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Effect of Isothermal Temperature on Growth Behavior of Nanostructured Bainite in Laser Cladded Coatings Yanbing Guo 1,2 , Chengwu Yao 2 , Kai Feng 2,3 , Zhuguo Li 2, *, Paul K. Chu 3 and Yixiong Wu 2 1 2

3

*

School of Mechanical Engineering, Shanghai Dianji University, Shanghai 201306, China; [email protected] Shanghai Key Lab of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] (C.Y.); [email protected] (K.F.); [email protected] (Y.W.) Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon 999077, Hong Kong, China; [email protected] Correspondence: [email protected]; Tel.: +86-21-54745878; Fax: +86-21-34203024

Received: 20 June 2017; Accepted: 8 July 2017; Published: 14 July 2017

Abstract: The growth and propagation behavior of austenite-to-bainite isothermal transformation in laser-cladded, Si-rich, and Fe-based coatings is investigated. The crystallographic features, orientation relationship at different isothermal temperatures, and the morphology of the nanostructured bainite are determined. The Nishiyama-Wassermann type orientation relationship is observed at a high temperature and at a low temperature, and mixed Nishiyama-Wassermann and Kurdjumov-Sach mechanisms are seen. The growth direction is investigated by the partial dislocation theory and an extrapolated model based on the repeated formation of lenticular-shaped subunits and pile-up along the close-packed directions of the close-packed planes. The variants of the bainite growth directions would be more selective at the high transformation temperature. Keywords: nanostructured bainite; sheaves; growth behavior; laser cladded coating; orientation relationship

1. Introduction A new generation of carbide-free nanostructured bainitic steel has been developed [1–3]. The large concentrations of carbon and silicon in the steel lead to slow bainitic transformation at a low temperature (200 ◦ C) and an ultra-fine nanoscaled structure composed of bainitic ferrite lathes and retained austenite films between these lathes [2,4,5]. Nanostructured bainitic steel, which possesses ultra-high tensile strength (>2 GPa) and a toughness of 30 MPa·m1/2 , has large industrial potential, but has the drawback of having a slow transformation process that usually requires several days. The transformation process can be accelerated by increasing the driving force of the bainitic transformation by adding Co and Al or by refining the austenite grain size to increase the initial nucleation sites [6]. For instance, laser cladding with a large cooling rate has been used to fabricate fine primary austenite grains [7] and aluminum and cobalt have been introduced to enhance the austenite-ferrite transformation [6]. Austenite “blocks” are distributed at the interdendritic boundaries in laser-cladded coatings due to microsegregation during solidification [8]. Segregation at the interdendritic boundaries affects the kenitics of bainitic transformation and, as a result, the morphology and distribution of the nanostructured bainite in the cladded coatings vary with the isothermal temperature [9]. The microstructural characteristics such as the crystallographic arrangement and the mutual crystallographic relationship between the bainite sheaves can be utilized to elucidate the nucleation and propagation processes and, furthermore, the mechanical properties are impacted by these Materials 2017, 10, 800; doi:10.3390/ma10070800

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microstrcutural characteristics [10–13]. Although the morphology and crystallography of nanostructured bainite have been studied [14,15], the evolution of the bainitic morphology and the bainitic microstructure disribution at different temperature has seldom been studied. As previously reported [15,16], the relationship between nanobainitic ferrite and austenite is Nishiyama-Wassermann (N-W) instead of Kuidjumov-Sachs (K-S), and there are 24 variants in the K-S relationship and 12 variants in the N-W relationship between bainitic ferrite and austenite [17,18]. In this work, the growth and propagation behavior of the bainitic transformation at different isothermal temperatures and the variants of the crystal orientations between the γ (austenite) and α (bainitic ferrite) phases are studied. A model based on the morphology and distribution of the nanobainitic sheaves and orientation relationship of the different phases is postulated and described. 2. Materials and Experimental Procedures The precursor materials used in laser cladding were 75–250 µm spherical powders fabricated by the plasma rotation electrode process. The composition of the powders was Fe-0.81 C-1.55 Si-1.96 Mn-1.13 Cr-0.32 Mo-1.53 Co-0.93 Al (wt. %). The substrate was mild steel [Fe-0.14 C-0.22 Si-0.58 Mn (wt. %)] with dimensions of 160 mm × 15 mm × 12 mm. The substrate was polished and then cleaned with acetone and alcohol prior to laser cladding. The cladding system (Rofin DL-035Q, Plymouth, MI, USA) consisted of a 3.5 kW diode laser, a five-axis robotic laser scanning control, and a powder coaxial nozzle feeding system with a gas shielding device to protect the coatings from oxidation. The argon was employed as shielding gas with a flow rate of 15 L/min. The spot size of the laser at the focal length (165 mm) is 2.5 mm × 3.5 mm. The substrate was pre-heated (by using the electronic hot plate, CT-946A) above the corresponding isothermal temperature before laser cladding and the sample was transferred to a furnace for isothermal heat treatment after laser cladding. During sample transferring, a surface digital thermometer (Anritsu HA-100K, Morgan Hill, CA, USA) was used to monitor the temperature of the specimens to make sure that the temperature was not below the isothermal temperature. The specimens were quenched at ambient temperature after a certain holding time. The important instrumental parameters in laser cladding and subsequent isothermal heat treatments are listed in Table 1. Optimized laser power of 2.2, 2.3, 2.4, or 2.5 kW was used for the cladded samples, which were preheated to 370, 320, 270, and 220 ◦ C, respectively. Table 1. Processing parameters for laser cladding and isothermal transformation. Laser Cladding

Isothermal Heat Treatment

Laser Power (W)

Preheating Temperature (◦ C)

Scanning Velocity (mm/s)

Powder Feed Rate (g/min)

Isothermal Temperature (◦ C)

Isothermal Times (h)

2500 2400 2300 2200

220 270 320 370

10 10 10 10

30 30 30 30

200 250 300 350

1, 2, 4, 8, 12, 24, 48, 1/2, 1, 2, 3, 4, 8, 12, 16 1/4, 1/2, 3/4, 1, 3/2, 2, 3, 4, 6 1/4, 1/3, 1/2, 3/4, 1, 1.5, 2, 3, 4

After cladding and heat treatment, the specimens were sectioned transversely, polished by a conventional metallographic technique, and etched by 4% nital to reveal the microstructure. The microstructure in the central area of the coatings was studied using field-emission scanning electron microscopy (NOVA NanoSEM 230, FEI, Hillsboro, OR, USA). The foils for transmission electron microscopy (TEM) were thinned to a thickness of 80 µm with emery paper and electropolished at −30 ◦ C in 5% perchloric acid/95% ethanol at 40 V. The thin foils were examined by transmission electron microscopy (TEM JEOL JEM-2100F, Tokyo, Japan) at 200 kV. The specimens for electron backscattered diffraction (EBSD) were prepared by mechanical grinding and vibration-polishing with a silica slurry. EBSD was performed at 20 kV, with a tilt angle of 70◦ and a step size of 0.1 µm.

