Multiscale Discrete Crystal Growth in the Solidification of 20SiMnMo5

Article pubs.acs.org/crystal

Multiscale Discrete Crystal Growth in the Solidification of 20SiMnMo5 Steel Xiaoping Ma* and Dianzhong Li Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 110016 Shenyang, China ABSTRACT: The solidified microstructures of metal alloys are produced by the process of crystal growth. However, some processes of crystal growth may remain uncovered because the solidification of metal alloys usually happens at high temperature with tiny growth scale and high growth speed. In the current work, based on the solidified microstructures, the crystal growth process of 20SiMnMo5 steel is investigated by abductive reasoning. The discrete crystal growth is identified at different scales. In the sub-microstructure scale, the basic elements of microstructures, such as single cellular grain, single dendritic arm, and single lamellar, are produced by the growth and convergence of discrete crystals. In the microstructure scale, the cellular microstructures, the lamellar microstructures, and dendritic microstructures are produced by discrete crystal growth. And the characteristic parameters of discrete crystal growth in this scale decide some characteristic parameters of solidified microstructures. The discrete crystal growth also happens in the mesoscale, which accounts for the formation of a channel zone. Besides, the faceted interface is observed in the crystal growth of 20SiMnMo5 steel.

1. INTRODUCTION For metal alloys, cellular and dendritic microstructures are the typical solidified microstructures.1,2 In the growth of cellular or dendritic microstructures, new crystal arms continuously appear and divide the melt into many small domains. In the subsequent solidification process, such small domains solidify independently. Thus, in the microstructure scale, the solidification of whole melt is fulfilled by the discrete crystal growth. However, for the small domain of single cellular grain or single dendritic arm, the solidification is recognized to be accomplished by the continuous growth of single crystal. It is generally accepted that there is no further discrete crystal growth in the sub-microstructure scale. Recently, the crystal growth in the solidification of metal alloys can be observed in real time by confocal scanning laser microscope3−6 or synchrotron radiation.7−10 The confocal laser microscope can observe the growth phenomena on the surface of the sample. And synchrotron radiation can observe the crystal growth in a slice sample. Although the solidification can be observed in real time and in situ, the crystal growth is seldom focused in the sub-microstructure scale. The crystal growth can also be analyzed by abductive reasoning according to the solidified microstructures and segregation. It is true that the dynamic crystal growth cannot be directly observed by the abductive reasoning. However, some crystal growth processes can also be reliably traced and identified by analyzing the solidified microstructures and segregation. As previously introduced, the solidification in the submicrostructure scale is generally recognized to be accomplished by the continuous crystal growth. However, because the © 2016 American Chemical Society

solidification usually happens at high temperature with tiny growth scale and high growth speed, the crystal growth in the sub-microstructure scale actually needs to be further investigated. In recent research, some substructures in the single dendritic arm were observed, and the discrete crystal growth in the sub-microstructure scale was revealed.11,12 According to the proposed mechanism,11,12 the discrete crystal growth is attributed to the non-equilibrium solute partition, the evolution of temperature gradient, and the evolution of liquidus temperature. The aforementioned mechanism for discrete crystal growth may not be limited to the growth of a single dendritic arm. Theoretically, the discrete crystal growth should also be possible for the growth of a single cellular crystal or planar crystal. Besides the sub-microstructure scale, the discrete crystal growth may also happen in the mesoscale and influence the mesostructure. Thus, the multiscale discrete crystal growth may exist in solidification. Based on the preceding consideration, it is interesting to explore the multiscale discrete crystal growth in different solidified microstructures. It is also worth investigating how the multiscale structures are produced by the multiscale discrete crystal growth. In current work, these questions are researched by abductive reasoning according to the solidified microstructures and segregation. Received: December 22, 2015 Revised: April 25, 2016 Published: April 29, 2016 3163

DOI: 10.1021/acs.cgd.5b01804 Cryst. Growth Des. 2016, 16, 3163−3169

Crystal Growth & Design

Article

Figure 1. Typical microstructures and phases in the upper part of the directionally solidified sample: (a) solidified microstructures, (b) microstructures constituted by two phases, and (c) XRD pattern of the constituent phases.

