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Effect of grain orientation on chemical etching
Micron 43 (2012) 349–351
Contents lists available at SciVerse ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Effect of grain orientation on chemical etching Peter J. Szabó a,∗ , Attila Bonyár b,1 a b
Budapest University of Technology and Economics, Department of Materials Science and Engineering, H-1111 Budapest, Bertalan L. u. 7., Hungary Budapest University of Technology and Economics, Department of Electronics Technology, H-1111 Budapest, Goldmann sqr. 3., Hungary
a r t i c l e
i n f o
Article history: Received 1 July 2011 Received in revised form 28 September 2011 Accepted 28 September 2011 Keywords: Color etching EBSD AFM Grain orientation
a b s t r a c t The effect of grain orientation on the effectiveness of pre-etching before color etching was investigated by the help of electron back scattering diffraction and atomic force microscopy in case of cast iron. Strong correlation was found between the angle between the specimen normal and the [0 0 1] orientation of the ferrite grains and the depth of the etching. If the angle between the specimen normal and the [0 0 1] direction of the ferritic grain is small, then the speed of the etching is low, but the lateral variation of the etching speed within the grain is larger. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sample preparation of metallic materials is the most important step when they are analyzed by optical microscope, scanning electron microscope or atomic force microscope. Their surface must be prepared carefully without introducing artifacts. The final microstructure can then be made visible by different etching techniques (Vander Voort, 1984). Color etching has become an increasingly popular and powerful technique, because phase and orientation specific coloring allows automated analysis with image analyzer software Some authors presented the application of several color etchants for identifying the phases in cast iron (Radzikowska, 2005) and in steel (San Martin et al., 2007). In most cases the different phases were well identified, therefore it was possible to characterize their microstructure by image analyzing methods. By determining the amount of different tissues elements and phases and by quantifying the diameter and shape of the spheroid graphite grains, the connection between the microstructure and mechanical properties can be well described, such as tensile strength vs. spheroidic graphite grain size, or tensile strength vs. amount of pearlite. In a former paper (Szabó and Kardos, 2010) the authors reported a correlation between grain orientation and the shade of the color etching. They found that there was a strong correlation between the luminescence and the R, G, B intensity of the color and the angle
∗ Corresponding author. Tel.: +36 1 463 3252; fax: +36 1 463 1366. E-mail addresses: [email protected] (P.J. Szabó), [email protected] (A. Bonyár). 1 Tel.: +36 1 463 4267; fax: +36 1 463 4118. 0968-4328/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2011.09.015
between the specimen normal and the 1 0 0 direction. Such correlation was not observed between the color parameters and the 1 1 0 and 1 1 1 directions, respectively. This indicates that film thickness is sensitive to the 1 0 0 direction of the crystal. The difference in color shade is expected to originate from the thickness variation of the film formed on the pre-etched surface of the specimen, but since the outer surface of the film appears to be smooth, the pre-etching must be uneven. Orientation dependent topography formation has also been reported for mechanical polishing (Zhu, 2004) and FIB cutting (Lenius et al., 2011). This paper shows the effect of grain orientation on the pre-etching. Surface topography was measured by atomic force microscopy (AFM), while crystal orientation was determined by electron backscattering diffraction (EBSD).
2. Experimental procedure Measurements were performed on a spheroidic cast iron specimen, its chemical composition and mechanical properties are listed in Table 1. In order to observe the microstructure of the material, a mechanical specimen preparation technique was applied (grinding, polishing), which was followed by a final polishing on 0.05 m colloid silica gel for 30 min in order to produce proper surface for EBSD measurements. As a positioning marker, a micro-Vickers hardness indent was used. The EBSD-measurements were carried out by a Philips XL-30 scanning electron microscope and an EDAX-TSL automated EBSD-system. After mechanical preparation of the specimen, it was chemically etched by 4% nital. Contact-mode AFM measurements were
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P.J. Szabó, A. Bonyár / Micron 43 (2012) 349–351
Table 1 Properties of the sample. Chemical composition (wt%)
Mechanical properties
C
Si
Mn
Mg
S
P
Rm (N/mm2 )
A5 (%)
HB
3.59
2.43
0.36
0.063
0.012
0.046
580
9.2
198
Fig. 1. (a) Inverse pole figure map of the specimen which indicates the crystal direction parallel to the sample normal. (b) Contact-mode AFM image of the specimen (scan size: 100 m × 80 m), where the heights of the individual points of the surface are gray scale coded. Grains are labeled according to a previous article (Szabó and Kardos, 2010).