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3. Results and Discussion 3.1. Characteristics of the Microstructure Microstructure of of Nanobainite Nanobainite Optical microstructures revealed by etching are illustrated in Figure 1a and show the structure structure ◦ C for 4 h in the coating alloy. Black thin acicular formed by the isothermal isothermal transformation at 200 °C in the coating alloy. Black thin acicular structures have already been been recognized recognized as bainitic bainitic ferrite ferrite (BF) (BF) sheaves sheaves in the the authors’ authors’ previous previous studies [9]. Bainitic ferrite sheaves nucleate at the austenite grain boundary initially and then grow from from them them into into a retained retained austenite austenite (RA) (RA) grain. grain. The propagation of these sheaves grow along several particular sheaves grow withwith the same direction are marked with the same colored particular directions directions(bainitic (bainitic sheaves grow the same direction are marked with the same ◦ C for 2 h dash lines, aslines, shown Figure In the decomposed at 200at◦ C for°C4 for h and colored dash as in shown in 1). Figure 1). coatings In the coatings decomposed 200 4 h 250 and 250 °C for typical acicular bainitebainite laths form, by polygonal areas composed of RA andof martensite 2 h typical acicular laths surrounded form, surrounded by polygonal areas composed RA and ◦ (M) between(M) them, as in Figure 1a,b. The 3001a,b. C transformed sheaves present a bamboo leaves shape. martensite between them, as in Figure The 300 °C transformed sheaves present a bamboo The width and length of bainitic sheavesof sharply increased the transformed temperature increased leaves shape. The width and length bainitic sheavesassharply increased as the transformed ◦ C grows to be long sheaves, while the from 200 ◦ C to 300 ◦ C. The bainite 350transformed temperature increased fromprimary 200 °C transformed to 300 °C. The primary bainite 350 °C grows to be secondary formed bainite plates withformed different growing directions interlacegrowing with each other. Island-like long sheaves, while the secondary bainite plates with different directions interlace shape bainite and martensite nearbainite the austenite grain boundaries can also grain be observed in Figure 1d. with each other. Island‐like shape and martensite near the austenite boundaries can also ◦ The scale of the sheaves1d. ofThe 350 scale C transformed bainite is almost unchanged compared withunchanged the 300 ◦ C be observed in Figure of the sheaves of 350 °C transformed bainite is almost transformed bainite. compared with the 300 °C transformed bainite.

Figure 1. Optical micrographs of the nanobaninitc coatings after different isothermal processing at Figure 1. Optical micrographs of the nanobaninitc coatings after different isothermal processing at 200 ◦°C for 4 h (a); 250 ◦°C for 2 h (b); 300 °C for 2 h (c); and 350 °C for 3 h (d). 200 C for 4 h (a); 250 C for 2 h (b); 300 ◦ C for 2 h (c); and 350 ◦ C for 3 h (d).

Additionally, FE‐SEM was employed to characterize the details of the microstructure and Additionally, FE-SEM wasbainite employed to characterize the isothermal details of the microstructure and morphology of nanostructured transformed at different temperatures, as shown morphology of nanostructured bainite transformed at different isothermal temperatures, as shown in Figure 2. The sheaves of bainite had a lamella morphology consisting of nanoscale or submicron in Figureferrite 2. Theplates sheaves bainite austenite. had a lamella consisting of nanoscale or submicron bainitic andof retained The morphology average width of the bainitic plate increased when bainitic ferrite plates and retained austenite. The average width of the bainitic plate increased when the transformation temperature increased. The growing directions of bainitic plates or sheaves are the transformation temperature growing directions bainitic orwith sheaves are marked by the colored dash lineincreased. and circledThe numbers. For instance,of the plates plates marked ① and yellow dash lines parallel with each other, and the plates circled with the red dash line and marked ③ propagate in the same direction, as shown in Figure 2a. The paralleled bainitic sheaves indicated

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marked by the colored dash line and circled numbers. For instance, the plates marked with À and yellow dash lines parallel with each other, and the plates circled with the red dash line and marked Â propagate in the same direction, as shown in Figure 2a. The paralleled bainitic sheaves indicated that Materials the propagation 2017, 10, 800 is not random but follows specific directions. Propagation of the bainitic 4 oflathes 16 or sheaves is in accordance with repeated nucleation and growth of the sub-units with a lenticular that the The propagation not random follows specific directions. Propagation bainitic as lathes shape [19]. sub-unitistraces of the but bainitic ferrite are separated by the films of ofthe austenite, shown or sheaves is in accordance with repeated nucleation and growth of the sub‐units with a lenticular by the red dash circled areas in Figure 2b. The bainitic sheaves consist of plates that are composed shape [19]. The sub‐unit traces of the bainitic ferrite are separated by the films of austenite, as shown of sub-units with the same growing direction. There are obvious angles between the sheaves with by the red dash circled areas in Figure 2b. The bainitic sheaves consist of plates that are composed of different growing directions, and the angles are the projections of the actual angles between the sub‐units with the same growing direction. There are obvious angles between the sheaves with different bainitic ferrite sheaves and on the are some huge sub-units thickness) different growing directions, thesection angles plane. are theThere projections of the actual angles (high between the at the edges between the bainitic and austenite blocks a high transformation temperature, different bainitic ferrite sheavessheaves on the section plane. There areatsome huge sub‐units (high thickness) as shown in Figure 2c,d. This because the thickness of the sub-unit neartransformation the austenite blocks increases at the edges between the is bainitic sheaves and austenite blocks at a high temperature, evenasafter lengthening has ceased and plastic relaxation of austenite enables the sub-units to grow shown in Figure 2c,d. This is because the thickness of the sub‐unit near the austenite blocks increases even after lengthening hasofceased and plastic relaxation of austenite enables sub‐units thicker [20]. In addition, the strength the austenite at high temperature is not high the enough to resist to grow thicker [20].ofInthe addition, the strength at is high temperature hightemperature enough the thickness increase sub-units. The sizeofofthe theaustenite sub-units much larger at is a not higher resist the increase sub‐units. Theso size ofplates the sub‐units is much larger at a higher the (for to example at thickness 350 ◦ C), as shownofinthe Figure 2d, and the are thicker [21,22]. Meanwhile, temperature (for example at 350 °C), as shown in Figure 2d, and so the plates are thicker [21,22]. average length of the lath becomes shorter, as propagating is easily stopped by the adjacent stable Meanwhile, the average length of the lath becomes shorter, as propagating is easily stopped by the austenite [23]. The edge line and contour surface can be clearly observed from the bainitic ferrite plates adjacent stable austenite [23]. The edge line and contour surface can be clearly observed from the after etching and the sub-unit of the bainitic ferrite has a lenticular morphology, as shown in Figure 2d. bainitic ferrite plates after etching and the sub‐unit of the bainitic ferrite has a lenticular morphology, Based on the above, one can draw the conclusion that the growth behavior and morphology of the as shown in Figure 2d. Based on the above, one can draw the conclusion that the growth behavior bainitic depended on thesheaves scale and growth of bainite and sheaves morphology of the bainitic depended ondirection the scale and growth sub-unit. direction of bainite sub‐unit.

Figure 2. The propagation morphology and sheaves of the nanostructured bainite in the laser cladded

Figure 2. Thetransformed propagation of°C the nanostructured bainite in the laser cladded coatings, atmorphology (a) 200 °C; (b)and 250 sheaves °C; (c) 300 and (d) 350 °C. coatings, transformed at (a) 200 ◦ C; (b) 250 ◦ C; (c) 300 ◦ C and (d) 350 ◦ C.


