A transmission electron microscopy and atom-probe tomography study of martensite morphology and composition in a dual-phase steel

Journal Pre-proof A transmission electron microscopy and atom-probe tomography study of martensite morphology and composition in a dual-phase steel

Dong An, Sung-Il Baik, Qingqiang Ren, Ming Jiang, Mingfang Zhu, Dieter Isheim, Bruce W. Krakauer, David N. Seidman PII:

S1044-5803(19)32324-1

DOI:

https://doi.org/10.1016/j.matchar.2020.110207

Reference:

MTL 110207

To appear in:

Materials Characterization

Received date:

26 August 2019

Revised date:

17 February 2020

Accepted date:

18 February 2020

Please cite this article as: D. An, S.-I. Baik, Q. Ren, et al., A transmission electron microscopy and atom-probe tomography study of martensite morphology and composition in a dual-phase steel, Materials Characterization (2018), https://doi.org/10.1016/ j.matchar.2020.110207

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© 2018 Published by Elsevier.

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A transmission electron microscopy and atom-probe tomography study of martensite morphology and composition in a dual-phase steel Dong Ana,b, Sung-Il Baikb,c, Qingqiang Renb, Ming Jianga, Mingfang Zhua,*, Dieter Isheimb,c, Bruce W. Krakauerd, David N. Seidmanb,c,* a

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Jiangsu Key Laboratory for Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China b Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA c Northwestern University Center for Atom Probe Tomography (NUCAPT), Evanston, IL 60208, USA d A. O. Smith Corporation, Milwaukee, WI 53224, USA * Corresponding authors: [email protected], [email protected]

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Abstract: Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom-probe tomography (APT) are utilized to study systematically the morphology and solute distributions in martensite formed after intercritical annealing in a dual-phase steel. The SEM observations demonstrate that the samples annealed at 760 °C for 5 min, and either water-quenched or air-cooled, contain martensite with a volume fraction of ~0.17 or ~0.10, respectively, in the vicinity of former ferrite/ferrite grain boundaries. APT measurements reveal that the levels of C and Mn in martensite are higher in the air-cooled sample than in the water-quenched sample. Combined TEM and APT analyses show that the water-quenched sample forms typical lath martensite with a high density of dislocations. In contrast, the air-cooled sample contains martensite with complex substructures, including fine twins, a mixture of laths and twins, and carbide precipitates, as well as a small portion of retained austenite that is highly enriched in C and Mn. The formation mechanisms of different martensites are discussed based on the experimental observations and thermodynamic calculations. The results provide insights into the phase transformations and corresponding microstructures formed during intercritical annealing, which are followed by either water quenching or air cooling at a moderate rate. Keywords: dual-phase steel; martensite; intercritical annealing; transmission electron microscopy; atom-probe tomography 1. Introduction Dual-phase (DP) steels are advanced high strength steels (AHSS) composed of two phases, normally a ferrite matrix and a dispersed second-phase of martensite [1,2]. Such materials have been widely utilized for automotive, architectural frame and boiler tank applications because they exhibit an excellent combination of strength and ductility [1–4]. DP steels are currently in high demand due to the broad range of mechanical properties that can be produced by various heat treatments, as a result of corresponding microstructural changes. The role of martensite (M), as a hardening and load-bearing phase, is the key to the design of DP steels [1–4]. Normally, the martensite in DP steels is produced in the ferrite (α)/austenite (γ) two-phase region by quenching, after annealing at an intercritical temperature. Since the martensitic 1

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transformation is a diffusionless and displacive transformation [5,6], martensite inherits the composition of the parent γ-phase formed during annealing. It is not difficult to make thermodynamic calculations to predict the composition of martensite produced by quenching from the austenite phase [2,7]. There are numerous articles in the archival literature discussing the dependence of martensite morphology and properties on the C content [5,6]; e.g., lath martensite is formed in low-carbon steels [8–10] and plate (twinned) martensite tends to be formed in high-carbon steels [11–13]. This information can be used to adjust the overall composition, as well as the annealing temperature and time required to obtain the martensite morphology that can provide the desirable mechanical properties in a DP steel. In addition to the annealing temperature and time, the subsequent cooling rate utilized in the processing also affects the volume fraction and composition of austenite, and hence the hardenability, which in turn determines the dual-phase microstructure and corresponding mechanical properties [1,14–16]. Cooling rates that are slower than water quenching have been shown to produce good strength-ductility combinations in low-alloyed DP steels [1]. An understanding of the phase transformations that take place during continuous cooling of a DP steel is, therefore, an important consideration in optimizing the intercritical annealing process. There are a number of archival literature reports concerning the transformation products resulting from different cooling rates in low-alloyed steels [14–21]. Most of these studies investigated continuous cooling transformations in samples annealed supercritically (above the Ac3 temperature) [17–21], while only a few have dealt with the intercritical temperature range [14–16]. Studies also discussed the substructures formed in different types of martensite [16–19]. Martensite produced by continuous cooling was shown to present morphologies that were somewhat different to those observed in quenched martensite [16]. Atom-probe tomography (APT) has recently emerged as a powerful technique for measuring the composition of materials [22,23]. The unparalleled combination of atomic imaging in three-dimensions and sub-nanometer scale spatial resolution, enables a more accurate mapping of solute atoms than was heretofore possible with more traditional methods [22,23]. APT has been applied extensively in intensive studies of different types of steels [24–34]. The current applications of APT to martensite are, however, mainly focused on quenched martensite in martensitic steels [29,30], DP steels [31,32] and quenching and partitioning (Q&P) steels [33,34]. Less attention has been given to detailed analyses of martensite produced by intercritical annealing followed by continuous cooling. Thus, the underlying mechanisms of martensite formation during continuous cooling, and the correlation between composition and morphology of this type of martensite still remain unclear. Herein we use scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom-probe tomography (APT), to investigate the morphology and atomic solute-distributions in different types of martensite formed in a low-alloyed DP steel produced by air cooling or water quenching, after intercritical annealing at 760 °C for 5 min. Based on a combination of TEM and APT analyses, as well as thermodynamic calculations, we now present and discuss solute partitioning between ferrite and austenite during intercritical annealing, the formation mechanisms of martensite, and the correlation between martensite composition and morphology. 2. Experimental procedures The subject of this study was a commercially produced, cold-rolled, DP steel with the nominal composition, Fe-0.323C-1.231Mn-0.849Si-0.091P-0.078Al-0.043Cr-0.001V (at.%). As-rolled samples with the dimensions 200×80×1.2 mm3, consisting of a ferrite matrix and dispersed 2

