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The transition from interphase-precipitated carbides to fibrous carbides in a vanadium-containing medium-carbon steel
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Scripta Materialia 68 (2013) 829–832 www.elsevier.com/locate/scriptamat
The transition from interphase-precipitated carbides to fibrous carbides in a vanadium-containing medium-carbon steel Meng-Yang Chen, Hung-Wei Yen and Jer-Ren Yang⇑ Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 12 December 2012; revised 11 January 2013; accepted 14 January 2013 Available online 26 January 2013
The precipitation of vanadium carbides in idiomorphic ferrite, which was deliberately produced at a large prior austenite grainsized condition, in a vanadium-containing medium-carbon steel has been investigated. Transmission electron microscopy reveals that in the idiomorphic ferrite matrix, the interphase-precipitated carbides can be intimately connected with the fibrous carbides. Through the analysis of orientation relationships, the correlation between these two precipitation modes has been proposed. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Fibrous carbides; Interphase precipitation; Idiomorphic ferrite; Medium-carbon steel; Transmission electron microscopy
Precipitation hardening by alloy carbides has been applied to develop advanced high-strength low-alloy steels for automotive and construction applications owing to the significantly enhanced strength offered by this approach [1–3]. The mechanical properties of these materials are strongly determined by the density, size, morphology and distribution of the carbides. In mediumcarbon steels containing V, Ti, Mo and Cr, in addition to pearlitic cementite (Fe3C), two distinctive morphological features of carbides, namely interphase-precipitated carbides and fibrous carbides, frequently exist in allotriomorphic ferrite [1,4–10] and in pearlitic ferrite [11–13]. In the case of interphase-precipitated carbides, during the austenite (c) ! ferrite (a) transformation the particles of alloy carbides nucleate densely on the a/c interface, which then move to a new position, where the nucleation repeats. This reaction continues for many cycles and leaves well-aligned particles with regular spacing in the ferrite matrix [5,14,15]. If the addition of carbon or manganese is increased, interphase-precipitated carbides can occur in pearlitic ferrite during the austenite (c) ! pearlite transformation [12,13]. Interphase-precipitated carbides in pearlitic ferrite are finer than those in allotriomorphic ferrite because most carbon atoms are consumed by pearlitic cementite during the formation of pearlite. In previous related works, transmission electron microscopy (TEM) evidence has illustrated that the repeated movement of ledges
⇑ Corresponding
author. Tel.: +886 2 23632756; fax: +886 2 23634562; e-mail: [email protected]
advancing an interphase boundary can result in row-like dispersions of carbides in both allotriomorphic ferrite [16] and pearlitic ferrite [17]. The morphology of fibrous carbide appears to be analogous to that of pearlitic cementite, but it is fine and slender rather than lamellar [6,10]. The fibrous carbide is about an order of magnitude finer than pearlitic cementite, and generally grows to be straight and branchless in a single direction away from the transformation interface. Edmonds [10] proposed that the embryos of fibrous carbides initially nucleate on the a/c interface and then cooperatively grow with ferrite in the direction away from the a/c interface. The fibrous precipitation of VC in pearlitic ferrite was first reported by Khalid and Edmonds [12], but has been rarely reported or highlighted elsewhere. For vanadium steels, researchers have observed [10,18] that interphase-precipitated carbides were changed into fibrous carbides in localized regions of the ferrite matrix, and suggested that the most coherent regions of the a/c interface are the most sluggish and thus have the best possibility of developing fibrous precipitate. However, the exact circumstances bringing about the formation of fibrous carbides in the face of competing interphase-precipitated carbides have not yet been clarified. Direct TEM information on the orientation relationships for the transition of precipitation modes has still not been reported. The correlation between these two precipitation modes is intriguing and worthy of further investigation. In the present work, a
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.01.020
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vanadium-containing medium-carbon steel has been studied. Through isothermal treatments, production of fibrous carbides adjacent to the interphase-precipitated carbides has been achieved. Thereby, TEM can provide direct orientation information about the transition. The as-received material was a hot-drawn steel bar (with a diameter of 43 mm), produced by Tung Ho Steel Enterprise Corporation. The chemical composition of the steel bar was Fe–0.36C–0.33Si–1.35Mn–0.33V–0.013S (wt.