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3.2. Crystal Orientation Relationship 3.2. Crystal Orientation Relationship The bainitic transformation is considered a diffusionless transformation similar to the martensitic The bainitic transformation is considered a diffusionless transformation the is transformation. There is an assumption that the close-packed plane {110} of the similar bainitic to ferrite martensitic transformation. There is an assumption that the close‐packed plane {110} of the bainitic parallel to the {111} plane of the austenite, meaning that propagation of the bainite sheaves has ferrite orientations is parallel to the {111} of phase the austenite, meaning that propagation of the bainite sheavesthe specific with theplane parent (austenite). The orientation relationship between has specific orientations with the parent phase (austenite). The orientation relationship between the ◦ ◦ bainitic ferrite and austenite in 200 C and 250 C transformed coatings have a K-S relationship bainitic ferrite and austenite in 200 °C and 250 °C transformed coatings have a K‐S relationship ({011}α//{111}γ, α//γ) and a N-W ({011}α//{111}γ, α//γ) relationship, ({011}α//{111}γ, α//γ) and a N‐W ({011}α//{111}γ, α//γ) relationship, as shown as shown in Figure 3a,b, respectively. The difference between the K-S and N-W orientations is about in◦ Figure 3a,b, respectively. The difference between the K‐S and N‐W orientations is about 5.26° [24] 5.26 [24] and the deviation of the orientation relationship between bainitic ferrite and film austenite and the deviation of the orientation relationship between bainitic ferrite and film austenite from K‐S from K-S and N-W in the circled areas of Figure 3a,c are about 2.9◦ and 2.5◦ . It can be inferred that and N‐W in the circled areas of Figure 3a,c are about 2.9° and 2.5°. It can be inferred that there is an there is an orientation relationship of the nanostructured bainitic andaustenite adjacent between austenitethe between orientation relationship of the nanostructured bainitic ferrite andferrite adjacent K‐S theand K-SN‐W and relationships. N-W relationships. These specific growth directions of the sheaves mentioned in Figure These specific growth directions of the sheaves mentioned in Figure 2 were 2 were determined by the specific orientation relationships between bainitic and austenite. determined by the specific orientation relationships between bainitic ferrite ferrite and austenite.

Figure 3. Transmission electron microscopy images of bainitic structure transformed at 200 °C (a) and Figure 3. Transmission electron microscopy images of bainitic structure transformed at 200 ◦ C (a) and 250 °C (c); the corresponding selected areas of electron diffraction (SAED) patterns are (b,d). 250 ◦ C (c); the corresponding selected areas of electron diffraction (SAED) patterns are (b,d).

Considering the symmetry of the cubic systems, there are crystallographic variants in the bainitic Considering of the cubic systems,evolving there arefrom crystallographic in the ferrite of the K‐Sthe andsymmetry N‐W orientation relationships a single grainvariants of austenite thatbainitic has ferrite of the K-S and N-W orientation relationships a single grain of austenite thatXhas specific growth orientations. The variants of the K‐Sevolving and N‐Wfrom relationships denoted as VX, where specific growth orientations. TheFigure variants of thesix K-S(V1–V6) and N-W denoted as VX, where indicates the variant number. 4 shows K‐Srelationships and three (V1–V3) N‐W variants evolving on (111) austenite The4 orange indicates austenite (fcc) plane and X indicates thethe variant number.plane. Figure shows triangle six (V1–V6) K-S the and(111) three (V1–V3) N-W variants the rectangles showaustenite the (011) plane. bainiticThe ferrite (bcc)triangle plane. The color the lines of the orange (fcc) triangle and evolving on the (111) orange indicates (111) austenite plane and indicate the parallel directions of the austenite ferrite. Because there are four therectangles rectangles show the (011) bainitic ferrite (bcc) plane. and The bainite color lines of the orange triangle and differentindicate {111} planes in austenite and sixof different directions onferrite. these planes, thethere number rectangles the parallel directions the austenite and bainite Because are of four


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different {111} planes in austenite and six different directions on these planes, the number of variants is 24. Similarly, there three different directions {111} plane K-SK‐S variants is 24. Similarly, there are are three different directions on on thethe {111} plane andand the the N-W N‐W variants number is 12. variants number is 12.

Figure 4. Illustration of the crystallographic variants evolving onon thethe (111) austenite plane inin thethe (a)(a) K-S Figure 4. Illustration of the crystallographic variants evolving (111) austenite plane relationship and (b) N–W relationship. K‐S relationship and (b) N–W relationship.

The orientation relationshipbetween betweenthe the bainitic bainitic ferrite ferrite and byby TEM, The orientation relationship and austenite austeniteisisinvestigated investigated TEM, and since the analytical area is quite small, EBSD is performed to characterize the phases of the fully and since the analytical area is quite small, EBSD is performed to characterize the phases of the fully transformed bainitic microstructures in a relatively large area. The orientation distribution of the transformed bainitic microstructures in a relatively large area. The orientation distribution of the {001} {001} pole figures of the austenite in the area (30 μm × 30 μm) of the nano bainitic coatings is presented pole figures of the austenite in the area (30 µm × 30 µm) of the nano bainitic coatings is presented in in Figure 5 (red scattered points). The orientation of the retained austenite in one single prior Figure 5 (red scattered points). The orientation of the retained austenite in one single prior austenite austenite grain at four isothermal transformation temperatures is determined and the average values grain at four isothermal transformation temperatures is determined and the average values are listed are listed in Table 2. The 24 variants of the K‐S and 12 variants of the N‐W orientation relationships in Table variants of the K-S and 125 variants of theby N-W based on based2.onThe the24 orientation results in Figure are calculated the orientation open sourcerelationships software (PTCLab), andthe orientation results in Figure 5 are calculated by the open source software (PTCLab), and the calculated the calculated results are listed in Tables 3 and 4, respectively [25]. results are listed in Tables 3 and 4, respectively [25]. Table 2. The orientation of the retained austenite of the selected area at different temperatures. Table 2. The orientation of the retained austenite of the selected area at different temperatures. Orientation of the Retained Austenite Isothermal Temperature (°C) Plane Direction Orientation of the Retained Austenite 200 (◦ C) (0.0898 0.2007 0.9755) [0.9955 0.0114 −0.094] Isothermal Temperature Plane0.7443) [−0.0624 −0.8489 Direction 250 (0.5175 0.4221 0.5248] 300 (0.6015 −0.6286] 200 (0.08980.4011 0.20070.6909) 0.9755) [0.7738 −0.0776 [0.9955 0.0114 −0.094] 350 (0.5122 0.3788] 250 (0.51750.4406 0.42210.7373) 0.7443) [0.2266 −0.8973 [−0.0624 −0.8489 0.5248] 300 (0.6015 0.4011 0.6909) [0.7738 −0.0776 −0.6286] 350 (0.5122 0.4406 0.7373)relationship[0.2266 −0.8973 0.3788] Table 3. The 24 variants for the K‐S orientation (V1–V24). Variant Parallel Plane Parallel Direction Rotation Axis in V1 Rotation Angle(deg) 3. The 24 variants for the K-S orientation relationship (V1–V24). V1 Table (111)γ//(011)α [101]γ//[1 11]α V2 (111)γ//(011)α [101]γ//[111]α [0.58 −0.58 0.58] 60.00 Variant Plane Parallel Direction Rotation Rotation 11]α [0.00 −0.71 Axis −0.71]in V1 60.00 Angle (deg) V3 Parallel (111)γ//(011)α [011]γ//[1 V4(111)γ//(011)α (111)γ//(011)α [011]γ//[111]α [0.00 0.71 0.71] 10.53 V1 [101]γ//[111]α [0.00 0.71 0.71]0.58] 60.00 60.00 V5(111)γ//(011)α (111)γ//(011)α [110]γ//[111]α V2 [0.58 −0.58 [101]γ//[111]α [0.00 −0.71 −0.71] 49.47 60.00 V6(111)γ//(011)α (111)γ//(011)α [110]γ//[111]α [0.00 −0.71 −0.71] V3 [011]γ//[111]α V7(111)γ//(011)α (111)γ//(011)α [101]γ//[111]α [−0.58 −0.58 0.58] 49.47 10.53 V4 [0.00 0.71 0.71] [011]γ//[111]α ]γ//[1 11 ]α [0.58 −0.58 0.58] 10.53 V8 (11 1)γ//(011)α [101 V5 (111)γ//(011)α [0.00 0.71 0.71] 60.00 [110]γ//[111]α [−0.19 0.61] 50.51 49.47 V9(111)γ//(011)α (111)γ//(011)α [110]γ//[111]α V6 [0.000.77 −0.71 −0.71] [110]γ//[111]α V10 (111)γ//(011)α [110]γ//[111]α [−0.49 −0.46 0.74] 50.51 [0.35 −0.93 −0.07] 14.88 V11 (111)γ//(011)α [011]γ//[111]α V12 (111)γ//(011)α [011]γ//[111]α [0.36 −0.71 0.60] 57.21 [0.93 0.35 0.07] 14.88 V13 (111)γ//(011)α [011]γ//[111]α