2. EXPERIMENTAL SECTION

following phase transition sequence: liquid−(austenite + liquid)−(austenite + δ ferrite)−(martensite + δ ferrite). Namely, the austenite forms from the melt first, and the δ ferrite forms from the residual melt later. Then, the martensite forms from the austenite in the cooling process. The preceding phase transition sequence is supported by the observation about porosities. As shown in Figure 2, the porosities and δ ferrite locate in the same positions. There is no doubt that the porosities form in the finally solidified positions. Therefore, the δ ferrite is transited from the residual melt in the later stage of solidification. In the lower part of the directionally solidified sample, the typical microstructures are constituted by the lateral cellular

In a previous publication, because the discrete crystal growth in the sub-microstructure scale has been revealed in the dendritic solidification of 20SiMnMo5 steel,11 20SiMnMo5 steel was still selected as the experimental material in the current work, in which chemical compositions (wt %) are as follows: C, 0.20; Si, 0.70; Mn, 1.0; Mo, 5; Fe, balance. The steel rod of diameter 6.5 mm × 100 mm was prepared. The steel rod was melted at 1600 °C in an alumina tube with i.d. 7 mm, and then the steel melt was held at 1550 °C. In order to produce different solidified microstructures, the steel melt was directionally solidified with the pulling velocity of 20 μm/s. After the sample was directionally solidified with 50 mm, the sample was quenched in cold water. The sample was cut along the longitudinal axis. After being ground and polished, the sample was etched with 4% HNO3−C2H5OH solution. The solidified microstructures and solute distribution were analyzed by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) spectroscopy, and the energy dispersive spectroscopy (EDS).

3. RESULTS AND DISCUSSION 3.1. Solidified Microstructures and Phase Transition Sequence. In the upper part of the directionally solidified sample, the microstructures are constituted by the tilted cellular grains and directionally solidified dendrite, as shown in Figure 1a. This microstructure is composed by two phases, as shown in Figure 1b. Identified by XRD as Figure 1c, the two phases have the same BCC crystal structure. The solute compositions of the two phases are tested by EDS. The compositions (wt %) of the dark phase are as follows: Si, 0.80; Mn, 1.09; Mo, 4.88; Fe, balance. The compositions (wt %) of the white phase are as follows: Si, 1.04; Mn, 1.08; Mo, 6.66; Fe, balance. According to the crystal structure, morphology, and compositions, the dark phase should be the martensite, and the white phase should be the δ ferrite. This microstructure is produced from the

Figure 2. Porosities surrounded by the δ ferrite. 3164



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Figure 3. Typical microstructures and phases in the lower part of the directionally solidified sample: (a) solidified microstructures, (b, c) microstructures constituted by three phases, and (d) XRD pattern of the constituent phases.

In the cellular microstructures produced in the quenching process, as shown in Figure 4, some interesting lamellar and zigzag δ ferrite appear in the interior of the cellular grain. Usually, the cellular grains are produced by the growth of a common cellular interface, as in situ observed by Shangguan and Hunt.13 However, in our experimental result, the complicated substructure in the single cellular grain implies complicated crystal growth in the sub-microstructure scale. It can be reasonably inferred that many discrete austenite branches grow laterally and converge in the center of a single cellular grain. When the austenite branches converge at the center of the cellular grain, the lamellar δ ferrites are left between the austenite branches, as shown in Figure 4b. It is interesting that the intercellular position solidifies earlier and the center of cellular grain solidifies finally. Besides, it is worthy to address the typical faceted and nonfaceted interfaces in the solidification of 20SiMnMo5 steel, as shown in Figure 4c. Through simulation, Tegze has proposed that porous structures can emerge in crystal growth in the crossover of nonfaceted and faceted crystallization modes.14 For the cellular microstructures produced in the directional solidification, the substructure is also observed in the interior of cellular grains, as shown in Figure 5a. Some parallelogram δ ferrites are trapped in the interior of cellular grains. The formation process of such parallelogram zones can be reasonably traced according to the microstructures at some special positions, as shown in Figure 5b, where the trapping of parallelogram zones is on the process. Actually, the trapped parallelogram zones are produced by the convergence of discrete austenite in the sub-microstructure scale. According to Figure 5b and Figure 6, the convergence of austenite happens in both directions along and perpendicular to the cellular grain. The preceding results reveal the important fact that the basic elements of solidified microstructures may further solidifiy with a complicated time and spatial sequence. In this case, the multiscale discrete crystal growth dominates the solidification process. The growth of cellular crystals goes through the

