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Microstructural analysis of the 〈111〉 and 〈110〉 nickel single crystals subjected to severe plastic deformation by hydroextrusion
Materials Science & Engineering A 569 (2013) 92–95
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructural analysis of the /111S and /110S nickel single crystals subjected to severe plastic deformation by hydroextrusion J. Zdunek a,n, J. Mizera a,b, K. Wa˛sik a, K.J. Kurzyd"owski a a b
Warsaw University of Technology, Materials Science and Engineering Faculty, Woloska 141, 02-507 Warsaw, Poland Functional Materials Research Centre, Warsaw University of Technology, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2012 Received in revised form 24 October 2012 Accepted 26 November 2012 Available online 22 December 2012
The effect of hydrostatic extrusion (HE) on the microstructure and crystalline orientation of the /111S and /110S nickel single crystals was examined. The crystals were deformed by two-step hydrostatic extrusion to achieve the true strain er ¼ 2.4. After the extrusion the samples had the form of cylindrical rods. The mechanical properties of the extruded samples (expressed in terms of their microhardness) were compared with the mechanical properties of ultra-fine grained nickel obtained by subjecting polycrystalline nickel to HE. The microhardness of the two deformed crystals appeared to be similar. The microstructure of the samples and the orientation evolution were examined using transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and X-Ray diffraction (XRD). Both the deformed crystals had an inhomogeneous ultra-fine-grained structure (as observed by TEM). The average grain diameter was 300 nm. The majority of the grain boundaries had high angle disorientations (EBSD). In both the deformed samples the predominating orientation was /111S (XRD). In the /111S oriented crystal 95% of the initial orientation was preserved whereas in the /110S oriented crystal the initial orientation was predominantly transformed into /111S and /100S. & 2012 Elsevier B.V. All rights reserved.
Keywords: EBSD X-ray diffraction Bulk deformation Orientation
1. Introduction Pure nickel is very rarely used as a structural material because of its high cost per unit. If however its structure is strongly refined, it exhibits good mechanical properties and quite good corrosion resistance. Thanks to these features nickel is an interesting material for the manufacture of the components of microelectromechanical systems. At present, ultrafine-grained and nanocrystalline nickel is usually produced by the two methods: severe plastic deformation (SPD) [1,2], and electro-deposition [1,3], or, recently, a combination of these two methods [4]. The crucial problem in the investigations of materials subjected to severe plastic deformation is to understand the cold deformation mechanisms active under high stresses. This deformation results in material consolidation and, in consequence, in an increase of its hardness and strength but a decrease of its ductility. These changes are due to the microstructural processes that proceed within the material. The main goal in the development of the SPD
n
Corresponding author. Tel.: þ48 22 234 8556; fax: þ 48 22 848 48 75. E-mail addresses: [email protected] (J. Zdunek), [email protected] (J. Mizera), [email protected] (K. Wa˛sik), [email protected] (K.J. Kurzyd"owski). 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.093
methods is to maximize the strength parameters of the material while preserving its good plastic properties. Among the available SPD methods the present authors have chosen hydrostatic extrusion (HE). The literature reports on the use of the SPD methods to deform single crystals of metals are very scarce (we can mention only ref. [5]). The present paper is concerned with the evolution of the microstructure of Ni monocrystals during plastic deformation realized by hydrostatic extrusion. The samples examined were two monocrystals extruded along their orientations (/110S and /111S). This enabled us to evaluate how the microstructure which begins to form and the changes of the orientation of the monocrystals which occur during severe plastic deformation depend on their initial orientation. The results have thrown some light on the processes which proceed during the production of ultra-fine-grained materials by SPD methods.
2. Material and experimental work Two nickel single crystals of orientation /111S and /110S were examined. The samples were cylindrical in shape and were cut out along the main crystallographic axes /111S and /110S of the monocrystals using a spark erosion saw. Fig. 1 shows schematically the two orientations of the samples.