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Table 3. Cont. Variant

Parallel Plane

V7 (111)γ//(011)α V8 (111)γ//(011)α V9 (111)γ//(011)α V10 (111)γ//(011)α V11 (111)γ//(011)α V12 (111)γ//(011)α V13 (111)γ//(011)α V14 (111)γ//(011)α V15 (111)γ//(011)α V16 Materials 2017, 10, 800 (111)γ//(011)α V17 (111)γ//(011)α V18 V14 (111)γ//(011)α (111)γ//(011)α V19 V15 (111)γ//(011)α (111)γ//(011)α (111)γ//(011)α V20 V16 (111)γ//(011)α (111)γ//(011)α V21 V17 (111)γ//(011)α V18 (111)γ//(011)α V22 (111)γ//(011)α (111)γ//(011)α V23 V19 (111)γ//(011)α V20 (111)γ//(011)α V24 (111)γ//(011)α V21 V22 V23 V24

Variant V1 Variant V2 V3V1 V4V2 V3 V5 V4 V6 V5 V7 V6 V8 V7 V9V8 V10V9 V11 V10 V12 V11 V12

Parallel Direction

Rotation Axis in V1

Rotation Angle (deg)

[−0.58 −0.58 0.58] [101]γ//[111]α [0.58 −0.58 0.58] [101]γ//[111]α [−0.19 0.77 0.61] [110]γ//[111]α [−0.49 −0.46 0.74] [110]γ//[111]α [0.35 −0.93 −0.07] [011]γ//[111]α [0.36 −0.71 0.60] [011]γ//[111]α [0.93 0.35 0.07] [011]γ//[111]α [0.74 0.46 −0.49] [011]γ//[111]α [−0.25 −0.63 −0.74] [101]γ//[111]α [0.66 0.66 0.36] [101]γ//[111]α [−0.66 0.36 −0.66] [110]γ//[111]α [−0.30 −0.63 −50.51 0.72] [110]γ//[111]α 11]α [0.74 0.46 −0.49] [011]γ//[1 [−0.61 0.19 −0.77] [110]γ//[111]α [101]γ//[1 11]α [−0.25 −0.63 −0.74] 57.21 11]α [0.66 0.66 0.36] [101]γ//[1 [−0.36 −0.60 −20.61 0.71] [110]γ//[111]α [110]γ//[1 11]α [−0.66 0.36 −0.66] 51.73 [0.96 0.00 −0.30] [011]γ//[111]α 11]α [−0.30 −0.63 −0.72] 47.11 [110]γ//[1 [−0.72 0.30 −0.63] [011]γ//[111]α [−0.61 0.19 −0.77] 50.51 [110]γ//[111]α [−0.74 −0.25 0.63] [101]γ//[111]α [110]γ//[111]α [−0.36 −0.60 −0.71] 57.21 [0.91 −0.41 0.00] [101]γ//[111]α

49.47 10.53 50.51 50.51 14.88 57.21 14.88 50.51 57.21 7 of 1620.61 51.73 47.11 50.51 57.21 20.61 47.11 57.21 21.06

[0.96 0.00 −0.30] 20.61 (111)γ//(011)α [011]γ//[111]α [−0.72 0.30 −0.63] 47.11 (111)γ//(011)α [011]γ//[111]α (111 )γ//(011)α 11]α [−0.74 orientation −0.25 0.63] relationship 57.21 (V1–V12). Table 4. The 12[101]γ//[1 variants for the N-W [0.91 −0.41 0.00] 21.06 (111)γ//(011)α [101]γ//[111]α

Parallel Plane

Parallel Direction

(111)γ//(011)α

[211]γ//[011]α

Rotation Axis in V1

Table 4. The 12 variants for the N‐W orientation relationship (V1–V12).

Rotation Angle (deg)

Parallel Plane Parallel Direction Rotation Axis[0.000 in V1 −0.707 Rotation Angle (deg) (111)γ//(011)α −0.707] [121]γ//[011]α (111)γ//(011)α [211]γ//[01[112]γ//[011]α 1]α (111)γ//(011)α [0.00 0.707 0.707] (111)γ//(011)α [121]γ//[011]α [0.000 −0.707 −0.707] 60.00 [1.000 0.000 0.000] (111)γ//(011)α [211]γ//[011]α [0.00 0.707 0.707] 60.00 (111)γ//(011)α [112]γ//[011]α [−0.223 −0.697 −0.681] (111)γ//(011)α [121]γ//[011]α [1.000 0.000 0.000] 49.47 (111)γ//(011)α [211]γ//[011]α [−0.223 0.697 0.681] (111)γ//(011)α [012]γ//[011]α (111)γ//(011)α [121]γ//[011]α [−0.223 −0.697 −0.681] 10.53 [0.706 −0.706 −0.060] (111)γ//(011)α [211]γ//[011]α [−0.223 0.697 0.681] 50.51 (111)γ//(011)α [012]γ//[011]α [−0.681 −0.223 0.697] (111)γ//(011)α [121]γ//[011]α (111)γ//(011)α [211]γ//[011]α [0.706 −0.706 −0.060] 14.88 [−0.624 0.471 −50.51 0.624] (111)γ//(011)α [112]γ//[011]α [−0.681 −0.223 0.697] (111)γ//(011)α [121]γ//[011]α [0.706 0.706 0.060] (111)γ//(011)α 1]α [−0.624 0.471 −0.624] 57.21 (111)γ//(011)α [112]γ//[01[211]γ//[011]α [−0.624 −0.471 50.51 0.624] (111)γ//(011)α (111)γ//(011)α [211]γ//[01[121]γ//[011]α 1]α [0.706 0.706 0.060] −0.681 0.223 −57.21 0.697] (111)γ//(011)α 1]α [−0.624 −0.471 [0.624] (111)γ//(011)α [121]γ//[01[112]γ//[011]α [−0.681 0.223 −0.697] 20.61 (111)γ//(011)α [112]γ//[011]α

