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The detection of local plastic strain in microscopic scale
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Materials Letters 62 (2008) 804 – 807 www.elsevier.com/locate/matlet
The detection of local plastic strain in microscopic scale Jian Li ⁎ Materials Technology Laboratory, CANMET, 568 Booth St. Ottawa, Ontario, Canada K1A 0G1 Received 28 February 2007; accepted 27 June 2007 Available online 4 July 2007
Abstract The detection and evaluation of plastic deformation in microscopic scale is very important under many circumstances. The conventional optical and scanning-electron microscope (SEM) do not provide crystallographic contrast with sufficient sensitivity. Site-specific transmission-electron microscope (TEM) analyses are usually difficult due to the limited size of the area being analyzed and the difficulties in sample preparation. This paper will demonstrate that small plastically deformed zones can be easily detected using focused-ion beam (FIB) secondary-electron images. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Plastic deformation; SEM; FIB
1. Introduction Theoretical models on plastic deformation of crystalline materials can be dated back to Taylor [1], and Bishop and Hill [2]. One of the most important assumptions of these models is that all crystallites are subjected to the same plastic strain (ɛijp = the prescribed macroscopic strain), and the plastic strain is achieved by multiple slip. However, in reality, this assumption does not apply to deformation of polycrystalline materials. Microscopic plastic strain varies significantly among crystallites depending on their crystallographic orientation and constrains from their neighbors [3]. In many circumstances, the state of local plastic deformation in microscopic scale controls the behavior of the bulk material. For example, the deformation of subsurface material is critical to the wear process [4]; the local plastic strain near crack tips could be a detrimental factor during crack propagation. However, the detection of these small plastic zones is not trivial. The conventional optical microscopy and scanning-electron microscopy do not provide enough crystallographic information. The TEM, although regarded as the preferred method, presents difficulties in sample preparation
and is also compromised by artifacts commonly introduced during sample preparation. There have been numerous reports on the applications of the focused-ion beam (FIB) microscope in recent years [5–8]. Most of its applications are focused on using the FIB as a micromachining tool, typically for site-specific TEM specimen preparation [9,10]. However, benefits of imaging with either secondary electrons or secondary ions are frequently ignored. In this paper, the author demonstrates that local plastic deformation can be conveniently detected by careful FIB imaging. 2. Experimental This report presents examples to illustrate that the FIB secondary imaging is a simple and effective way to detect local plastic deformation in microscopic scale. All samples were carefully polished using a typical metallographic polishing routine prior to FIB ion-beam imaging using a Micrion-2500 FIB system, and no metallographic etching is required. For the purpose of comparison, optical and SEM images were taken from the etched samples. 3. Results and discussion
⁎ Tel.: +1 613 992 3878; fax: +1 613 992 8735. E-mail address: [email protected].
FIB images are very sensitive to crystallographic orientation [11,12]. This sensitivity is demonstrated using a series of FIB secondary-electron (SE) images of a low-carbon steel sample taken
0167-577X/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.06.065
J. Li / Materials Letters 62 (2008) 804–807
805
Fig. 1. FIB SE images of an area on a metallographic polished low-carbon steel surface with various specimen tilt angles.
from the same location with different tilt angles. Fig. 1 shows that, with as little as a 2° difference in tilt, grey levels of some grains start to show noticeable changes. Suppose that, at no specimen tilt, the ion beam strikes a grain with crystallographic plane (h1k1l1) parallel to the surface. By tilting the specimen, the incident angle changes which is equivalent to imaging a different grain with crystallographic plane (h2k2l2) parallel to the surface. Such high sensitivities arise because certain grains with less dense atomic planes parallel to the imaging surface could result in the Ga ions channeling deeper into the substrate thus reducing both secondary ion and secondary-electron emissions; these grains appear “darker”. For example, in face-centered cubic (FCC) aluminum, grains with {100} and {110} planes parallel to the sample surface appear to be
“dark” [13]. However, partly due to the size of the Ga ion, the range of incident angles for channeling to occur is very small. As soon as the incident angle changes (within 2° in this case), the contrast of some grains starts to show noticeable change. This high sensitivity is very useful to detect a small amount of plastic deformation. Each individual grain in a fully annealed crystalline material has its designated crystallographic orientation, and the orientation of which should not vary across each grain. Upon application of a certain amount of plastic deformation, dislocations sweep across the grains, and form cell walls or subgrains depending on the deformation condition [14]. This results in slight changes in crystallographic orientation within the original grain. Usually, the change in orientation across each grain is cumulative due to the local stress tensor (at each point within the grain) can be
Fig. 2. FIB images of a polished IF steel (a) annealed, (b) the same steel plastically deformed.
