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Irradiation damage in commercial purity magnesium
Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Irradiation damage in commercial purity magnesium A.K. Khan ⇑, Z. Yao, M.R. Daymond, R.A. Holt Department of Mechanical and Materials Engineering, Queen’s University, Nicol Hall, 60 Union Street, Kingston, Ontario, Canada K7L 3N6
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Irradiation Magnesium Dislocations TEM
a b s t r a c t In-situ electron irradiation damage within the transmission electron microscope has been studied and compared with ion irradiation on commercial purity magnesium. Magnesium has the same crystal structure (hcp) and a similar c/a ratio to that of zirconium and is considered a suitable model system and useful analogue to zirconium (an important structural material for nuclear reactors). The results presented in this paper suggest that microstructure developed during irradiation is modified by the nature of the irra 3i are formed by electron diating particles. The basal plane dislocation loops with Burgers vector 1=6h2 0 2 irradiation and analysis showed that all of these dislocation loops are interstitial in nature. Vacancy type 0i also form but to a lesser degree than the prism plane dislocation loops with Burgers vector 1=3h1 1 2 basal plane dislocation loops. Ion irradiation for a dose up to 0.1 dpa produces a majority of interstitial 0i. Void formation also takes place in type prism plane dislocation loops with Burgers vector 1=3h1 1 2 ion irradiated samples subjected to a higher dose of 0.7 dpa. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The effects of particle irradiation on the properties of materials have been studied for more than half a century. The semiconductor industry uses the beneficial effects of ion beam processing of semiconductors whereas in the nuclear industry irradiation effects are detrimental and result in the degradation of structural materials. The microscopic processes responsible for these effects are not completely understood limiting researchers ability to predict the consequences of irradiation. It is therefore important to analyze the microstructural development during irradiation to understand the various effects of irradiation damage. Materials with hcp crystal structure such as zirconium and its alloys are very important structural materials used in nuclear reactors. Magnesium has the same crystal structure and its c/a ratio (1.623) is also close to that of zirconium (1.593). Development of microstructures similar to zirconium is also expected in magnesium during irradiation therefore magnesium is considered a suitable model system to help understand irradiation damage development in zirconium. Magnesium is selected in this study because it can be easily irradiated in a conventional transmission electron microscope operating at 200 keV due to its low displacement energy (12 eV) and atomic mass (24.3 amu) as compared to zirconium (25 eV, 91.2 amu) [1,2]. The irradiation damage structure appears in the form of dislocation loops and voids and in-situ transmission electron micros-
⇑ Corresponding author. Tel.: +1 613 533 3236; fax: +1 613 533 6610. E-mail address: [email protected] (A.K. Khan). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.072
copy permits direct observation of the microstructural changes taking place during irradiation. It is believed that in hexagonal close packed structures evolution of microstructure can be correlated with the c/a ratio [3]. Nucleation of basal plane pffiffiffi loops takes place in metals with c/a ratio greater than 1.732 ð 3Þ and prism plane loops form in metals with c/a ratio less than 1.732. According to this rule the principal habit plane of dislocations in all hcp metals should be prismatic except in zinc and cadmium. There are many exceptions to this rule be 3i cause basal plane dislocations with burgers vector 1=6h2 0 2 were observed in magnesium [4,5] and zirconium [6,7] where the c/a ratio is less than 1.732. The formation of basal plane loops in metals other than zinc and cadmium has been attributed to the presence of impurities [8,9], cascade collapse [10] and stresses in thin foils [11]. In this investigation preliminary results of electron and ion irradiation on commercial purity magnesium are presented to clarify the effect of irradiating particles on microstructural evolution during irradiation.