%) with a slightly high level of sulfur (as the steel bar was made from scrap). The heat treatments for isothermal transformations were carried out in a Dilatronic III RDP deformation dilatometer manufactured by Theta Industries, Inc. All dilatometry specimens were prepared from the half-radius position of the original steel bars and machined into 3 mm diameter cylindrical rods 6 mm in length. The specimens were homogenized at 1200 °C for 3 days in quartz capsules containing pure argon, with subsequent quenching to room temperature. The homogenized specimens were austenitized at 1200 °C for 3 min then cooled to 650 °C at a cooling rate of 20 °C s1, held at that temperature for 1 h and finally quenched to room temperature at a cooling rate of 100 °C s1. The specimens for optical metallography and transmission electron microscopy were prepared from dilatometry specimens. The volume percentage of product phases in the optical specimens were determined by point counting method via the freeware, Image J [19]. TEM specimens were examined on a FEI Tecnai G2 20 and a Tecnai F30. The optical metallograph shown in Figure 1 was obtained from the dilatometry specimen. The high austenitization temperature led to the extremely large grain size of prior c (380 lm). The white-etched phase includes allotriomorphic ferrite (6 vol.%) and idiomorphic ferrite (22 vol.%), while the dark-etched phase is pearlite (72 vol.%). Allotriomorphic ferrite is the first phase to form during the cooling of austenite to the temperature below the Ar3 temperature; it usually nucleates heterogeneously at the boundaries of the austenite grains, and these boundaries rapidly become decorated with virtually continuous layers of polycrystalline ferrite. The morphology of this ferrite is like a layer and does not reflect its crystal symmetry. The newly formed allotriomorphic ferrite always has a rational orientation relationship with one of the adjacent austenite grains (leading to a partially coherent interface), but usually has a random orientation relationship with the other neighboring austenite grain (leading to an incoherent interface). As seen in Figure 1,
Figure 1. Optical metallograph taken from the dilatometry sample.
the former case with a low-energy interface possesses immobile broad facets at the prior austenite grain boundaries. For latter case, an irrational orientation relationship between allotriomorphic ferrite and austenite, the interface with a high energy is more mobile and becomes irregularly curved; allotriomorphic ferrite grows into the austenite with which it should have a random orientation relationship, while the retained austenite transforms to pearlite during further isothermal holding. It is worth noting that there is a large amount of idiomorphic ferrite (22 vol.%) within the prior austenite. Idiomorphic ferrite has a roughly equiaxed morphology and forms intragranularly at the inclusion. In this work, the inclusions are regarded as an important factor for the development of microstructure. The related TEM/EDX investigation has identified the inclusions to be MnS particles ranging in size from 10 to 80 nm (as shown in Supplementary Fig. 1). The optical metallograph of Figure 1 reflects that idiomorphic ferrite readily occurs in the steel as a result of an extremely large prior austenite grain size and a high density of inclusions. The large prior austenite grain size leads to the restriction of allotriomorphic ferrite formation, because the prior austenite grain boundary area per unit volume diminishes. Consequently, the intergranular nucleation sites available for allotriomorphic ferrite decreases dramatically, whereas the intragranular nucleation sites available for idiomorphic ferrite develop gradually (as seen in Supplementary Fig. 2). It is noted that, besides pearlite, idiomorphic ferrite becomes the main product, rather than allotriomorphic ferrite. The Vickers hardness of allotrimorphic ferrite grains and idiomorphic ferrite grains have been determined to be HV 249 ± 8 and 251 ± 12, respectively; each value was obtained from 50 measurements. The result shows that idiomorphic ferrite and allotriomorphic ferrite have nearly the same hardness. As the value of Vickers hardness for the normal ferrite is only about HV 180 [20], it is supposed that interphase-precipitated carbides and/or fibrous carbides have made a significant contribution to the strength of ferrite in the steel studied. Although the precipitation hardening in allotriomorphic ferrite via interphase-precipitated carbides [1,16,21] or fibrous carbides [22] has been investigated intensively, the related research work on precipitation hardening in idiomorphic ferrite has not yet been reported. In the present work the TEM investigation has focused on the carbide precipitation in idiomorphic ferrite. The montage of TEM micrographs in Figure 2 was taken from the region covering the interphase-precipitated carbides, the fibrous carbides and pearlite in the dilatometer specimen. It is revealed that the interphase-precipitated carbides are intimately connected with the fibrous carbides in the grain of idiomorphic ferrite. The development of fibrous carbides in idiomorphic ferrite can presumably be attributed to the large prior austenite grain size deterring the formation of allotriomorphic ferrite and the subsequent reaction for idiomorphic ferrite formation being rather sluggish. As analyzed in Figure 3, these carbides have been identified as VC with NaCl-type crystal structure. Different mechanisms must be involved with the transition of VC morphology from precipitation as rows of discrete particles to that as slender fibers. In previous work [23], it was suggested that a/c orientations affect the precipitation modes. However, this proposition
M.-Y. Chen et al. / Scripta Materialia 68 (2013) 829–832
Figure 2. A montage of TEM bright-field images taken from the specimen isothermally transformed at 650 °C for 1 h.