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3. Results

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spheroidized carbides, were annealed at an intercritical temperature of 760 °C for 5 min in a box-type furnace, followed by air cooling (AC, ~6 °C s-1) to room temperature. Other samples were water quenched (WQ) after annealing at 760 °C for 5 min for comparison with the AC samples. The microstructures of the polished and 4% Nital etched specimens were observed with a Hitachi SU8030 field-emission scanning electron microscope (SEM). The volume fraction of the second-phase was measured using ImageTool software [35]. Samples for TEM were prepared from sectioned rectangular samples, which were mechanically polished from both sides to 40 μm. Discs with a 3 mm diameter were punched out and electropolished with an MTP-1A electropolisher until perforation occurred. The electrolyte consisted of 8% perchloric acid and 92% ethanol. The microstructure and crystallography of the thinned TEM samples were characterized utilizing a FEI Tecnai G2 TEM operating at 200 kV. Selective area diffraction patterns (SADPs) were indexed using the programs CrystalMaker and SingleCrystal [36]. Nanotips for APT were prepared by Ga+ ion milling employing an FEI Helios dual-beam focused-ion beam (FIB) microscope utilizing a final Ga+ energy of 2 keV at an ion-beam current of 46 pA. Site-specific preparation of APT nanotips was performed by lifting-out specimens from selected regions from the bulk sample, and then transferring them to Si micro-posts on coupons [37,38]. A LEAP4000X Si local-electrode atom-probe tomograph (Cameca, Madison, WI) was employed to analyze the compositions of APT nanotips. Picosecond pulses from an ultraviolet laser (355 nm wavelength) were utilized to activate field-evaporation of individual atoms, at a pulse repetition rate of 250 kHz, a laser pulse energy of 20 pJ per pulse, and an average detection rate of 0.01 ions per pulse. The specimen stage was maintained at 54 K (nanotip temperature = 60 K), and the gauge pressure was < 2.0×10−11 Torr. Data analyses and three-dimensional (3-D) reconstructions were performed utilizing the program IVAS 3.6.12 (Cameca, Madison, WI). Compositional information was obtained by employing the proximity histogram concentration profile methodology for APT [39].

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3.1 Scanning electron microscope microstructures Fig. 1 displays SEM images of the WQ and AC samples, and locally highly magnified regions are displayed in the insets. A phase map of the AC sample detected by electron back-scattered diffraction (EBSD) is presented in Fig. 1(b). The WQ and AC microstructures consist of polygonal ferrite grains labeled as α and second-phases distributed at the α/α grain boundaries (GBs). The second-phases in the WQ sample, (Fig. 1(a)), are determined to be martensite (M) and are labeled as M. The fine substructure is considered to be parallel laths within martensitic grains [5,6]. The volume fraction of martensite in the WQ sample is measured to be ~0.17, which is slightly lower than the 0.20 value for the equilibrium γ-fraction at 760 °C predicted by Pandat software thermodynamic calculation [40]. With respect to the AC sample, the values for the volume fraction (~0.10) and average dimensions of the second-phase are lower than those of the WQ sample. Since the morphology is different from the pearlite or bainite that are typically observed in bulk samples [6], these second-phases are probably also martensite. The marked differences in the martensite morphology between the AC and WQ sample indicates that the martensites are formed by different mechanisms. In the EBSD phase map, the α- and γ-phases are indexed according to the standard patterns of BCC and FCC iron, while zero solutions (indexing failures) are regarded as martensite, due to the complex substructure and lattice distortion in martensite [41]. As shown in Fig. 1(b), most of the second-phase in the AC sample is martensite with only a minor fraction (~ 1%) identified as the γ-phase retained in the microstructure. 3

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Figure 1. SEM images of the: (a) water-quenched (WQ) sample; and (b) air-cooled (AC) sample. Insets show the magnified microstructures of the regions indicated by the white outlined rectangles. An EBSD phase map is also shown in the inset of (b).