2. Material and methods Commercial purity magnesium (99.8%) was used for this investigation. The major impurities present in this composition in wt. ppm are 370 Al, 100 Pb, 48 Cu and 40 Zn. For transmission electron microscopy 1 mm thick slices were cut and thinned to 100 lm thickness using a series of silicon carbide papers down to 1200 grit. Three millimeter discs punched from these slices were perforated in Struers TenuPol-3 using a solution of 6% perchloric acid and 94% methanol at 40 °C. The electron irradiation was carried out
232
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
using a CM 20 electron microscope operated at 200 keV. In-situ ion beam irradiation was carried out at IVEM-Tandem Facility at Argonne National Laboratory using 1 MeV Kr2+ ions. The electron and ion irradiation both were carried out at room temperature. The rise in temperature during irradiation is considered negligible due to the high thermal conductivity of magnesium and therefore dynamic annealing effects can be neglected during irradiation [12]. The damage profile of 1 MeV Kr2+ ions on magnesium calculated by SRIM 2008 is shown in Fig. 1. After irradiation, analysis of defect structures formed was carried out in TEM operating at 80 keV or 100 keV.
3. Experimental results 3.1. Electron irradiation The microstructure developed after electron irradiation of 0.1 dpa consisted of two types of dislocation loops i.e. hexagonal
shaped and the elliptical shaped loops. All the images were taken in strict two beam conditions and keeping the deviation parameter(s) positive. In the detailed analysis (not shown here) it was observed that these hexagonal loops show faulted contrast when 0 type diffraction vector and also remain in imaged with 1 0 1 strong contrast when imaged in g = (0 0 0 2). This contrast behavior suggests that hexagonal dislocation loops have Burgers vector 3i. These loops also demonstrate edge-on morof the type 1=6h2 0 2 0Þ when beam direction is close to ½1 2 1 0. The phology in g = ð1 0 1 trace analysis suggests that these loops are lying on the basal planes and hence will be called basal plane loops hereafter. Fig. 2 displays a series of micrographs taken in beam direction close to [0 0 0 1] to specify the Burgers vector of individual loops. 1 3. The foil orientation in this case is close to ½1 2 The g.b analysis is listed in Table 1and according to this 3i, ± 1=6h2 2 0 3i, and analysis hexagonal loops with b = ±1=6h0 2 2 3i are present in the microstructure. Consider loop A, ± 1=6h2 0 2 1 0Þ and therefore its b = ± it is out of contrast when g = ð2 1 3i. The method proposed by Kelly and Blake [13] was 1=6h0 2 2 used to identify the nature of the loops. The inside–outside contrast experiment shown in Fig. 3 suggest that these basal plane 1Þ and ð1 1 0 1Þ loops demonstrate outside contrast in g = ð0 1 1 3Þ. The beam direction is close therefore loop at A have b = 1=6ð0 2 2 1 3 and upward pointing loop plane normal n = [0 0 0 1] to ½1 2 gives the result that loop is interstitial in character. All the hexagonal loops demonstrate similar behavior therefore it can be concluded that all these loops are interstitial in nature. The elliptical loops were also characterized and g.b analysis shows that these loops have Burgers vector of the type 0i and were found to lie on prism planes. Consider the 1=3h1 1 2 elliptical loop at the bottom right in Fig. 3 at point B. This loop is 0 1 1Þ therefore b must be ±1=3h1 2 1 0i. out of contrast in g = ð1 The outside contrast is observed in g = ð0 1 1 1Þ and ð1 1 0 1Þ there 21 0i. According to the inside–outside contrast fore b = 1=3h1 method proposed by Kelly and Blake [13] in this safe orientation 1 3 this dislocation loop is vacancy in nature. Other ellipof B = ½1 2 1 1 0i and were tical loops analyzed were found to have b = 1=3h2 also vacancy in nature. 3.2. Ion irradiation
Fig. 1. Damage profile of 1 MeV Kr2+ ions on magnesium calculated by SRIM-2008. The displacement threshold energy of magnesium is taken as 12 eV.
The damage microstructure developed during ion irradiation up to 0.1 dpa on a different foil of the same composition as used in electron irradiation is shown Fig. 4. The foil orientation in this case was close to [0 0 0 1]. The Burgers vector analysis was also carried
3. (a) g = ð1 2 1 0Þ, (b) g = ð2 1 1 0Þ and (c) g = ð1 1 2 0Þ. Fig. 2. Contrast of the hexagonal dislocation loops when the beam direction is close to [0 0 0 1]. Foil orientation ½1 21 1 0Þ and therefore its b = ±1=6h0 2 2 3i. Elliptical dislocation loops are also present such as a loop at point B. For example dislocation loop at A is out of contrast in g = ð2 1
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235 Table 1 g.b analysis of the dislocation loops. Beam direction?