cannot explain the transition of precipitation modes in the present work as shown in Figures 2 and 3; as the interphase-precipitated carbides are connected with the fibrous carbides in the same idiomorphic ferrite grain, the a/c orientation should not change during the transformation. Direct crystallographic information about the transition from interphase-precipitated carbides to fibrous carbides within an idiomorphic ferrite grain is presented in Figure 3, which provides TEM bright-field and darkfield images with corresponding diffraction patterns. As analyzed in Figure 3d–f, both interphase-precipitated carbides and fibrous carbides are identified as VC with NaCltype crystal structure; both carbides are also within the same idiomorphic ferrite grain. In the case of precipitation from the supersaturated ferrite (e.g. tempered martensite), these metal carbides can adopt three variants of the Baker–Nutting orientation relationship (B-N OR)
Figure 3. (a) TEM bright-field image showing that interphase-precipitated carbides are intimately connected with fibrous carbides within a idiomorphic ferrite grain. (b) Dark-field image using the (0 0 2)VC reflection of interphase-precipitated carbides. (c) Dark-field image using the (0 0 2)VC reflection of fibrous carbides. (d) The corresponding selected-area diffraction pattern (SADP) revealing two variants of the Baker–Nutting orientation relationship (B-N OR). (e) SADP analysis showing that interphase-precipitated vanadium carbides adopt the first variant of the B-N OR with ferritic matrix, where subscript “i” represents interphase-precipitated vanadium carbides. (f) Other SADP analysis showing that fibrous vanadium carbides adopt the second variant of the B-N OR with the same ferritic matrix, where subscript “ii” represents the fibrous vanadium carbides.
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[24,25] with respect to the ferrite matrix. However, interphase-precipitated carbides adopt only one variant of the B-N OR with respect to the idiomorphic ferrite: (0 0 1)VC || (0 0 1)ferrite and ½1 1 0VC jj ½0 1 0ferrite , as identified in Figure 3e. Fibrous carbides also exhibit only one variant of the B-N OR with respect to the same idiomorphic ferrite: (0 0 1)VC || (1 0 0)ferrite and ½1 1 0VC jj ½0 1 0ferrite , as identified in Figure 3f. The latter variant can be produced from the former variant by a 90° rotation of VC crystal around ½1 1 0VC , as illustrated via stereographic projections in Supplementary Figure 3. The interphase precipitation is related with the discrete carbides distributed in planar sheets (also called the terrace planes), which lie approximately parallel to the c ! a transformation front. Previous investigators [26] have proposed a convincing suggestion for the formation of interphase-precipitated carbides with the single variant B-N OR as follows. The variant, of which the carbide broad plane (0 0 1)MC || (0 0 1)ferrite has the smallest angle with the a/c interface, is preferable to other variants during the interphase precipitation, and thereby the growth kinetic of carbide itself can be highly enhanced by interfacial diffusion and the interfacial energy between carbide/ ferrite can also be reduced. As will be seen later, this mechanism is not suitable for the fibrous carbides. The preferential development of carbide precipitation mode must depend on its broad plane being adopted during the carbide formation at the a/c interface. The TEM results in Figure 3e and f clearly indicate that the transition from the mode of interphase-precipitated carbides to that of fibrous carbides leads to a new selected variant via a 90° rotation of VC crystal around ½1 1 0VC . As a result, the carbide broad plane (0 0 1)MC || (0 0 1)a shifts from the position most closely aligned to the a/c interface to that almost perpendicular to the a/c interface. The TEM micrograph in Figure 2 indicates that the interphase-precipitated carbides first form and then fibrous carbides, followed by pearlite. It is therefore appropriate to conclude that the formation of fibrous carbides is preferable at the final stage of c ! a transformation. The sluggish reaction at the final stage of c ! a transformation may promote the formation of the fibrous carbide as the long-range diffusion will favor the growth of newly formed carbide. The addition of further alloying elements (e.g. Ni, Mn or Cr) has been proven to have the effect of slowing down the transformation, and at the same time increasing the amount of fibrous VC [5,10]. In this work, the large prior austenite grain size (380 lm) had a strong impact on the kinetics of the c ! a transformation. The intergranular nucleation sites available for allotriomorphic ferrite are expected to be retarded. Although significant numbers of intragranular nucleation sites available for idiomorphic ferrite occur, the growth rate of this intragranular ferrite is slow. It is obvious that the transition from interphase-precipitated carbides to fibrous carbides must be controlled by the mobility of the a/c interface. The TEM evidence in Figure 3 shows that these two precipitation modes possess their own unique variant of the B-N OR with respect to the same idiomorphic ferrite grain matrix. This crystallographic constraint is directly connected with the development of the most favorable habit plane of VC during precipitation. This selected habit plane of VC will grow and become the broad
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direct evidence, it is suggested that the transition involves a change of nucleation site from the terrace plane (for interphase-precipitated carbides) to the ledge (for fibrous carbides).
