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Structural refinement of chromium by severe plastic deformation
Fusion Engineering and Design 66 /68 (2003) 265 /269 www.elsevier.com/locate/fusengdes
Structural refinement of chromium by severe plastic deformation R. Wadsack a, R. Pippan a,*, B. Schedler b a
Austrian Academy of Sciences, Erich Schmid Institute of Materials Science, Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, Jahnstrasse 12, 8700 Leoben, Austria b Plansee AG, 6600 Reutte, Austria
Abstract In the present study, pure chromium (99.97%) has been deformed by high pressure torsion (HPT). The deformation has been performed clearly above the ductile to brittle transition temperature (320 /390 8C) of the material in its recrystallized condition and at room temperature. Specimens with different degrees of deformation have been produced. A reduction of grain size from :/80 mm to the submicrometer region with typical structural sizes between 50 and :/500 nm has been found by examinations in the scanning electron microscope (SEM) by means of back scattered electrons (BSE) images and by electron back scatter diffraction (EBSD) measurements. The thermal stability of the microstructures has been investigated after annealing the samples in vacuum for 10 h at 500, 600 and 700 8C. Furthermore, micro hardness measurements have been carried out. The hardness after an applied degree of deformation of 25.8 (von Mises) at room temperature of the deformed sample has been about four times higher than the hardness of the undeformed material. After the heat treatment at 700 8C, the hardness remains about two times higher. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Deformation; Thermal stability
1. Introduction Since chromium is superior to most materials with regard to the low neutron-induced radioactivity, it is considered as a suitable material for fusion technology. Limitations of the application in industrial design are the low ductility at room temperature and a ductile to brittle transition
* Corresponding author. Tel.: /43-3842-804-205; fax: /433842-804-116. E-mail address: [email protected] (R. Pippan).
temperature (DBTT) which lies significantly above room temperature. In the last few years, intense research has started to produce nanostructured materials by severe plastic deformation (SPD). Compared with the undeformed materials, these materials with grain sizes clearly smaller than 1 mm are distinguished by an increase in strength without loosing ductility. In the present study, unirradiated chromium with a purity of 99.97% (DUCROPUR) has been deformed by high pressure torsion (HPT). The aim of the study is to investigate the structural refinement of chromium, the possible improvement in
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00136-4
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ductility and the thermal stability of such microstructures. This paper is mainly focused on the structural refinement and the thermal stability. The chemical composition and the mechanical characteristics of the undeformed chromium are given in Ref. [1].
2. HPT of DUCROPUR HPT is used for the fabrication of disk samples with a nanostructured microstructure. The undeformed sample is held between two anvils which are pressed together. The lower anvil rotates and due to the friction between the anvils and the sample it is strained in torsion. A schematic illustration of the HPT process is depicted in Fig. 1. For the deformation process it is very important to avoid slipping between the sample and the anvils. This has been ensured by sandblasting both the sample and the anvils before rotating. In this study, the deformation by HPT has been performed above the DBTT (between 320 and 390 8C) of DUCROPUR in the undeformed condition. To investigate the influence of temperature on the development of microstructure, the same degrees of deformation have been applied at
Fig. 1. Schematic representation of the HPT process.
room temperature. Specimens with a half and two revolutions have been produced under an applied stress of 7.8 GPa.
3. Developed microstructures during HPT The microstructure of samples which have been deformed by SPD have been investigated in the SEM by means of different techniques. Micrographs have been taken with a detector for back scattered electrons (BSE). With a BSE image it is possible to determine the typical sizes of the microstructure. In a single phase material, the intensity of the back scattered electrons depends on the crystallographic orientation of the scanned area and the dislocation density. It is not possible to identify the degree of misorientation between the grains from such micrographs. To distinguish if the boundaries are large or low angle boundaries, the degree of misorientation has been determined with the electron back scatter diffraction (EBSD) method [2 /4]. BSE micrographs of the developed microstructures during HPT are depicted in Fig. 2. A reduction of the size of the structural elements from :/80 mm to below 1 mm is observed. In all microstructures, an irregular distribution of the size of the structural elements has been found. All microstructures contain some larger grains and between the larger grains, regions with smaller structural elements are observed. The final size of the microstructure depends on the degree of deformation and the deformation temperature. It can be observed in Fig. 2 that the larger the degree of deformation and the lower the deformation temperature, the finer the developed microstructure. The finest microstructure (sample G) has been developed at room temperature at a strain of 25.8. The largest grains are in the range of 500 nm, the finest structural elements have sizes below 100 nm. To identify if the boundaries are large or low angle boundaries, the degree of misorientation has been determined with the EBSD method. The samples which have been deformed to a strain of 5.55 already show boundaries with a misorientation larger than 208, but still many boundaries
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267
Fig. 2. BSE micrographs of the microstructures developed during HPT (p/7.8 GPa).