13 12
0001
1 23 1
b; g?
10 12
1 0 21
1 20 1
1 0 11
101 1
011 1
3i 1=6h2 0 2 2 0 3i 1=6h2 3i 1=6h0 2 2 1 1 0i 1=3h2 1 0i 1=3h1 2 0i 1=3h1 1 2
0 1 1 1 2 1
1 1 0 2 1 1
1 0 1 1 1 2
5/6 5/6 7/6 0 1 1
1/6 7/6 5/6 1 1 0
7/6 1/6 5/6 1 0 1
out, not shown here, and it demonstrated that all the dislocation 0i. The inloops are prism plane with Burgers vector 1=3h1 1 2 side–outside contrast experiments also suggested that majority of these dislocation loops are interstitial in character. Very few vacancy dislocation loops were also present in the microstructure, shown by symbol V in Fig. 4. Basal plane dislocation loops with 3i were not observed in the microstructure Burgers vector 1=6h2 0 2 up to irradiation of 0.1 dpa. Fig. 5 displays the damage microstructure in a prism foil. In the beginning, up to dose of 0.1 dpa, microstructure mainly consisted of prism dislocation loops with Burgers
233
0i as these dislocation loops are out of contrast vector 1=3h1 1 2 when g = (0 0 0 2). The irradiation experiment conducted on another prism foil suggested that as the irradiation dose is increased to 0.7 dpa, the basal plane dislocation loops and voids became dominant in the microstructure as shown in Fig. 6. These basal plane dislocation loops appear edge-on in prism foils. The prism type dislocation loops were not observed suggesting their instability as compared to the basal plane dislocation loops at higher doses. The voids present in the microstructure also appear to align themselves in rows parallel to the edge-on orientation of the basal plane dislocation loops suggesting that the voids are also aligned in rows parallel to the basal plane. Fig. 6 also displays a denuded zone about 100 nm in size along the grain boundary that is free of voids. 4. Discussion Prism plane dislocations are expected in magnesium due to the lower c/a ratio (1.623) than ideal because of higher packing density 0} planes. Hossain and Brown [1] have in fact reof prism {1 01 ported the formation of interstitial type prism plane dislocations in high purity (99.998%) magnesium after electron irradiation. The formation of basal plane dislocations after electron irradiation
1 3 and in (d) B = ½1 1 2 3. Diffraction vectors are shown on each Fig. 3. Inside–outside contrast of the dislocation loops formed during electron irradiation. In (a–c) B = ½1 2 1Þ and ð1 1 0 1Þ and is interstitial in nature. The elliptical loop at point B is out of contrast in g = ð1 0 1 1Þ micrograph. Hexagonal loop at A show outside contrast in g = ð0 1 1 21 0i. This loop shows outside contrast in g = ð0 1 1 1Þ and is vacancy in nature. have b = 1=3h1
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A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
3. (a) g = ð0 1 1 1Þ and (b) g = ð0 1 1 1Þ. Inside–outside contrast shows that most of the dislocation Fig. 4. Dislocations in a basal foil after ion irradiation up to 0.1 dpa. B ½1 1 2 loops are interstitial in nature. Few vacancy loops are also present and shown by the symbol V.
0. (a) g = ð1 0 1 1Þ, (b) g = (0 0 0 2). The loops present in g = ð1 0 1 1Þ becomes out of contrast when Fig. 5. Dislocations in a prism foil after ion irradiation of 0.1 dpa. B ½1 21 0i. The encircled edge-on loops are basal plane imaged in g = (0 0 0 2) except few encircled loops suggesting that these dislocations have a Burgers vector of the type 1=3h1 1 2 3i. loops with Burgers vector 1=6h2 0 2
Fig. 6. Formation of voids in a prism foil after ion irradiation of 0.7 dpa. The basal plane dislocation loops appear edge-on and the voids also appear to align parallel to the edge-on orientation of the basal plane dislocation loops. Void denuded zone also exists along the grain boundary.