Figure 4. (a) HRTEM image and IFFT lattice image of the fibrous carbide; (b) FFT diffractogram obtained from the area of moire´ fringe; and (c) EDX of the fibrous carbide embedded in the ferrite matrix.
plane. For interphase-precipitated carbides, the characteristic banded dispersion of carbides has long been accepted to be associated with the ledge mechanism; the carbides nucleate on immobile terrace planes of the a/c interface, while ferrite grows via the movement of the mobile step planes (also called ledges). The sites of the ledges would be energetically favorable for nucleation of carbides, but the ledges move too rapidly to allow nuclei to form there. For interphase-precipitated carbides, that the ledges, which are of higher energy, are free from precipitates is an interesting behavior, as opposed to that of normal nucleation. After having compared the morphologies and orientation relationships for the two precipitation modes investigated here, it seems appropriate to suggest that for nucleation of fibrous carbides, the sites of the ledges become energetically preferable. A carbide nucleated at a ledge may grow nearly along the side of the ledge by an effective diffusion path, if the growth of carbide can be compatible with the movement of the transformation front. One is forced to conclude that the transformation kinetics dominates the crystallographic development. As the c ! a transformation becomes sluggish, the fibrous carbide may cooperatively grow with ferrite in the direction parallel to the direction of movement of the a/c interface. A HRTEM investigation of the fibrous carbide has been performed in this study, and an example of a lattice image of the fibrous carbide is presented in Figure 4. The fibrous carbide is so slender that it lies within the foil thickness. The overlapping of carbide and ferrite lattices leads to the development of a moire´ fringe contrast as illustrated by the dashed band in Figure 4a. The fast Fourier transformation (FFT) diffractogram of Figure 4b, obtained from the examined area as indicated by the marked rectangle in Figure 4a, shows that the fibrous carbide adopts the B-N OR with the ferrite matrix. By using inverse fast Fourier transformation (IFFT) for only the fibrous carbide, the lattice image of the fibrous carbide (as shown in the inset in Fig. 4a) demonstrates a slender morphology with a thickness of 3.06 nm. The nanoprobe EDX spectrum of the fibrous carbide is displayed in Figure 4c, and shows vanadium to be the main alloying element. In summary, TEM has provided morphological and crystallographic information about the transition from interphase-precipitated carbides to fibrous carbides in a vanadium-containing medium-carbon steel. From the
This work was carried out with financial support from the National Science Council of the Republic of China, Taiwan, under Contract NSC 98-2221-E-002055-MY3. The authors thank Tung Ho Steel Enterprise Corporation for providing the alloy steel.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.scriptamat.2013.01.020. [1] Y. Funakawa, T. Shiozaki, K. Tomita, T. Yamamoto, E. Maeda, ISIJ Int. 44 (2004) 1945. [2] S. Shanmugam, M. Tanniru, R.D.K. Misra, D. Panda, S. Jansto, Mater. Sci. Technol. 21 (2005) 165. [3] H. Aoyama, Design of Modern Highrise Reinforced Concrete Structures, Imperial College Press, London, 2001, 88. [4] A.D. Batte, R.W.K. Honeycombe, Met. Sci. 7 (1973) 160. [5] R.W.K. Honeycombe, Metall. Trans. A 7 (1976) 915. [6] D.V. Edmonds, R.W.K. Honeycombe, J. Iron Steel Inst. 211 (1973) 209. [7] F.G. Berry, R.W.K. Honeycombe, Metall. Trans. 1 (1970) 3279. [8] K.H. Campbell, R.W.K. Honeycombe, Met. Sci. 8 (1974) 197. [9] J.V. Bee, P.R. Howell, R.W.K. Honeycombe, Metall. Trans. 10A (1979) 1207. [10] D.V. Edmonds, J. Iron Steel Inst. 210 (1972) 363. [11] G. Fourlaris, A.J. Baker, G.D. Papadimitriou, Acta Metall. Mater. 43 (1995) 3733. [12] F.A. Khalid, D.V. Edmonds, Mater. Sci. Technol. 9 (1993) 384. [13] S.A. Parsons, D.V. Edmonds, Mater. Sci. Technol. 3 (1987) 894. [14] A.T. Davenpor, R.W.K. Honeycome, Proc. R. Soc. London, Ser. A 322 (1971) 191. [15] R.A. Ricks, P.R. Howell, Acta Metall. 31 (1983) 853–861. [16] H.W. Yen, P.Y. Chen, C.Y. Huang, J.R. Yang, Acta Mater. 59 (2011) 6264. [17] D.S. Zhou, G.J. Shiflet, Metall. Trans. A 22 (1991) 1349. [18] A.D. Batte, R.W.K. Honeycombe, J. Iron Steel Inst. 211 (1973) 284. [19]
Scripta Materialia 68 (2013) 829–832 www.elsevier.com/locate/scriptamat
The transition from interphase-precipitated carbides to fibrous carbides in a vanadium-containing medium-carbon steel Meng-Yang Chen, Hung-Wei Yen and Jer-Ren Yang⇑ Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 12 December 2012; revised 11 January 2013; accepted 14 January 2013 Available online 26 January 2013
