# Morphology and Crystallography of Ausferrite in Austempered Ductile Iron

^{1}

^{2}

^{*}

## Abstract

**:**

_{α}(i.e., {111}

_{γ}) plane. When two γ grains are twinned, the twins share a {111}

_{γ}plane and have seven packets. The adjacent acicular bainitic ferrite plates (or laths) sharing a ${\u27e8001\u27e9}_{\mathsf{\gamma}}$ axis have small misorientation of about 5.7°. The adjacent acicular bainitic ferrite plates (or laths) not sharing a ${\u27e8001\u27e9}_{\mathsf{\gamma}}$ axis have two high misorientation angles of ~54.3° and ~60.0°. Further, the low angle boundary to high angle boundary ratio is far less than the ratio of the variant pairs with small misorientation to the ones with large misorientation. This work is available for structures obtained as a consequence of the heat treatment of austempering.

## 1. Introduction

_{γ}plane is not exactly parallel to the {110}

_{α}plane in G–T relation. However, the G–T relationship of bainite has not been found in irons and steels except low-carbon, high-alloy steel [25]. The Pitsch and N–W are two complementary orientation relations (the parallel planes and directions of martensite and those of γ are interchanged). The G–T and G–T′ are also complementary to each other.

## 2. Materials and Methods

_{8}RE

_{3}alloy, followed by innoculation with 1.3 wt % FeSi

_{75}alloy. Then, the melt was poured into sand molds in the form of Y-blocks according to the GB/T 24733-2009 [34]. The samples for isothermal heat treatment were machined from the Y-blocks. The austempering heat treatment process consists of austenitizing the samples at 900 °C for 90 min to get a full austenite matrix, and then quenching them at 270 °C for 90 min in a salt bath. After being withdrawn from the salt bath, the samples were air cooled to room temperature.

## 3. Results

#### 3.1. Analysis of Ferrite/Austenite

_{γ}// {011}

_{α}, ${\langle 51217\rangle}_{\mathsf{\gamma}}$ // ${\langle 71717\rangle}_{\mathsf{\alpha}}$.

_{α}parallel to ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$)

_{γ}or ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$)

_{γ}. The six G–T variants on the same close-packed planes of austenite form a packet. For example, the variants from GT1 to GT6 on the γ close-packed planes ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$)

_{γ}form one packet (denoted as P1). A prior γ grain can be divided into four packets because there are four close-packed planes ((111)

_{γ}, ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$)

_{γ}, ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$)

_{γ}, and ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$)

_{γ}) in it. Furthermore, each ${\langle 001\rangle}_{\mathsf{\gamma}}$ axis in a packet is shared by two variants, e.g., X is shared by GT1 and GT2. The two variants sharing a ${\langle 001\rangle}_{\mathsf{\gamma}}$ axis in a packet (denoted as Bain paired variants) are misoriented by low angles of about 5.7°.

_{a}(labeled in Figure 1a) projected onto the (111)

_{γ}plane, with the poles from the ideal G–T relation of a single γ grain superimposed on it. The four packets in the ideal pole figure are expressed using green, violet, gold, and red colors. As can be seen, most of the experimental poles agree well with the theoretical ones. The K–S and G–T relations are quite similar to each other, so we have also superimposed an ideal K–S pole figure on the poles of the acicular bainitic ferrite in prior γ grain I

_{a}to test whether the orientation relation is exactly a G–T or K–S relation, as shown in Figure 2b. Clearly, the experimental data is not well fit by the K–S relationship. Of course, the experimental data is also far away from the N–W relation which only has the half of G–T variants and much less poles. Though the G–T' relation cannot be distinguished from the G–T using commercial EBSD software such as TSL, it usually appears in precipitates [4]. Therefore, we think the orientation relationship between α and γ is the G–T relationship.