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3.2 Transmission electron microscopic analyses Fig. 2 displays a bright-field (BF) TEM image of a typical martensitic grain obtained from the WQ sample, and the corresponding selected-area diffraction pattern (SADP) of the region indicated by the white circle. The martensitic grain contains a lath-type substructure. The SADP also confirms that the two sets ([001] and [111] zone axes) of BCC α-Fe spots are derived from two adjacent laths. The widths of the laths vary from 200 to 500 nm. The BF image obtained using a different tilting angle reveals that the laths comprise a high density of dislocations, Fig. 2(b). TEM analyses confirm that the martensitic grains observed in the WQ sample correspond to the typical lath martensite (LM) formed in low-carbon steels [5,6].

Figure 2. TEM bright-field images obtained from the water-quenched (WQ) sample: (a) a lath martensite (LM) grain and the corresponding SADP; and (b) the same LM-grain with a different tilting angle. Fig. 3 presents the BF TEM images of second-phase grains in the AC sample. As shown in Fig. 3(a) and (b), fine twins with a spacing of 10 to 20 nm can be observed inside the grain and the corresponding SADP confirms the existence of a twin structure. Thus, this grain is considered to be twinned martensite (TM), which is normally found in high-carbon steels [5,6]. The elongated diffraction spots observed in the SADP indicate that stacking faults may coexist with the twins inside the twinned martensite. Fig. 3(c) shows the bright-field image of a martensitic grain containing a mixture of both laths and twins. The SADP in the inset clearly shows the diffraction spots resulting from the twin structure. The corresponding dark-field image of the (01̅1̅)t spot, Fig. 4

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3(d), confirms the existence of twins inside the bottom lath. Moreover, the bright-phase in the top lath may be a fine carbide precipitated in the lath, and some diffraction spots in the complex SADP may come from carbides (cementite, orthorhombic (Pnma), a = 4.517 Å, b = 5.084 Å, c = 6.748 Å [42]).

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Figure 3. TEM images obtained from the air-cooled (AC) sample: (a) bright-field image and SADP of a twinned martensite (TM); (b) a magnified image displaying the fine twins in the same martensitic grain; (c) bright-field image and SADP of another martensitic grain containing both laths and twins; and (d) corresponding dark-field image of (01̅1̅)t reflection exhibiting fine twins. 3.3 Atom-probe tomographic analyses The SEM and TEM images reveal that while the martensite in the WQ sample is typical LM, there are different types of martensite morphologies, ranging from fine twins to a mixture of laths and twins in the AC sample. These phenomena are likely related to the solute distributions within the martensite [16–19]. Representative APT analyses for one nanotip from the WQ sample (Nanotip WQ) and four different nanotips from the AC sample (Nanotips AC-1 to AC-4) are shown in Fig. 4 and Figs. 5 to 8, respectively. Fig. 4(a) displays a 3-D reconstruction of Nanotip WQ from the LM in the WQ sample. The overall composition of this APT specimen is 1.509±0.002C, 1.465±0.002Mn and 0.774±0.001Si (at.%). The 6.0 at.% C (dark red) iso-concentration surfaces delineate the C segregated regions, which are thought to be dislocations decorated by C atoms, because their morphologies are consistent with those of dislocations described in previous APT reports [43,44]. The high-density dislocation structure in the LM in the current APT result is considered to be consistent with that presented in the TEM image, Fig 2. The concentration profiles of the major solute atoms (Fe, C, Mn, Si) across the 6.0 at.% C iso-concentration surfaces are displayed in Fig. 4(b), using proximity 5

Journal Pre-proof histogram (proxigram) analysis. The C concentration can be clearly seen to increase from a relatively low value (~1.5 at.% C) in the martensite, to a high level (~12.3 at.% C) in the dislocation core. There is essentially no segregation detected between the dislocation and martensite for the substitutional elements, Mn and Si.

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Figure 4. (a) 3-D APT reconstruction of Nanotip WQ obtained from the water-quenched (WQ) sample, containing ~38 million atoms. Dislocations and lath martensite (LM) are indicated by C (black), and interfaces between the LM and dislocations are delineated by 6.0 at.% C iso-concentration surfaces (dark red); (b) proximity histogram (proxigram) concentration profiles across the interface between LM and a selected dislocation (indicated by the arrow) for Fe, C, Mn, and Si.

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Fig. 5(a) shows the 3-D reconstruction of Nanotip AC-1 obtained from the AC sample, containing an α/M heterophase interface delineated by a 2.0 at.% C iso-concentration surface (dark red). The martensite phase is the C-rich region (LM) on the left-hand side of the iso-concentration surface, and the α-phase is on the right-hand side. Fig. 5(b) displays the spatial distribution of C segregated regions in the martensite as delineated by 7.0 at.% C iso-concentration surfaces (orange) from a different viewing direction obtained by rotating the 3-D reconstruction in Fig. 5(a) by 120o. This nanotip is still considered to be LM, and contains dislocation structures, although the morphology differs somewhat from the dislocation structures observed in Nanotip WQ, Fig. 4(a). Fig. 5(c) displays the concentration profiles across the α/LM heterophase interface for Fe, C, Mn, and Si. The enrichment of C in the martensite is obvious (~4.3 at.% in LM vs. ~0.016 at.% in the α-phase). Additionally, there is a slight C segregation (+0.74 at.%) near the α/LM heterophase interface on the LM-side. Mn partitions strongly to the martensite (~1.9 at.% in LM vs. ~1.0 at.% in the α-phase) with interfacial segregation (+0.66 at.%). Conversely, there is preferential partitioning of Si to the α-phase (~0.74 at.% in LM vs. ~1.0 at.% in the α-phase) with a slight depletion (-0.45 at.%) near the α/M heterophase interface. This partitioning behavior of solute elements between the α- and LM-phases is consistent with that between the α- and γ-phases and confirms that the C-rich phase in this nanotip is martensite, which has transformed from the γ-phase during cooling. Fig. 5(d) shows the proxigram concentration profiles across the interface between LM and a selected dislocation for major solute atoms (Fe, C, Mn, Si). The C concentration near the dislocation core is ~10.2 at.%, higher than that in the LM (~4.7 at.%) and there is essentially no segregation of Mn and Si between the dislocation and LM. These features are consistent with those observed in Fig. 4(b). 6