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
The HE process was carried out in two steps (from F10 mm to F6 mm and then to F3 mm) so as to be able to observe better the evolution of the microstructure and changes of the mechanical properties. The total deformation value was 2.4. Changes of the mechanical properties were observed by measuring the microhardness on cross-sections of the initial single crystals and of each sample at each stage of deformation using the Vickers method. The microhardness was measured in a ZWICK microhardness tester equipped with a microscope which permitted measuring precisely the indentation diagonal. After the severe plastic deformation (SPD), the microstructure was examined in thin films of the material (specially prepared for transmission electron microscopy (TEM)) using a Hitachi S5500 scanning electron microscope/scanning–transmission electron microscope (SEM/STEM) and the local texture was determined by the electron backscatter diffraction (EBSD) measurement (Hitachi SU70 SEM). The global texture of the materials after SPD was analyzed (qualitatively and quantitatively) by the X-ray diffraction method
Fig. 1. Schematic representation of the sample orientation /111S (a) and /110S (b).
Fig. 2. Comparison of the microhardness of polycrystalline Ni and the two single crystals /111S and /110S in the initial state and after HE.
93
using a Bruker D8 DISCOVER powder diffractometer equipped with a Cr X-ray source.
3. Results The HV 0.2 microhardness of the two single crystals /110S and /111S was measured on their cross-sections in the initial state and after the hydrostatic extrusion Fig. 2 compares the microhardness of polycrystalline Ni [6,7] and of the two single crystals /111S and /110S in both the initial state and after HE. Microstructure of the single crystals after the first deformation step contains strongly developed dislocation substructures. The large number of dislocation tangles is typical of plastically deformed materials. The dislocations are agglomerated within the grains and form subgrains defined by the dislocation boundaries. Fig. 3 shows the microstructure after the first step of HE (F10 mm-F6 mm, e ¼1) and Fig. 4—this microstructure after the second deformation step (F6 mm-F3 mm, e ¼2.4 accumulated deformation). The microstructural observations allowed us to determine the average grain diameter deq using an image analysis. The average subgrain diameter after e ¼2.4 was 305 nm in the /110S crystal and 295 nm in the /111S crystal. The microstructure observations also included the determination of the grain misorientations in the deformed material by the EBSD method. Two types of maps were obtained: the distribution of misorientations of the grain boundaries and the distribution of the orientations of the subgrains. Fig. 5 shows the maps of the grain orientation distribution in the two crystals (/111S and /110S) after the second step of deformation (er ¼2.4). EBSD maps reveal that, in both cases, the misorientation angles above 151 are in majority. The local texture was examined by the same method and the results indicate that the orientations in the investigated area are greatly varied. After HE with e ¼2.4, the subgrains in the /111S crystal are mostly oriented along the /111S direction whereas those in the /110S crystal have two main orientations—/111S and /001S. The global texture was measured by the X-ray diffraction method. The pole figures indicate that, in both samples, the texture is strong. In the /110S oriented sample we have two main texture components /111S and /100S, so that the original texture of the initial single crystal was strongly transformed during the hydrostatic extrusion. In the /111S oriented sample the strongest texture component is /111S which indicates that the HE process yields a substructure with the original orientation. This was confirmed by the quantitative analysis of the texture. Fig. 6 shows the pole figures {111} obtained experimentally for the two extruded materials and Table 1 gives the results of the quantitative texture analysis.
Fig. 3. Microstructure of the single crystals: (a) /110S, and (b) /111S after HE, er ¼ 1.
94
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
Fig. 4. Microstructure of the single crystals: (a) /110S, and (b) /111S after HE, er ¼ 2.4.
Fig. 5. Distribution of grain orientations in the crystals: (a) /110S and (b) /111S, er ¼2.4.
Fig. 6. {111} pole figures determined experimentally for /110S (a) and /111S (b) Ni crystal deformed to e ¼2.4.