_{a}are presented in Figure 2c,d, respectively. In Figure 2c, the poles sharing a common close-packed plane are tinted in green, violet, gold, and red, respectively. Therefore, each color stands for one packet. Note that each rosette of poles is focused by the poles of one particular packet, indicating that the ${\langle 001\rangle}_{\mathsf{\alpha}}$ axis is shared by the acicular bainitic ferrite plates (or laths) in the packet. The four ${\langle 011\rangle}_{\mathsf{\alpha}}$ axes of the four packets are perpendicular to the {011}

_{α}planes and correspond to the four ${\langle 111\rangle}_{\mathsf{\gamma}}$ directions in the prior γ grain (normal to the four {111}

_{γ}planes). This means that the six variants in a particular packet share a common {011}

_{α}(i.e., {111}

_{γ}) plane. From Figure 2d, it can also be seen that there are four packets in I

_{a}, which are highlighted in green (P1), violet (P2), gold (P3), and red (P4), respectively. Further, any packet (P1, P2, P3, or P4) is subdivided into several disconnected areas.

_{a}and I

_{b}(white dotted line) is very straight, as shown in the highlighted map of the retained austenite (Figure 3a). This means that the prior γ grains I

_{a}and I

_{b}could be twins. Figure 3b presents the highlighted (111) pole figure of the retained austenite in Figure 3a. It is apparent that the prior γ grains I

_{a}and I

_{b}are really twins and the twinning plane (twin boundary) is ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$)

_{γ}. In order to investigate the bainitic transformation of twinned austenite, the highlighted (011) pole figure and the corresponding packets of the acicular bainitic ferrite in I

_{a}and I

_{b}are displayed in Figure 3c,d, respectively. Note that the data were rotated. In Figure 3c, the poles of the acicular bainitic ferrite in I

_{a}and I

_{b}are pigmented in seven colors (green, violet, gold, red, pink, brown, and blue) with each color representing one packet. Clearly, there are seven packets in I

_{a}and I

_{b}. Further, we can see that there are two sets of pole figures with each similar to the calculated one, and the “violet” rosette (circled in Figure 3c) is shared by them. The shared rosette (shared packet) make the total number of the packets in I

_{a}and I

_{b}be one less than that it should be (eight), being the same as the previous results [24,36]. As mentioned above, each rosette of poles is concentrated by the poles of one particular packet and the six variants in one packet share a {011}

_{α}(i.e., {111}

_{γ}) plane. The “violet” rosette means that the six variants in the “violet” packet share the (011)

_{α}plane (violet color corresponds to the packet of P2 listed in Table 1). Therefore, the two sets of poles share the (011)

_{α}(i.e., ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$)

_{γ}) plane. The colors in Figure 3d correspond to those in Figure 3c. From Figure 3d, it can be seen that the three packets highlighted in green, gold, and red lie in I

_{a}, and the three packets highlighted in pink, brown, and blue lie in I

_{b}. However, the “violet” packet lies both in I

_{a}and I

_{b}.

#### 3.2. Ferrite/Austenite Boundaries

_{α}. For example, the misorientation angle of GT7-GT12 in the “violet” packet (P2) is 60°, but the twin misorientation between them is 54.3°/[011]

_{α}.

_{a}. Here, we investigated the boundaries only based on misorientation larger than 3.5° because there is small misorientation less than 3.5° in acicular bainitic ferrite plates (or laths). From Figure 4, we can see that the length of red and black boundaries are 0.08 and 0.78 mm, respectively. This means that the boundary ratio of group 1 to group 2 + group 3 is about 1:10, which is far less than the ratio of the variant pairs in group 1 to the ones in group 2 + group 3 (1:4).

## 4. Discussion

_{α}(i.e., {111}

_{γ}) plane. Furthermore, the four packets interpenetrate and each is further divided into several disconnected areas in the two-dimension cross section. The three Bain variant pairs in one packet have significantly distinct orientations (a mean rotation of 120° around the common ${\langle 011\rangle}_{\mathsf{\alpha}}$ axis), while the orientation angles between the variants sharing a ${\langle 001\rangle}_{\mathsf{\gamma}}$ axis are less than 6°.