The precipitation of vanadium carbides in idiomorphic ferrite, which was deliberately produced at a large prior austenite grainsized condition, in a vanadium-containing medium-carbon steel has been investigated. Transmission electron microscopy reveals that in the idiomorphic ferrite matrix, the interphase-precipitated carbides can be intimately connected with the fibrous carbides. Through the analysis of orientation relationships, the correlation between these two precipitation modes has been proposed. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Fibrous carbides; Interphase precipitation; Idiomorphic ferrite; Medium-carbon steel; Transmission electron microscopy
Precipitation hardening by alloy carbides has been applied to develop advanced high-strength low-alloy steels for automotive and construction applications owing to the significantly enhanced strength offered by this approach [1–3]. The mechanical properties of these materials are strongly determined by the density, size, morphology and distribution of the carbides. In mediumcarbon steels containing V, Ti, Mo and Cr, in addition to pearlitic cementite (Fe3C), two distinctive morphological features of carbides, namely interphase-precipitated carbides and fibrous carbides, frequently exist in allotriomorphic ferrite [1,4–10] and in pearlitic ferrite [11–13]. In the case of interphase-precipitated carbides, during the austenite (c) ! ferrite (a) transformation the particles of alloy carbides nucleate densely on the a/c interface, which then move to a new position, where the nucleation repeats. This reaction continues for many cycles and leaves well-aligned particles with regular spacing in the ferrite matrix [5,14,15]. If the addition of carbon or manganese is increased, interphase-precipitated carbides can occur in pearlitic ferrite during the austenite (c) ! pearlite transformation [12,13]. Interphase-precipitated carbides in pearlitic ferrite are finer than those in allotriomorphic ferrite because most carbon atoms are consumed by pearlitic cementite during the formation of pearlite. In previous related works, transmission electron microscopy (TEM) evidence has illustrated that the repeated movement of ledges
⇑ Corresponding
author. Tel.: +886 2 23632756; fax: +886 2 23634562; e-mail: [email protected]
advancing an interphase boundary can result in row-like dispersions of carbides in both allotriomorphic ferrite [16] and pearlitic ferrite [17]. The morphology of fibrous carbide appears to be analogous to that of pearlitic cementite, but it is fine and slender rather than lamellar [6,10]. The fibrous carbide is about an order of magnitude finer than pearlitic cementite, and generally grows to be straight and branchless in a single direction away from the transformation interface. Edmonds [10] proposed that the embryos of fibrous carbides initially nucleate on the a/c interface and then cooperatively grow with ferrite in the direction away from the a/c interface. The fibrous precipitation of VC in pearlitic ferrite was first reported by Khalid and Edmonds [12], but has been rarely reported or highlighted elsewhere. For vanadium steels, researchers have observed [10,18] that interphase-precipitated carbides were changed into fibrous carbides in localized regions of the ferrite matrix, and suggested that the most coherent regions of the a/c interface are the most sluggish and thus have the best possibility of developing fibrous precipitate. However, the exact circumstances bringing about the formation of fibrous carbides in the face of competing interphase-precipitated carbides have not yet been clarified. Direct TEM information on the orientation relationships for the transition of precipitation modes has still not been reported. The correlation between these two precipitation modes is intriguing and worthy of further investigation. In the present work, a
1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.01.020
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M.-Y. Chen et al. / Scripta Materialia 68 (2013) 829–832
vanadium-containing medium-carbon steel has been studied. Through isothermal treatments, production of fibrous carbides adjacent to the interphase-precipitated carbides has been achieved. Thereby, TEM can provide direct orientation information about the transition. The as-received material was a hot-drawn steel bar (with a diameter of 43 mm), produced by Tung Ho Steel Enterprise Corporation. The chemical composition of the steel bar was Fe–0.36C–0.33Si–1.35Mn–0.33V–0.013S (wt.