on commercial purity (99.8%) magnesium in the present study suggests that evolution of microstructure is influenced by the presence of impurities. Griffiths [8] has also reported that increasing the concentration of impurities enhances the formation of basal plane defects in magnesium and zirconium. The possible effect of impurities can be to reduce the stacking fault energy resulting in the formation of faulted basal plane dislocations. Comparison of the electron and ion irradiation results suggests that interstitial basal loops are more prevalent in the microstructure during electron irradiation for about 0.1 dpa and interstitial prism plane loops are more dominant for the same dose during ion irradiation. The widespread nucleation of prism plane dislocation loops during ion irradiation in this low purity magnesium seems surprising and suggests that nature of irradiating particles influences the microstructure development. The point defects produced during electron irradiation are in the form of Frenkel pairs, whereas ion irradiation produces cascades and residual clusters of vacancies and interstitials as well as Frenkel pairs. Faulted basal plane dislocation loops were also observed in neutron irradiated zirconium but these were vacancy in nature [6,7]. The different nature of basal plane faults in these metals suggests that opposite diffusional anisotropy difference exists in magnesium compared with zirconium.
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
Formation of voids was not observed during electron irradiation up to 0.5 dpa because of the fact that most of the vacancies were found to be condensed to form dislocation loops at low doses. The observation of voids in ion irradiated samples is consistent with the observation that the majority of loops are interstitial at low doses as supersaturation of vacancies is required for the nucleation and/or growth of voids during subsequent irradiation at higher doses. Alternatively vacancies created during cascade damage may form relatively stable three dimensional clusters (which subsequently grow into voids) leaving an excess of self-interstitial atoms (SIAs) to nucleate loops. The voids formed in samples irradiated with ions were seen to grow with increasing the irradiation dose and voids were preferably arranged in rows and lying parallel to the basal planes. Jostsons and Farrell [14] have also observed the pronounced clustering of voids on basal planes in neutron irradiated magnesium. The findings in the present investigation seem consistent with the previous observation of void formation in magnesium however detailed investigation of void morphology and their growth behavior during irradiation would be required to ascertain this. 5. Summary Electron irradiation in commercial purity magnesium results in the formation of majority of basal plane dislocation loops with Bur 3i whereas ion irradiation results in the forgers vector 1=6h2 0 2 mation of majority of prism plane dislocation loops with Burgers 0i. The results presented in this investigation also vector 1=3h1 1 2 suggest the biased migration of interstitials towards the basal planes in electron irradiation and towards the prism planes in ion irradiation. All the basal plane dislocation loops formed during
235
electron irradiation were interstitial in character whereas majority of prism plane dislocation loops were vacancy in character. The majority of the prism plane dislocation loops formed during ion irradiation was interstitial in character. Voids observed during ion irradiation appear to lie in rows parallel to the basal planes. Acknowledgements This work is sponsored by NSERC, UNENE (COG, OPG) and NuTech Precision Metals under the Industrial Research Chair Program in Nuclear Materials at Queen’s University. The ion irradiation was accomplished at the electron Microscopy Centre for Materials Research at Argonne National Laboratory, under Contract No. DEAC02-06CH11357 by UChicago Argonne, LLC. The authors are grateful to Dr. Mark Kirk and Pete Boldo for their help on the ion beam facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