with smaller misorientations are observed. Sample E, which has been deformed to a strain of 25.8, shows much more large angle boundaries than small angle boundaries. An orientation analysis of sample G failed because the structural sizes in the fine grained regions are smaller than the resolution of the EBSD system used ( :/50 nm). The development of microstructure during SPD is summarised in Fig. 3. At small strains, a steep decrease of structural size with increasing strain takes place during deformation. After a certain strain, a saturation of the structural size is obtained. At larger strains, the size of the micro-
Fig. 3. Dependence of developed grain size during SPD on degree of deformation.
structure decreases only slightly, but the misorientation between neighbouring structural elements still increases. The saturation structural
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size depends on the temperature. The lower the temperature the finer the minimum structural size which is obtained. Beside the investigations in the SEM, micro hardness measurements have been performed. The obtained results are summarised in Fig. 4. The measured hardnesses are related to the hardness of the undeformed material. The finer the developed microstructure, the larger the measured micro hardness.
4. Thermal stability of HPT deformed samples The deformed samples have been annealed in vacuum for 10 h at 500, 600 and 700 8C. After the heat treatment at 500 8C, a coarsening of the microstructure has set only in the fine structured regions. The fraction of ‘recrystallized’ microstructure can be observed by the amount of the change of micro hardness. Since in sample G the coarsening processes are very pronounced (many fine structured regions), here the largest decrease of micro hardness is found. In sample B (almost no fine structured regions) the inverse behaviour is observed. During the heat treatments at 600 and 700 8C, different structural coarsening behaviours are observed. The ‘recrystallization’ can start in triple points of grain boundaries or very small structural elements with large misorientations or by bulging
of boundaries in the direction of subgrains (grain growth). Samples with a lower number of boundaries with large misorientations (sampled deformed to a strain of 5.55) have a lower number of nucleuses and hence tend to a non-uniform ‘recrystallization’. In these samples, some large grains have developed, surrounded by small grains (see Fig. 5a). In samples where boundaries with large misorientations prevail (samples deformed to a strain of 25.8), a very uniform ‘recrystallization’ is observed (see Fig. 5b). These samples show finer microstructures and larger micro hardness after the heat treatments. After the heat treatments at 700 8C, the micro hardness is still about two times higher than the hardness of the undeformed samples and compared with the original grain size (80 mm), we still find a fine grained microstructure with sizes below 5 mm.
5. Summary Pure chromium with microstructures smaller than 1 mm have been produced by HPT. The larger the strain and the lower the deformation temperature, the finer the developed microstructure. After heat treatments at 500 8C only the very fine structured regions have coarsened. The coarsening behaviour at 600 and 700 8C depends on the number of boundaries with large misorienta-
Fig. 4. Micro hardness after deformation and after different heat treatments.
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269
Fig. 5. BSE micrographs of the microstructures after heat treatment at 700 8C of (a) sample B and (b) sample G.
tions and hence on the applied degree of deformation.
Acknowledgements This work has been carried out within Associa¨ AW and is supported by the tion EURATOM-O Bundesministerium fu¨r Wissenschaft, Forschung und Kunst.
References [1] R. Wadsack, R. Pippan, B. Schedler, Chromium */a material for fusion technology, Fusion Engineering and Design 58 /59 (2001) 743 /748. [2] C. Semprimoschnig, Die kristallographische Fraktometrie */Entwicklung einer Methode zur quantitativen Analyse von Spaltbruchfla¨chen, Doctor thesis, Institute of Metal Physics, University of Leoben, 1996. [3] A. Tatschl, Neue experimentelle Methoden zur Charakterisierung von Verformungsvorga¨ngen, Doctor thesis, Institute of Metal Physics, University of Leoben, 2000. [4] T. Hebesberger, Entwicklung der Mikrostruktur bei Hochverformung kubisch fla¨chenzentrierter Metalle, Doctor thesis, Institute of Metal Physics, University of Leoben, 2001.