Table 1 Results of quantitative texture measurements in 110 and 111 Ni crystals after e ¼ 2.4. Crystal orientation
/110S /111S
Texture components [%] /111S
/100S
/235S
Random
57.3 94.2
28.0 –
6.8 –
Rest Rest
4. Discussion The earliest papers describing the texture (examined by X-ray diffraction method) in conventionally extruded rods were
published in the mid-20th century by Hill [8] and Hosford and Backofen [9]. The authors found that, in the case of a non-textured starting material or a material with fibrous texture, deformation of a cylinder with a large diameter into a cylindrical rod with a much smaller diameter leads to the formation of texture which varies depending on the distance from the sample axis. The relationship between the texture and the distance from the rod head (the end at which the extrusion begins) can also be attributed to the possible partial recrystallization due to the increase of the temperature during extrusion. The crystallographic texture of rods also depends on the deformation method employed. The texture components strongly affect the mechanical properties of the material, such as the yield stress, ultimate strength, etc. In rods in which the dominant component is /111S the values of these parameters are doubled compared to
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
those measured in the material with the dominant component /001S. The changes of plasticity go in the opposite direction [10]. Most investigators who examined texture in aluminum and nickel found that the /111S component predominated. In copper and silver, which crystallize in the same system as Al or Ag, the results are different. The components /111S and /100S are partially superimposed on the initial texture which remains unchanged within a wide range of plastic deformation. To summarize the results one can conclude that it is necessary to take into account the initial texture, the deformation conditions and the possibility of recrystallization during the deformation process. The above examinations have allowed us to compare the microstructure, orientation and microhardness of the /110S and /111S Ni single crystals subjected to severe plastic deformation by hydroextrusion. It was also possible to compare these results with the results obtained for polycrystalline Ni subjected to the same process of severe deformation. The microstructure and properties of hydroextruded polycrystalline Ni are well known and described [6,7]. The authors of these papers report that the greatest increase of the microhardness is achieved after the first step of plastic deformation during which the reduction is the greatest (in their experiments the maximum microhardness values were obtained at er ¼3 HV0.2 whereas with a further increase of er the HV0.2 values increased only slightly). They also observed that the spread of the microhardness values measured on a sample cross-section is the largest at medium deformation values (er ¼2.4) and decreases when the deformation is increased. Another observation of these authors was that above er ¼2.4 the microstructure begins to become ordered [6]. In our experiments the results of microhardness measurements indicate that the single crystal /111S is harder than the /110S crystal, which is in good agreement with the theory of slip deformation in single crystals. Also the microstructure analysis is in good agreement with conclusions given by Kulczyk [7]. The grains after er ¼2.4 seem to be much more ordered with less defects in the form of dislocation. The analysis of the dislocations density is of course necessary and this topic is in the field of interest in near future paper. But determination of average grain size revealed that average grain diameter after er ¼1 is about 500 nm while after er ¼2.4 it decreases to about 300 nm. It is also noticeable that after er ¼1 the microstructure consists of elongated grains (in the direction of extrusion) and after next step of deformation this banding type structure is rebuilt in the equiaxed microstructure without any noticeable morphological texture. A similar tendency is noticeable for both orientations /111S and /110S. Our results are also in agreement with the results reported by Reed and McHargue [11]. Table 2 gives the percent proportions of the /111S and /001S texture components in extrusiondeformed poly- and monocrystals of an FCC metal with the initial orientation /001S and their percent proportion in a polycrystal with a random orientation. TEM observations revealed that the microstructure contains a dislocation substructure. The average grain size in the deformed /111S and /110S crystals was about 300 nm, which leads to the conclusion that the orientation has not much influence on the final grain refinement. The EBSD investigations have shown that the microstructure produced by the HE SPD process mostly contains high angle
95
Table 2 Texture components in extruded FCC poly- and monocrystals [11]. Initial orientation
Polycrystal /001S Monocrystal /001S Random
Volume fraction [%] /111S
/001S
71 70 77
17 15 18
boundaries. As a result of severe plastic deformation the accumulated dislocations form configurations with high-angle boundaries which further evolve to become homogenous high angle boundaries [12]. The emerging dislocations can be divided into two groups—the boundaries which are geometrically necessary (most often in the form of shear bands) and boundaries built of dislocation (random boundaries which form agglomerates between the previous ones). The increase of deformation results in an increase of the surface fraction of the dislocation boundaries, which greatly improves the material properties such as the yield stress. The global texture measurements confirm the EBSD observations of the grain orientations. The /111S crystal contains above 90% of the /111S texture component, which means that the original orientation of the single crystal has been saved. The /110S crystal also contains the /111S component (about 58%), but in addition it contains 28% of the /100S component— we can therefore see that its original /110S orientation has been transformed during the SPD process.