## 5. Conclusions

_{α}(i.e., {111}

_{γ}) plane. When two γ grains are twinned, the twins share a {111}

_{γ}plane and have seven packets. In a packet, the adjacent acicular bainitic ferrite plates (or laths) which do not share a ${\langle 001\rangle}_{\mathsf{\gamma}}$ axis have two high misorientation angles of ~54.3° and ~60.0°. The adjacent acicular bainitic ferrite plates (or laths) sharing a ${\langle 001\rangle}_{\mathsf{\gamma}}$ axis have small misorientation of about 5.7°, but most of the included angle between them is about 30°. Further, the low angle boundary to high angle boundary ratio is far less than the number percentage of the variant pairs with small misorientation to the ones with large misorientation. These results showed that the adjacent acicular bainitic ferrite plates or laths could be considered as the basic element of austempered ductile iron.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**(

**a**) Inverse pole figure (IPF) orientation map combined with image quality (IQ) map of ausferrite; (

**b**) IPF orientation map combined with IQ map of the acicular bainitic ferrite in (

**a**); (

**c**) IPF orientation map combined with IQ map of the retained austenite in (

**a**); (

**d**) phase map of the ausferrite in (

**a**). Prior γ grain I

_{a}and I

_{b}is labeled in (

**a**). The α grain circled in (

**b**) crossing the straight one (the twin boundaries).

**Figure 2.**Gray-scale (011) pole figure of the acicular bainitic ferrite in prior γ grain I

_{a}(labeled in Figure 1a) compared with the ideal pole figure for (

**a**) the G–T relationship and (

**b**) the K–S relationship, (

**c**) (011) pole figure of the acicular bainitic ferrite in γ grain I

_{a}colored to show the packets, and (

**d**) corresponding packet map of the acicular bainitic ferrite. Black lines in (

**d**) show the boundaries with misorientation angles larger than 10°. Each color stands for one packet.

**Figure 3.**(

**a**) Highlighted orientation map of the retained austenite in prior γ grains I

_{a}and I

_{b}, (

**b**) (111) pole figure of the retained austenite in (

**a**) colored to show the twins, (

**c**) (011) pole figure of the acicular bainitic ferrite in γ grains I

_{a}and I

_{b}colored to show the packets, (

**d**) corresponding packet map of the acicular bainitic ferrite. Black lines in (

**d**) show the boundaries with misorientation angles larger than 10°. Each color stands for one packet.

**Figure 4.**(

**a**) (011) pole figure colored to show the acicular-ferrite orientations of the “green” packet (P1) in Figure 2b, (

**b**) highlighted acicular-ferrite structure of the “green” packet (P1), (

**c**) (011) pole figure colored to show the acicular-ferrite orientations of the “violet” packet (P2), (

**d**) highlighted acicular-ferrite of the “violet” packet (P2). The variant names are further abbreviated as number, e.g., GT1 is abbreviated as 1. The B1, B2, B3, B4, B5, and B6 are the Bain variant pairs in (

**a**,

**c**). The lines from C to D in (

**b**) and from E to F in (

**d**) show the paths of point-to-point misorientation in Figure 5. The included angles between the adjacent acicular bainitic ferrite plates in the red circle of (

**b**) and blue circle of (

**d**) are about 30°.

**Table 1.**The 24 G–T variants keeping (011)

_{α}parallel to ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$)

_{γ}or ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$ )

_{γ}.

G–T Variant | {111}_{γ} | {011}_{α} | ${\langle 51217\rangle}_{\mathsf{\gamma}}$ | ${\langle 71717\rangle}_{\mathsf{\alpha}}$ | Bain Variant | Packet |
---|---|---|---|---|---|---|

GT1 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{17}\text{\hspace{0.17em}}\overline{5}\text{\hspace{0.17em}}12$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | X | P1 |

GT2 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{17}\text{\hspace{0.17em}}\overline{12}\text{\hspace{0.17em}}5$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | X | |

GT3 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$12\text{\hspace{0.17em}}17\text{\hspace{0.17em}}5$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Y | |

GT4 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$5\text{\hspace{0.17em}}17\text{\hspace{0.17em}}12$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Y | |