grains and directionally solidified lamellar structures, as shown in Figure 3a. The microstructures are composed by three BCC phases, as shown and identified in Figure 3b−d. Tested by EDS, the compositions (wt %) of the A phase are as follows: Si, 0.77; Mn, 0.90; Mo, 6.94; Fe, balance. The compositions (wt %) of the B phase are as follows: Si, 0.78; Mn, 1.20; Mo, 4.81; Fe, balance. The compositions (wt %) of the C phase are as follows: Si, 0.71; Mn, 1.02; Mo, 4.88; Fe, balance. According to the crystal structure, morphology, and compositions, the A phase should be the δ ferrite, the B phase should be the ferrite, and the C phase should be the martensite. This microstructure is produced by the following phase transition sequence: liquid− (austenite + liquid)−(austenite + δ ferrite)−(ferrite + austenite + δ ferrite)−(ferrite + martensite + δ ferrite). Namely, the austenite forms from the melt first, and the δ ferrite forms from the residual melt later. Subsequently, the ferrite transits from the austenite in the cooling process. And the martensite forms from the residual austenite in the subsequent cooling process. In the quenching process, some cellular grains along the pulling direction are produced in front of the mushy zone, as shown in Figure 4. The microstructures are composed by the black martensite and the white δ ferrite. This microstructure is produced from the following phase transition sequence: liquid− (austenite + liquid)−(austenite + δ ferrite)−(martensite + δ ferrite). 3.2. Multiscale Discrete Crystal Growth in the Formation Cellular Microstructures. Generally, the formation of cellular microstructures is accomplished by the synchronous growth of many cellular grains. However, because the sample in the current work was directionally solidified, the different cellular grains along the pulling direction in Figure 1a or in Figure 3a should form with a certain time sequence. Anyway, as proposed in the Introduction, for the solidification of cellular microstructures, the whole melt is actually divided into many small domains coinciding with the single cellular grain. The key question is whether discrete crystal growth exists in the sub-microstructure scale and mesoscale. 3165



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Figure 4. (a) Cellular grains with lamellar δ ferrite produced in the quenching process; (c) cellular grains with zigzag δ ferrite produced in the quenching process; (b, d−f) images amplified for the squared zones b, d, e, and f, respectively.

following important steps. First, the discrete crystals with the faceted interface grow in the sub-microstructure scale. Second, the discrete crystals converge to form the single cellular grain. Third, the cellular grains continue the discrete crystal growth in the microstructural scale. 3.3. Multiscale Discrete Crystal Growth in the Formation of Dendrite and Lamella Microstructures. In the upper and center parts of the directionally solidified sample, some dendrites are observed, as shown in Figure 1a and Figure 7. The formation mechanism of dendrite has been wellexplained by the theory of constitutional undercooling.15,16 Because the melt in the enriched boundary layer is undercooled, the solid penetrated into the constitutional under-

cooling zone will grow preferentially to form the new dendritic arms. Thus, in the microstructure scale, the dendrite microstructures actually divide the melt into many small domains and solidify through discrete crystal growth. In our observation, it should be highlighted that some dendritic arms are constituted by several discrete crystals, as shown in Figure 7a. Although some dendritic arms are constituted by one continuous crystal, such single continuous dendritic arm is actually converged by several discrete crystals, as shown in Figure 7b−d. These evidence imply that the discrete crystal growth also exists in the sub-microstructure scale for the dendritic growth. Besides, it is worthy to address that the typical faceted interface also appears in the dendritic crystals, as shown in Figure 8. 3166



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Figure 5. Substructure in the interior of cellular grains produced by directional solidification: (a) substructure in the cellular grain, (b, c) formation of substructure revealed at special positions. The red square of panel b is amplified as panel c.

Figure 6. (a) Cellular grains and channel zones in the mesoscale; (b, c) amplified channel zones.

Figure 7. Dendrite produced by the discrete crystal growth. The red square of panel b is amplified as panel c, and the red square of panel c is further amplified as panel d.

the solid−liquid interface, the changes of the growth rate, and the nucleation transition. Compared with the band-like microstructures in the literature, the lamellar microstructures with much smaller scale were observed in the lower and center parts of the sample, as shown in Figure 3a and Figure 9. As introduced previously, the δ ferrites in the lamellar microstructures are transited from the original residual melt among original lamellar austenite. Hence, the lamellar microstructures were produced by the discrete growth of austenite in the microstructural scale. It should also be noted that some δ ferrites appear in the inner of single martensite lamella, as shown in Figure 9. This result means that the single austenite lamella is further produced by the growth and convergence of discrete austenite in the sub-microstructure scale. The mechanism for discrete growth of austenite should be attributed to the non-equilibrium interface, solute diffusion, and the temperature gradient. Especially, the concentrated multicomponent effect23−26 can result in the significant nonequilibrium interface. The liquidus temperature in front of the non-equilibrium interface may be lower than the solidus temperature, which is crucial for the discrete crystal growth.11

Figure 8. Faceted interface in the dendrite.

