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Three-dimensional analysis of ferrite allotrimorphs nucleated on grain boundary faces, edges and corners
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0–5 8 3
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Short communication
Three-dimensional analysis of ferrite allotrimorphs nucleated on grain boundary faces, edges and corners L. Cheng, X.L. Wan, K.M. Wu⁎ Hubei Provincial Key Laboratory for Systems Science on Metallurgical Processing, Wuhan University of Science and Technology, Wuhan 430081, China
AR TIC LE D ATA
ABSTR ACT
Article history:
The ferrite allotriomorphs nucleated on austenite grain boundary faces exhibit complex
Received 27 November 2009
morphologies. Most of them are of oblate ellipsoids in shape and some of them are plate-like
Received in revised form
or pyramid-like. Ferrite allotriomorphs nucleated on austenite grain boundary edges take
12 February 2010
the form of triangular pyramids or triangular bi-pyramids. Ferrite allotriomorphs nucleated
Accepted 16 February 2010
on austenite grain boundary corners are of irregular shapes. © 2010 Elsevier Inc. All rights reserved.
Keywords: Steels Phase transformation Microstructure Nucleation and growth Three-dimensional morphology
1.
Introduction
Grain boundary ferrite has a significant influence on the hardenability and mechanical properties of low and medium carbon steels [1,2]. The influence of precipitate morphology on mechanical properties is well established [2]. It is recently reported that the mechanical properties of steel weldments used in ship construction are particularly sensitive to the amount and morphology of the proeutectoid ferrite that forms [3]. The mechanical properties and microstructural evolution are thus strongly related to the nucleation sites, growth kinetics and morphologies of grain boundary ferrite precipitates. Serial sectioning and computer-aided reconstruction and visualization allow one to determine the overall shape, size and connectivity of microstructures [4,5]. The present work is focused on the three-dimensional morphology of ferrite
allotriomorphs nucleated on grain boundary faces, edges and corners in a low-carbon low alloy steel.
2.
Experimental
An Fe-0.09wt%C-1.48wt%Mn alloy was prepared by vacuum induction melting. The heat treatment and computer-aided 3-D reconstruction procedures are described elsewhere [4,5].
3.
Results and Discussion
Fig. 1 shows optical micrographs of ferrite allotriomorphs nucleated on austenite grain boundary faces in the specimen isothermally reacted at 690 °C for 7 s and 10 s. It is seen that
⁎ Corresponding author. Tel.: +86 27 68862772, 68893270; fax: + 86 27 68893261. E-mail addresses: [email protected], [email protected] (K.M. Wu). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.02.013
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0 –5 8 3
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Fig. 1 – Optical micrographs and 3-D reconstructed images of ferrite allotriomorphs nucleated at the grain boundary faces in the specimen isothermally reacted at 690 °C for (a) and (d) 7 s, and (b), (c), (e) and (f) 10 s and (g)–(l). 3-D reconstructed images of ferrite allotriomorphs designated as F1–F6 in (a)–(f). (a), (b), (d) and (e) at a magnification of 500×; (c) and (f) at a magnification of 200×. ferrite allotriomorphs follow the austenite grain boundary contours. Ferrite allotriomorphs marked with F1 and F2 have smooth interfaces with two adjacent austenite grains. Ferrite allotriomorphs marked with F4 and F5 have smooth interfaces with one adjacent austenite grain and grow into the interior of the other austenite grain. Ferrite allotriomorphs marked with F3 and F6 grow into the interior of two adjacent austenite grains. It is also seen that ferrite allotriomorphs do not impinge against each other due to a short holding time, which is beneficial to study the morphologies of individual
ferrite precipitates. The 3-D reconstructed images of ferrite allotriomorphs F1–6 are shown in Fig. 1g–l. Obviously, ferrite allotriomorph F1 is a plate (Fig. 1g) and those marked with F2 and F3 are oblate ellipsoids (Fig. 1h–i). Serial sectioning revealed that the long axis of them is perpendicular to the polished plane (Fig. 1b–c). F4 is clearly a triangle pyramid (Fig. 1j). It is evident that F5 and F6 are large in one dimension but small in the other two dimensions (Fig. 1k–l). Fig. 2 presents optical micrographs and 3-D reconstructed images of two individual ferrite allotriomorphs nucleated on
582
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0–5 8 3
Fig. 2 – Optical micrographs and 3-D reconstructed images of ferrite allotriomorphs nucleated at austenite grain boundary edges in the specimen isothermally held at 690 °C (a) for 7 s at a magnification of 500× and (b) for 10 s at a magnification of 200×; and (c) and (d). 3-D reconstructed images of ferrite allotriomorphs designated as F7 and F8 in (a) and (b).