performed with a Veeco diInnova scanning probe microscope (SPM) with Veeco MSCT-AUNM-10 probes utilizing the longest tip with small spring constant (k = 0.01 N/m). The sampling rate of the image presented in the paper is 521 × 512 obtained with 1 Hz scan rate. For data evaluation the Gwyddion 2.20 software was used [http://gwyddion.net]. The surface tilt angle of the grains was measured in the following way: first the tilt direction of the grain was assessed based on the 3D representation of the AFM image and then by cross sectional analysis the precise angle was measured. Due to the tilting of the grains the given height is measured at the centre of the grain which is taken as the average grain height. 3. Results and discussion Fig. 1a shows the inverse pole figure map of the specimen which indicates the crystal direction parallel to the sample normal. The AFM image of the same area can be seen in Fig. 1b. Based on the AFM image in Fig. 1b the different phases (pearlite, ferrite) can be easily distinguished due to morphological differences as well as the height differences between the ferrite grains. Due to the variation of the pre-etching speed the surface of the ferrite grains show tilting with
respect to the specimen surface, which vary both in direction and in magnitude of angle between the different grains. As an illustration of the grain tilt Fig. 2a shows a closer image of the specimen and Fig. 2b presents a cross sectional profile of grain 7 along the marker. For all the grains the surface tilt direction with the highest tilt angle was selected as grain tilt angle. After the EBSD-measurement, a clean-up procedure was applied: orientation of every pixel within a grain was substituted by the average orientation of that grain, thus local inhomogenity of orientation was eliminated. Angular differences between the specimen normal and the [0 0 1], [0 1 1] and [1 1 1] direction were measured. Results can be seen in Table 2, together with the results of grain surface tilt and height measurements by AFM. The best correlation between the angle and specimen height was observed in the case of [0 0 1] direction. Here the correlation coefficient is −0.9536. The correlation between the angle between specimen normal and the [0 0 1] direction and the grain tilt shows also an excellent correlation with correlation coefficient of −0.9238. Fig. 3 shows the graphs of angle-grain height and anglegrain tilt correlations.
Fig. 2. (a) Contact-mode AFM image of the specimen (scan size: 50 m × 50 m), where the heights of the individual points of the surface are gray scale coded. Grains are labeled according to a previous article (Szabó and Kardos, 2010). (b) Cross-sectional profile along the marker No. 1 in Fig. 2a.
P.J. Szabó, A. Bonyár / Micron 43 (2012) 349–351
351
Table 2 Angular differences between the specimen normal and the [0 0 1], [0 1 1] and [1 1 1] direction, and grain surface tilt and height. Grain ID
Angle with [0 0 1] (◦ )
Angle with [0 1 1] (◦ )
Angle with [1 1 1] (◦ )
Grain tilt (◦ )
Grain height (m)
1 2 7 8 10 11 12 13
20.9 30.5 12.1 35.2 43.3 5.9 35.1 28.2
30.1 20 36.6 9.8 13.9 39.1 14.2 18.6
34.2 28.2 42.7 35.8 22 50.3 28.1 32.9
0.33 0.26 0.81 0.26 0.15 0.85 0.25 0.29
300 310 490 250 170 520 220 350
irregular according to individual grain orientations. Grain heights and tilts correlate, which means that a grain which is etched faster, will have a less tilted surface. By other words, the etching is faster in certain crystallographic directions, but the lateral variation of the etching speed within the grain is smaller. According to AFM and EBSD measurements, the etching is slower if the grain normal is closer to the [0 0 1] direction. However, the gradient of etching speed is larger in this case. Besides this no strong correlation could be observed between the etching and the other crystallographic directions. 4. Conclusion
Fig. 3. Correlation between the angle between specimen normal and [0 0 1] direction and grain height and tilt.