Structural refinement of chromium by severe plastic deformation R. Wadsack a, R. Pippan a,*, B. Schedler b a
Austrian Academy of Sciences, Erich Schmid Institute of Materials Science, Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, Jahnstrasse 12, 8700 Leoben, Austria b Plansee AG, 6600 Reutte, Austria
Abstract In the present study, pure chromium (99.97%) has been deformed by high pressure torsion (HPT). The deformation has been performed clearly above the ductile to brittle transition temperature (320 /390 8C) of the material in its recrystallized condition and at room temperature. Specimens with different degrees of deformation have been produced. A reduction of grain size from :/80 mm to the submicrometer region with typical structural sizes between 50 and :/500 nm has been found by examinations in the scanning electron microscope (SEM) by means of back scattered electrons (BSE) images and by electron back scatter diffraction (EBSD) measurements. The thermal stability of the microstructures has been investigated after annealing the samples in vacuum for 10 h at 500, 600 and 700 8C. Furthermore, micro hardness measurements have been carried out. The hardness after an applied degree of deformation of 25.8 (von Mises) at room temperature of the deformed sample has been about four times higher than the hardness of the undeformed material. After the heat treatment at 700 8C, the hardness remains about two times higher. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Chromium; Deformation; Thermal stability
1. Introduction Since chromium is superior to most materials with regard to the low neutron-induced radioactivity, it is considered as a suitable material for fusion technology. Limitations of the application in industrial design are the low ductility at room temperature and a ductile to brittle transition
* Corresponding author. Tel.: /43-3842-804-205; fax: /433842-804-116. E-mail address: [email protected] (R. Pippan).
temperature (DBTT) which lies significantly above room temperature. In the last few years, intense research has started to produce nanostructured materials by severe plastic deformation (SPD). Compared with the undeformed materials, these materials with grain sizes clearly smaller than 1 mm are distinguished by an increase in strength without loosing ductility. In the present study, unirradiated chromium with a purity of 99.97% (DUCROPUR) has been deformed by high pressure torsion (HPT). The aim of the study is to investigate the structural refinement of chromium, the possible improvement in
0920-3796/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0920-3796(03)00136-4
266
R. Wadsack et al. / Fusion Engineering and Design 66 /68 (2003) 265 /269
ductility and the thermal stability of such microstructures. This paper is mainly focused on the structural refinement and the thermal stability. The chemical composition and the mechanical characteristics of the undeformed chromium are given in Ref. [1].
2. HPT of DUCROPUR HPT is used for the fabrication of disk samples with a nanostructured microstructure. The undeformed sample is held between two anvils which are pressed together. The lower anvil rotates and due to the friction between the anvils and the sample it is strained in torsion. A schematic illustration of the HPT process is depicted in Fig. 1. For the deformation process it is very important to avoid slipping between the sample and the anvils. This has been ensured by sandblasting both the sample and the anvils before rotating. In this study, the deformation by HPT has been performed above the DBTT (between 320 and 390 8C) of DUCROPUR in the undeformed condition. To investigate the influence of temperature on the development of microstructure, the same degrees of deformation have been applied at
Fig. 1. Schematic representation of the HPT process.
room temperature. Specimens with a half and two revolutions have been produced under an applied stress of 7.8 GPa.
3. Developed microstructures during HPT The microstructure of samples which have been deformed by SPD have been investigated in the SEM by means of different techniques. Micrographs have been taken with a detector for back scattered electrons (BSE). With a BSE image it is possible to determine the typical sizes of the microstructure. In a single phase material, the intensity of the back scattered electrons depends on the crystallographic orientation of the scanned area and the dislocation density. It is not possible to identify the degree of misorientation between the grains from such micrographs. To distinguish if the boundaries are large or low angle boundaries, the degree of misorientation has been determined with the electron back scatter diffraction (EBSD) method [2 /4]. BSE micrographs of the developed microstructures during HPT are depicted in Fig. 2. A reduction of the size of the structural elements from :/80 mm to below 1 mm is observed. In all microstructures, an irregular distribution of the size of the structural elements has been found. All microstructures contain some larger grains and between the larger grains, regions with smaller structural elements are observed. The final size of the microstructure depends on the degree of deformation and the deformation temperature. It can be observed in Fig. 2 that the larger the degree of deformation and the lower the deformation temperature, the finer the developed microstructure. The finest microstructure (sample G) has been developed at room temperature at a strain of 25.8. The largest grains are in the range of 500 nm, the finest structural elements have sizes below 100 nm. To identify if the boundaries are large or low angle boundaries, the degree of misorientation has been determined with the EBSD method. The samples which have been deformed to a strain of 5.55 already show boundaries with a misorientation larger than 208, but still many boundaries
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267
Fig. 2. BSE micrographs of the microstructures developed during HPT (p/7.8 GPa).