60.00 60.00 49.47 10.53 50.51 14.88 50.51 57.21 50.51 57.21 20.61

The2424K‐S K-S variants 12 N-W variants are calculated based on therelationship orientationofrelationship of The variants andand 12 N‐W variants are calculated based on the orientation the prioraustenite austenite and directions shown in the {001}pole austenite polecolor figure. The color squares the prior and the the directions shown in the {001} austenite figure. The squares and trianglesrepresent represent K-S N‐W and ideal N-Wrelationships, ideal relationships, respectively. The and triangles the the K‐S and respectively. The numbers of 1numbers to 24 and of 1 to 24 and ① to ⑫ represent serial number of theof 24the K‐S 24 variants and 12 N‐W variants of the bainiticof ferrite, 1 to representthethe serial number K-S variants and 12 N-W variants the bainitic ferrite,

respectively, corresponding to the numbers in Tables 3 and 4. respectively, corresponding to the numbers in Tables 3 and 4. Figure 6 indicates the inverse pole figure (IPF) of the bainitic ferrite formed at different Figure 6 indicates the inverse pole figure (IPF) of the bainitic ferrite formed at different temperatures and the calculated K‐S orientation relationship variants. The scattered points with temperatures the the calculated K-S orientation relationship Theindicating scattered points with different colors and illustrate orientation distributions of the transformedvariants. bainitic ferrite different colors illustrate the orientation of match the transformed bainitic ferrite indicating that the actual orientation relationships of α anddistributions γ do not exactly the K‐S relationship when compared to the EBSD and calculated variants. The actual orientations of the bainitic ferrite deviate that the actual orientation relationships of α and γ do not exactly match the K-S relationship when from the ideal orientation relationship. instance, small deviation between V24 ferrite deviate compared to calculated the EBSDK‐S and calculated variants.For The actualthe orientations of the bainitic to the actual orientation is a blue arrow, as shown in the blue dashed box in Figure 6a, from a to b. from the ideal calculated K-S orientation relationship. For instance, the small deviation between V24 Not all 24 variants are observed from the pole figures. This is due to the fact that the interaction to the actual orientation is a blue arrow, as shown in the blue dashed box in Figure 6a, from a to b. energy of each of the K‐S variants is different when associated with the calculation result of the γ→α Not all 24 variants thenumber pole figures. This is same due to that the interaction transformation (Kunduare andobserved Bhadeshia from [26]). The of variants in the unitthe areafact decreases energy of each of the K-S variants is different when associated with the calculation result of the γ→α with increasing temperature, and hence the green, cyan, and purple regions illustrate that V1, V16, V19, and V20 are the dominant variants in the K‐S orientation relationship of the 350 °C specimen. transformation (Kundu and Bhadeshia [26]). The number of variants in the same unit area decreases All the determinedtemperature, orientations of and the bainitic deviatecyan, from the K‐S variants based that V1, V16, with increasing henceferrite the green, andcalculated purple regions illustrate on the average orientation of the retained austenite, as consistency with the TEM results and the V19, and V20 are the dominant variants in the K-S orientation relationship of the 350 ◦ C specimen. All deviation from the calculated orientations is more obvious at a high temperature.

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the determined orientations of the bainitic ferrite deviate from the calculated K-S variants based on the average orientation of the retained austenite, as consistency with the TEM results and the deviation from the2017, calculated Materials 10, 800 orientations is more obvious at a high temperature. 8 of 16

Figure 5. The {001} pole figures of the retained austenite in the (a) 200 °C; (b) 250 °C; (c) 300 °C and Figure 5. The {001} pole figures of the retained austenite in the (a) 200 ◦ C; (b) 250 ◦ C; (c) 300 ◦ C and (d) 350 ◦°C isothermal transformed microstructures, and the corresponding calculated K‐S (colored (d) 350 C isothermal transformed microstructures, and the corresponding calculated K-S (colored squares) and N‐W (colored triangles) variants of the bainitic ferrite. squares) and N-W (colored triangles) variants of the bainitic ferrite.

Figure 7 compares the bainitic ferrite orientations in the (001) inverse pole figure with the Figure 7 compares the bainitic ferrite orientations in the (001) inverse pole figure with the calculated results of the 12 variants of the N‐W orientation relationship based on the average calculated results of the 12 variants of the N-W orientation relationship based on the average orientation orientation of the retained austenite showing that the calculated N‐W variants do not follow the of the retained austenite showing that the calculated N-W variants do not follow the inverse pole inverse pole figures of the bainitic ferrite very well at a low temperature. For instance, the upper left figures of the bainitic ferrite very well at a low temperature. For instance, the upper left of the of the inverse pole figure of the bainitic ferrite transformed at 200 °C is divided into eight parts, and inverse pole figure of the bainitic ferrite transformed at 200 ◦ C is divided into eight parts, and the the calculated N‐W variants of the V3, V6, V9, and V12 are distributed between the two determined calculated N-W variants of the V3, V6, V9, and V12 are distributed between the two determined regions. regions. The orientation distribution of bainitic ferrite is closer to the calculated results of the N‐W The orientation distribution of bainitic ferrite is closer to the calculated results of the N-W variants as variants as the isothermal temperature is increased. Therefore, the orientation of the bainitic ferrite the isothermal temperature is increased. Therefore, the orientation of the bainitic ferrite corresponds to corresponds to the 12 variants at 300 °C and 350 °C, as shown in Figure 7c,d, implying that the the 12 variants at 300 ◦ C and 350 ◦ C, as shown in Figure 7c,d, implying that the orientation relationship orientation relationship between the bainitic ferrite and austenite is closer to N‐W than K‐S. In between the bainitic ferrite and austenite is closer to N-W than K-S. In addition, the orientation addition, the orientation exhibits continuous gradients between the N‐W and K‐S as the isothermal exhibits continuous gradients between the N-W and K-S as the isothermal transformation temperature transformation temperature is reduced. is reduced.