The effect of crystal orientation on the effectiveness of preetching was investigated on gray cast iron specimens. Close correlation was shown between the angle of the surface normal and the [0 0 1] orientation and the amount of removed material. The gradient of the speed of the etching also strongly correlates with grain orientation. These effects elucidate the unevenness of the pre-etched surface, which causes the slight color differences of the specimen surface after color etching. Acknowledgement This work is connected to the scientific program of the “Development of quality-oriented and harmonized R+D+I strategy and functional model at BME” project. This project is supported by the New Hungary Development Plan (Project ID: TÁMOP-4.2.1/B09/1/KMR-2010-0002). P.J. Szabó is grateful for the support of Bolyai János Scholarship. References
Fig. 4. Correlation between grain height vs. grain tilt and angle between specimen normal and [0 0 1] direction.
A strong correlation was observed also between the grain height and grain tilt. Here the correlation coefficient was 0.9448. Fig. 4 shows the corresponding graph. According to these results, the following conclusions can be drawn. During pre-etching with nital, the surface becomes
Lenius, M., Kree, R., Volkert, C.A., 2011. Influence of crystal orientation on pattern formation of focused-ion-beam milled Cu surfaces. Physical Review B 84 (3), 035451. Radzikowska, Janina M., 2005. Effect of specimen preparation on evaluation of cast iron microstructures. Materials Characterization 54 (May (4–5)), 287–304. San Martin, D., Rivera Diaz del Castillo, P.E.J., Peekstok, E., van der Zwaag, S., 2007. A new etching route for revealing the austenite grain boundaries in an 11.4% Cr precipitation hardening semi-austenitic stainless steel. Materials Characterization 58 (May (5)), 455–460. Szabó, P.J., Kardos, I., 2010. Correlation between grain orientation and the shade of color etching. Materials Characterization 61 (08), 814–817. Vander Voort, G., 1984. Metallography. In: Principles and Practice. McGraw-Hill Book Co, New York, ASM International, Materials Park, OH. Zhu, H., 2004. Chemical mechanical polishing (CMP) anisotropy in sapphire. Applied Surface Science 236 (1–4), 120–130.
Contents lists available at SciVerse ScienceDirect
Micron journal homepage: www.elsevier.com/locate/micron
Effect of grain orientation on chemical etching Peter J. Szabó a,∗ , Attila Bonyár b,1 a b
Budapest University of Technology and Economics, Department of Materials Science and Engineering, H-1111 Budapest, Bertalan L. u. 7., Hungary Budapest University of Technology and Economics, Department of Electronics Technology, H-1111 Budapest, Goldmann sqr. 3., Hungary
a r t i c l e
i n f o
Article history: Received 1 July 2011 Received in revised form 28 September 2011 Accepted 28 September 2011 Keywords: Color etching EBSD AFM Grain orientation
a b s t r a c t The effect of grain orientation on the effectiveness of pre-etching before color etching was investigated by the help of electron back scattering diffraction and atomic force microscopy in case of cast iron. Strong correlation was found between the angle between the specimen normal and the [0 0 1] orientation of the ferrite grains and the depth of the etching. If the angle between the specimen normal and the [0 0 1] direction of the ferritic grain is small, then the speed of the etching is low, but the lateral variation of the etching speed within the grain is larger. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Sample preparation of metallic materials is the most important step when they are analyzed by optical microscope, scanning electron microscope or atomic force microscope. Their surface must be prepared carefully without introducing artifacts. The final microstructure can then be made visible by different etching techniques (Vander Voort, 1984). Color etching has become an increasingly popular and powerful technique, because phase and orientation specific coloring allows automated analysis with image analyzer software Some authors presented the application of several color etchants for identifying the phases in cast iron (Radzikowska, 2005) and in steel (San Martin et al., 2007). In most cases the different phases were well identified, therefore it was possible to characterize their microstructure by image analyzing methods. By determining the amount of different tissues elements and phases and by quantifying the diameter and shape of the spheroid graphite grains, the connection between the microstructure and mechanical properties can be well described, such as tensile strength vs. spheroidic graphite grain size, or tensile strength vs. amount of pearlite. In a former paper (Szabó and Kardos, 2010) the authors reported a correlation between grain orientation and the shade of the color etching. They found that there was a strong correlation between the luminescence and the R, G, B intensity of the color and the angle
∗ Corresponding author. Tel.: +36 1 463 3252; fax: +36 1 463 1366. E-mail addresses: [email protected] (P.J. Szabó), [email protected] (A. Bonyár). 1 Tel.: +36 1 463 4267; fax: +36 1 463 4118. 0968-4328/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2011.09.015
between the specimen normal and the 1 0 0 direction. Such correlation was not observed between the color parameters and the 1 1 0 and 1 1 1 directions, respectively. This indicates that film thickness is sensitive to the 1 0 0 direction of the crystal. The difference in color shade is expected to originate from the thickness variation of the film formed on the pre-etched surface of the specimen, but since the outer surface of the film appears to be smooth, the pre-etching must be uneven. Orientation dependent topography formation has also been reported for mechanical polishing (Zhu, 2004) and FIB cutting (Lenius et al., 2011). This paper shows the effect of grain orientation on the pre-etching. Surface topography was measured by atomic force microscopy (AFM), while crystal orientation was determined by electron backscattering diffraction (EBSD).