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Figure 5. (a) 3-D APT reconstruction of Nanotip AC-1 obtained from the air-cooled (AC) sample, containing ~43 million atoms. Ferrite (α) and lath martensite (LM) are indicated by C (black, 40% displayed), and the α/LM heterophase interface is delineated by a 2 at.% C iso-concentration surface (dark red); (b) a selected view of the 3D reconstruction displaying dislocations inside the LM obtained by rotating (a) ~120° counterclockwise around the z-axis, delineated by 7.0 at.% C iso-concentration surfaces (orange); (c) proxigram concentration profiles across the α/LM heterophase interface for Fe, C, Mn and Si; (d) proxigram concentration profiles across the interface between LM and a selected dislocation (indicated by an arrow) for Fe, C, Mn, and Si. Fig. 6(a) shows the 3-D reconstruction of Nanotip AC-2 obtained from the AC sample. It is clear that the C-enriched phase at the bottom right is martensite. The α/M heterophase interface is delineated by a 4.0 at.% C iso-concentration surface (dark red). According to the proxigram concentration profiles across the α/TM heterophase interface displayed in Fig. 6(b), the partitioning behavior of solute elements between the α- and M-phases is similar to that displayed in Fig. 5(c). In martensite, the C concentration is ~3.6 at.% with segregation of +1.6 at.% near the α/M heterophase interface, while the Mn concentration is ~1.7 at.% with a strong segregation of +4.1 at.%, and the Si concentration is ~0.90 at.%. The C, Mn, and Si concentrations in the α-phase are ~0.027 at.%, ~1.6 at.%, and ~0.91 at.%, respectively. The segregation of C and Mn near the α/M heterophase interface, Fig. 6 (b), is greater than that displayed in Fig. 5(c), while the Si depletion is significantly weaker. Moreover, fine parallel C-enriched plates are observed in martensite, Fig. 6(a). The 1-D 7

Journal Pre-proof concentration profiles along the red arrow (from A to B) in the cylinder are displayed in Fig. 6(c). Fluctuations in the C concentration, ranging from the peak value (~4.5 at.%) to the trough (~2.9 at.%), are seen in the direction normal to the axial direction of the cylinder. The fluctuations in the Mn and Si concentrations in Fig. 6(c) are less obvious than the changes in the C concentrations. From the 1D concentration profiles, we could measure the interlamellar spacing of the C enriched plates, approximately 8~12 nm apart, which is comparable to the spacing of the twins measured by TEM analyses (10-20 nm), Fig. 3(a). These values are similar to those reported by Caballero et al. [45] for APT analyses of the micro-twins (5-10 nm) inside retained austenite in a high-carbon steel. Thus, the fine C enriched plates can be considered to be twins formed in the martensite and the martensite is identified as TM. 95 90 85 6

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Figure 6. (a) 3-D APT reconstruction of Nanotip AC-2 obtained from the air-cooled (AC) sample, containing ~40 million atoms. Ferrite (α) and twinned martensite (TM) are indicated by C (black, 50% displayed) and the α/TM heterophase interface is delineated by a 4.0 at.% C iso-concentration surface (dark red); (b) proxigram concentration profiles across the α/TM heterophase interface for Fe, C, Mn, and Si; (c) 1D concentration profile along the red arrow from A to B displayed in (c) for Fe, C, Mn and Si. Fig. 7(a) shows the 3-D reconstruction of Nanotip AC-3 obtained from the AC sample. Note that the majority of this nanotip has a high C content, while the small region at the top left is highly enriched in C. The interface between these two regions is delineated by an iso-concentration surface 8

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of 12.5 at.% C (dark red), indicating that the top-left phase is most likely cementite (θ). The detailed solute concentrations across the interface for Fe, C, Mn, and Si are displayed in Fig. 7(b). The respective concentrations in the major phase and the θ-carbide are: 5.8 vs. 23.5 at.% for C, 4.2 vs. 4.4 at.%, with a depletion of -0.58 at.% near the interface for Mn, and 0.48 vs. 0 at.% with a strong segregation of +2.7 at.% near the interface for Si. The C concentration of the carbide is close to the stoichiometry of Fe3C and the Si concentration is extremely low, observations that are consistent with the features of equilibrium cementite [46,47]. The reasons why the measured C concentration of cementite is slightly lower than 25 at.% will be discussed in detail in Section 4.4. Compared to the martensite composition (0.016-0.027C, 1.0-1.6Mn, 0.9-1.0Si, at.%) determined in Nanotips AC-1 and AC-2 (Figs. 5 and 6), the major phase in Nanotip AC-3 has much higher concentrations of C and Mn (5.8 at.% C, 4.2 at.% Mn), but less Si (0.48 at.% Si). We speculate that this phase is more likely to be retained austenite than martensite, because both C and Mn significantly stabilize austenite. The details are discussed in Section 4.3. (b) 90