5. Conclusions The SPD process realized by hydrostatic extrusion (HE, e ¼ 2.4) applied to the Ni /111S and /110S single crystals yielded a strongly refined microstructures—the average subgrain diameter in both the single crystals was about 300 nm with a majority of high angle boundaries. After the hydrostatic extrusion, the /111S single crystal preserved its original orientation, whereas the orientation /110S of the other single crystal has been transformed into two principal orientations /111S and /100S.
References + [1] A´. Czira´ki, I. Gerocs, E. To´th-Ka´da´r, I. Bakonyi, Nanostruct. Mater. 6 (5–8) (1995) 547. [2] A.R. Kilmametov, A.V. Khristoforov, G. Wilde, R.Z. Valiev, Z. Kristallogr 26 (2007) 339. [3] C. Gu, J. Lian, Z. Jiang, Q. Jiang, Scr. Mater. 54 (4) (2006) 579. [4] B. Yang, H. Vehoff, A. Hohenwarter, M. Hafok, R. Pippan, Scr. Mater. 58 (2008) 790. [5] W. Han, S. Li, S. Wu, Z. Zhang, Mater. Sci. Forum 633–634 (2010) 511. [6] M. Kulczyk, W. Pachla, A. Mazur, R. Diduszko, H. Garbacz, M. Lewandowska, W. Łojkowski, K.J. Kurzyd"owski, Mater. Sci. 23 (2005)839 23 (2005). [7] M. Kulczyk, Application of Hydrostatic Extrusion a Method of Microstructure Refinement of Pure Ni, Ph.D. Thesis, Warsaw University of Technology, Materials Science and Engineering Faculty, 2007. [8] R. Hill, Proc. R. Soc., 193A, 1948, 281. [9] W.F. Hosford, W.A. Backofen, Fundamentals of Deformation Processing, 1964. [10] D. Lee, W.A. Backofen, Trans. Metall. Soc. AIME 236 (1966) 1077. [11] R.E. Reed, C.J. McHargue, Trans. Metall. Soc. AIME 242 (1988) 180. [12] Y.T. Zhu, T.G. Langdon, J. Microsc 56 (2004) 58.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructural analysis of the /111S and /110S nickel single crystals subjected to severe plastic deformation by hydroextrusion J. Zdunek a,n, J. Mizera a,b, K. Wa˛sik a, K.J. Kurzyd"owski a a b
Warsaw University of Technology, Materials Science and Engineering Faculty, Woloska 141, 02-507 Warsaw, Poland Functional Materials Research Centre, Warsaw University of Technology, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2012 Received in revised form 24 October 2012 Accepted 26 November 2012 Available online 22 December 2012
The effect of hydrostatic extrusion (HE) on the microstructure and crystalline orientation of the /111S and /110S nickel single crystals was examined. The crystals were deformed by two-step hydrostatic extrusion to achieve the true strain er ¼ 2.4. After the extrusion the samples had the form of cylindrical rods. The mechanical properties of the extruded samples (expressed in terms of their microhardness) were compared with the mechanical properties of ultra-fine grained nickel obtained by subjecting polycrystalline nickel to HE. The microhardness of the two deformed crystals appeared to be similar. The microstructure of the samples and the orientation evolution were examined using transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and X-Ray diffraction (XRD). Both the deformed crystals had an inhomogeneous ultra-fine-grained structure (as observed by TEM). The average grain diameter was 300 nm. The majority of the grain boundaries had high angle disorientations (EBSD). In both the deformed samples the predominating orientation was /111S (XRD). In the /111S oriented crystal 95% of the initial orientation was preserved whereas in the /110S oriented crystal the initial orientation was predominantly transformed into /111S and /100S. & 2012 Elsevier B.V. All rights reserved.