grain boundary edges in the specimen isothermally reacted at 690 °C for 7 s and 10 s. The 3-D reconstructed image of the ferrite allotriomorph F7 is shown in Fig.2c. This ferrite allotriomorph shows a form of triangular bipyramid in its 3-D form. Fig. 2d illustrates an image of the ferrite allotriomorph F8 nucleated on another grain boundary edge, which is in a form of a triangular bi-pyramid, but it is more elongated as
compared with ferrite allotriomorph F7. It is found that ferrite allotriomorphs nucleated at grain boundary edges are more like triangular bi-pyramids than prolate ellipsoids. On a random 2-D section, it is very difficult to identify a ferrite allotriomorph formed on grain boundary corners. Serial sectioning provides a useful tool to do that. Fig. 3 shows optical micrographs and 3-D reconstructed images of a ferrite
Fig. 3 – (a), (b) and (c) optical micrographs at a magnification of 500× and (d), (e) and (f). 3-D reconstructed images, viewed from different perspectives, of a ferrite allotriomorph nucleated at a grain boundary corner in the specimen isothermally reacted at 690 °C for 7 s.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0 –5 8 3
allotriomorph nucleated on a grain boundary corner in the specimen isothermally reacted at 690 °C for 7 s. Three separate sections selected from a 2-D stack are shown in Fig. 3a–c. The grains marked with A1–A4 are four adjacent austenite grains, which form a grain boundary corner. As shown in Fig. 3a, A4 is surrounded by A1, A2 and A3. As serial sectioning proceeded, A4 gradually became smaller (see Fig. 3b), and finally disappeared (see Fig. 3c). Therefore, it is deduced that A1, A2, A3 and A4 form a grain boundary corner. In Fig. 3d–f are shown different views of a 3-D reconstructed ferrite allotriomorph nucleated at the above-mentioned grain boundary corner. It is clear that the ferrite allotriomorph nucleated on a grain boundary corner is of a complex shape. Ferrite allotriomorphs nucleated on grain boundary faces are often modeled as oblate ellipsoids [6]. Based on these 3-D observations, it is found that the morphologies of grain boundary ferrite allotriomorphs are different from each other, even when they are nucleated at the same austenite grain boundary face. Actually, according to the present work, ferrite allotriomorphs nucleated at the grain boundary faces are of complex morphologies with the presence of oblate ellipsoids, prolate ellipsoids, pyramids and plates. The pyramid-like or spike-like ferrite precipitates formed on the grain boundary were firstly revealed by means of 3-D reconstruction by Spanos and Kral in an Fe–C–Ni alloy [1]. The prolate ellipsoid was also observed by breaking the specimen along the prior austenite boundary [7]. Grain boundary corners and edges are considered to be the preferential sites of ferrite allotriomorph nucleation. Ferrite precipitates nucleated at grain boundary corners and edges gradually grow at the expense of grain boundary corners and edges. Three-dimensional reconstruction observations and analyses demonstrate that ferrite allotriomorphs nucleated at grain boundary corners and edges usually take the shape formed by the adjacent austenite grain boundaries.
4.