M.K. Hossain, L.M. Brown, Acta Metall. 25 (1977) 257. M. Griffiths, J. Nucl. Mater. 165 (1989) 315. H. Foll, M. Wilkens, Phys. Status Solid A 39 (1977) 561. M. Griffiths, J. Nucl. Mater. 205 (1993) 225. V. Levy, J. Microsc.-Oxf. 19 (1974) 1. M. Griffiths, R.W. Gilbert, J. Nucl. Mater. 150 (1987) 169. A. Jostsons, R.G. Blake, J.G. Napier, P.M. Kelly, K. Farrell, J. Nucl. Mater. 68 (1977) 267. M. Griffiths, Philos. Mag. A 63 (1991) 835. J. Hillaire, C. Mairy, J. Espinass, V. Levy, Acta Metall. 18 (1970) 1285. W.J. Phythian, C.A. English, D.H. Yellen, D.J. Bacon, Philos. Mag. A 63 (1991) 821. M. Griffiths, M.H. Loretto, R.E. Smallman, Philos. Mag. A 49 (1984) 613. S.B. Fisher, Radiat. Eff. Defects Solids 5 (1970) 239. P.M. Kelly, R.G. Blake, Philos. Mag. 28 (1973) 415. A. Jostsons, K. Farrell, Radiat. Eff. Defects Solids 15 (1972) 217.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Irradiation damage in commercial purity magnesium A.K. Khan ⇑, Z. Yao, M.R. Daymond, R.A. Holt Department of Mechanical and Materials Engineering, Queen’s University, Nicol Hall, 60 Union Street, Kingston, Ontario, Canada K7L 3N6
a r t i c l e
i n f o
Article history: Available online 2 February 2011 Keywords: Irradiation Magnesium Dislocations TEM
a b s t r a c t In-situ electron irradiation damage within the transmission electron microscope has been studied and compared with ion irradiation on commercial purity magnesium. Magnesium has the same crystal structure (hcp) and a similar c/a ratio to that of zirconium and is considered a suitable model system and useful analogue to zirconium (an important structural material for nuclear reactors). The results presented in this paper suggest that microstructure developed during irradiation is modified by the nature of the irra 3i are formed by electron diating particles. The basal plane dislocation loops with Burgers vector 1=6h2 0 2 irradiation and analysis showed that all of these dislocation loops are interstitial in nature. Vacancy type 0i also form but to a lesser degree than the prism plane dislocation loops with Burgers vector 1=3h1 1 2 basal plane dislocation loops. Ion irradiation for a dose up to 0.1 dpa produces a majority of interstitial 0i. Void formation also takes place in type prism plane dislocation loops with Burgers vector 1=3h1 1 2 ion irradiated samples subjected to a higher dose of 0.7 dpa. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The effects of particle irradiation on the properties of materials have been studied for more than half a century. The semiconductor industry uses the beneficial effects of ion beam processing of semiconductors whereas in the nuclear industry irradiation effects are detrimental and result in the degradation of structural materials. The microscopic processes responsible for these effects are not completely understood limiting researchers ability to predict the consequences of irradiation. It is therefore important to analyze the microstructural development during irradiation to understand the various effects of irradiation damage. Materials with hcp crystal structure such as zirconium and its alloys are very important structural materials used in nuclear reactors. Magnesium has the same crystal structure and its c/a ratio (1.623) is also close to that of zirconium (1.593). Development of microstructures similar to zirconium is also expected in magnesium during irradiation therefore magnesium is considered a suitable model system to help understand irradiation damage development in zirconium. Magnesium is selected in this study because it can be easily irradiated in a conventional transmission electron microscope operating at 200 keV due to its low displacement energy (12 eV) and atomic mass (24.3 amu) as compared to zirconium (25 eV, 91.2 amu) [1,2]. The irradiation damage structure appears in the form of dislocation loops and voids and in-situ transmission electron micros-
⇑ Corresponding author. Tel.: +1 613 533 3236; fax: +1 613 533 6610. E-mail address: [email protected] (A.K. Khan). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.01.072
copy permits direct observation of the microstructural changes taking place during irradiation. It is believed that in hexagonal close packed structures evolution of microstructure can be correlated with the c/a ratio [3]. Nucleation of basal plane pffiffiffi loops takes place in metals with c/a ratio greater than 1.732 ð 3Þ and prism plane loops form in metals with c/a ratio less than 1.732. According to this rule the principal habit plane of dislocations in all hcp metals should be prismatic except in zinc and cadmium. There are many exceptions to this rule be 3i cause basal plane dislocations with burgers vector 1=6h2 0 2 were observed in magnesium [4,5] and zirconium [6,7] where the c/a ratio is less than 1.732. The formation of basal plane loops in metals other than zinc and cadmium has been attributed to the presence of impurities [8,9], cascade collapse [10] and stresses in thin foils [11]. In this investigation preliminary results of electron and ion irradiation on commercial purity magnesium are presented to clarify the effect of irradiating particles on microstructural evolution during irradiation.