%) with a slightly high level of sulfur (as the steel bar was made from scrap). The heat treatments for isothermal transformations were carried out in a Dilatronic III RDP deformation dilatometer manufactured by Theta Industries, Inc. All dilatometry specimens were prepared from the half-radius position of the original steel bars and machined into 3 mm diameter cylindrical rods 6 mm in length. The specimens were homogenized at 1200 °C for 3 days in quartz capsules containing pure argon, with subsequent quenching to room temperature. The homogenized specimens were austenitized at 1200 °C for 3 min then cooled to 650 °C at a cooling rate of 20 °C s1, held at that temperature for 1 h and finally quenched to room temperature at a cooling rate of 100 °C s1. The specimens for optical metallography and transmission electron microscopy were prepared from dilatometry specimens. The volume percentage of product phases in the optical specimens were determined by point counting method via the freeware, Image J [19]. TEM specimens were examined on a FEI Tecnai G2 20 and a Tecnai F30. The optical metallograph shown in Figure 1 was obtained from the dilatometry specimen. The high austenitization temperature led to the extremely large grain size of prior c (380 lm). The white-etched phase includes allotriomorphic ferrite (6 vol.%) and idiomorphic ferrite (22 vol.%), while the dark-etched phase is pearlite (72 vol.%). Allotriomorphic ferrite is the first phase to form during the cooling of austenite to the temperature below the Ar3 temperature; it usually nucleates heterogeneously at the boundaries of the austenite grains, and these boundaries rapidly become decorated with virtually continuous layers of polycrystalline ferrite. The morphology of this ferrite is like a layer and does not reflect its crystal symmetry. The newly formed allotriomorphic ferrite always has a rational orientation relationship with one of the adjacent austenite grains (leading to a partially coherent interface), but usually has a random orientation relationship with the other neighboring austenite grain (leading to an incoherent interface). As seen in Figure 1,
Figure 1. Optical metallograph taken from the dilatometry sample.
the former case with a low-energy interface possesses immobile broad facets at the prior austenite grain boundaries. For latter case, an irrational orientation relationship between allotriomorphic ferrite and austenite, the interface with a high energy is more mobile and becomes irregularly curved; allotriomorphic ferrite grows into the austenite with which it should have a random orientation relationship, while the retained austenite transforms to pearlite during further isothermal holding. It is worth noting that there is a large amount of idiomorphic ferrite (22 vol.%) within the prior austenite. Idiomorphic ferrite has a roughly equiaxed morphology and forms intragranularly at the inclusion. In this work, the inclusions are regarded as an important factor for the development of microstructure. The related TEM/EDX investigation has identified the inclusions to be MnS particles ranging in size from 10 to 80 nm (as shown in Supplementary Fig. 1). The optical metallograph of Figure 1 reflects that idiomorphic ferrite readily occurs in the steel as a result of an extremely large prior austenite grain size and a high density of inclusions. The large prior austenite grain size leads to the restriction of allotriomorphic ferrite formation, because the prior austenite grain boundary area per unit volume diminishes. Consequently, the intergranular nucleation sites available for allotriomorphic ferrite decreases dramatically, whereas the intragranular nucleation sites available for idiomorphic ferrite develop gradually (as seen in Supplementary Fig. 2). It is noted that, besides pearlite, idiomorphic ferrite becomes the main product, rather than allotriomorphic ferrite. The Vickers hardness of allotrimorphic ferrite grains and idiomorphic ferrite grains have been determined to be HV 249 ± 8 and 251 ± 12, respectively; each value was obtained from 50 measurements. The result shows that idiomorphic ferrite and allotriomorphic ferrite have nearly the same hardness. As the value of Vickers hardness for the normal ferrite is only about HV 180 [20], it is supposed that interphase-precipitated carbides and/or fibrous carbides have made a significant contribution to the strength of ferrite in the steel studied. Although the precipitation hardening in allotriomorphic ferrite via interphase-precipitated carbides [1,16,21] or fibrous carbides [22] has been investigated intensively, the related research work on precipitation hardening in idiomorphic ferrite has not yet been reported. In the present work the TEM investigation has focused on the carbide precipitation in idiomorphic ferrite. The montage of TEM micrographs in Figure 2 was taken from the region covering the interphase-precipitated carbides, the fibrous carbides and pearlite in the dilatometer specimen. It is revealed that the interphase-precipitated carbides are intimately connected with the fibrous carbides in the grain of idiomorphic ferrite. The development of fibrous carbides in idiomorphic ferrite can presumably be attributed to the large prior austenite grain size deterring the formation of allotriomorphic ferrite and the subsequent reaction for idiomorphic ferrite formation being rather sluggish. As analyzed in Figure 3, these carbides have been identified as VC with NaCl-type crystal structure. Different mechanisms must be involved with the transition of VC morphology from precipitation as rows of discrete particles to that as slender fibers. In previous work [23], it was suggested that a/c orientations affect the precipitation modes. However, this proposition