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Figure Figure6. 6.The Thedetermined determined{001} {001}inverse inversepole polefigure figureof ofthe thebainitic bainiticferrite ferriteand andthe thecomparison comparisonwith withthe the calculated variants of K‐S relationship of the selected areas of microstructures transformed calculated variants of K-S relationship of the selected areas of microstructures transformed at (a) 200 ◦at C; (a) and ◦ C;(b) ◦ C (c) ◦ C.(d) 350 °C. (b)200 250°C; (c)250 300°C; and300 (d)°C 350

Figure 8 presents the inverse pole figures of the nanobainitic microstructures transformed at Figure 8 presents the inverse pole figures of the nanobainitic microstructures transformed at different temperatures. The regions plotted with the same color represent the bainitic sheaves with different temperatures. The regions plotted with the same color represent the bainitic sheaves with the same growth orientations and the variant colors illustrate the different orientations of the bainitic the same growth orientations and the variant colors illustrate the different orientations of the bainitic ferrite corresponding to the color of the scattered regions in the inverse pole in Figures 6 and 7. All ferrite corresponding to the color of the scattered regions in the inverse pole in Figures 6 and 7. 12 variants of the N‐W relationship can be observed from the four figures and the bainitic lathes with All 12 variants of the N-W relationship can be observed from the four figures and the bainitic lathes different colors in the IPF map are numbered with the corresponding variants. The white regions in with different colors in the IPF map are numbered with the corresponding variants. The white regions the microstructures are the retained austenite and the surface fraction of the retained austenite that in the microstructures are the retained austenite and the surface fraction of the retained austenite goes up with increasing isothermal temperature. The variant selection is weakened at a low that goes up with increasing isothermal temperature. The variant selection is weakened at a low temperature because plastic relaxation of austenite is difficult. Hence, the variants of the bainitic temperature because plastic relaxation of austenite is difficult. Hence, the variants of the bainitic ferrite ferrite becomes multiple as a result of the internal stress relaxation caused by the bainitic becomes multiple as a result of the internal stress relaxation caused by the bainitic transformation transformation misfit strain [27]. Consequently, there is no obvious dominant variant sheaf at 200 °C. misfit strain [27]. Consequently, there is no obvious dominant variant sheaf at 200 ◦ C. V7, V8, V9, and V7, V8, V9, and V10 are dominant at 250 °C, whereas V8, V9, and V11 are dominant at 300 °C, and V10 are dominant at 250 ◦ C, whereas V8, V9, and V11 are dominant at 300 ◦ C, and V11 in the N-W V11 in the N‐W relationship is the only dominant one at 350 °C. relationship is the only dominant one at 350 ◦ C.

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Figure 7. The determined {001} inverse pole figure of the bainitic ferrite and the comparison with the

Figure 7. The determined {001} inverse pole figure of the bainitic ferrite and the comparison with calculated variants of N‐W relationship of the selected areas of microstructures transformed at the calculated variants of N-W relationship of the selected areas of microstructures transformed at (a) 200 °C; (b) determined 250 °C; (c) 300 °C inverse and (d) pole 350 °C. Figure 7. The {001} figure of the bainitic ferrite and the comparison with the (a) 200 ◦ C; (b) 250 ◦ C; (c) 300 ◦ C and (d) 350 ◦ C. calculated variants of N‐W relationship of the selected areas of microstructures transformed at (a) 200 °C; (b) 250 °C; (c) 300 °C and (d) 350 °C.

Figure 8. Cont.


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Figure 8. The map of bainitic microstructure transformed transformed atat(a) °C;◦ C; (b)(b) 250250 °C; ◦(c) ◦ C and Figure 8. The IPF IPF map of bainitic microstructure (a)200 200 C;300 (c) °C 300and (d)◦350 °C. (d) 350 C.

The orientation relationship of γ and α is between the K‐S and N‐W relationships at the low

The orientation of γ and isαhomogeneously is between thedistributed K-S and N-W the low temperature of 200relationship °C. The bainitic ferrite in the relationships prior austenite at grain. ◦ C. The bainitic The orientation becomes closer the N‐W relationship as the isothermal temperature temperature of 200 relationship ferrite is to homogeneously distributed in the prior austeniteisgrain. increased, and some sheaves with specific N‐W are dominant in aisothermal given austenite grain. The orientation relationship becomes closer to the variants N-W relationship as the temperature is More retained austenite is obtained at a higher temperature. increased, and some sheaves with specific N-W variants are dominant in a given austenite grain. More When the bainitic transformation occurs in a single austenite crystal (grain), there are 12 or 24 retained austenite is obtained at a higher temperature. crystallographic variants in the N‐W or K‐S orientation relationships due to the symmetry of the cubic When the bainitic transformation occurs in a single austenite crystal (grain), there are 12 or systems. There are five and ten possible misorientation angles between the different variants of the 24 crystallographic variants in the N-W or K-S orientation relationships due to the symmetry of bainitic ferrite in the N‐W relationship (13.76°, 19.47°, 50.05°, 53.69°, and 60°) and the K‐S relationship the cubic systems. There are 47.11°, five and ten50.51°, possible misorientation between the different (10.53°, 14.88°, 20.61°, 21.06°, 49.47°, 51.73°, 57.21°, and 60°),angles respectively [28], as shown ◦ ◦ ◦ ◦ variants of the3 and bainitic ferrite in the N-W relationship , 19.47 , 50.05 , 53.69 , and 60◦ ) and in Tables 4. The misorientation angles for a single (13.76 austenite grain transformed to nanobainite ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ the K-S relationship (10.53 , 14.88 , 20.61 , 21.06 , 47.11 , 49.47 , 50.51 , 51.73 57.21◦ ,inand at different isothermal temperatures are shown in Figure 9. The misorientation angles,marked the 60◦ ), figures and 42.85° and 46° in represent interface of the bainitic ferrite and retained that respectively [28], as shown Tables the 3 and 4. The misorientation angles for aaustenite single austenite have the K‐S or N‐W relationship [14]. grain transformed to nanobainite at different isothermal temperatures are shown in Figure 9. The misorientation angles marked in the figures and 42.85◦ and 46◦ represent the interface of the bainitic ferrite and retained austenite that have the K-S or N-W relationship [14].

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Figure 9.9. The The misorientation misorientation angle/axis angle/axis distribution given austenite austenite grain grain transformed transformed to to Figure distribution for for the the given bainite at at different different temperatures temperaturesof of(a) (a)200 200◦°C; (b) 250 250 ◦°C; (c) 300 300 ◦°C and (d) (d) 350 350 ◦°C. bainite C; (b) C; (c) C and C.

3.3. Variants Variants of of Growth Growth Directions Directions 3.3. The misorientation from nucleation of the newnew formed lath.lath. The The misorientation of ofthe thegrowth growthdirection directioninitiates initiates from nucleation of the formed newly formed bainitic sub‐unit nucleates at the phase boundary of the existing bainite and austenite The newly formed bainitic sub-unit nucleates at the phase boundary of the existing bainite and austenite and grows grows according according to to the the N-W N‐W or or K-S K‐S relationship relationship with with austenite, austenite, which which is is called called autocatalysis autocatalysis and (Figure 10). The initial bainite lath or sheaf propagates according to the mechanism of repeated repeated (Figure 10). The initial bainite lath or sheaf propagates according to the mechanism of nucleation and growth of the sub‐units. When the new embryo of the bainite forms on defects such nucleation and growth of the sub-units. When the new embryo of the bainite forms on defects such as dislocations, bainitic ferrite grows at aatmisorientation angles (θ) with the prior as dislocations, the thenew newnucleus nucleusofof bainitic ferrite grows a misorientation angles (θ) with the propagation direction of the bainitic lath. The misorientation angles (θ) only have five possible values prior propagation direction of the bainitic lath. The misorientation angles (θ) only have five possible of 13.76°, 19.47°, 50.05°, 53.69°, and 60,° as 60, discussed previously, and autocatalytic nucleation of the ◦ , 19.47 ◦ , 50.05 ◦ , 53.69 ◦ , and ◦ as discussed values of 13.76 previously, and autocatalytic nucleation sub‐unit is affected by the previously formed sub‐units as a result of the strain of bainitic of the sub-unit is affected by the previously formed sub-units as a result of the strain of bainitic transformation[29]. [29]. In In addition, addition, nucleation nucleation occurs occurs preferentially preferentially in in the the dislocations dislocations on on the the slip slip planes planes transformation near the thepreviously previouslyformed formedbainitic bainiticsub-units. sub‐units.The Thebainitic bainiticorormartensitic martensitic transformation with N‐ near transformation with thethe N-W W relationship of the parent and product phases is related to partial dislocations [30] and the effect relationship of the parent and product phases is related to partial dislocations [30] and the effect of the of the partial dislocation on autocatalysis is shown in the following: partial dislocation on autocatalysis is shown in the following: (1) a ∙ [101] = a ∙ [211] +a [112] 2 ·[101] = 6·[211] + 6[112] (1) 2 6 6 The perfect dislocation [101 ] in the austenite plane (111) splits into two Shockley partial a The perfect in the plane (111) splits into two dislocations. Thedislocation lattice change during theaustenite bainitic transformation process withShockley V1 of thepartial N‐W 2 [101] dislocations. lattice change relationship isThe shown in Figure 11.during the bainitic transformation process with V1 of the N-W relationship is shown in Figure 11.