with smaller misorientations are observed. Sample E, which has been deformed to a strain of 25.8, shows much more large angle boundaries than small angle boundaries. An orientation analysis of sample G failed because the structural sizes in the fine grained regions are smaller than the resolution of the EBSD system used ( :/50 nm). The development of microstructure during SPD is summarised in Fig. 3. At small strains, a steep decrease of structural size with increasing strain takes place during deformation. After a certain strain, a saturation of the structural size is obtained. At larger strains, the size of the micro-
Fig. 3. Dependence of developed grain size during SPD on degree of deformation.
structure decreases only slightly, but the misorientation between neighbouring structural elements still increases. The saturation structural
268
R. Wadsack et al. / Fusion Engineering and Design 66 /68 (2003) 265 /269
size depends on the temperature. The lower the temperature the finer the minimum structural size which is obtained. Beside the investigations in the SEM, micro hardness measurements have been performed. The obtained results are summarised in Fig. 4. The measured hardnesses are related to the hardness of the undeformed material. The finer the developed microstructure, the larger the measured micro hardness.
4. Thermal stability of HPT deformed samples The deformed samples have been annealed in vacuum for 10 h at 500, 600 and 700 8C. After the heat treatment at 500 8C, a coarsening of the microstructure has set only in the fine structured regions. The fraction of ‘recrystallized’ microstructure can be observed by the amount of the change of micro hardness. Since in sample G the coarsening processes are very pronounced (many fine structured regions), here the largest decrease of micro hardness is found. In sample B (almost no fine structured regions) the inverse behaviour is observed. During the heat treatments at 600 and 700 8C, different structural coarsening behaviours are observed. The ‘recrystallization’ can start in triple points of grain boundaries or very small structural elements with large misorientations or by bulging
of boundaries in the direction of subgrains (grain growth). Samples with a lower number of boundaries with large misorientations (sampled deformed to a strain of 5.55) have a lower number of nucleuses and hence tend to a non-uniform ‘recrystallization’. In these samples, some large grains have developed, surrounded by small grains (see Fig. 5a). In samples where boundaries with large misorientations prevail (samples deformed to a strain of 25.8), a very uniform ‘recrystallization’ is observed (see Fig. 5b). These samples show finer microstructures and larger micro hardness after the heat treatments. After the heat treatments at 700 8C, the micro hardness is still about two times higher than the hardness of the undeformed samples and compared with the original grain size (80 mm), we still find a fine grained microstructure with sizes below 5 mm.
5. Summary Pure chromium with microstructures smaller than 1 mm have been produced by HPT. The larger the strain and the lower the deformation temperature, the finer the developed microstructure. After heat treatments at 500 8C only the very fine structured regions have coarsened. The coarsening behaviour at 600 and 700 8C depends on the number of boundaries with large misorienta-
Fig. 4. Micro hardness after deformation and after different heat treatments.
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269
Fig. 5. BSE micrographs of the microstructures after heat treatment at 700 8C of (a) sample B and (b) sample G.
tions and hence on the applied degree of deformation.
Acknowledgements This work has been carried out within Associa¨ AW and is supported by the tion EURATOM-O Bundesministerium fu¨r Wissenschaft, Forschung und Kunst.
References [1] R. Wadsack, R. Pippan, B. Schedler, Chromium */a material for fusion technology, Fusion Engineering and Design 58 /59 (2001) 743 /748. [2] C. Semprimoschnig, Die kristallographische Fraktometrie */Entwicklung einer Methode zur quantitativen Analyse von Spaltbruchfla¨chen, Doctor thesis, Institute of Metal Physics, University of Leoben, 1996. [3] A. Tatschl, Neue experimentelle Methoden zur Charakterisierung von Verformungsvorga¨ngen, Doctor thesis, Institute of Metal Physics, University of Leoben, 2000. [4] T. Hebesberger, Entwicklung der Mikrostruktur bei Hochverformung kubisch fla¨chenzentrierter Metalle, Doctor thesis, Institute of Metal Physics, University of Leoben, 2001.