Keywords: EBSD X-ray diffraction Bulk deformation Orientation
1. Introduction Pure nickel is very rarely used as a structural material because of its high cost per unit. If however its structure is strongly refined, it exhibits good mechanical properties and quite good corrosion resistance. Thanks to these features nickel is an interesting material for the manufacture of the components of microelectromechanical systems. At present, ultrafine-grained and nanocrystalline nickel is usually produced by the two methods: severe plastic deformation (SPD) [1,2], and electro-deposition [1,3], or, recently, a combination of these two methods [4]. The crucial problem in the investigations of materials subjected to severe plastic deformation is to understand the cold deformation mechanisms active under high stresses. This deformation results in material consolidation and, in consequence, in an increase of its hardness and strength but a decrease of its ductility. These changes are due to the microstructural processes that proceed within the material. The main goal in the development of the SPD
n
Corresponding author. Tel.: þ48 22 234 8556; fax: þ 48 22 848 48 75. E-mail addresses: [email protected] (J. Zdunek), [email protected] (J. Mizera), [email protected] (K. Wa˛sik), [email protected] (K.J. Kurzyd"owski). 0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.11.093
methods is to maximize the strength parameters of the material while preserving its good plastic properties. Among the available SPD methods the present authors have chosen hydrostatic extrusion (HE). The literature reports on the use of the SPD methods to deform single crystals of metals are very scarce (we can mention only ref. [5]). The present paper is concerned with the evolution of the microstructure of Ni monocrystals during plastic deformation realized by hydrostatic extrusion. The samples examined were two monocrystals extruded along their orientations (/110S and /111S). This enabled us to evaluate how the microstructure which begins to form and the changes of the orientation of the monocrystals which occur during severe plastic deformation depend on their initial orientation. The results have thrown some light on the processes which proceed during the production of ultra-fine-grained materials by SPD methods.
2. Material and experimental work Two nickel single crystals of orientation /111S and /110S were examined. The samples were cylindrical in shape and were cut out along the main crystallographic axes /111S and /110S of the monocrystals using a spark erosion saw. Fig. 1 shows schematically the two orientations of the samples.
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
The HE process was carried out in two steps (from F10 mm to F6 mm and then to F3 mm) so as to be able to observe better the evolution of the microstructure and changes of the mechanical properties. The total deformation value was 2.4. Changes of the mechanical properties were observed by measuring the microhardness on cross-sections of the initial single crystals and of each sample at each stage of deformation using the Vickers method. The microhardness was measured in a ZWICK microhardness tester equipped with a microscope which permitted measuring precisely the indentation diagonal. After the severe plastic deformation (SPD), the microstructure was examined in thin films of the material (specially prepared for transmission electron microscopy (TEM)) using a Hitachi S5500 scanning electron microscope/scanning–transmission electron microscope (SEM/STEM) and the local texture was determined by the electron backscatter diffraction (EBSD) measurement (Hitachi SU70 SEM). The global texture of the materials after SPD was analyzed (qualitatively and quantitatively) by the X-ray diffraction method
Fig. 1. Schematic representation of the sample orientation /111S (a) and /110S (b).
Fig. 2. Comparison of the microhardness of polycrystalline Ni and the two single crystals /111S and /110S in the initial state and after HE.
93
using a Bruker D8 DISCOVER powder diffractometer equipped with a Cr X-ray source.
3. Results The HV 0.2 microhardness of the two single crystals /110S and /111S was measured on their cross-sections in the initial state and after the hydrostatic extrusion Fig. 2 compares the microhardness of polycrystalline Ni [6,7] and of the two single crystals /111S and /110S in both the initial state and after HE. Microstructure of the single crystals after the first deformation step contains strongly developed dislocation substructures. The large number of dislocation tangles is typical of plastically deformed materials. The dislocations are agglomerated within the grains and form subgrains defined by the dislocation boundaries. Fig. 3 shows the microstructure after the first step of HE (F10 mm-F6 mm, e ¼1) and Fig. 4—this microstructure after the second deformation step (F6 mm-F3 mm, e ¼2.4 accumulated deformation). The microstructural observations allowed us to determine the average grain diameter deq using an image analysis. The average subgrain diameter after e ¼2.4 was 305 nm in the /110S crystal and 295 nm in the /111S crystal. The microstructure observations also included the determination of the grain misorientations in the deformed material by the EBSD method. Two types of maps were obtained: the distribution of misorientations of the grain boundaries and the distribution of the orientations of the subgrains. Fig. 5 shows the maps of the grain orientation distribution in the two crystals (/111S and /110S) after the second step of deformation (er ¼2.4). EBSD maps reveal that, in both cases, the misorientation angles above 151 are in majority. The local texture was examined by the same method and the results indicate that the orientations in the investigated area are greatly varied. After HE with e ¼2.4, the subgrains in the /111S crystal are mostly oriented along the /111S direction whereas those in the /110S crystal have two main orientations—/111S and /001S. The global texture was measured by the X-ray diffraction method. The pole figures indicate that, in both samples, the texture is strong. In the /110S oriented sample we have two main texture components /111S and /100S, so that the original texture of the initial single crystal was strongly transformed during the hydrostatic extrusion. In the /111S oriented sample the strongest texture component is /111S which indicates that the HE process yields a substructure with the original orientation. This was confirmed by the quantitative analysis of the texture. Fig. 6 shows the pole figures {111} obtained experimentally for the two extruded materials and Table 1 gives the results of the quantitative texture analysis.