Summary (1) Ferrite allotriomorphs nucleated at austenite grain boundary faces are oblate ellipsoids in shape on usual two-dimensional planar sections. They have complex
583
morphologies, even on the same grain boundary face. Most of them are oblate ellipsoids or prolate ellipsoids. Some of them have a plate-like shape or like a pyramid. (2) Ferrite allotriomorphs nucleated at austenite grain boundary edges take the form of triangular pyramids or triangular bi-pyramids. Ferrite allotriomorphs nucleated at austenite grain boundary corners exhibit complex shapes formed by adjacent austenite grain boundaries.
Acknowledgements The authors express their thanks to Professor M. Enomoto, Ibaraki University, for providing them with alloy specimen. The authors gratefully acknowledge the support from NSFC (National Natural Science Foundation of China) under Grant 50734004.
REFERENCES [1] Kral MV, Spanos G. Three-dimensional analysis and classification of grainboundary- nucleated proeutectoid ferrite precipitate. Metall Mater Trans A 2005;36A:1199–207. [2] Dieter GE. Mechanical metallurgy. New York: McGraw-Hill; 1961. [3] Spanos G, Moon DW, Fonda RW, Menon ESK, Fox AG. Microstructural, compositional, and microhardness variations across a gas–metal arc weldment made with an ultralow-carbon consumable. Metall Mater Trans A 2001;32:3043–54. [4] Cheng L, Wu KM. Three-dimensional morphology of grain boundary Widmanstätten ferrite in a low carbon low alloy steel. Mater Des 2010;61:192–7. [5] Cheng L, Wu KM. New insight into intragranular ferrite in a low-carbon low-alloy steel. Acta Mater 2009;57:3754–62. [6] Atkinson C, Aaron HB, Kinsman KR, Aaronson HI. On the growth kinetics of grain boundary ferrite allotriomorphs. Metall Trans 1973;4:783–92. [7] Enomoto M, Lange III WF, Aaronson HI. The kinetics of ferrite nucleation at austenite grain edges in Fe–C and Fe–C–X alloys. Metall Trans A 1986;17:1399–407.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Short communication
Three-dimensional analysis of ferrite allotrimorphs nucleated on grain boundary faces, edges and corners L. Cheng, X.L. Wan, K.M. Wu⁎ Hubei Provincial Key Laboratory for Systems Science on Metallurgical Processing, Wuhan University of Science and Technology, Wuhan 430081, China
AR TIC LE D ATA
ABSTR ACT
Article history:
The ferrite allotriomorphs nucleated on austenite grain boundary faces exhibit complex
Received 27 November 2009
morphologies. Most of them are of oblate ellipsoids in shape and some of them are plate-like
Received in revised form
or pyramid-like. Ferrite allotriomorphs nucleated on austenite grain boundary edges take
12 February 2010
the form of triangular pyramids or triangular bi-pyramids. Ferrite allotriomorphs nucleated
Accepted 16 February 2010
on austenite grain boundary corners are of irregular shapes. © 2010 Elsevier Inc. All rights reserved.
Keywords: Steels Phase transformation Microstructure Nucleation and growth Three-dimensional morphology
1.
Introduction
Grain boundary ferrite has a significant influence on the hardenability and mechanical properties of low and medium carbon steels [1,2]. The influence of precipitate morphology on mechanical properties is well established [2]. It is recently reported that the mechanical properties of steel weldments used in ship construction are particularly sensitive to the amount and morphology of the proeutectoid ferrite that forms [3]. The mechanical properties and microstructural evolution are thus strongly related to the nucleation sites, growth kinetics and morphologies of grain boundary ferrite precipitates. Serial sectioning and computer-aided reconstruction and visualization allow one to determine the overall shape, size and connectivity of microstructures [4,5]. The present work is focused on the three-dimensional morphology of ferrite
allotriomorphs nucleated on grain boundary faces, edges and corners in a low-carbon low alloy steel.
2.