GT5 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$5\text{\hspace{0.17em}}\overline{12}\text{\hspace{0.17em}}\overline{17}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Z | |

GT6 | ($1\text{\hspace{0.17em}}\overline{1}\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$12\text{\hspace{0.17em}}\overline{5}\text{\hspace{0.17em}}\overline{17}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Z | |

GT7 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{17}\text{\hspace{0.17em}}5\text{\hspace{0.17em}}\overline{12}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | X | P2 |

GT8 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{17}\text{\hspace{0.17em}}12\text{\hspace{0.17em}}\overline{5}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | X | |

GT9 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$12\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}\overline{5}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Y | |

GT10 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$5\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}\overline{12}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Y | |

GT11 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$5\text{\hspace{0.17em}}12\text{\hspace{0.17em}}17$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Z | |

GT12 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}\overline{1}$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$12\text{\hspace{0.17em}}5\text{\hspace{0.17em}}17$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Z | |

GT13 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$17\text{\hspace{0.17em}}5\text{\hspace{0.17em}}12$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | X | P3 |

GT14 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$17\text{\hspace{0.17em}}12\text{\hspace{0.17em}}5$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | X | |

GT15 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{12}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}5$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Y | |

GT16 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{5}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}12$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Y | |

GT17 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{5}\text{\hspace{0.17em}}12\text{\hspace{0.17em}}\overline{17}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Z | |

GT18 | ($\overline{1}\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{12}\text{\hspace{0.17em}}5\text{\hspace{0.17em}}\overline{17}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Z | |

GT19 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$17\text{\hspace{0.17em}}\overline{5}\text{\hspace{0.17em}}\overline{12}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | X | P4 |

GT20 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$17\text{\hspace{0.17em}}\overline{12}\text{\hspace{0.17em}}\overline{5}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | X | |

GT21 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{12}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{5}$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Y | |

GT22 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{5}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{12}$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Y | |

GT23 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{5}\text{\hspace{0.17em}}\overline{12}\text{\hspace{0.17em}}17$] | [$\overline{7}\text{\hspace{0.17em}}17\text{\hspace{0.17em}}\overline{17}$] | Z | |

GT24 | ($1\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | ($0\text{\hspace{0.17em}}1\text{\hspace{0.17em}}1$) | [$\overline{12}\text{\hspace{0.17em}}\overline{5}\text{\hspace{0.17em}}17$] | [$\overline{7}\text{\hspace{0.17em}}\overline{17}\text{\hspace{0.17em}}17$] | Z |

**Table 2.**The misorientation angles between adjacent variants in the “violet” packet (P2) and the number percentage of the variant pairs in the packet.

Group | Variant Pair | Misorientation Angle | Number Percentage |
---|---|---|---|

1 | GT7-GT8, GT9-GT10, GT11-GT12 | 5.7° | 20% |

2 | GT7-GT10, GT8-GT11, GT9-GT12 | 54.3° | 20% |

3 | GT7-GT9, GT7-GT11, GT7-GT12 | 60° | 60% |

GT8-GT9, GT8-GT10, GT8-GT12 | |||

GT9-GT11, GT10-GT11, GT10-GT12 |

© 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/).

## Share and Cite

**MDPI and ACS Style**

Wang, C.; Du, X.; Li, S.; Sun, Y.; Yang, P.
Morphology and Crystallography of Ausferrite in Austempered Ductile Iron. *Metals* **2017**, *7*, 238.
https://doi.org/10.3390/met7070238

**AMA Style**

Wang C, Du X, Li S, Sun Y, Yang P.
Morphology and Crystallography of Ausferrite in Austempered Ductile Iron. *Metals*. 2017; 7(7):238.
https://doi.org/10.3390/met7070238

**Chicago/Turabian Style**

Wang, Chengduo, Xueshan Du, Songjie Li, Yufu Sun, and Peixu Yang.
2017. "Morphology and Crystallography of Ausferrite in Austempered Ductile Iron" *Metals* 7, no. 7: 238.
https://doi.org/10.3390/met7070238