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Figure thethe growth intersection angle of the bainitic lath and Figure10. 10.Schematic Schematicillustration illustrationdescribing describing growth intersection angle of initial the initial bainitic lath Figure 10. caused Schematic illustration describing the growth intersection angle of the initial bainitic lath the lath by autocatalysis. and thecaused lath by autocatalysis. and the lath caused by autocatalysis.

Figure 11. Cont.


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Figure Thelattice latticechange changeduring duringthe the bainitic bainitic transformation transformation (fcc Figure 11.11.The (fcc to to bcc); bcc); (a) (a)lattice latticerotation rotationininV1 11 ]γ//[0 1 1]α; (b) asShockley Shockleypartial partial dislocation V1relationship, relationship, (111)γ//(111)α, (111)γ//(111)α,[2[211]γ//[011]α; (b) lattice lattice distortion as dislocation mechanism (011) plane. mechanism onon thethe (011) fccfcc plane.

The blue spheres and blue lines in Figure 11 indicate the fcc structure of the previous austenite The blue spheres and blue lines in Figure 11 indicate the fcc structure of the previous austenite and the purple spheres and lines indicate the bcc structure of bainitic ferrite. There is an assumed and the purple spheres and lines indicate the bcc structure of bainitic ferrite. There is an assumed temporary bct structure in the fcc to bcc transformation, as shown by the red spheres and lines, and temporary bct structure in the fcc to bcc transformation, as shown by the red spheres and lines, and there is strain for the 9.7 ° rotation along the [100] axis. The triangle plotted as orange (ABC) shows there is strain for the 9.7 ◦ rotation along the [100] axis. The triangle plotted as orange (ABC) shows the the plane of (111) fcc which is paralleled to (011) bcc (plotted as a green parallelogram). The direction plane of (111) fcc which is paralleled to (011) bcc (plotted as a green parallelogram). The direction of of [211] on the (111) plane of the fcc structure (DB) is parallel to the direction of [011] on the (011) [211] on the (111) plane of the fcc structure (DB) is parallel to the direction of [011] on the (011) plane of plane of the bcc structure (E′G). The lattice of the austenite is distorted and displaced along the the bcc structure (E’G). The lattice of the austenite is distorted and displaced along the Burgers vector Burgers vector of the Shockley partial dislocation. The lattice displaces the plane of (011) fcc when of the Shockley partial dislocation. The lattice displaces the plane of (011) fcc when atom D moves to atom D moves to D́, E moves to É, and F moves to F́, as shown in Figure 11b. It is obvious that the bct D’, E moves to E’, and F moves to F’, as shown in Figure 11b. It is obvious that the bct lattice should lattice should change to the bcc lattice by contraction of the [001] axis and extension of the [010] axis, change to the bcc lattice by contraction of the [001] axis and extension of the [010] axis, thereby making thereby making lattice distortion happen easily through V1 of the NW relationship. lattice distortion happen easily through V1 of the NW relationship. There are 24 equivalent Burgers vectors in the austenite slip system dissociated to 48 equivalent There are 24 equivalent Burgers vectors in the austenite slip system dissociated to 48 equivalent Shockley partial dislocations. Considering the symmetry of the cubic structures, there are 12 variants Shockley partial dislocations. Considering the symmetry of the cubic structures, there are 12 variants of the N‐W orientation relationship between the austenite and bainitic ferrite. Apparently, the bainitic of the N-W orientation relationship between the austenite and bainitic ferrite. Apparently, the bainitic microstructure propagates with the specific variants orientation from one original austenite grain, microstructure propagates with the specific variants orientation from one original austenite grain, and and this is also the reason why there are sheaves with different growth directions, as shown in Figure 2. this is also the reason why there are sheaves with different growth directions, as shown in Figure 2. Conclusions 4. 4. Conclusions The orientation relationship of bainitic ferrite and austenite doesfollow not follow theK-S ideal and The orientation relationship of bainitic ferrite and austenite does not the ideal andK‐S N-W N‐W relationship. Rather, the orientation relationship falls between the ideal K‐S and N‐W. The relationship. Rather, the orientation relationship falls between the ideal K-S and N-W. The orientation orientationisrelationship closer to the N‐W at a hightemperature transformation temperature and relationship closer to the is N-W relationship at arelationship high transformation and the mixed N-W the mixed N‐W and K‐S relationships are observed as the transformation temperature decreases. The and K-S relationships are observed as the transformation temperature decreases. The propagation propagation directions the bainitic sheaves are by specific variants as the temperature directions of the bainitic of sheaves are dominated bydominated specific variants as the temperature increases. increases. The phenomenon stems from decomposition of the dislocation structures during repeated The phenomenon stems from decomposition of the dislocation structures during repeated autocatalytic autocatalytic nucleation during transformation. The variants of the propagation directions of the nucleation during transformation. The variants of the propagation directions of the bainitic sheaves are caused Shockley partial dislocations and the smaller dislocation densities arebainitic causedsheaves by Shockley partialbydislocations and the smaller dislocation densities cause enhanced cause enhanced variant selection of the nanobainitic microstructures at high isothermal temperature. variant selection of the nanobainitic microstructures at high isothermal temperature. The morphology oflath the bainitic lath changes fromto long‐and‐slender to short‐and‐thicker as the ofThe the morphology bainitic ferrite changesferrite from long-and-slender short-and-thicker as the transformation transformation temperature decreases. A smaller lengthening bainitic lathes is caused temperature decreases. A smaller lengthening of the bainitic lathesofis the caused by propagation haltedby propagation halted by adjacent stable austenite and the larger thickness results from the lower by adjacent stable austenite and the larger thickness results from the lower strength of the primary strengthtoofresist the primary austenite resist thickening theedge bainitic The edge line of the austenite thickening of theto bainitic sub-units. of The line sub‐units. of the bainitic lath provides bainitic lath provides evidence that the morphology of the sub‐unit has a lenticular shape and the growth follows the pile‐up mechanism.