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J. Li / Materials Letters 62 (2008) 804–807
Fig. 3. Comparison of (a) optical, (b) SEM and (c) FIB images on a stress-corrosion crack tip.
assumed to be identical, and the mechanical properties at each location within the grain should be the same. Thus, with a certain degree of deformation, the change in crystallographic orientation within the grain can be sufficient to be detected by FIB secondary-electron imaging. Grains with a certain degree of plastic strain appear as non-uniform in contrast. Examples of FIB images of a plastically deformed microstructure are shown in Fig. 2. Two samples were cut out from a fully annealed interstitial-free (IF) steel sheet. Plastic deformation was applied to one of the samples by means of a cup drawing test. The test imposed 53% major strain along the drawing direction (vertical) and 25% minor strain in the tangential direction (horizontal) of the sheet. Both samples were mounted and polished using a metallographic polishing routine. Fig. 2(a), shows a typical equi-axed ferrite grain in the annealed IF steel. The microstructure is completely different in the deformed sample [Fig. 2(b)]. Within each grain, the contrast varies indicating changes in crystallographic orientation as a result of the formation of some kind of dislocation structure or subgrains. The high sensitivity to the crystallographic orientation of individual grains is due not only to the high sensitivity of the gallium ion channeling into the substrate but also to the extremely small ion-specimen interaction volume. In contrast, the electron-specimen interaction volume in an SEM (for secondary and backscattered-electron emission) is relatively large which reduces its efficiency in detecting the sub-micrometer sized cell structures. In addition, the SEM backscattered-electron (BSE) images, although capable of showing crystallographic contrast, require electropolished surfaces that are problematic to many kind of analysis. For example, in the study of stress-corrosion cracking (SCC), electropolishing may
dissolve the corrosion product in the crack. Electropolishing is also difficult to use for multi-phased materials due to potential local galvanic reaction. FIB imaging is used to evaluate the effect of pipeline hydrostatic testing on existing cracks. Concerns about SCC in the pipeline industry have increased over the past few years due to an increase in the frequency of pipeline failures. The SCC-related failures have occurred not only in natural gas pipelines but also in pipelines transporting oil. Pipeline SCC inspections/assessments have been commonly carried out by hydrostatic testing for decades. In a typical hydrostatic test, sections of pipeline are pressurized using water up to 110% of the material's specified minimum yield strength (SMYS) and held for a designated length of time. The hydrostatic test is very effective in detecting near-critical cracks [15,16], however there have been concerns that sub-critical sized cracks may grow larger, and some blunt dormant cracks may be re-activated during hydrostatic tests. To investigate the effect of hydrostatic tests on existing SCCs, a small sample containing SCC was cut out, mounted in low-shrinkage Epoxy resin, and carefully polished using a metallographic polishing routine. Subsequent to FIB imaging, the sample was etched in 2% Nital solution, and the same crack tip was imaged using both optical microscope and SEM for comparison purposes. Features near a selected crack tip are shown in Fig. 3. Both optical and SEM images provide little information on the highly stressed crack-tip zone, while the FIB image shows that the pipeline hydrostatic test has created a small plastic zone (in microscopic scale) ahead this particular crack tip. The significance and implications of these small plastic zones are extensively discussed elsewhere [17].
Fig. 4. SEM and FIB images of the polished cross-section of an aluminum alloy subjected to dry sliding wear. (a) SEM backscattered-electron image, (b) FIB SE image showing the subsurface deformation zone.
J. Li / Materials Letters 62 (2008) 804–807
In a recent study to evaluate wear characteristics of aluminum A390 alloy, worn surfaces were carefully examined for damage after aggressive dry-sliding wear tests. Fig. 4(a) shows a typical SEM image taken from a polished cross section of an aluminum alloy subjected to dry sliding wear; it shows little contrast in the aluminum substrate. The FIB SE image [Fig. 4(b)] taken from a similar area shows the formation of very fine subgrains beneath the worn surface. This fine-grained structure is closely related to the wear behavior of this material [4].