Experimental
An Fe-0.09wt%C-1.48wt%Mn alloy was prepared by vacuum induction melting. The heat treatment and computer-aided 3-D reconstruction procedures are described elsewhere [4,5].
3.
Results and Discussion
Fig. 1 shows optical micrographs of ferrite allotriomorphs nucleated on austenite grain boundary faces in the specimen isothermally reacted at 690 °C for 7 s and 10 s. It is seen that
⁎ Corresponding author. Tel.: +86 27 68862772, 68893270; fax: + 86 27 68893261. E-mail addresses: [email protected], [email protected] (K.M. Wu). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.02.013
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0 –5 8 3
581
Fig. 1 – Optical micrographs and 3-D reconstructed images of ferrite allotriomorphs nucleated at the grain boundary faces in the specimen isothermally reacted at 690 °C for (a) and (d) 7 s, and (b), (c), (e) and (f) 10 s and (g)–(l). 3-D reconstructed images of ferrite allotriomorphs designated as F1–F6 in (a)–(f). (a), (b), (d) and (e) at a magnification of 500×; (c) and (f) at a magnification of 200×. ferrite allotriomorphs follow the austenite grain boundary contours. Ferrite allotriomorphs marked with F1 and F2 have smooth interfaces with two adjacent austenite grains. Ferrite allotriomorphs marked with F4 and F5 have smooth interfaces with one adjacent austenite grain and grow into the interior of the other austenite grain. Ferrite allotriomorphs marked with F3 and F6 grow into the interior of two adjacent austenite grains. It is also seen that ferrite allotriomorphs do not impinge against each other due to a short holding time, which is beneficial to study the morphologies of individual
ferrite precipitates. The 3-D reconstructed images of ferrite allotriomorphs F1–6 are shown in Fig. 1g–l. Obviously, ferrite allotriomorph F1 is a plate (Fig. 1g) and those marked with F2 and F3 are oblate ellipsoids (Fig. 1h–i). Serial sectioning revealed that the long axis of them is perpendicular to the polished plane (Fig. 1b–c). F4 is clearly a triangle pyramid (Fig. 1j). It is evident that F5 and F6 are large in one dimension but small in the other two dimensions (Fig. 1k–l). Fig. 2 presents optical micrographs and 3-D reconstructed images of two individual ferrite allotriomorphs nucleated on
582
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0–5 8 3
Fig. 2 – Optical micrographs and 3-D reconstructed images of ferrite allotriomorphs nucleated at austenite grain boundary edges in the specimen isothermally held at 690 °C (a) for 7 s at a magnification of 500× and (b) for 10 s at a magnification of 200×; and (c) and (d). 3-D reconstructed images of ferrite allotriomorphs designated as F7 and F8 in (a) and (b).
grain boundary edges in the specimen isothermally reacted at 690 °C for 7 s and 10 s. The 3-D reconstructed image of the ferrite allotriomorph F7 is shown in Fig.2c. This ferrite allotriomorph shows a form of triangular bipyramid in its 3-D form. Fig. 2d illustrates an image of the ferrite allotriomorph F8 nucleated on another grain boundary edge, which is in a form of a triangular bi-pyramid, but it is more elongated as
compared with ferrite allotriomorph F7. It is found that ferrite allotriomorphs nucleated at grain boundary edges are more like triangular bi-pyramids than prolate ellipsoids. On a random 2-D section, it is very difficult to identify a ferrite allotriomorph formed on grain boundary corners. Serial sectioning provides a useful tool to do that. Fig. 3 shows optical micrographs and 3-D reconstructed images of a ferrite
Fig. 3 – (a), (b) and (c) optical micrographs at a magnification of 500× and (d), (e) and (f). 3-D reconstructed images, viewed from different perspectives, of a ferrite allotriomorph nucleated at a grain boundary corner in the specimen isothermally reacted at 690 °C for 7 s.