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evidence that the morphology of the sub-unit has a lenticular shape and the growth follows the pile-up mechanism. Acknowledgments: The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51171116), the Ministry of Science and Technology of the People’s Republic of China (Grant No. 2009DFB50350), City University of Hong Kong Applied Research Grant (ARG) No. 9667104, and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 11301215. Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Y.G. and Z.L. conceived and designed the experiments; Y.G., K.F. and C.Y. performed the experiments; Y.G. and C.Y. analyzed the data; Y.W. contributed reagents/materials/analysis tools; Y.G. and P.K.C. wrote the paper.” Authorship must be limited to those who have contributed substantially to the work reported. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2.

3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15.

16. 17.

Caballero, F.G.; Bhadeshia, H.K.D.H.; Mawella, K.J.A.; Jones, D.G.; Brown, P. Design of novel high strength bainitic steels: Part 1. Mater. Sci. Technol. 2001, 17, 512–516. [CrossRef] Caballero, F.G.; Allain, S.; Cornide, J.; Velásquez, J.P.; Garcia-Mateo, C.; Miller, M.K. Design of cold rolled and continuous annealed carbide-free bainitic steels for automotive application. Mater. Des. 2013, 49, 667–680. [CrossRef] Caballero, F.G.; Garcia-Mateo, C.; Miller, M.K. Design of Novel Bainitic Steels: Moving from UltraFine to Nanoscale Structures. JOM 2014, 66, 747–755. [CrossRef] Caballero, F.G.; Garcia-Mateo, C.; Miller, M.K. Modern steels at atomic and nanometre scales. Mater. Sci. Technol. 2015, 31, 764–772. [CrossRef] Garcia-Mateo, C.; Caballero, F.G.; Sourmail, T.; Smanio, V.; De Andres, C.G. Industrialised nanocrystalline bainitic steels. Design approach. Int. J. Mater. Res. 2014, 105, 725–734. [CrossRef] Garcia-Mateo, C.; Bhadeshia, H.K.D.H.; Caballero, F.G. Acceleration of Low-temperature Bainite. ISIJ Int. 2003, 43, 1821–1825. [CrossRef] Mingxi, L.; Yizhu, H.; Xiaomin, Y. Effect of nano-Y2O3 on microstructure of laser cladding cobalt-based alloy coatings. Appl. Surf. Sci. 2006, 252, 2882–2887. [CrossRef] Guo, Y.; Li, Z.; Yao, C.; Zhang, K.; Lu, F.; Feng, K.; Huang, J.; Wang, M.; Wu, Y. Microstructure evolution of Fe-based nanostructured bainite coating by laser cladding. Mater. Des. 2014, 63, 100–108. [CrossRef] Guo, Y.; Feng, K.; Lu, F.; Zhang, K.; Li, Z.; Hosseini, S.R.E.; Wang, M. Effects of isothermal heat treatment on nanostructured bainite morphology and microstructures in laser cladded coatings. Appl. Surf. Sci. 2015, 357, 309–316. [CrossRef] Morito, S.; Yoshida, H.; Maki, T.; Huang, X. Effect of block size on the strength of lath martensite in low carbon steels. Mater. Sci. Eng. A 2006, 438, 237–240. [CrossRef] Gourgues, A.F.; Flower, H.M.; Lindley, T.C. Electron backscattering diffraction study of acicular ferrite, bainite, and martensite steel microstructures. Mater. Sci. Technol. 2000, 16, 26–40. [CrossRef] Bakhtiari, R.; Ekrami, A. The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel. Mater. Sci. Eng. A 2009, 525, 159–165. [CrossRef] Abbaszadeh, K.; Saghafian, H.; Kheirandish, S. Effect of Bainite Morphology on Mechanical Properties of the Mixed Bainite-martensite Microstructure in D6AC Steel. J. Mater. Sci. Technol. 2012, 28, 336–342. [CrossRef] Beladi, H.; Adachi, Y.; Timokhina, I.; Hodgson, P.D. Crystallographic analysis of nanobainitic steels. Scr. Mater. 2009, 60, 455–458. [CrossRef] Gong, W.; Tomota, Y.; Adachi, Y.; Paradowska, A.M.; Kelleher, J.F.; Zhang, S.Y. Effects of ausforming temperature on bainite transformation, microstructure and variant selection in nanobainite steel. Acta Mater. 2013, 61, 4142–4154. [CrossRef] Beladi, H.; Timokhina, I.B.; Hodgson, P.D.; Adachi, Y. Characterization of nanostructured bainitic steel. Int. J. Mod. Phys. 2012, 5, 1–8. Furuhara, T.; Kawata, H.; Morito, S.; Miyamoto, G.; Maki, T. Variant Selection in Grain Boundary Nucleation of Upper Bainite. Metall. Mater. Trans. A 2008, 39, 1003–1013. [CrossRef]

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18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

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Guo, Z.; Lee, C.S.; Morris, W., Jr. On coherent transformations in steel. Acta Mater. 2004, 52, 5511–5518. [CrossRef] Gaude-Fugarolas, D.; Jacques, P.J. A New Physical Model for the Kinetics of the Bainite Transformation. ISIJ Int. 2006, 46, 712–717. [CrossRef] Tszeng, T.C. Autocatalysis in bainite transformations. Mater. Sci. Eng. A 2000, 293, 185–190. [CrossRef] Matsuzaki, A.; Bhadeshia, H.K.D.H. Effect of austenite grain size and bainite morphology on overall kinetics of bainite transformation in steels. Mater. Sci. Technol. 1999, 15, 518–522. [CrossRef] Bhadeshia, H.K.D.H.; Christian, J.W. Bainite in steels. MTA 1990, 21, 767–797. [CrossRef] Rees, G.I.; Bhadeshia, H.K.D.H. Bainite transformation kinetics Part 1 Modified model. Mater. Sci. Technol. 1992, 8, 985–993. [CrossRef] Bhadeshia, H.K.D.H. Bainite in Steels, 3rd ed.; Taylor & Francis Ltd.: Oxford, UK, 2015; pp. 19–60. Gu, X.F.; Furuhara, T.; Zhang, W.Z. PTCLab: A python program to calculate phase transformation crystallography. J. Appl. Cryst. 2014, 49, 1099–1106. [CrossRef] Kundu, S.; Bhadeshia, H.K.D.H. Crystallographic texture and intervening transformations. Scr. Mater. 2007, 57, 869–872. [CrossRef] Takayama, N.; Miyamoto, G.; Furuhara, T. Effects of transformation temperature on variant pairing of bainitic ferrite in low carbon steel. Acta Mater. 2012, 60, 2387–2396. [CrossRef] Kitahara, H.; Ueji, R.; Ueda, M.; Tsuji, N.; Minamino, Y. Crystallographic analysis of plate martensite in Fe–28.5 at.% Ni by FE-SEM/EBSD. Mater. Charact. 2005, 54, 378–386. [CrossRef] Van Bohemen, S.M.; Sietsma, J. Modeling of isothermal bainite formation based on the nucleation kinetics. Int. J. Mater. Res. 2008, 99, 739–747. [CrossRef] Abbasi-Bani, A.; Zarei-Hanzaki, A.; Pishbin, M.H.; Haghdadi, N. A comparative study on the capability of Johnson-Cook and Arrhenius-type constitutive equations to describe the flow behavior of Mg-6Al-1Zn alloy. Mech. Mater. 2014, 71, 52–61. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).