2. Material and methods Commercial purity magnesium (99.8%) was used for this investigation. The major impurities present in this composition in wt. ppm are 370 Al, 100 Pb, 48 Cu and 40 Zn. For transmission electron microscopy 1 mm thick slices were cut and thinned to 100 lm thickness using a series of silicon carbide papers down to 1200 grit. Three millimeter discs punched from these slices were perforated in Struers TenuPol-3 using a solution of 6% perchloric acid and 94% methanol at 40 °C. The electron irradiation was carried out
232
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
using a CM 20 electron microscope operated at 200 keV. In-situ ion beam irradiation was carried out at IVEM-Tandem Facility at Argonne National Laboratory using 1 MeV Kr2+ ions. The electron and ion irradiation both were carried out at room temperature. The rise in temperature during irradiation is considered negligible due to the high thermal conductivity of magnesium and therefore dynamic annealing effects can be neglected during irradiation [12]. The damage profile of 1 MeV Kr2+ ions on magnesium calculated by SRIM 2008 is shown in Fig. 1. After irradiation, analysis of defect structures formed was carried out in TEM operating at 80 keV or 100 keV.
3. Experimental results 3.1. Electron irradiation The microstructure developed after electron irradiation of 0.1 dpa consisted of two types of dislocation loops i.e. hexagonal
shaped and the elliptical shaped loops. All the images were taken in strict two beam conditions and keeping the deviation parameter(s) positive. In the detailed analysis (not shown here) it was observed that these hexagonal loops show faulted contrast when 0 type diffraction vector and also remain in imaged with 1 0 1 strong contrast when imaged in g = (0 0 0 2). This contrast behavior suggests that hexagonal dislocation loops have Burgers vector 3i. These loops also demonstrate edge-on morof the type 1=6h2 0 2 0Þ when beam direction is close to ½1 2 1 0. The phology in g = ð1 0 1 trace analysis suggests that these loops are lying on the basal planes and hence will be called basal plane loops hereafter. Fig. 2 displays a series of micrographs taken in beam direction close to [0 0 0 1] to specify the Burgers vector of individual loops. 1 3. The foil orientation in this case is close to ½1 2 The g.b analysis is listed in Table 1and according to this 3i, ± 1=6h2 2 0 3i, and analysis hexagonal loops with b = ±1=6h0 2 2 3i are present in the microstructure. Consider loop A, ± 1=6h2 0 2 1 0Þ and therefore its b = ± it is out of contrast when g = ð2 1 3i. The method proposed by Kelly and Blake [13] was 1=6h0 2 2 used to identify the nature of the loops. The inside–outside contrast experiment shown in Fig. 3 suggest that these basal plane 1Þ and ð1 1 0 1Þ loops demonstrate outside contrast in g = ð0 1 1 3Þ. The beam direction is close therefore loop at A have b = 1=6ð0 2 2 1 3 and upward pointing loop plane normal n = [0 0 0 1] to ½1 2 gives the result that loop is interstitial in character. All the hexagonal loops demonstrate similar behavior therefore it can be concluded that all these loops are interstitial in nature. The elliptical loops were also characterized and g.b analysis shows that these loops have Burgers vector of the type 0i and were found to lie on prism planes. Consider the 1=3h1 1 2 elliptical loop at the bottom right in Fig. 3 at point B. This loop is 0 1 1Þ therefore b must be ±1=3h1 2 1 0i. out of contrast in g = ð1 The outside contrast is observed in g = ð0 1 1 1Þ and ð1 1 0 1Þ there 21 0i. According to the inside–outside contrast fore b = 1=3h1 method proposed by Kelly and Blake [13] in this safe orientation 1 3 this dislocation loop is vacancy in nature. Other ellipof B = ½1 2 1 1 0i and were tical loops analyzed were found to have b = 1=3h2 also vacancy in nature. 3.2. Ion irradiation
Fig. 1. Damage profile of 1 MeV Kr2+ ions on magnesium calculated by SRIM-2008. The displacement threshold energy of magnesium is taken as 12 eV.