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 5 8 0 –5 8 3
allotriomorph nucleated on a grain boundary corner in the specimen isothermally reacted at 690 °C for 7 s. Three separate sections selected from a 2-D stack are shown in Fig. 3a–c. The grains marked with A1–A4 are four adjacent austenite grains, which form a grain boundary corner. As shown in Fig. 3a, A4 is surrounded by A1, A2 and A3. As serial sectioning proceeded, A4 gradually became smaller (see Fig. 3b), and finally disappeared (see Fig. 3c). Therefore, it is deduced that A1, A2, A3 and A4 form a grain boundary corner. In Fig. 3d–f are shown different views of a 3-D reconstructed ferrite allotriomorph nucleated at the above-mentioned grain boundary corner. It is clear that the ferrite allotriomorph nucleated on a grain boundary corner is of a complex shape. Ferrite allotriomorphs nucleated on grain boundary faces are often modeled as oblate ellipsoids [6]. Based on these 3-D observations, it is found that the morphologies of grain boundary ferrite allotriomorphs are different from each other, even when they are nucleated at the same austenite grain boundary face. Actually, according to the present work, ferrite allotriomorphs nucleated at the grain boundary faces are of complex morphologies with the presence of oblate ellipsoids, prolate ellipsoids, pyramids and plates. The pyramid-like or spike-like ferrite precipitates formed on the grain boundary were firstly revealed by means of 3-D reconstruction by Spanos and Kral in an Fe–C–Ni alloy [1]. The prolate ellipsoid was also observed by breaking the specimen along the prior austenite boundary [7]. Grain boundary corners and edges are considered to be the preferential sites of ferrite allotriomorph nucleation. Ferrite precipitates nucleated at grain boundary corners and edges gradually grow at the expense of grain boundary corners and edges. Three-dimensional reconstruction observations and analyses demonstrate that ferrite allotriomorphs nucleated at grain boundary corners and edges usually take the shape formed by the adjacent austenite grain boundaries.
4.
Summary (1) Ferrite allotriomorphs nucleated at austenite grain boundary faces are oblate ellipsoids in shape on usual two-dimensional planar sections. They have complex
583
morphologies, even on the same grain boundary face. Most of them are oblate ellipsoids or prolate ellipsoids. Some of them have a plate-like shape or like a pyramid. (2) Ferrite allotriomorphs nucleated at austenite grain boundary edges take the form of triangular pyramids or triangular bi-pyramids. Ferrite allotriomorphs nucleated at austenite grain boundary corners exhibit complex shapes formed by adjacent austenite grain boundaries.
Acknowledgements The authors express their thanks to Professor M. Enomoto, Ibaraki University, for providing them with alloy specimen. The authors gratefully acknowledge the support from NSFC (National Natural Science Foundation of China) under Grant 50734004.
REFERENCES [1] Kral MV, Spanos G. Three-dimensional analysis and classification of grainboundary- nucleated proeutectoid ferrite precipitate. Metall Mater Trans A 2005;36A:1199–207. [2] Dieter GE. Mechanical metallurgy. New York: McGraw-Hill; 1961. [3] Spanos G, Moon DW, Fonda RW, Menon ESK, Fox AG. Microstructural, compositional, and microhardness variations across a gas–metal arc weldment made with an ultralow-carbon consumable. Metall Mater Trans A 2001;32:3043–54. [4] Cheng L, Wu KM. Three-dimensional morphology of grain boundary Widmanstätten ferrite in a low carbon low alloy steel. Mater Des 2010;61:192–7. [5] Cheng L, Wu KM. New insight into intragranular ferrite in a low-carbon low-alloy steel. Acta Mater 2009;57:3754–62. [6] Atkinson C, Aaron HB, Kinsman KR, Aaronson HI. On the growth kinetics of grain boundary ferrite allotriomorphs. Metall Trans 1973;4:783–92. [7] Enomoto M, Lange III WF, Aaronson HI. The kinetics of ferrite nucleation at austenite grain edges in Fe–C and Fe–C–X alloys. Metall Trans A 1986;17:1399–407.