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Figure 7. (a) 3-D APT reconstruction of Nanotip AC-3 obtained from the air-cooled (AC) sample, containing ~23 million atoms. Cementite (θ) and retained austenite (RA) or martensite (M) are represented by C (black, 40% shown) and the θ/RA heterophase interface is delineated by a 12.5 at.% C iso-concentration surface (dark red); (b) proxigram concentration profiles across the θ/RA heterophase interface for Fe, C, Mn, and Si. Fig. 8(a) presents the 3-D reconstruction of Nanotip AC-4 obtained from the AC sample, which has a more complex morphology than the structures observed in the other nanotips. The region at the top-right with a low C content is the α-phase, while the major part of this nanotip contains θ-carbides with different dimensions and is considered to be auto-tempered martensite (ATM) or bainite (B) [6,48–50]. The heterophase interfaces between the θ-carbide and auto-tempered martensite are delineated by iso-concentration surfaces of 12.5 at.% C (dark red). When using iso-concentration surfaces of 3.0 at.% C (orange) in Fig. 8(b), more carbides or C clusters are visible, which are preferentially distributed at the α/M heterophase interface and different C-rich planes (indicated by black dotted- and dashed-lines, respectively). Fig. 8(c) shows a selected view displaying the spatial distribution of θ-carbides by rotating the 3-D reconstruction in Fig. 8(a) by ~150° clockwise around the z-axis. It is confirmed that the θ-carbides are located in different C-rich planes. Moreover, the C-rich planes displayed in Fig. 8(b) and (c) have different orientations, which are considered to be subgrain or lath boundaries. This phenomenon can be explained by the model of auto-tempering [6,48–50], which states that when the Ms temperature is relatively high, C atoms may have sufficient mobility to redistribute themselves and therefore carbides may precipitate after 9

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the formation of martensite. A more detailed explanation is presented in the Discussion. Fig. 8(d) and (e) display the corresponding concentration profiles across the θ/ATM heterophase interfaces for the largest (θ1) and the second largest carbides (θ2) observed in Nanotip AC-4, respectively. As displayed, θ1 has a high C concentration of ~24.2 at.% compared to ~0.9 at.% in the auto-tempered martensite. However, Mn is slightly depleted in θ1 with an average value of 1.3 at.% compared to ~1.7 at.% in martensite. The Si concentrations in θ1 and martensite are 0.62 at.% and 0.89 at.%, respectively, with a slight segregation of +0.11 at.% near the heterophase interface (Fig. 8(d)). With respect to θ2 (Fig. 8(e)), the C concentration has a small value of ~0.68 at.% on the M-side far from the heterophase interface and it increases gradually, towards the heterophase interface, to reach a high value of 23.8 at% at the core of θ2. Except for a slight Si depletion in the θ-carbide, there is negligible substitutional elemental partitioning between θ2 and auto-tempered martensite.

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Figure 8. (a) 3-D APT reconstruction of Nanotip AC-4 obtained from the air-cooled (AC) sample, containing ~18 million atoms. Cementite (θ) and auto-tempered martensite (ATM) or bainite (B) are indicated by C (black), and the θ/ATM heterophase interfaces are delineated by 12.5 at.% C iso-concentration surfaces (dark red); (b) 3.0 at.% C iso-concentration surfaces (orange) indicating the spatial distribution of the α/ATM heterophase interface (black dotted-line) and C-rich planes (black dashed-lines); (c) a selected view of the 3-D reconstruction showing the spatial distribution of θ-carbides in a C-rich plane obtained by rotating (a) ~150° clockwise around the z-axis; (d), (e) proxigram concentration profiles for Fe, C, Mn, and Si across the θ/ATM heterophase interfaces for 10

Journal Pre-proof the largest and second largest θ-carbides (labeled 1 and 2), respectively.

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4. Discussion 4.1 Solute partitioning during isothermal holding Since the martensitic transformation is diffusionless, the martensite formed inherits the composition of the parent γ-phase [6]. To understand the compositional features in martensite, it is crucial to analyze solute partitioning between the α- and γ-phases during the entire heat treatment process, including the isothermal holding at an intercritical temperature and subsequent continuous cooling. Fig. 9 presents the equilibrium phase volume fractions and compositions of the α- and γ-phases as a function of temperature, based on thermodynamic calculations performed utilizing Pandat software [40] (PanFe2014 database). Table 1 presents the calculated volume fractions and compositions of the α- and γ-phases at 760 °C, and experimental data measured utilizing SEM and APT from the WQ sample. The results reveal that C and Mn partition strongly to the γ-phase, while Si only partitions slightly to the α-phase. APT analysis of the martensite in the WQ sample (Nanotip WQ, Fig. 4), yields a C concentration of 1.509±0.002 at.%, which is close to the equilibrium value of 1.46 at.%. In contrast, APT analysis estimates the Mn concentration to be 1.465±0.002 at.%, which is much lower than the equilibrium value of 2.40 at.%, and slightly higher than the nominal concentration (1.231 at.%). This is because the C diffusivities in the α- and γ-phases at 760 °C (1.4×10-10 and 9.8×10-13 m2 s-1, respectively) are much higher than those of Mn (2.3×10-16 and 9.4×10-19 m2 s-1 in the α- and γ- phases, respectively) [51]. After holding at 760 °C for 5 min, the root-mean-square C diffusion distances in the α- and γ-phases are 500 and 40 μm, respectively; these values are much larger than the average grain size (~6 μm). Notably, the root-mean-square Mn diffusion distances in the α- and γ-phase are only ~0.6 and ~0.04 μm; these values are about three orders of magnitude smaller than those of C. This implies that the redistribution of Mn is localized in a narrow region near the interface, where the Mn concentration remains close to its nominal concentration in most areas. The results presented in Fig. 9 (b) and Table 1 demonstrate that the Si partitioning between the α- and γ-phases is much smaller than that of C and Mn. The Si concentration in the martensite as measured by APT, is 0.774±0.001 at.%, which is slightly higher than the equilibrium value (0.72 at.%) in the γ-phase, but somewhat lower than the nominal concentration (0.849 at.%). Since the Si diffusivities in the α- and γ- phases (4.4×10-16 and 3.6×10-18 m2 s-1 respectively), have similar orders of magnitude as the Mn diffusivities [52], the redistribution of Si is probably also localized in the interfacial region. Table 1 shows that the γ-volume fraction measured from SEM micrographs (~0.17) is smaller than the equilibrium value (~0.20), which is attributed to the incomplete α-γ transformation, as the distributions of Mn and Si deviate from the equilibrium state.