In some literature,17−22 band-like microstructures have been reported, which are related to the solute distribution ahead of 3167



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convergence of discrete crystals in the mesoscale. It should be clarified why the nonuniform distribution or nonuniform convergence of discrete crystals can happen in the mesoscale along the channel zones. The channel zone in the current work is very similar to the well-known channel segregation that appeared in the steel ingot. Generally, the channel segregation is recognized by most researchers to be aroused by the interdendritic flow in the mushy zone.27−30 Some researchers suggest that the channel segregation forms because the interdendritic flow can remelt the dendrite.31 Some researchers suggest that the channel segregation is caused by the different interdendritic melt flow and different crystal growth phenomena in the mesoscale,32 which does not necessarily need to remelt the dendrite. Although the channel zones in the current work appear in the cellular crystals other than dendritic crystals, the channel zones in the current work may actually share a similar principle with the channel segregation. Both the channel zone and the channel segregation form because enriched melt restricts the crystal growth in the channels. For the channel segregation, the enriched melt is produced by the interdendritic flow. For the channel zone in the current work, the enriched melt should be produced by the enriched boundary layers. As introduced in our previous work,11,12 the enriched boundary layer exists in front of the solid−liquid interface, and part of the enriched boundary layer is trapped by the discrete crystal growth. If the trapped part of the enriched boundary layer is identical with the same dimension and solute content, the distribution of the discrete crystals and the convergence of the discrete crystals should be uniform. If the trapped part of the enriched boundary layer is with larger dimension and higher solute content along a mesoscale solidification front, the channel zone may be produced.

Figure 9. Lamellar microstructure produced by the discrete crystal growth.

Besides, the non-equilibrium solute diffusion in the melt can produce the enriched boundary layer in front of the crystal− melt interface. The liquidus temperature of the boundary layer decreases with the increase of solute enrichment, which can further restrict the growth of the crystal interface. Because of the decrease of actual temperature in the melt, the melt in front of the original crystal interface is undercooled and solidifies preferentially. At the same time, part of the enriched boundary layer is trapped in the discrete crystals. The preceding discrete crystal growth will significantly influence the distribution of solute in the sub-microstructure scale and significantly decrease the solute enrichment in the bulk melt. Based on the solute distribution produced by discrete crystal growth, different solid phases are produced in the final microstructures. 3.4. Characteristic Parameters of Discrete Crystal Growth. Two characteristic parameters are crucial for describing the discrete crystal growth. The one is the dimension of melt between two discrete crystals, and the other is the distance between two discrete crystals. These two characteristic parameters can decide the characteristic parameters of final solidified microstructures. In the microstructure scale, for the cellular microstructures in Figure 5 and Figure 6, the dimension of melt between two discrete cellular grains decides the thickness of δ ferrite, and the distance between two discrete cellular grains decides the cellular grain spacing. For the dendritic microstructures in Figure 7, the dimension of melt between two discrete dendritic arms decides the thickness of interdendritic δ ferrite, and the distance between two discrete dendritic arms decides the dendritic arm spacing. In the sub-microstructure scale, for the single cellular grain in Figure 4, the dimension of melt between two discrete crystals decides the thickness of lamellar δ ferrite, and the distance between two discrete crystals decides the lamellar spacing of δ ferrite. However, in most situations other than quenching, the relation between the characteristic parameters of discrete crystal growth and the substructures is concealed because of significant convergence of discrete crystals. Only some trace about the substructures can be observed, as shown in Figures 5−9. 3.5. Discrete Crystal Growth in the Mesoscale. The two channel zones in the mesoscale are observed, as shown in Figure 6. The channel zones are induced either by the nonuniform distribution of discrete crystals or the nonuniform

4. CONCLUSIONS Based on the solidified microstructures of 20SiMnMo5 steel, the time and space sequence of crystal growth is investigated. The discrete crystal growth in three scales was revealed. In the sub-microstructure scale, the complicated time and space sequence is revealed by the substructures. And the growth and convergence of discrete crystals produce the basic element of solidified microstructures, such as single cellular grain, single dendritic arm, and single lamella. In the microstructure scale, the diverse cellular microstructures, dendritic microstructures, and lamellar microstructures are produced by discrete crystal growth. And the characteristic parameters of discrete crystal growth decide the characteristic parameters of solidified microstructures. In the mesoscale, the discrete crystal growth produces the channel zone. The formation mechanism for the channel zone is proposed, which emphasizes the nonuniform trapping of the enriched boundary layer.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-024-83970106. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Multiscale Discrete Crystal Growth in the Solidification of 20SiMnMo5

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