Fig. 3. Microstructure of the single crystals: (a) /110S, and (b) /111S after HE, er ¼ 1.
94
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
Fig. 4. Microstructure of the single crystals: (a) /110S, and (b) /111S after HE, er ¼ 2.4.
Fig. 5. Distribution of grain orientations in the crystals: (a) /110S and (b) /111S, er ¼2.4.
Fig. 6. {111} pole figures determined experimentally for /110S (a) and /111S (b) Ni crystal deformed to e ¼2.4.
Table 1 Results of quantitative texture measurements in 110 and 111 Ni crystals after e ¼ 2.4. Crystal orientation
/110S /111S
Texture components [%] /111S
/100S
/235S
Random
57.3 94.2
28.0 –
6.8 –
Rest Rest
4. Discussion The earliest papers describing the texture (examined by X-ray diffraction method) in conventionally extruded rods were
published in the mid-20th century by Hill [8] and Hosford and Backofen [9]. The authors found that, in the case of a non-textured starting material or a material with fibrous texture, deformation of a cylinder with a large diameter into a cylindrical rod with a much smaller diameter leads to the formation of texture which varies depending on the distance from the sample axis. The relationship between the texture and the distance from the rod head (the end at which the extrusion begins) can also be attributed to the possible partial recrystallization due to the increase of the temperature during extrusion. The crystallographic texture of rods also depends on the deformation method employed. The texture components strongly affect the mechanical properties of the material, such as the yield stress, ultimate strength, etc. In rods in which the dominant component is /111S the values of these parameters are doubled compared to
J. Zdunek et al. / Materials Science & Engineering A 569 (2013) 92–95
those measured in the material with the dominant component /001S. The changes of plasticity go in the opposite direction [10]. Most investigators who examined texture in aluminum and nickel found that the /111S component predominated. In copper and silver, which crystallize in the same system as Al or Ag, the results are different. The components /111S and /100S are partially superimposed on the initial texture which remains unchanged within a wide range of plastic deformation. To summarize the results one can conclude that it is necessary to take into account the initial texture, the deformation conditions and the possibility of recrystallization during the deformation process. The above examinations have allowed us to compare the microstructure, orientation and microhardness of the /110S and /111S Ni single crystals subjected to severe plastic deformation by hydroextrusion. It was also possible to compare these results with the results obtained for polycrystalline Ni subjected to the same process of severe deformation. The microstructure and properties of hydroextruded polycrystalline Ni are well known and described [6,7]. The authors of these papers report that the greatest increase of the microhardness is achieved after the first step of plastic deformation during which the reduction is the greatest (in their experiments the maximum microhardness values were obtained at er ¼3 HV0.2 whereas with a further increase of er the HV0.2 values increased only slightly). They also observed that the spread of the microhardness values measured on a sample cross-section is the largest at medium deformation values (er ¼2.4) and decreases when the deformation is increased. Another observation of these authors was that above er ¼2.4 the microstructure begins to become ordered [6]. In our experiments the results of microhardness measurements indicate that the single crystal /111S is harder than the /110S crystal, which is in good agreement with the theory of slip deformation in single crystals. Also the microstructure analysis is in good agreement with conclusions given by Kulczyk [7]. The grains after er ¼2.4 seem to be much more ordered with less defects in the form of dislocation. The analysis of the dislocations density is of course necessary and this topic is in the field of interest in near future