2. Experimental procedure Measurements were performed on a spheroidic cast iron specimen, its chemical composition and mechanical properties are listed in Table 1. In order to observe the microstructure of the material, a mechanical specimen preparation technique was applied (grinding, polishing), which was followed by a final polishing on 0.05 m colloid silica gel for 30 min in order to produce proper surface for EBSD measurements. As a positioning marker, a micro-Vickers hardness indent was used. The EBSD-measurements were carried out by a Philips XL-30 scanning electron microscope and an EDAX-TSL automated EBSD-system. After mechanical preparation of the specimen, it was chemically etched by 4% nital. Contact-mode AFM measurements were
350
P.J. Szabó, A. Bonyár / Micron 43 (2012) 349–351
Table 1 Properties of the sample. Chemical composition (wt%)
Mechanical properties
C
Si
Mn
Mg
S
P
Rm (N/mm2 )
A5 (%)
HB
3.59
2.43
0.36
0.063
0.012
0.046
580
9.2
198
Fig. 1. (a) Inverse pole figure map of the specimen which indicates the crystal direction parallel to the sample normal. (b) Contact-mode AFM image of the specimen (scan size: 100 m × 80 m), where the heights of the individual points of the surface are gray scale coded. Grains are labeled according to a previous article (Szabó and Kardos, 2010).
performed with a Veeco diInnova scanning probe microscope (SPM) with Veeco MSCT-AUNM-10 probes utilizing the longest tip with small spring constant (k = 0.01 N/m). The sampling rate of the image presented in the paper is 521 × 512 obtained with 1 Hz scan rate. For data evaluation the Gwyddion 2.20 software was used [http://gwyddion.net]. The surface tilt angle of the grains was measured in the following way: first the tilt direction of the grain was assessed based on the 3D representation of the AFM image and then by cross sectional analysis the precise angle was measured. Due to the tilting of the grains the given height is measured at the centre of the grain which is taken as the average grain height. 3. Results and discussion Fig. 1a shows the inverse pole figure map of the specimen which indicates the crystal direction parallel to the sample normal. The AFM image of the same area can be seen in Fig. 1b. Based on the AFM image in Fig. 1b the different phases (pearlite, ferrite) can be easily distinguished due to morphological differences as well as the height differences between the ferrite grains. Due to the variation of the pre-etching speed the surface of the ferrite grains show tilting with
respect to the specimen surface, which vary both in direction and in magnitude of angle between the different grains. As an illustration of the grain tilt Fig. 2a shows a closer image of the specimen and Fig. 2b presents a cross sectional profile of grain 7 along the marker. For all the grains the surface tilt direction with the highest tilt angle was selected as grain tilt angle. After the EBSD-measurement, a clean-up procedure was applied: orientation of every pixel within a grain was substituted by the average orientation of that grain, thus local inhomogenity of orientation was eliminated. Angular differences between the specimen normal and the [0 0 1], [0 1 1] and [1 1 1] direction were measured. Results can be seen in Table 2, together with the results of grain surface tilt and height measurements by AFM. The best correlation between the angle and specimen height was observed in the case of [0 0 1] direction. Here the correlation coefficient is −0.9536. The correlation between the angle between specimen normal and the [0 0 1] direction and the grain tilt shows also an excellent correlation with correlation coefficient of −0.9238. Fig. 3 shows the graphs of angle-grain height and anglegrain tilt correlations.