4. Conclusions Local plastic deformation can be effectively imaged using the secondary-electron signal generated by the primary gallium ion beam in focused-ion beam microscopes. The detection of such plastic zones using the FIB is much more effective and reliable than the conventional TEM investigation and the more recent electron backscattered pattern (EBSD) analysis. References [1] G.I. Taylor, Journal of the Institute of Metals 62 (1938) 307. [2] J.F.W. Bishop, R. Hill, Philadelphia Magazine 42 (1951) 1298. [3] Jian Li, PhD thesis, Queen's University, Kingston, Ontario, Canada, 1997.
807
[4] Jian Li, E. Mustafa, V.Y. Guertsman, J. Lo, A.T. Alpas, Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing 421 (1–2) (2006) 317–327. [5] M.W. Phaneuf, Micron 30 (1999) 277–288. [6] M.W. Phaneuf, Jian Li, T. Malis, Microscopy and Microanalysis 4 (1998) 492–493. [7] Jian Li, Journal of Metals 58 (3) (2006) 27–31. [8] Jian Li, G.S. McMahon, M.W. Phaneuf, Proceedings of the Microscopical Society of Canada XXVIII (2001) 26–27. [9] L.A. Giannuzzi, F.A. Stevie, Micron 30 (1999) 197–204. [10] R. Anderson, Proceeding of Microscopy and Microanalysis 8 (2002) 44–45. [11] M.W. Phaneuf, Jian Li, R.F. Shuman, K. Noll and J.D. Casey Jr., Apparatus and method for reducing differential sputter rates, US patent #6,641,705, issued November 4, 2003. [12] M.W. Phaneuf, Jian Li, J.D. Casey Jr., Microscopy and Microanalysis 8 (Suppl. 2) (2002) 52. [13] Jian Li, M.W. Phaneuf, S. Saimoto, Presentation in Conference of Microscopy Society of Canada, July, 1998, Ottawa, Ontario, Canada. [14] M.A. Brown, Journal of the Institute of Metals 80 (1952) 115–124. [15] M. Wood, A. Cosham, Pipes & Pipelines International 45 (4) (2000) 1–19. [16] W. Zheng, W.R. Tyson, R.W. Revie, G. Shen, J.E.M. Braid, International Pipeline Conference 1 (1998) 459–472. [17] Jian Li, M. Elboujdaini, R.W. Revie and M. Gao, Effect of hydrostatic tests on existing SCC, submitted for publication.
Materials Letters 62 (2008) 804 – 807 www.elsevier.com/locate/matlet
The detection of local plastic strain in microscopic scale Jian Li ⁎ Materials Technology Laboratory, CANMET, 568 Booth St. Ottawa, Ontario, Canada K1A 0G1 Received 28 February 2007; accepted 27 June 2007 Available online 4 July 2007
Abstract The detection and evaluation of plastic deformation in microscopic scale is very important under many circumstances. The conventional optical and scanning-electron microscope (SEM) do not provide crystallographic contrast with sufficient sensitivity. Site-specific transmission-electron microscope (TEM) analyses are usually difficult due to the limited size of the area being analyzed and the difficulties in sample preparation. This paper will demonstrate that small plastically deformed zones can be easily detected using focused-ion beam (FIB) secondary-electron images. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Keywords: Plastic deformation; SEM; FIB
1. Introduction Theoretical models on plastic deformation of crystalline materials can be dated back to Taylor [1], and Bishop and Hill [2]. One of the most important assumptions of these models is that all crystallites are subjected to the same plastic strain (ɛijp = the prescribed macroscopic strain), and the plastic strain is achieved by multiple slip. However, in reality, this assumption does not apply to deformation of polycrystalline materials. Microscopic plastic strain varies significantly among crystallites depending on their crystallographic orientation and constrains from their neighbors [3]. In many circumstances, the state of local plastic deformation in microscopic scale controls the behavior of the bulk material. For example, the deformation of subsurface material is critical to the wear process [4]; the local plastic strain near crack tips could be a detrimental factor during crack propagation. However, the detection of these small plastic zones is not trivial. The conventional optical microscopy and scanning-electron microscopy do not provide enough crystallographic information. The TEM, although regarded as the preferred method, presents difficulties in sample preparation
and is also compromised by artifacts commonly introduced during sample preparation. There have been numerous reports on the applications of the focused-ion beam (FIB) microscope in recent years [5–8]. Most of its applications are focused on using the FIB as a micromachining tool, typically for site-specific TEM specimen preparation [9,10]. However, benefits of imaging with either secondary electrons or secondary ions are frequently ignored. In this paper, the author demonstrates that local plastic deformation can be conveniently detected by careful FIB imaging. 2. Experimental This report presents examples to illustrate that the FIB secondary imaging is a simple and effective way to detect local plastic deformation in microscopic scale. All samples were carefully polished using a typical metallographic polishing routine prior to FIB ion-beam imaging using a Micrion-2500 FIB system, and no metallographic etching is required. For the purpose of comparison, optical and SEM images were taken from the etched samples. 3. Results and discussion
⁎ Tel.: +1 613 992 3878; fax: +1 613 992 8735. E-mail address: [email protected].