paper. But determination of average grain size revealed that average grain diameter after er ¼1 is about 500 nm while after er ¼2.4 it decreases to about 300 nm. It is also noticeable that after er ¼1 the microstructure consists of elongated grains (in the direction of extrusion) and after next step of deformation this banding type structure is rebuilt in the equiaxed microstructure without any noticeable morphological texture. A similar tendency is noticeable for both orientations /111S and /110S. Our results are also in agreement with the results reported by Reed and McHargue [11]. Table 2 gives the percent proportions of the /111S and /001S texture components in extrusiondeformed poly- and monocrystals of an FCC metal with the initial orientation /001S and their percent proportion in a polycrystal with a random orientation. TEM observations revealed that the microstructure contains a dislocation substructure. The average grain size in the deformed /111S and /110S crystals was about 300 nm, which leads to the conclusion that the orientation has not much influence on the final grain refinement. The EBSD investigations have shown that the microstructure produced by the HE SPD process mostly contains high angle
95
Table 2 Texture components in extruded FCC poly- and monocrystals [11]. Initial orientation
Polycrystal /001S Monocrystal /001S Random
Volume fraction [%] /111S
/001S
71 70 77
17 15 18
boundaries. As a result of severe plastic deformation the accumulated dislocations form configurations with high-angle boundaries which further evolve to become homogenous high angle boundaries [12]. The emerging dislocations can be divided into two groups—the boundaries which are geometrically necessary (most often in the form of shear bands) and boundaries built of dislocation (random boundaries which form agglomerates between the previous ones). The increase of deformation results in an increase of the surface fraction of the dislocation boundaries, which greatly improves the material properties such as the yield stress. The global texture measurements confirm the EBSD observations of the grain orientations. The /111S crystal contains above 90% of the /111S texture component, which means that the original orientation of the single crystal has been saved. The /110S crystal also contains the /111S component (about 58%), but in addition it contains 28% of the /100S component— we can therefore see that its original /110S orientation has been transformed during the SPD process.
5. Conclusions The SPD process realized by hydrostatic extrusion (HE, e ¼ 2.4) applied to the Ni /111S and /110S single crystals yielded a strongly refined microstructures—the average subgrain diameter in both the single crystals was about 300 nm with a majority of high angle boundaries. After the hydrostatic extrusion, the /111S single crystal preserved its original orientation, whereas the orientation /110S of the other single crystal has been transformed into two principal orientations /111S and /100S.
References + [1] A´. Czira´ki, I. Gerocs, E. To´th-Ka´da´r, I. Bakonyi, Nanostruct. Mater. 6 (5–8) (1995) 547. [2] A.R. Kilmametov, A.V. Khristoforov, G. Wilde, R.Z. Valiev, Z. Kristallogr 26 (2007) 339. [3] C. Gu, J. Lian, Z. Jiang, Q. Jiang, Scr. Mater. 54 (4) (2006) 579. [4] B. Yang, H. Vehoff, A. Hohenwarter, M. Hafok, R. Pippan, Scr. Mater. 58 (2008) 790. [5] W. Han, S. Li, S. Wu, Z. Zhang, Mater. Sci. Forum 633–634 (2010) 511. [6] M. Kulczyk, W. Pachla, A. Mazur, R. Diduszko, H. Garbacz, M. Lewandowska, W. Łojkowski, K.J. Kurzyd"owski, Mater. Sci. 23 (2005)839 23 (2005). [7] M. Kulczyk, Application of Hydrostatic Extrusion a Method of Microstructure Refinement of Pure Ni, Ph.D. Thesis, Warsaw University of Technology, Materials Science and Engineering Faculty, 2007. [8] R. Hill, Proc. R. Soc., 193A, 1948, 281. [9] W.F. Hosford, W.A. Backofen, Fundamentals of Deformation Processing, 1964. [10] D. Lee, W.A. Backofen, Trans. Metall. Soc. AIME 236 (1966) 1077. [11] R.E. Reed, C.J. McHargue, Trans. Metall. Soc. AIME 242 (1988) 180. [12] Y.T. Zhu, T.G. Langdon, J. Microsc 56 (2004) 58.