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Table 1. Volume fractions and compositions (at.%) of the α- and γ-phases at 760 °C calculated using Pandat and measured experimentally utilizing SEM and APT from the water-quenched (WQ) sample. Method Phase Vol. Frac. C Mn Si Nominal / / 0.323 1.231 0.849 α 0.80 0.034 0.93 0.88 Pandat γ 0.20 1.46 2.40 0.72 Experimental LM (γ) 0.17 1.509±0.002 1.465±0.002 0.774±0.001

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4.2 Solute partitioning during continuous cooling When the temperature decreases during continuous cooling, the γ-phase transforms back to the α-phase. This explains why the volume fraction (~0.10) of martensite in the AC sample is lower than that of the WQ sample. Thermodynamic calculations demonstrate that C and Mn partition more to the γ-phase during cooling, although there is little effect on the partitioning of Si, Fig. 9(b). The overall compositions of the α- and martensite-phases obtained from APT analyses of Nanotips AC-1 to AC-4 from the AC sample (Figs. 5-8) are summarized in Table 2. The average composition was obtained from mass spectrum analyses of each phase. Regarding the martensite or cementite enriched in C, the peak decomposition algorithm was adopted to estimate the contributions of different molecular ions ((C2)+ and (C4)2+) to the peak at 24 Da [53–55]. Results show that over 80% of contributions are from (C4)2+, and thus the peak at 24 Da was assigned to (C4)2+. The partitioning of C, Mn, and Si between the α- and γ(M)-phases in Nanotips AC-1 and AC-2 is consistent with that predicted by thermodynamic calculations. The detected martensite (or retained austenite) in Nanotips AC-1 to AC-4 have concentrations of C in the range 2-6 at.% with Mn in the range 1.6-1.9 at.% (with exception of the extremely high Mn content of 4.16 at.% in Nanotip AC-3). These values are higher than those in the WQ sample (1.51 at.% for C and 1.47 at.% for Mn). There are a number of mechanisms that may explain this phenomenon. The lack of pearlite in the AC sample, Fig. 1(b), suggests that the pearlitic transformation is constrained during cooling. As a consequence, C and Mn partition to the γ-phase continuously, even when the temperature is below the eutectoid temperature. This yields a more significant enrichment of both elements in the γ-phase compared to the values at the end of the isothermal holding period. Concerning the Si 12

Journal Pre-proof concentration in martensite, and with the exception of a particularly low value of 0.483 at.% Si in Nanotip AC-3, the Si content in the other three nanotips is in the range 0.7-0.9 at.%, which is similar to the 0.774 at.% value in the WQ sample. This confirms that the Si concentration in the γ-phase is relatively unchanged due to its weak partitioning between the α- and γ-phases.

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Table 2. Average compositions of ferrite (α) and martensite (M) measured by APT for Nanotips AC-1 to AC-4 obtained from the air-cooled (AC) sample (at.%). Nanotip Phase C Mn Si No. 0.016±0.0008 α 0.983±0.006 1.047±0.007 AC-1 LM 4.308±0.007 1.892±0.005 0.736±0.003 α 0.027±0.0008 1.598±0.007 0.912±0.005 AC-2 TM 3.637±0.009 1.688±0.006 0.896±0.005 AC-3 RA (or M) 5.837±0.012 4.162±0.010 0.483±0.003 AC-4 ATM (or B) 2.112±0.004 1.689±0.003 0.893±0.002