Fig. 2. (a) Contact-mode AFM image of the specimen (scan size: 50 m × 50 m), where the heights of the individual points of the surface are gray scale coded. Grains are labeled according to a previous article (Szabó and Kardos, 2010). (b) Cross-sectional profile along the marker No. 1 in Fig. 2a.
P.J. Szabó, A. Bonyár / Micron 43 (2012) 349–351
351
Table 2 Angular differences between the specimen normal and the [0 0 1], [0 1 1] and [1 1 1] direction, and grain surface tilt and height. Grain ID
Angle with [0 0 1] (◦ )
Angle with [0 1 1] (◦ )
Angle with [1 1 1] (◦ )
Grain tilt (◦ )
Grain height (m)
1 2 7 8 10 11 12 13
20.9 30.5 12.1 35.2 43.3 5.9 35.1 28.2
30.1 20 36.6 9.8 13.9 39.1 14.2 18.6
34.2 28.2 42.7 35.8 22 50.3 28.1 32.9
0.33 0.26 0.81 0.26 0.15 0.85 0.25 0.29
300 310 490 250 170 520 220 350
irregular according to individual grain orientations. Grain heights and tilts correlate, which means that a grain which is etched faster, will have a less tilted surface. By other words, the etching is faster in certain crystallographic directions, but the lateral variation of the etching speed within the grain is smaller. According to AFM and EBSD measurements, the etching is slower if the grain normal is closer to the [0 0 1] direction. However, the gradient of etching speed is larger in this case. Besides this no strong correlation could be observed between the etching and the other crystallographic directions. 4. Conclusion
Fig. 3. Correlation between the angle between specimen normal and [0 0 1] direction and grain height and tilt.
The effect of crystal orientation on the effectiveness of preetching was investigated on gray cast iron specimens. Close correlation was shown between the angle of the surface normal and the [0 0 1] orientation and the amount of removed material. The gradient of the speed of the etching also strongly correlates with grain orientation. These effects elucidate the unevenness of the pre-etched surface, which causes the slight color differences of the specimen surface after color etching. Acknowledgement This work is connected to the scientific program of the “Development of quality-oriented and harmonized R+D+I strategy and functional model at BME” project. This project is supported by the New Hungary Development Plan (Project ID: TÁMOP-4.2.1/B09/1/KMR-2010-0002). P.J. Szabó is grateful for the support of Bolyai János Scholarship. References
Fig. 4. Correlation between grain height vs. grain tilt and angle between specimen normal and [0 0 1] direction.
A strong correlation was observed also between the grain height and grain tilt. Here the correlation coefficient was 0.9448. Fig. 4 shows the corresponding graph. According to these results, the following conclusions can be drawn. During pre-etching with nital, the surface becomes
Lenius, M., Kree, R., Volkert, C.A., 2011. Influence of crystal orientation on pattern formation of focused-ion-beam milled Cu surfaces. Physical Review B 84 (3), 035451. Radzikowska, Janina M., 2005. Effect of specimen preparation on evaluation of cast iron microstructures. Materials Characterization 54 (May (4–5)), 287–304. San Martin, D., Rivera Diaz del Castillo, P.E.J., Peekstok, E., van der Zwaag, S., 2007. A new etching route for revealing the austenite grain boundaries in an 11.4% Cr precipitation hardening semi-austenitic stainless steel. Materials Characterization 58 (May (5)), 455–460. Szabó, P.J., Kardos, I., 2010. Correlation between grain orientation and the shade of color etching. Materials Characterization 61 (08), 814–817. Vander Voort, G., 1984. Metallography. In: Principles and Practice. McGraw-Hill Book Co, New York, ASM International, Materials Park, OH. Zhu, H., 2004. Chemical mechanical polishing (CMP) anisotropy in sapphire. Applied Surface Science 236 (1–4), 120–130.