M.-Y. Chen et al. / Scripta Materialia 68 (2013) 829–832
Figure 2. A montage of TEM bright-field images taken from the specimen isothermally transformed at 650 °C for 1 h.
cannot explain the transition of precipitation modes in the present work as shown in Figures 2 and 3; as the interphase-precipitated carbides are connected with the fibrous carbides in the same idiomorphic ferrite grain, the a/c orientation should not change during the transformation. Direct crystallographic information about the transition from interphase-precipitated carbides to fibrous carbides within an idiomorphic ferrite grain is presented in Figure 3, which provides TEM bright-field and darkfield images with corresponding diffraction patterns. As analyzed in Figure 3d–f, both interphase-precipitated carbides and fibrous carbides are identified as VC with NaCltype crystal structure; both carbides are also within the same idiomorphic ferrite grain. In the case of precipitation from the supersaturated ferrite (e.g. tempered martensite), these metal carbides can adopt three variants of the Baker–Nutting orientation relationship (B-N OR)
Figure 3. (a) TEM bright-field image showing that interphase-precipitated carbides are intimately connected with fibrous carbides within a idiomorphic ferrite grain. (b) Dark-field image using the (0 0 2)VC reflection of interphase-precipitated carbides. (c) Dark-field image using the (0 0 2)VC reflection of fibrous carbides. (d) The corresponding selected-area diffraction pattern (SADP) revealing two variants of the Baker–Nutting orientation relationship (B-N OR). (e) SADP analysis showing that interphase-precipitated vanadium carbides adopt the first variant of the B-N OR with ferritic matrix, where subscript “i” represents interphase-precipitated vanadium carbides. (f) Other SADP analysis showing that fibrous vanadium carbides adopt the second variant of the B-N OR with the same ferritic matrix, where subscript “ii” represents the fibrous vanadium carbides.
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[24,25] with respect to the ferrite matrix. However, interphase-precipitated carbides adopt only one variant of the B-N OR with respect to the idiomorphic ferrite: (0 0 1)VC || (0 0 1)ferrite and ½1 1 0VC jj ½0 1 0ferrite , as identified in Figure 3e. Fibrous carbides also exhibit only one variant of the B-N OR with respect to the same idiomorphic ferrite: (0 0 1)VC || (1 0 0)ferrite and ½1 1 0VC jj ½0 1 0ferrite , as identified in Figure 3f. The latter variant can be produced from the former variant by a 90° rotation of VC crystal around ½1 1 0VC , as illustrated via stereographic projections in Supplementary Figure 3. The interphase precipitation is related with the discrete carbides distributed in planar sheets (also called the terrace planes), which lie approximately parallel to the c ! a transformation front. Previous investigators [26] have proposed a convincing suggestion for the formation of interphase-precipitated carbides with the single variant B-N OR as follows. The variant, of which the carbide broad plane (0 0 1)MC || (0 0 1)ferrite has the smallest angle with the a/c interface, is preferable to other variants during the interphase precipitation, and thereby the growth kinetic of carbide itself can be highly enhanced by interfacial diffusion and the interfacial energy between carbide/ ferrite can also be reduced. As will be seen later, this mechanism is not suitable for the fibrous carbides. The preferential development of carbide precipitation mode must depend on its broad plane being adopted during the carbide formation at the a/c interface. The TEM results in Figure 3e and f clearly indicate that the transition from the mode of interphase-precipitated carbides to that of fibrous carbides leads to a new selected variant via a 90° rotation of VC crystal around ½1 1 0VC . As a result, the carbide broad plane (0 0 1)MC || (0 0 1)a shifts from the position most closely aligned to the a/c interface to that almost perpendicular to the a/c interface. The TEM micrograph in Figure 2 indicates that the