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Considering the composition of the steel used in this study and the cooling rate utilized, the small amounts of pearlite in the AC sample are not unexpected. Increasing the concentrations of C and Mn in the γ-phase is known to increase the stability of the γ-phase and to suppress the pearlitic transformation. Correspondingly, the critical cooling rate required for martensitic transformation is decreased [56]. According to the data reported by Vander Voort [57], the critical cooling rate is around 3 °C/s for an Fe-4.4C-1.8Mn (at.%) alloy, which has a similar composition to the martensite formed in the AC sample presented herein. Hence, it appears reasonable that, during air cooling (6 °C/s), the γ-phase has sufficient hardenability to transform to martensite, rather than pearlite. The variability in the martensite composition detected in Nanotips AC-1 to AC-4 (Table 2) can be attributed to the presence of different specific surface areas of γ-grains with different sizes, which influence directly the enrichment of solute atoms [58]. Additionally, the solute distributions of substitutional elements in the γ-phase may be non-uniform due to the small diffusivities, as discussed in Section 4.1. 4.3 Ms temperatures and formation mechanisms of different martensite morphologies Krauss [6] summarized the martensite morphologies and Ms temperatures as a function of the C content of the martensite in Fe-C alloys. A low-carbon (<2.7 at.%) content has been shown to lead to a high Ms temperature (>400 °C) and to produce lath martensite after quenching, while a high-carbon (>4.5 at.%) content and a low Ms temperature (<200 °C) favor the formation of plates including fine twins in the martensite. When the C concentration lies between 2.7 and 4.5 at.%, the resultant martensite may contain a mixture of laths and twins. In alloyed steels, the γ-stabilizing elements, e.g. Mn, may also influence the Ms temperature and thereby affect the martensite morphology [1]. Fig. 10 presents a plot of the Ms temperature with varying concentrations of C and Mn (at.%), using the data obtained from the work of Capdevila et al. [59]. The results demonstrate clearly that the Ms temperature decreases with increasing concentrations of C and Mn.

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The Ghosh–Olson thermodynamic and kinetic approach [60–63], are used to estimate the Ms temperature and to investigate the formation mechanisms of martensite with different morphologies. (i) Lath martensite in the WQ sample According to the martensite composition measured in Nanotip WQ (~1.51 at.% C, ~1.47 at.% Mn, ~0.77 at.% Si, Table 1), the calculated Ms temperature is greater than 600 °C. Under these conditions, the γ-phase formed after holding at the intercritical temperature is most likely to transform to lath martensite, comprising a high density of dislocations, during water quenching. This prediction can be confirmed by TEM imaging (Fig. 2) and APT analysis (Fig. 4). (ii) Lath and twinned martensite (Nanotips AC-1 and AC-2) in the AC sample With respect to the AC sample, the martensite in Nanotips AC-1 and AC-2 contain similar concentrations of C and Mn with values in the ranges 2.7-4.5 and 1.6-1.9 at.%, respectively, Table 2. The calculated Ms temperatures for these two nanotips are 85 and 159 °C, respectively, which both fall in the range 50-200 °C. These conditions might be expected to produce a mixed substructure that contains both laths (dislocations) and twins, or even a complete twin structure, which correspond to the situations seen in TEM images in Fig. 3(c), (d) and Fig. 3(a), (b), respectively. (iii) Retained austenite (Nanotip AC-3) in the AC sample The major phase detected in Nanotip AC-3 contained much higher concentrations of C and Mn than measured in the three other nanotips obtained from the AC sample. The calculated Ms temperature is -218 °C, which should provide the γ-phase with enough hardenability to be retained at room temperature. Thus, the major phase in Nanotip AC-3 is most likely to be retained as austenite, rather than martensite even though the EBSD analysis in Fig. 1(b) indicates that the volume fraction of retained austenite in this sample is small. iv) Auto-tempered martensite (Nanotip AC-4) in the AC sample The microstructure in Nanotip AC-4 has the smallest C content (~2.11 at.%) and the highest Ms temperature (292 °C) among all the four analyzed nanotips obtained from the AC sample. Martensite formed at a relatively high temperature may be expected to contain laths or even carbide precipitates. This is because after the formation of martensite, the C atoms inside the material retain sufficient mobility to redistribute during the time required to cool to room temperature. Carbides may precipitate at imperfections, a finding known as auto-tempering [6,48–50]. In Nanotip AC-4, 14

Journal Pre-proof different carbides may precipitate inside the martensite, producing a type of material known as auto-tempered martensite. Another explanation for the observed morphology could be that carbides form through a bainitic transformation at a temperature higher than the Ms temperature. There is, however, insufficient evidence supporting this mechanism, since no obvious bainitic structure is observed in TEM. 4.4 Formation mechanisms of different carbides The average compositions of cementite (θ-carbide) and adjacent martensite as measured by APT in Nanotips AC-3 and AC-4, are summarized in Table 3. It should be noted that the average compositions of martensite displayed in Table 2 were measured over the entire martensitic region, while in Table 3 they were obtained from the martensitic region adjacent to the analyzed cementite.

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Table 3. Average compositions of cementite (θ) and adjacent martensite (M) measured by APT for Nanotips AC-3 and AC-4 obtained from the air-cooled (AC) sample (at.%). Nanotip Phase C Mn Si No. θ 23.47±0.075 4.368±0.030 0 AC-3 RA (or M) 6.677±0.092 4.690±0.078 0.519±0.025 θ ATM (or B)