FIB images are very sensitive to crystallographic orientation [11,12]. This sensitivity is demonstrated using a series of FIB secondary-electron (SE) images of a low-carbon steel sample taken
0167-577X/$ - see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.06.065
J. Li / Materials Letters 62 (2008) 804–807
805
Fig. 1. FIB SE images of an area on a metallographic polished low-carbon steel surface with various specimen tilt angles.
from the same location with different tilt angles. Fig. 1 shows that, with as little as a 2° difference in tilt, grey levels of some grains start to show noticeable changes. Suppose that, at no specimen tilt, the ion beam strikes a grain with crystallographic plane (h1k1l1) parallel to the surface. By tilting the specimen, the incident angle changes which is equivalent to imaging a different grain with crystallographic plane (h2k2l2) parallel to the surface. Such high sensitivities arise because certain grains with less dense atomic planes parallel to the imaging surface could result in the Ga ions channeling deeper into the substrate thus reducing both secondary ion and secondary-electron emissions; these grains appear “darker”. For example, in face-centered cubic (FCC) aluminum, grains with {100} and {110} planes parallel to the sample surface appear to be
“dark” [13]. However, partly due to the size of the Ga ion, the range of incident angles for channeling to occur is very small. As soon as the incident angle changes (within 2° in this case), the contrast of some grains starts to show noticeable change. This high sensitivity is very useful to detect a small amount of plastic deformation. Each individual grain in a fully annealed crystalline material has its designated crystallographic orientation, and the orientation of which should not vary across each grain. Upon application of a certain amount of plastic deformation, dislocations sweep across the grains, and form cell walls or subgrains depending on the deformation condition [14]. This results in slight changes in crystallographic orientation within the original grain. Usually, the change in orientation across each grain is cumulative due to the local stress tensor (at each point within the grain) can be
Fig. 2. FIB images of a polished IF steel (a) annealed, (b) the same steel plastically deformed.
806
J. Li / Materials Letters 62 (2008) 804–807
Fig. 3. Comparison of (a) optical, (b) SEM and (c) FIB images on a stress-corrosion crack tip.
assumed to be identical, and the mechanical properties at each location within the grain should be the same. Thus, with a certain degree of deformation, the change in crystallographic orientation within the grain can be sufficient to be detected by FIB secondary-electron imaging. Grains with a certain degree of plastic strain appear as non-uniform in contrast. Examples of FIB images of a plastically deformed microstructure are shown in Fig. 2. Two samples were cut out from a fully annealed interstitial-free (IF) steel sheet. Plastic deformation was applied to one of the samples by means of a cup drawing test. The test imposed 53% major strain along the drawing direction (vertical) and 25% minor strain in the tangential direction (horizontal) of the sheet. Both samples were mounted and polished using a metallographic polishing routine. Fig. 2(a), shows a typical equi-axed ferrite grain in the annealed IF steel. The microstructure is completely different in the deformed sample [Fig. 2(b)]. Within each grain, the contrast varies indicating changes in crystallographic orientation as a result of the formation of some kind of dislocation structure or subgrains. The high sensitivity to the crystallographic orientation of individual grains is due not only to the high sensitivity of the gallium ion channeling into the substrate but also to the extremely small ion-specimen interaction volume. In contrast, the electron-specimen interaction volume in an SEM (for secondary and backscattered-electron emission) is relatively large which reduces its efficiency in detecting the sub-micrometer sized cell structures. In addition, the SEM backscattered-electron (BSE) images, although capable of showing crystallographic contrast, require electropolished surfaces that are problematic to many kind of analysis. For example, in the study of stress-corrosion cracking (SCC), electropolishing may
dissolve the corrosion product in the crack. Electropolishing is also difficult to use for multi-phased materials due to potential local galvanic reaction. FIB imaging is used to evaluate the effect of pipeline hydrostatic testing on existing cracks. Concerns about SCC in the pipeline industry have increased over the past few years due to an increase in the frequency of pipeline failures. The SCC-related failures have occurred not only in natural gas pipelines but also in pipelines transporting oil. Pipeline SCC inspections/assessments have been commonly carried out by hydrostatic testing for decades. In a typical hydrostatic test, sections of pipeline are pressurized using water up to 110% of the material's specified minimum yield strength (SMYS) and held for a designated length of time. The hydrostatic test is very effective in detecting near-critical cracks [15,16], however there have been concerns that sub-critical sized cracks may grow larger, and some blunt dormant cracks may be re-activated during hydrostatic tests. To investigate the effect of hydrostatic tests on existing SCCs, a small sample containing SCC was cut out, mounted in low-shrinkage Epoxy resin, and carefully polished using a metallographic polishing routine. Subsequent to FIB imaging, the sample was etched in 2% Nital solution, and the same crack tip was imaged using both optical microscope and SEM for comparison purposes. Features near a selected crack tip are shown in Fig. 3. Both optical and SEM images provide little information on the highly stressed crack-tip zone, while the FIB image shows that the pipeline hydrostatic test has created a small plastic zone (in microscopic scale) ahead this particular crack tip. The significance and implications of these small plastic zones are extensively discussed elsewhere [17].