The damage microstructure developed during ion irradiation up to 0.1 dpa on a different foil of the same composition as used in electron irradiation is shown Fig. 4. The foil orientation in this case was close to [0 0 0 1]. The Burgers vector analysis was also carried
3. (a) g = ð1 2 1 0Þ, (b) g = ð2 1 1 0Þ and (c) g = ð1 1 2 0Þ. Fig. 2. Contrast of the hexagonal dislocation loops when the beam direction is close to [0 0 0 1]. Foil orientation ½1 21 1 0Þ and therefore its b = ±1=6h0 2 2 3i. Elliptical dislocation loops are also present such as a loop at point B. For example dislocation loop at A is out of contrast in g = ð2 1
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235 Table 1 g.b analysis of the dislocation loops. Beam direction?
13 12
0001
1 23 1
b; g?
10 12
1 0 21
1 20 1
1 0 11
101 1
011 1
3i 1=6h2 0 2 2 0 3i 1=6h2 3i 1=6h0 2 2 1 1 0i 1=3h2 1 0i 1=3h1 2 0i 1=3h1 1 2
0 1 1 1 2 1
1 1 0 2 1 1
1 0 1 1 1 2
5/6 5/6 7/6 0 1 1
1/6 7/6 5/6 1 1 0
7/6 1/6 5/6 1 0 1
out, not shown here, and it demonstrated that all the dislocation 0i. The inloops are prism plane with Burgers vector 1=3h1 1 2 side–outside contrast experiments also suggested that majority of these dislocation loops are interstitial in character. Very few vacancy dislocation loops were also present in the microstructure, shown by symbol V in Fig. 4. Basal plane dislocation loops with 3i were not observed in the microstructure Burgers vector 1=6h2 0 2 up to irradiation of 0.1 dpa. Fig. 5 displays the damage microstructure in a prism foil. In the beginning, up to dose of 0.1 dpa, microstructure mainly consisted of prism dislocation loops with Burgers
233
0i as these dislocation loops are out of contrast vector 1=3h1 1 2 when g = (0 0 0 2). The irradiation experiment conducted on another prism foil suggested that as the irradiation dose is increased to 0.7 dpa, the basal plane dislocation loops and voids became dominant in the microstructure as shown in Fig. 6. These basal plane dislocation loops appear edge-on in prism foils. The prism type dislocation loops were not observed suggesting their instability as compared to the basal plane dislocation loops at higher doses. The voids present in the microstructure also appear to align themselves in rows parallel to the edge-on orientation of the basal plane dislocation loops suggesting that the voids are also aligned in rows parallel to the basal plane. Fig. 6 also displays a denuded zone about 100 nm in size along the grain boundary that is free of voids. 4. Discussion Prism plane dislocations are expected in magnesium due to the lower c/a ratio (1.623) than ideal because of higher packing density 0} planes. Hossain and Brown [1] have in fact reof prism {1 01 ported the formation of interstitial type prism plane dislocations in high purity (99.998%) magnesium after electron irradiation. The formation of basal plane dislocations after electron irradiation
1 3 and in (d) B = ½1 1 2 3. Diffraction vectors are shown on each Fig. 3. Inside–outside contrast of the dislocation loops formed during electron irradiation. In (a–c) B = ½1 2 1Þ and ð1 1 0 1Þ and is interstitial in nature. The elliptical loop at point B is out of contrast in g = ð1 0 1 1Þ micrograph. Hexagonal loop at A show outside contrast in g = ð0 1 1 21 0i. This loop shows outside contrast in g = ð0 1 1 1Þ and is vacancy in nature. have b = 1=3h1
234
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
3. (a) g = ð0 1 1 1Þ and (b) g = ð0 1 1 1Þ. Inside–outside contrast shows that most of the dislocation Fig. 4. Dislocations in a basal foil after ion irradiation up to 0.1 dpa. B ½1 1 2 loops are interstitial in nature. Few vacancy loops are also present and shown by the symbol V.
