may occur during SPS, namely, detwinning (inverse process of twinning leading to annihilation of TBs) [5,18] and TB migration [19,20], which are deformation mechanisms known for metals possessing high density of nanoscale twins with extremely small thicknesses. Considering that 100 MPa is relatively low stress and that the temperature is high during SPS, the mechanically induced processes are less probable than the thermally activated processes. Sole TB migration, whether thermally activated or mechanically induced, will lead to an increase in average twin thickness, but would not cause a reduction in twin volume fraction. GB migration eliminates TBs in the grains that shrink, but may extend TBs in the grains that grow [4]. However, the TBs in the growing grains do not always get extended, and therefore GB migration tends to decrease the twin volume fraction. Recrystallization consumes deformed grains with a high density of deformation twins and generates recrystallized grains without twins or with annealing twins. In essence, recrystallization reduces twin volume fraction and increases twin thickness. Creation of annealing twins increases the twin volume fraction and leads to an increase in the total average twin thickness, since annealing twins are usually thick. Therefore, considering all the processes discussed, the increase in average twin thickness from 4 nm in the cryomilled powders to 30 nm in the sintered bulk samples is attributed to thermally activated TB migration of original deformation twins and generation of annealing twins. Mechanically induced TB migration may be also responsible for the increase in twin thickness, but it is less probable. The significant reduction in twin volume fraction from the cryomilled powders to sintered bulk samples is ascribed to GB migration, recrystallization and detwinning during SPS. Because of the relatively low stress applied, the probability for detwinning is lower than that for GB migration and recrystallization. Special twins with unique morphology are observed in the sintered bulk samples, as illustrated in Figure 3. The entire area of Figure 3a is in a large grain of 600 nm in size. A wide twin lamella is evident, with its two TBs depicted by curve A and line B. Note that there are several steps on TB A, and the lower left part of the boundary is significantly shifted with respect to the upper right portion. Therefore, the thickness of the lower left part of the twin lamella is much smaller than that of the upper right part. The steps on TB A can be
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part. The complex and unique twin morphology illustrated in Figure 3 has not heretofore been reported in the literature. The specific mechanisms responsible for the formation of the twin morphology are presently unknown and require further study. The simultaneous occurrence of the multiple processes described above, i.e. GB migration, TB migration, detwinning and formation of annealing twins, results in complex GB–TB and TB–TB interactions, and may lead to the special twin morphology documented here. In summary, cryomilling induces in brass 260 powders high density of deformation twins with extremely small average thickness of 4 nm. The thermo-mechanical process of SPS results in a significant reduction in twin density and formation of unique twin morphology in the sintered bulk samples. The twin volume fraction decreases to 10%, which occurs as a result of GB migration, recrystallization and detwinning. The average twin thickness increases to 30 nm, which is attributed to TB migration and formation of annealing twins. Financial support from the Office of Naval Research (N00014-08-1-0405 & N00014-12-1-0237) is gratefully acknowledged.
Figure 3. TEM images of sintered bulk brass samples showing special twin morphology: (a) the whole area; (b) a magnified view of the yellow circle in (a) showing the intersection of two thin twin lamellae; (c) a magnified view of the turquoise ellipse in (a) showing steps on twin boundary A and intersection of several twins with twin boundary A. A, B, C, D, E, F, G denote twin boundaries. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
clearly seen in Figure 3c, a magnified view of the green ellipse in Figure 3a. Distinctively, there is a very thin twin lamella inside the wide twin lamella in Figure 3a, and it intersects with another twin lamella in the matrix with very similar thickness. The intersection of the two thin twin lamellae is magnified in Figure 3b, in which TBs C, D, E and F are depicted and twin relationships are labeled. Note that the part of TB A at the intersection is curved, and the twin relationship requirement is not met across this segment of the boundary, which may be composed of dislocations. Furthermore, the lower left part of TB A is shifted by 1 nm across the twin intersection with respect to the upper right part of the boundary. Another distinctive feature to be noted is that in the matrix below TB A there are several twins with their TBs intersecting TB A, Figure 3c. Additionally, there is also a step on TB G, and the lower part of the boundary is notably shifted with reference to the upper
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