24.19±0.202 0.879±0.022

1.334±0.053 1.670±0.031

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20.17±0.555 0.788±0.055

1.576±0.171 1.623±0.079

0.658±0.111 0.888±0.059

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As shown in Table 3, the carbide in Nanotip AC-3 has a C concentration slightly lower than the stoichiometric value of 25 at.%, which may result from the systematic errors from the LEAP instrument and multiple events when using a MCP detector [64-66]. The local magnification or demagnification factor caused by C atoms in the matrix and carbide may also contribute to the low C detection issue [67]. Regarding the Si content, the carbide exhibits an extremely small Si concentration, thus it is close chemically to equilibrium cementite. As presented in Fig. 7(b), there is also strong Si segregation at the interface. This is because Si is rejected from the cementite to the adjacent phase, and accumulates at the interface due to its small diffusivity. Since the redistribution of Si requires the Si atoms to be sufficiently mobile, the cementite precipitation in Nanotip AC-3 probably takes place through an incomplete pearlitic or bainitic transformation at a high temperature, far above the Ms temperature. Additionally, the high concentration of C and Mn, and small concentration of Si detected in the major phase (retained austenite) in Nanotip AC-3 may also favor cementite precipitation [46]. In Nanotip AC-4, which we conclude to exhibit auto-tempered martensite, there are carbides of different sizes distributed around the α/M heterophase interface or C enriched plates (subgrains or lath boundaries, or other imperfections). According to Table 3, the second largest carbide (θ2) is likely to be far from the equilibrium state since it contains a lower C concentration compared to the largest carbide (θ1). On the other hand, the two carbides both contain finite amounts of Si. It is considered that there is insufficient time for substitutional elements to redistribute during auto-tempering, the carbides observed in Nanotip AC-4 may still be in transition from the paraequilibrium state to local equilibrium. 15

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5. Conclusions SEM, TEM, and APT analyses are employed to study systematically the morphologies and solute distributions of martensite (M) in a dual-phase steel, produced by annealing at 760 °C for 5 min, followed by water quenching (WQ) or air cooling (AC) to room temperature. The results obtained permit us to reach the following conclusions:  The WQ sample contains about 0.17 volume fraction of martensite distributed at α/α GBs. TEM and APT analyses indicate that the martensite in the WQ sample is the typical lath martensite comprising a high dislocation density The C content (1.51 at.%) of martensite measured by APT is comparable with that predicted by thermodynamic calculations at the annealing temperature (1.46 at.%), although the Mn content (1.47 at.%) is smaller than the anticipated value (2.40 at.%).  The volume fraction (~0.10) of the transformed regions in the AC sample is smaller than that of the WQ sample, due to the partial decomposition of the γ-phase into the α-phase during cooling. EBSD, TEM and APT analyses demonstrate that most of the transformed regions are martensite, with only a small portion being retained austenite. APT analyses demonstrate that C and Mn are more enriched in the martensite in the AC sample than in the WQ sample, as a result of extended C and Mn partitioning to the γ-phase during cooling. This stabilizes the γ-phase and enhances its hardenability, leading to the formation of martensite under conditions of a moderate cooling rate in the AC sample.  According to the APT analyses, the major microstructural features seen in Nanotips AC-1 and AC-2 obtained from the AC sample, are high densities of dislocations and fine twins. These morphologies are consistent with TEM analyses. Martensite detected in these two nanotips contain C and Mn in the ranges 2.7-4.5 and 1.6-1.9 at.%, respectively. The Ms temperatures for these two nanotips are estimated to be in the range 50-200 °C, which explains the complex martensite morphologies formed in those nanotips.  The major phase in Nanotip AC-3 obtained from the AC sample contains extremely high levels of C and Mn (5.8 at.% C, 4.2 at.% Mn). The microstructure is considered to be retained austenite as the calculated Ms temperature is -218 °C. Nanotip AC-4 contains less C (~2.1 at.%) and has a higher Ms temperature (~300 °C) than the other three nanotips. This nanotip is thought to contain auto-tempered martensite, which explains the carbides with different sizes distributed preferentially at heterophase interfaces or C segregated planes, as observed in the APT reconstruction. The detected carbides have a C content close to the stoichiometry of cementite, but there is less evidence for Si redistribution.  Compared to the typical lath martensite observed in the WQ sample, the second-phase in the AC sample contains a small portion of retained austenite and martensite with complex substructures including fine twins, a mixture of laths and twins, and carbide precipitates. This can be attributed to the continued solute partitioning during air cooling after annealing at an intercritical temperature. These findings may provide insights into the correlation between heat treatment process and corresponding microstructure and solute distribution during the production of DP steels. Acknowledgments The authors wish to thank CompuTherm LLC, Middleton, Wisconsin, for providing us with the license to use Pandat software. This work was financially supported by A. O. Smith Corporation, USA, NSFC (Grant Nos. 51371051), the Jiangsu Key Laboratory for Advanced Metallic Materials (BM2007204). Dong An is grateful for the financial support from the China Scholarship Council 16

Journal Pre-proof (CSC) and Mr. Lichu Zhou in Southeast University for the help on TEM analyses. D. Isheim and S.-I. Baik received support from the Office of Naval Research for this research; Dr. W. Mullins, grant monitor. APT was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. This work made use of the EPIC facility of Northwestern University’s NUANCE Center. NUCAPT and NUANCE received support through the MRSEC program (NSF DMR-1720139) at the Materials Research Center and the SHyNE Resource (NSF ECCS-1542205), NUCAPT from the Initiative for Sustainability and Energy (ISEN), at Northwestern University; NUANCE from the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

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Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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Declaration of interests

lP

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Graphical abstract

Highlights 

Different martensites are found in an intercritically annealed dual-phase steel.



Morphologies and solute distributions of martensites are studied by TEM/APT analyses.



Solute partitioning is analyzed using APT and thermodynamic calculations.



Formation mechanisms of different martensites are discussed. 21

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