interphase-precipitated carbides first form and then fibrous carbides, followed by pearlite. It is therefore appropriate to conclude that the formation of fibrous carbides is preferable at the final stage of c ! a transformation. The sluggish reaction at the final stage of c ! a transformation may promote the formation of the fibrous carbide as the long-range diffusion will favor the growth of newly formed carbide. The addition of further alloying elements (e.g. Ni, Mn or Cr) has been proven to have the effect of slowing down the transformation, and at the same time increasing the amount of fibrous VC [5,10]. In this work, the large prior austenite grain size (380 lm) had a strong impact on the kinetics of the c ! a transformation. The intergranular nucleation sites available for allotriomorphic ferrite are expected to be retarded. Although significant numbers of intragranular nucleation sites available for idiomorphic ferrite occur, the growth rate of this intragranular ferrite is slow. It is obvious that the transition from interphase-precipitated carbides to fibrous carbides must be controlled by the mobility of the a/c interface. The TEM evidence in Figure 3 shows that these two precipitation modes possess their own unique variant of the B-N OR with respect to the same idiomorphic ferrite grain matrix. This crystallographic constraint is directly connected with the development of the most favorable habit plane of VC during precipitation. This selected habit plane of VC will grow and become the broad
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direct evidence, it is suggested that the transition involves a change of nucleation site from the terrace plane (for interphase-precipitated carbides) to the ledge (for fibrous carbides).
Figure 4. (a) HRTEM image and IFFT lattice image of the fibrous carbide; (b) FFT diffractogram obtained from the area of moire´ fringe; and (c) EDX of the fibrous carbide embedded in the ferrite matrix.
plane. For interphase-precipitated carbides, the characteristic banded dispersion of carbides has long been accepted to be associated with the ledge mechanism; the carbides nucleate on immobile terrace planes of the a/c interface, while ferrite grows via the movement of the mobile step planes (also called ledges). The sites of the ledges would be energetically favorable for nucleation of carbides, but the ledges move too rapidly to allow nuclei to form there. For interphase-precipitated carbides, that the ledges, which are of higher energy, are free from precipitates is an interesting behavior, as opposed to that of normal nucleation. After having compared the morphologies and orientation relationships for the two precipitation modes investigated here, it seems appropriate to suggest that for nucleation of fibrous carbides, the sites of the ledges become energetically preferable. A carbide nucleated at a ledge may grow nearly along the side of the ledge by an effective diffusion path, if the growth of carbide can be compatible with the movement of the transformation front. One is forced to conclude that the transformation kinetics dominates the crystallographic development. As the c ! a transformation becomes sluggish, the fibrous carbide may cooperatively grow with ferrite in the direction parallel to the direction of movement of the a/c interface. A HRTEM investigation of the fibrous carbide has been performed in this study, and an example of a lattice image of the fibrous carbide is presented in Figure 4. The fibrous carbide is so slender that it lies within the foil thickness. The overlapping of carbide and ferrite lattices leads to the development of a moire´ fringe contrast as illustrated by the dashed band in Figure 4a. The fast Fourier transformation (FFT) diffractogram of Figure 4b, obtained from the examined area as indicated by the marked rectangle in Figure 4a, shows that the fibrous carbide adopts the B-N OR with the ferrite matrix. By using inverse fast Fourier transformation (IFFT) for only the fibrous carbide, the lattice image of the fibrous carbide (as shown in the inset in Fig. 4a) demonstrates a slender morphology with a thickness of 3.06 nm. The nanoprobe EDX spectrum of the fibrous carbide is displayed in Figure 4c, and shows vanadium to be the main alloying element. In summary, TEM has provided morphological and crystallographic information about the transition from interphase-precipitated carbides to fibrous carbides in a vanadium-containing medium-carbon steel. From the
This work was carried out with financial support from the National Science Council of the Republic of China, Taiwan, under Contract NSC 98-2221-E-002055-MY3. The authors thank Tung Ho Steel Enterprise Corporation for providing the alloy steel.
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