0. (a) g = ð1 0 1 1Þ, (b) g = (0 0 0 2). The loops present in g = ð1 0 1 1Þ becomes out of contrast when Fig. 5. Dislocations in a prism foil after ion irradiation of 0.1 dpa. B ½1 21 0i. The encircled edge-on loops are basal plane imaged in g = (0 0 0 2) except few encircled loops suggesting that these dislocations have a Burgers vector of the type 1=3h1 1 2 3i. loops with Burgers vector 1=6h2 0 2
Fig. 6. Formation of voids in a prism foil after ion irradiation of 0.7 dpa. The basal plane dislocation loops appear edge-on and the voids also appear to align parallel to the edge-on orientation of the basal plane dislocation loops. Void denuded zone also exists along the grain boundary.
on commercial purity (99.8%) magnesium in the present study suggests that evolution of microstructure is influenced by the presence of impurities. Griffiths [8] has also reported that increasing the concentration of impurities enhances the formation of basal plane defects in magnesium and zirconium. The possible effect of impurities can be to reduce the stacking fault energy resulting in the formation of faulted basal plane dislocations. Comparison of the electron and ion irradiation results suggests that interstitial basal loops are more prevalent in the microstructure during electron irradiation for about 0.1 dpa and interstitial prism plane loops are more dominant for the same dose during ion irradiation. The widespread nucleation of prism plane dislocation loops during ion irradiation in this low purity magnesium seems surprising and suggests that nature of irradiating particles influences the microstructure development. The point defects produced during electron irradiation are in the form of Frenkel pairs, whereas ion irradiation produces cascades and residual clusters of vacancies and interstitials as well as Frenkel pairs. Faulted basal plane dislocation loops were also observed in neutron irradiated zirconium but these were vacancy in nature [6,7]. The different nature of basal plane faults in these metals suggests that opposite diffusional anisotropy difference exists in magnesium compared with zirconium.
A.K. Khan et al. / Nuclear Instruments and Methods in Physics Research B 272 (2012) 231–235
Formation of voids was not observed during electron irradiation up to 0.5 dpa because of the fact that most of the vacancies were found to be condensed to form dislocation loops at low doses. The observation of voids in ion irradiated samples is consistent with the observation that the majority of loops are interstitial at low doses as supersaturation of vacancies is required for the nucleation and/or growth of voids during subsequent irradiation at higher doses. Alternatively vacancies created during cascade damage may form relatively stable three dimensional clusters (which subsequently grow into voids) leaving an excess of self-interstitial atoms (SIAs) to nucleate loops. The voids formed in samples irradiated with ions were seen to grow with increasing the irradiation dose and voids were preferably arranged in rows and lying parallel to the basal planes. Jostsons and Farrell [14] have also observed the pronounced clustering of voids on basal planes in neutron irradiated magnesium. The findings in the present investigation seem consistent with the previous observation of void formation in magnesium however detailed investigation of void morphology and their growth behavior during irradiation would be required to ascertain this. 5. Summary Electron irradiation in commercial purity magnesium results in the formation of majority of basal plane dislocation loops with Bur 3i whereas ion irradiation results in the forgers vector 1=6h2 0 2 mation of majority of prism plane dislocation loops with Burgers 0i. The results presented in this investigation also vector 1=3h1 1 2 suggest the biased migration of interstitials towards the basal planes in electron irradiation and towards the prism planes in ion irradiation. All the basal plane dislocation loops formed during
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electron irradiation were interstitial in character whereas majority of prism plane dislocation loops were vacancy in character. The majority of the prism plane dislocation loops formed during ion irradiation was interstitial in character. Voids observed during ion irradiation appear to lie in rows parallel to the basal planes. Acknowledgements This work is sponsored by NSERC, UNENE (COG, OPG) and NuTech Precision Metals under the Industrial Research Chair Program in Nuclear Materials at Queen’s University. The ion irradiation was accomplished at the electron Microscopy Centre for Materials Research at Argonne National Laboratory, under Contract No. DEAC02-06CH11357 by UChicago Argonne, LLC. The authors are grateful to Dr. Mark Kirk and Pete Boldo for their help on the ion beam facility. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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