Fig. 4. SEM and FIB images of the polished cross-section of an aluminum alloy subjected to dry sliding wear. (a) SEM backscattered-electron image, (b) FIB SE image showing the subsurface deformation zone.
J. Li / Materials Letters 62 (2008) 804–807
In a recent study to evaluate wear characteristics of aluminum A390 alloy, worn surfaces were carefully examined for damage after aggressive dry-sliding wear tests. Fig. 4(a) shows a typical SEM image taken from a polished cross section of an aluminum alloy subjected to dry sliding wear; it shows little contrast in the aluminum substrate. The FIB SE image [Fig. 4(b)] taken from a similar area shows the formation of very fine subgrains beneath the worn surface. This fine-grained structure is closely related to the wear behavior of this material [4].
4. Conclusions Local plastic deformation can be effectively imaged using the secondary-electron signal generated by the primary gallium ion beam in focused-ion beam microscopes. The detection of such plastic zones using the FIB is much more effective and reliable than the conventional TEM investigation and the more recent electron backscattered pattern (EBSD) analysis. References [1] G.I. Taylor, Journal of the Institute of Metals 62 (1938) 307. [2] J.F.W. Bishop, R. Hill, Philadelphia Magazine 42 (1951) 1298. [3] Jian Li, PhD thesis, Queen's University, Kingston, Ontario, Canada, 1997.
807
[4] Jian Li, E. Mustafa, V.Y. Guertsman, J. Lo, A.T. Alpas, Materials Science and Engineering. A, Structural Materials: Properties, Microstructure and Processing 421 (1–2) (2006) 317–327. [5] M.W. Phaneuf, Micron 30 (1999) 277–288. [6] M.W. Phaneuf, Jian Li, T. Malis, Microscopy and Microanalysis 4 (1998) 492–493. [7] Jian Li, Journal of Metals 58 (3) (2006) 27–31. [8] Jian Li, G.S. McMahon, M.W. Phaneuf, Proceedings of the Microscopical Society of Canada XXVIII (2001) 26–27. [9] L.A. Giannuzzi, F.A. Stevie, Micron 30 (1999) 197–204. [10] R. Anderson, Proceeding of Microscopy and Microanalysis 8 (2002) 44–45. [11] M.W. Phaneuf, Jian Li, R.F. Shuman, K. Noll and J.D. Casey Jr., Apparatus and method for reducing differential sputter rates, US patent #6,641,705, issued November 4, 2003. [12] M.W. Phaneuf, Jian Li, J.D. Casey Jr., Microscopy and Microanalysis 8 (Suppl. 2) (2002) 52. [13] Jian Li, M.W. Phaneuf, S. Saimoto, Presentation in Conference of Microscopy Society of Canada, July, 1998, Ottawa, Ontario, Canada. [14] M.A. Brown, Journal of the Institute of Metals 80 (1952) 115–124. [15] M. Wood, A. Cosham, Pipes & Pipelines International 45 (4) (2000) 1–19. [16] W. Zheng, W.R. Tyson, R.W. Revie, G. Shen, J.E.M. Braid, International Pipeline Conference 1 (1998) 459–472. [17] Jian Li, M. Elboujdaini, R.W. Revie and M. Gao, Effect of hydrostatic tests on existing SCC, submitted for publication.